Methods and apparatus for moving and separating materials exhibiting different physical properties

Methods and apparatus for controlling the movement of materials having different physical properties when one of the materials is a fluid. The invention does not rely on flocculation, sedimentation, centrifugation, the buoyancy of the materials, or any other gravity dependent characteristic, in order to achieve its desired results. The methods of the present invention provide that a first acoustic wave is progpagated through a vessel containing the materials. A second acoustic wave, at a frequency different than the first acoustic wave, is also propagated through the vessel so that the two acoustic waves are superimposed upon each other. The superimposition of the two waves creates a beat frequency wave. The beat frequency wave comprises pressure gradients dividing regions of maximum and minimum pressure. The pressure gradients and the regions of maximum and minimum pressure move through space and time at a group velocity. The moving pressure gradients and regions of maximum and minimum pressure act upon the marterials so as to move one of the materials towards a predetermined location in the vessel. The present invention provides that the materials may be controllably moved toward a location, aggreated at a particular location, or physically separated from each other.

BACKGROUND 
1. The Field of the Invention 
The present invention is related to methods and apparatus for controlling 
the movement of materials exhibiting different physical properties by the 
application of acoustical energy to the materials. More particularly, the 
present invention is directed to methods and apparatus capable of 
continuously separating various materials from a fluid flow system when 
the materials exhibit physical properties, such as acoustical properties, 
different than the fluid medium. 
2. The Prior Art 
Numerous fields of modern technology require that materials which are being 
carried by a fluid system be separated from the liquid. For example, many 
industrial processes generate waste water which is contaminated by 
particulate matter. Separation of the particulate matter from the fluid 
allows the water to be easily disposed of and the particulate matter, if 
valuable, put to a good use. Furthermore, it is often desirable to 
separate an immiscible liquid or undissolved gas from a liquid. 
The number of occasions in which it is necessary to separate particulates 
from a fluid medium is so pervasive that an extraordinary amount of 
attention has been devoted to the development of methods and apparatus to 
effect such separations. 
One of the most rudimentary, yet pervasive, of separation techniques 
involves simple sedimentation. Sedimentation is the natural settling 
process wherein the particulates, gas bubbles, or immiscible liquids are 
separated due to gravitational force. The medium may then be removed by 
decanting or suction, while taking care not to disturb the particulates 
which have settled out of the medium. 
Sedimentation techniques have the advantage of being simple and 
inexpensive. Unfortunately, the characteristics of the medium and the 
particles to be separated are often such that the time required for 
separation by sedimentation can be so long as to make this technique 
entirely impractical. Furthermore, if the particles are of a very small 
size, the particles will never "settle out" due to the Brownian motion of 
the molecules. Still further, if the carrying liquid is not kept free of 
any turbulence until sedimentation is complete, the particles will become 
resuspended. As a result, simple sedimentation techniques are practical 
only in certain limited situations. 
In recognition of the fact that gravitational forces are too weak to effect 
rapid sedimentation in many instances, a frequent approach utilized in the 
prior art in order to increase the sedimentation rate of the material is 
to increase the gravitational force. This may be accomplished by 
subjecting the particle and medium mixture to centrifugation. 
Centrifugation is a technique in which a container holding the particle and 
medium mixture is spun about a central axis in order to create centrifugal 
forces extending radially from the central axis. Increasing the speed of 
rotation will increase the centrifugal force applied to suspended 
particles, thereby increasing the rate of sedimentation. Modern 
centrifuges are capable of generating forces many thousands of times 
greater than gravity. 
Yet another general technique used to separate some types of particles from 
a medium is filtration. Filtration involves the use of a porous filter 
that allows passage of the medium, while forming a barrier to the 
particles to be separated out. The speed of filtration can be enhanced by 
the application of pressure. However, the speed of filtration markedly 
decreases as a layer of filtered material builds up against the filter. 
For optimum performance, the filter must be replaced or cleaned 
frequently. 
Each of the foregoing techniques is widely practiced and is extremely 
useful in many applications. Yet, each technique suffers significant 
drawbacks which limits its application to many situations. 
For example, as mentioned above, gravitational sedimentation is not 
effective in many instances when the particles or the medium exhibit 
particular characteristics, such as when the medium is extremely viscous. 
Although centrifugation often speeds up the process of separation in such 
cases, centrifugation is often not completely effective; moreover, 
centrifugation is ill suited either for processing large quantities of a 
medium and particle mixture or for processing in continuous flow systems. 
Filter techniques also suffer ineffectiveness when the particles to be 
separated from the medium begin to significantly build up on the filter. 
This build-up, or "caking", reduces the efficiency of the filter; at some 
point in the filtration process, this caking may completely stop the flow 
of the medium through the filter. If additional pressure is applied to the 
medium in order to improve the flow through the filter, damage to some 
types of separated material, e.g., blood cells, may occur. 
Furthermore, filtration is generally ineffective when separating two 
immiscible liquids or when separating undissolved gases from a liquid. 
Some additional shortcomings of these traditional approaches may be better 
appreciated by reference to certain specific applications. 
One area in which it is important to separate particles from a medium is in 
the medical arts. Numerous medical treatments and diagnostic tests, for 
example, require that blood (or other body fluids) be separated into their 
particulate and liquid components. Centrifugation has long been used for 
processing small amounts of blood in test tube sized containers. Such 
containers are typically filled with blood and placed in a small 
centrifuge, and then spun so that the blood cells accumulate in one 
portion of the container, leaving plasma in the upper portion of the 
container. The plasma is then decanted or suctioned off. 
It will be readily appreciated that the use of test tube-sized containers 
is not very practical when a large amount of blood is to be separated into 
its plasma and cellular components. Yet, several medical procedures 
require separation of substantial volumes of blood into the cellular and 
plasma components. 
One such procedure, generally known as "plasma phoresis", involves 
replacement of most of a patient's plasma with donor plasma or other 
suitable plasma substitute. This procedure involves removing whole blood 
from a patient, separating the cellular components from the plasma, 
discarding the plasma, and resuspending the cellular components in donor 
plasma. The reconstituted blood is then returned to the patient. Plasma 
exchange therapy has been successfully used to treat a variety of clinical 
conditions such as toxemias, drug overdoses, certain types of cancer, 
rheumatoid arthritis, and disseminated intravascular coagulation. 
One attempt to improve the usefulness of centrifugation for use in plasma 
phoresis has lead to the development of continuous flow centrifuges. 
Unfortunately, continuous flow centrifuge processes also have serious 
drawbacks. 
For example, the equipment necessary to perform continuous centrifugation 
is large, bulky, and also relatively expensive. Further, continuous 
centrifuges require relatively large volumes of blood to operate properly, 
and blood passing therethrough has a substantial residence time. This 
characteristic, in turn, mean that the patient must either do without a 
substantial volume of blood for an extended period of time, or must be 
provided with a whole blood substitute. Use of a whole blood substitute 
dilutes the patient's blood, and thus partially negates the aim of plasma 
phoresis to replace plasma, but not to replace the cellular components of 
the patient's blood. 
Yet another disadvantage when using centrifugation to separate plasma from 
cellular blood components is that centrifugation causes the cellular 
components to become very tightly packed which may in itself cause damage 
to the blood cells. Subsequent reconstitution to whole blood by the 
addition of donor plasma is difficult to accomplish without causing 
hemolysis (i.e., damage) of the relatively delicate red blood cells. In 
any procedure in which biological materials are to be separated for reuse, 
extreme care must be taken so that the biological materials to be 
separated are not damaged by the process. 
Another example of an area in which it is commonly important to separate 
another material from a medium involves petroleum-based materials. 
Oftentimes, a petroleum based product, hereinafter generally referred to 
as "oil," will be introduced into water during a processing step. 
For example, in order to retrieve the maximum amount of oil possible from a 
particular amount of oil shale (rock having a high oil content), high 
temperature steam will be applied to the shale so as to extract the oil 
out from the nonpetroleum substances. 
After the process is completed, the condensed steam contains a significant 
percentage of the oil that has been extracted from the oil shade. Since 
oil and water are immiscible, these liquids might be separated by the use 
of sedimentation or centrifugation. However, the same difficulties that 
were mentioned above are compounded when sedimentation or centrifugation 
are used to separate two immiscible liquids. 
Another example of an area in which there is a need to separate material 
from the medium is liquid purification. Many times a liquid must be 
"purified" before it is used. While many applications do not require a 
degree of purification that is available when distillation purification 
procedures are used, many applications require that a significant amount 
of particulate matter be removed from the liquid. 
In many applications, this particulate matter will be microscopic-sized 
particles of dirt. Removal of these dirt particles by sedimentation is 
impractical for the reasons mentioned earlier. 
Filtration techniques are often used to remove such microscopic sized 
particles of dirt. However, the use of conventional filters to remove 
particles requires that, as mentioned above, the filter be replaced or 
cleaned as the particles build up on the filter media. Removal or cleaning 
of filters is often a time-consuming procedure requiring that the 
processing of the fluid be discontinued. 
Because of the limitations of conventional techniques for separating 
particles from a medium, a great deal of effort has been directed to 
developing new techniques as well as improving the conventional 
techniques. One technique of relatively recent origin is shown in U.S. 
Pat. No. 4,055,491 issued to Porath-Furedi. 
According to the Porath-Furedi patent, ultrasonic standing waves are used 
to cause flocculation of small particles, such as blood or algae, so that 
they will settle out of the carrying liquid. The Porath-Furedi patent 
describes a separation process which submerges an ultrasonic wave 
generator within a liquid having particles suspended therein and 
energizing it so that standing wave is established. 
The establishment of a standing wave in the medium results in formation of 
pressure nodes to which the particles tend to migrate; these nodes and 
antinodes are at right angles to the direction of propagation of the 
ultrasonic waves, and the nodes are spaced from adjacent nodes by a 
distance equal to one-half of the wavelength of the ultrasonic wave. 
The Porath-Furedi patent utilizes the accumulation of solid particles at 
the nodes or antinodes to cause flocculation, thereby assisting in simple 
gravitational sedimentation of the suspended particles when the ultrasonic 
standing wave is discontinued. 
While the use of ultrasonic waves to flocculate particles as disclosed by 
the Porath-Furedi patent does substantially increase the sedimentation 
rate of those particles, the process is still quite slow. It also appears 
that the Porath-Furedi process is limited to intermittent flow "batch" 
operations. In particular, this process would not be practical in a high 
volume, or relatively rapid flow, process because of the extended 
residence time in the device that would be required to remove all of the 
particulate matter. 
A variation of the Porath-Furedi process appears in U.S. Pat. No. 4,398,925 
to Trinh et al. relating to the removal of air bubbles from a liquid, such 
as molten glass. The Trinh et al. process involves application of a 
particular ultrasonic frequency capable of establishing a standing wave 
having a single pressure well at a location half way between the bottom 
and the top of the container of liquid. Bubbles suspended in the liquid 
are pushed toward the pressure well, where they coalesce to form larger 
bubbles. 
The ultrasonic wave is then interrupted so that the bubbles begin to float 
upward due to their buoyancy. After the coalesced bubbles have risen above 
the level of the pressure well, a second ultrasonic frequency is applied 
so that a second standing wave pattern is established--the second standing 
wave pattern having two pressure wells. The bubbles are then urged 
upwardly to the closest of the two pressure wells. 
The foregoing process is then repeated. After the bubbles reach the upper 
pressure well, the ultrasonic generator is switched off so that bubbles 
continue to rise above the level of that well, and then yet a third 
ultrasonic frequency is applied, this one having three pressure wells. 
Again, the bubbles will be urged toward the highest pressure well, to 
which point the process can be repeated with progressively higher 
ultrasonic frequencies. 
It will be readily appreciated that the Trinh et al. process relies on the 
buoyancy of the suspended bubbles to move the bubbles between wells during 
periods when the ultrasonic generator is switched off. Failure of the 
particles to move beyond the well will result in splitting of the 
particles and formation of multiple bands. Additionally, as with the 
Porath-Furedi process, it appears that the Trinh et al. process is 
primarily a batch process and is not well suited for use in situations 
such as plasma phoresis where a continuous supply of a medium must be 
subjected to the process. 
Ultrasonic processes also have application in other fluid processing 
situations. For example, U.S. Pat. No. 4,013,552, issued to Creuter, shows 
the use of ultrasonic energy transmitted through sewage in order to reduce 
the size of the particles in the liquid by cavitation. Such cavitation 
enhances the ability of the particles to be exposed to oxygen and thus 
accelerate the action of aerobic bacteria. (The term "cavitation" refers 
to the creation of disturbances in a fluid caused by formation of gas 
bubbles by the application of acoustic energy.) 
U.S. Pat. No. 4,346,011, issued to Brownstein, discloses a process which 
utilizes ultrasonic waves to flocculate particulate matter so as to 
prevent the particles from fouling a filter screen. The Brownstein patent, 
similar to the Creuter patent, appears to use cavitation to achieve its 
desired result. 
In view of the foregoing, it will be appreciated that it would be a 
significant advancement in the art if methods and apparatus could be 
provided which are capable of effecting movement and separation of 
particles from liquids, immiscible liquids from each other, and 
undissolved gases from a liquid, that avoided the disadvantages of the 
techniques found in the prior art. It would also be of particular 
significance if methods and apparatus could be provided which have a high 
volume throughput, a relatively short residence time, and the ability to 
effect movement and rapid separation of the particles from the medium. 
It would also be a significant improvement in the art to provide methods 
and apparatus for separating two materials without requiring physical 
contact with the materials and without causing significant damage to the 
materials, for example, blood. Furthermore, providing methods and 
apparatus for controllably moving, agitating, or separating materials of 
different physical properties, such as size or density, as well as methods 
which are adaptable to either batch mode or continuous flow systems, would 
be an important advancement in the art. 
BRIEF SUMMARY AND OBJECTS OF THE INVENTION 
The present invention is directed to methods and apparatus for moving and 
separating materials of different physical properties. Examples of such 
properties include the velocity of acoustic pressure waves through the 
materials, the size of the particles when one of the materials is 
particulate matter, and the density of the materials. The methods of the 
present invention use acoustic pressure waves in the ultrasonic region to 
aggregate materials having similar physical properties in a location where 
they may be separated from the remaining material using techniques known 
in the art. 
According to the preferred embodiments of the present invention, two 
acoustic waves having different frequencies are propagated in opposite 
directions such that the two waves are superimposed upon one another to 
form a beat frequency wave. This beat frequency wave exhibits pressure 
gradients which separate regions of pressure maxima and pressure minima. 
The materials are segregated since they tend to migrate to the regions of 
either pressure maxima or pressure minima. 
Furthermore, the pressure gradients are capable of moving materials 
suspended within a fluid medium in a predetermined direction. The movement 
of the pressure gradients is controlled such that the materials are moved 
toward a predetermined location. After materials of similar properties 
have been aggregated at the predetermined location, they may be physically 
separated from the remaining materials. 
It is, therefore, an object of the present invention to provide methods and 
apparatus capable of separating materials possessing different physical 
properties from one another. 
Another important object of the present invention is to provide methods and 
apparatus to separate materials having different physical properties from 
one another without causing damage to the materials, such as the 
separation of blood cells from plasma. 
Still another object of the present invention is to provide methods and 
apparatus which allows materials having different physical properties to 
be separated without requiring physical contact within an isolated system 
containing the materials. 
A further object of the present invention is to provide methods and 
apparatus for separating particulate matter from a fluid medium in either 
a batch mode or a continuous flow fluid system. 
A still further object of the present invention is to provide methods and 
apparatus for separating immiscible liquids from one another. 
Another object of the present invention is to provide methods and apparatus 
for moving or agitating materials of different physical properties. 
Still another object of the present invention is to provide methods and 
apparatus for separating particles of different sizes, which are contained 
within a fluid, from each other. 
These and other objects of the present invention will become apparent 
throughout the following description, taken in connection with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
A. Introduction 
In order to achieve the above-mentioned objectives, the present invention 
utilizes the propagation of acoustic pressure waves propagated through a 
medium at frequencies in the ultrasonic region. The ultrasonic region is 
generally considered to be those frequencies which are greater than 20 
kilohertz (kHz). However, the portion of the ultrasonic region that will 
be discussed herein will principally be concerned with those frequencies 
in the range from about 500 kHz to as high as about 20 megahertz (mHz). 
However, frequencies higher or lower than this range may be used according 
to the present invention with the realization that the introduction of 
cavitation, as will be explained later, is detrimental to the present 
invention. 
In the following general discussion, some of the principles of acoustics 
are presented as background for gaining an understanding of the present 
invention. Following the general discussion section, an explanation of the 
structure and operation of the various embodiments of the present 
invention is presented. Included with the explanation of the structure and 
operation of the embodiments are specific examples showing results 
obtainable when using the embodiments. When reference is made to the 
drawings, like structures in the drawings will be designated with like 
reference numerals throughout. 
As will be appreciated from the foregoing discussion, the present invention 
has application in the movement and separation of particles, immiscible 
liquids, and undissolved gases within a liquid. However, for purposes of 
clarity, the following description generally refers only to particles; 
however, it should be understood that an immiscible liquid or gas bubble 
could also be involved. Thus, when the term "particle" or "particulate" is 
used, it is not intended to be limiting, but merely representative. 
Furthermore, whenever the term "medium", or "media" is used hereinafter, it 
shall be understood to mean the fluid which is carrying the particles 
which are desired to be separated from the medium. The embodiments of the 
present invention which are disclosed herein generally use a liquid as the 
medium. Further, since the present invention includes the capability of 
separating materials of different physical properties, such as having 
different densities or sizes, from a single medium, the following 
description may refer to a denser and a lighter particle being carried in 
a particular medium. 
B. General Discussion 
As will be appreciated by those familiar with the fundamental principles of 
sound and acoustics, the most basic principles can be readily understood 
by reference to diagrams schematically showing the propagation and 
interaction of the acoustic pressure waves within a system. FIGS. 1-5 will 
now be referred to in order to explain these fundamental principles of 
sound and acoustics. 
FIGS. 1-4 show the response of acoustic pressure waves in a closed system, 
in this case a closed rectangularly shaped system, represented by 10 in 
FIGS. 1-4. Within system 10 is contained a propagation material, generally 
designated 12, through which the acoustic pressure waves are propagated. 
In FIGS. 1-4, the selected propagation material 12 is water; however, the 
principles discussed hereinafter apply to whatever material is used as the 
propagation material. 
In FIG. 1, an acoustic pressure wave 14 has been introduced into system 10 
by transducer 16 located at a first end of system 10. Transducer 16, 
converts electrical energy, generated by frequency generator 18, into 
acoustical energy. Acoustic pressure wave 14 created by transducer 16 
travels in a direction, shown by arrow 23, from the first end of the 
system, generally designated 20, to a second end of the system, generally 
designated 22. 
It will be appreciated that the sine wave representation of the acoustic 
pressure wave follows from the conventional method of showing an acoustic 
pressure wave propagating through a medium. However, it should be 
understood that the acoustic pressure wave is propagating, (i.e., 
traveling) through the system and that all of the figures schematically 
represent the acoustic pressure wave at a particular moment in time. In 
FIG. 1, the particular moment in time shown is when the leading pressure 
gradient reaches second end 22 of system 10. 
It should also be understood that the sine wave representation is meant to 
indicate areas of increased and decreased pressure within the medium. 
Also, acoustic pressure waves in a fluid such as water, are actually 
longitudinal waves, not transverse waves as are generally indicated by the 
sine waves used in the figures. This principle is best illustrated in the 
lower portion of system 10 which is separated by the dashed box marked 24. 
In dashed box 24, areas of increased pressure are represented by denser 
stipling, while areas of decreased pressure are represented by lighter 
stipling. The dots of the stipling are generally representative of the 
molecules of the medium and their spacing relative one to another. 
Within the medium, various particles, gas bubbles, or droplets of an 
immiscible liquid, are represented by circles and triangles. Particles 
possessing a density greater than that of the medium are represented by 
circles, a few of which are designated 26, and particles possessing a 
density less than that of the medium are represented by triangles, a few 
of which are designated 28. 
FIG. 1 shows the acoustic pressure wave 14 having propagated across the 
length of system 10. The length of system 10, as shown in FIGS. 1-4 by 
line 30, is two wavelengths long. The wavelength of a particular frequency 
is related to the speed at which the acoustic pressure wave 14 propagates 
through the medium. In the present example, the medium is water which 
exhibits a longitudinal wave velocity of about 1480 meters per second. The 
wavelength is related to the frequency and the wave velocity through the 
medium by Equation A, as set forth below: 
##EQU1## 
Where: .lambda.=Wavelength 
c=Velocity of the wave in the medium 
f=Frequency of the wave 
Thus, if the frequency of the wave propagated through the medium of FIG. 1 
is 3 mHz, then the wavelength is about 493 microns and the length of the 
system, designated by line 30, is about 986 microns. 
Reference will now be made to FIG. 2. In FIG. 2, system length 30 is the 
same as in FIG. 1. At the second end 22 of the system 10 is placed a 
surface 32 which reflects a high percentage of acoustic pressure wave 14 
incident upon it. Since the length of the system 10 is an even multiple of 
wavelengths long, the reflected wave, shown by dashed sine wave 34, 
interferes with incident wave 14 to create a standing wave. 
FIG. 2 shows reflected wave 34, superimposed upon incident wave 14. By 
creating a standing wave, areas of maximum pressure, commonly referred to 
as antinodes and generally designated 36, and areas of minimum pressure, 
commonly referred to as nodes and generally designated 38, are formed. The 
graph located at the bottom of FIG. 2 represents the root mean square 
pressure distribution with system 10. The vertical axis represents 
pressure levels which correspond to the standing wave within system 10. 
The wave form on the graph represents the pressure gradients within the 
chamber. The pressure gradient representation shown in the graph will be 
used several times in the following description of the embodiments of the 
present invention, rather than the sine wave representation. 
As shown in FIG. 2, denser particles 26 and lighter particles 28, which 
were scattered throughout system 10 in FIG. 1, migrate to nodes 38 and 
antinodes 36, respectively. Generally stated another way, the denser 
particles move to regions of pressure minima while lighter particles move 
to regions of pressure maxima, as shown by the graph. In this fashion, 
particles of dissimilar physical properties, such as particles of 
dissimilar densities, may be segregated from the medium whose density is 
intermediate between the densities of lighter and denser particles. 
With the particles segregated as shown in FIG. 2, it is possible to 
separate the particles from the medium. Indeed, various attempts in the 
prior art have been made to do so by way of providing structures that 
cause the physical separation of the particles and the medium after the 
segregation shown in FIG. 2 has occurred. 
Unfortunately, such methods of physical separation as shown in the prior 
art do not lend themselves to allowing significant volumes of media to be 
quickly processed. However, the discussion that follows will explain how 
the present invention allows for extremely efficient aggregation and 
separation of particles from a medium. 
The regions between the nodes and antinodes may best be described as 
pressure gradients, i.e., areas in which the pressure changes over a 
specified distance. In the standing wave, as shown in FIG. 2, the pressure 
gradients shown in the graph do not move. These stationary pressure 
gradients are an inherent characteristic of a standing wave. The present 
invention, while not using standing waves, uses a similar and related 
phenomenon, as well as exploiting additional principles, to move, 
aggregate, and separate particles from the medium as will now be 
explained. 
In FIG. 3, incident acoustic pressure wave 40, created by transducer 16 is 
of a first frequency. The reflected wave 42 has been altered so that its 
frequency, a second frequency, is slightly different than the first 
frequency. Methods in which the frequency of the reflected wave may be 
slightly shifted from the frequency of the incident wave will be explained 
later in this disclosure. For the present, this analysis assumes that the 
reflected wave has been shifted in frequency. When the two waves are 
superimposed, as shown in system 10, a beat frequency wave 44 may be 
observed as shown in FIG. 3. 
As can be seen by the representation in FIG. 3, the resultant beat 
frequency is expressed by Equation B: 
##EQU2## 
Where: F.sub.Beat =Frequency of the Beat Wave 
F.sub.inc =Frequency of the Incident Wave 
F.sub.ref =Frequency of the Reflected Wave 
As shown by the dashed line outlining what is termed the envelope of beat 
frequency 46, the envelope of the beat frequency forms a wave pattern 
similar to that as with a standing wave. As explained in connection with 
FIG. 2, it must be appreciated that envelope 46 of the beat frequency 
represents areas of decreased and increased pressure in the medium as 
represented in dashed box 24 of FIG. 1. Furthermore, as mentioned in 
connection with the analysis of the standing wave, the beat frequency must 
actually be thought of as plane wave whose pressure gradients propagate 
through the medium parallel to the face of transducer 16. For ease of 
reference, and for clarity, the beat frequency wave form 46, as shown in 
FIG. 3, is hereinafter sometimes referred to as a "pseudo-standing wave". 
The term "pseudo-standing wave" is adopted because of the similarity 
between the wave form of the envelope of the beat frequency wave and wave 
form of the "true" standing wave. Likewise, the node and antinode regions 
of the pseudo-standing wave envelope are termed "pseudo-nodes" and 
"pseudo-antinodes," as generally indicated at 48 and 50 in FIG. 3, 
respectively. 
It should be appreciated that even though FIG. 3, as well as FIG. 4, show 
that system 10 is resonant at all the wavelengths represented, the present 
invention does not require that system 10 be resonant. In fact, the 
majority of the embodiments disclosed herein operate principally at 
frequencies which do not resonate when used with the embodiments 
illustrated herein. 
Reference will now be made to FIG. 4 to explain how the present invention 
effects the movement of the particles. FIG. 4 shows a pseudo-standing wave 
46. The wavelength of the acoustic pressure waves, (only one wavelength of 
the envelope of the beat wave is represented in the system 10 of FIG. 4) 
which are superimposed upon each other to form the pseudo-standing wave, 
are much shorter than those shown in FIGS. 1, 2, and 3. One portion of 
FIG. 4 shows the acoustic pressure wave in much greater detail. The root 
mean square pressure gradients formed by the superposition of the acoustic 
pressure waves is represented in the lower portion of FIG. 4, just as the 
pressure gradients are shown in the lower portion of FIG. 2. 
The enlarged portion of FIG. 4 shows the pressure gradients, with their 
associated nodes 38 and antinodes 36, as discussed in connection with FIG. 
2. Also, the enlarged portion of FIG. 4 shows denser particles 26 and 
lighter particles 28 having migrated to nodes 38 and antinodes 36, 
respectively. 
As the particles segregate as shown in FIG. 4, at nodes 38 and antinodes 36 
of the pressure gradients, the segregated particles move as a group in the 
direction indicated by arrow 52. This is due to the fact that 
pseudo-standing wave 46 moves through time and space. Stated another way, 
the pressure gradients shown in the enlarged portion of FIG. 4, if viewed 
in real time, would appear to move in the direction indicated by arrow 52. 
Furthermore, it is not necessary that a pressure gradient move only in the 
direction indicated by arrow 52. It is possible to cause the pressure 
gradients to move in the opposite direction to that shown by arrow 52. How 
this is accomplished will become clearer later in this description. 
The movement and velocity of the pressure gradients of the acoustic 
pressure wave is termed the "group velocity," which will be mathematically 
described later in this description. It will be appreciated that, even 
though for simplicity of analysis, the preceding discussion refers to the 
migration or movement of particles to nodes 38 and antinodes 36, such 
migration or movement is not required by the present invention to separate 
the particles, as contrasted by some schemes presented in the prior art. 
However, if movement of the particles is all that is desired, such as when 
particles are being agitated or circulated in a liquid, movement of the 
particles to nodes 38 and antinodes 36 is not required, but only that the 
particles make some motion towards the pressure minima or pressure maxima. 
Also, since nodes 38 and antinodes 36 merely represent the minima and 
maxima of the pressure gradients which move through space and time, as 
explained above, there is no need for "waiting" for particles 26 and 28 to 
migrate to nodes 38 and antinodes 36, but the moving pressure gradients 
will move towards a particle and "collect" it into a node or antinode. 
This phenomenon causes particles 26 and 28 to be moved or "swept" along 
until they are halted in their motion by a structure or until the acoustic 
pressure wave dissipates. 
While the above description generally explains the methods of the present 
invention, it will also be appreciated that additional forces such as the 
Stokes viscous drag force and the Bjerkness force, may effect the movement 
of the particles. For example, the Stokes viscous drag force effects the 
available choices for a group velocity. If the group velocity is too fast, 
the pressure exerted on the particles by the pressure gradients will not 
overcome the Stokes force and no, or very little, particle movement will 
result. Alternatively, the group velocity must be fast enough so that the 
movement of the particles occurs in a reasonable period of time. 
The Stokes force may be utilized to assist the separation process. The 
action of the moving pressure gradients on a particle is an exponential 
function related to the radius of the particle. The Stokes force is a 
linear function. Thus, because of the Stokes force, two particles 
possessing, for example, the same density, but of different radii, will 
generally be subject to different forces if acted upon by the same 
pressure gradients in the same medium. Making use of this characteristic, 
it is possible to effectuate the separation of particles from each other 
when the particles only differ in the volume that they occupy. 
As the action of the pressure gradients upon the particles continues, 
particles 26 and 28 will be swept in direction 52 until they strike the 
exposed transducer face 56 where they will aggregate. It will be 
appreciated that the separation of particles 26 and 28 from the medium is 
greatly simplified using the above-described procedure as compared to 
methods used in the prior art wherein the particles were left in the 
arrangement as shown in FIG. 2 above. 
An important consideration in the method of the present invention is the 
avoidance of cavitation in the propagating material or in the medium. As 
mentioned earlier, cavitation is the creation of disturbances in the 
medium due to the formation of gas bubbles caused by the application of 
acoustic pressure waves. It will be appreciated that cavitation in either 
the propagating material or the medium is counter productive to the 
objective of the present invention. Cavitation in either the the 
propagating material or medium disrupts the propagation of acoustic 
pressure waves as well as causing turbulence within the medium, thus being 
counter productive to any segregation or aggregation which has taken place 
in the medium. 
Since the majority of the applications of the presently preferred 
embodiments of the present invention deal with propagating materials and 
mediums which are mainly water, FIG. 5 has been included so as to indicate 
the approximate cavitation threshold of water. It will be appreciated that 
similar cavitation thresholds may be obtained for different propagating 
materials and media. 
As can be seen from the chart of FIG. 5, gas-free water exhibits a 
cavitation threshold generally higher than, at least at frequencies below 
1 mHz, aerated water. Since the introduction of cavitation is counter 
productive to the method of the present invention, a frequency and power 
level must be chosen so as to avoid the introduction of cavitation. 
Choosing frequencies and power levels well below the thresholds indicated 
in FIG. 5 are preferred. 
An additional concern when choosing an operating frequency and power level, 
however, must be avoiding damage to, or the destruction of, the materials 
to be separated. This concern is especially applicable when subjecting 
biological materials to the method of the present invention. Damage to 
biological materials may be caused by physical deformation, increased 
temperatures, or several other possible effects of ultrasonic acoustic 
pressure waves. Generally, frequencies in the range of from about 1 mHz to 
about 10 mHz may be used in the present invention, however, for most 
applications frequencies in the range from about 2 mHz to about 3 mHz are 
preferred. 
Having explained the fundamental principles which allow the present 
invention to operate so efficiently, several representative examples of 
the specific embodiments in which the present invention is incorporated, 
and examples of their use, will be explained. 
C. Variable Frequency Transducer Embodiment 
The variable frequency transducer embodiment of the present invention is 
shown in the perspective view of FIG. 6. The embodiment illustrated in 
FIG. 6 generally comprises a propagation chamber 102. The interior 
dimensions of the chamber, which must be considered in the present 
invention, are its width, designated by line 104, its height, designated 
by line 106, and its length, designated by line 108 in FIG. 6. 
Length 108 of propagation chamber 102, while not critical to the present 
invention, must be within reasonable limits in the present embodiment. The 
reason the length is not critical in the embodiment illustrated in FIG. 6 
is that the embodiment is not intended to operate in a resonant mode, that 
is, the embodiment is not intended to create standing waves. 
However, the fact that the embodiment illustrated in FIG. 6 will operate 
through a range of frequencies which may include a frequency which will 
resonate at the length of propagation chamber 102 that is chosen, requires 
that transducer 110 used to create the acoustic pressure waves be capable 
of providing a range of acoustic power to propagation chamber 102. This is 
because transducer 110 will "see" the acoustic impedance of the chamber 
change as the input frequency changes. 
Later in this disclosure embodiments will be described using the present 
invention which overcome the requirement of using broad range transducers, 
which are characteristically less efficient, i.e., less acoustic output 
power per unit of electrical input power, than transducers which have an 
output variable over a very limited range of frequencies. The transducer 
as used in the embodiment illustrated in FIG. 6, and the embodiments 
illustrated in FIGS. 7-10, is preferably a piezoelectric transducer 10. 
However, other types of transducers could be used. 
Height 106 of propagation chamber 102 shown in FIG. 6, is dependent upon 
the length of time that the particulate and medium mixture are to be 
subjected to ultrasonic treatment. If the height 106 of propagation 
chamber 102 is increased, then the particle and medium mixture may be 
subjected to the action of the ultrasonic waves for a longer period of 
time, even though the feed flow rate remains constant. This period of time 
is termed the "time in residence" or "residence time." 
Generally, the longer the residence time, the greater the likelihood that 
complete aggregation and separation will occur. However, extending 
residence time indefinitely is not practical, and a balance must be struck 
between obtaining practical residence time durations and obtaining the 
desired separation. Other means of varying the residence time will be 
explained later. 
Width 104 of propagation chamber 102, as well as height 106, are limited by 
the dimensions of available transducers suitable for use with the 
embodiment. Since a wave front is preferably to be propagated as a plane 
wave through propagation chamber 102 (that is, the wave front ideally 
should present equal pressure at all points in any plane which is parallel 
to the ends of chamber 102) transducer 110 must be of a size so as to 
create a uniform pressure wave across the width of propagation chamber 
102. 
Alternatively, a transducer which is not capable of producing a uniform 
plane wave may be used with a corresponding reduction in efficiency. 
However, multiple transducers could be mounted so as to present a single 
transmitting surface, with all of the transducers driven in phase, if 
greater transducer surface area were desired to increase residence time. 
Transducer 110 is mounted onto propagation chamber 102 at a first end 112. 
A quarter wave acoustic impedance matching section 114 is disposed between 
transducer 110 and propagation chamber 102. Impedance matching section 114 
acts to match the impedance of transducer 110 to the propagating material, 
generally designated 116, contained within the chamber 102. While 
impedance matching section 114 is not required in the embodiment shown in 
FIG. 6, the operation of transducer 110 may in some cases be improved if 
impedance matching section 114 is used. Impedance matching sections may 
also be used with the other embodiments described herein. 
The propagating material of the embodiment illustrated in FIG. 6 is water. 
However, other propagating materials, such as oils, glass, or alcohol, 
could be used as particular conditions require. It will be appreciated 
that the propagation material acoustic impedance will preferably match the 
impedance of the medium and the structure containing the medium for 
maximum efficiency. However, an exact match is not required. The important 
property of propagating material 116 is that the acoustic pressure waves 
created by transducer 110 and coupled to propagating material 116 by 
impedance matching section 114 be carried accurately by the propagating 
material and that little cavitation will be induced. 
In the embodiment illustrated in FIG. 6, the transducer is excited by 
variable frequency generator 118 whose signal is amplified to the 
necessary power levels by power amplifier 120. The particular frequencies 
used and the operation of the generator, transducer, and the embodiment 
illustrated in FIG. 6, will be explained later in this section. Wires 122 
carry the electrical signal to transducer 110. 
At a second end 123 of propagation chamber 102 is placed a reflector 124. 
The reflector is constructed so as to absorb very little of the incident 
acoustic pressure wave. For example, where a zinc reflector is used, it 
will reflect about 97% of the sound pressure wave incident upon it. 
Reflector 124 is precisely constructed so that the reflected wave is 
propagated back in the direction of the incident wave. 
In the embodiment shown in FIG. 6, a separation vessel 126 is inserted 
through propagation chamber 102. Separation vessel 126 preferably extends 
the entire width, along line 104, of propagation chamber 102, and divides 
propagation chamber 102 into two separate cavities. In the illustrated 
embodiment, it is preferable that the separation vessel be located about 
halfway between the ends of propagation chamber 102. However, separation 
vessel 126 may be located either closer to propagation chamber first end 
112 or second end 123 if desired as will become clear later. 
The walls 130 and 132 of separation vessel 102 should be impermeable to the 
propagating material and medium, but transparent to the acoustic pressure 
waves traveling through propagation chamber 102 at the frequencies at 
which transducer 110 is operated. A medium and particle mixture is 
introduced into separation vessel 126 at a first end 134 of separation 
chamber 126, as indicated by arrow 136. 
The mixture is introduced into separation vessel 126 at a predetermined 
flow rate. Since the flow rate partially determines the residence time, 
the flow rate is one factor which determines the percentage of separation 
which will be achieved using the embodiments of the present invention 
illustrated in the figures. 
When separation vessel 126 is filled with a mixture of medium and 
particles, acoustic pressure waves created by transducer 110 will travel 
from propagation chamber first end 112 through separation vessel wall 130 
and the medium, through separation chamber wall 132 to reflector 124. 
Separation chamber walls 130 and 132, while transparent to the acoustic 
pressure waves transmitted by transducer 110, are rigid such that the 
pressure of the medium and particle mixture being introduced into 
separation vessel 126 does not cause any significant deformation of 
separation chamber walls 130 and 132. Preferably, separation chamber walls 
130 and 132 are both uniformly parallel to the face of transducer 110. 
As will be explained shortly, when in operation, the particles contained in 
the mixture are moved adjacent to one wall of separation chamber 126. For 
example, in the embodiment illustrated in FIG. 6, the particles are moved 
adjacent to wall 130. As particles are aggregated next to wall 130, the 
medium, without particles, is displaced so as to be adjacent the opposite 
wall 132. As additional particle and medium mixture is moved into 
separation vessel 126, the particles and medium are separated as they are 
moved into separation collectors, generally designated 138 and 140. 
As can be seen from the perspective view of FIG. 6, separation collectors 
138 and 140 are joined at the point marked 142 called a separation 
surface. Separation surface 142 is preferably constructed so that the 
particles in the medium are separated without causing excessive turbulence 
in either the particles or the medium. The introduction of excessive 
turbulence is counter productive to the separation process since 
turbulence may cause the particles to be remixed with the medium. 
Furthermore, the flow rate into separation vessel 126 must also be chosen 
so as to avoid excessive turbulence. 
As will be appreciated by an understanding of the operation of the 
embodiment, the distance between separation surface 142 and separation 
vessel walls 130 and 132 may be varied according to what particles and 
medium are to be separated and according to the degree of separation 
desired. For example, if it is desired to remove particulate matter from 
water in order to provide particulate-free water, separation surface 142 
may be positioned between separation vessel walls 130 and 132 such that 
the bulk of the particulate matter enters separation collector 138. 
By moving separation surface 142 closer to separation vessel wall 130, the 
percentage of particles removed may be reduced. However, by making this 
adjustment, the percentage of water diverted to separation collector 138 
will also be reduced. Thus, the position of separation surface may be 
altered according to the particular application of the embodiment. 
Still further, it will be appreciated that the percentage of separation may 
be varied by altering the feed flow rate through separation vessel 126. 
Furthermore, the percentage of separation may also be varied by including 
valves, shown schematically at 139 and 141 in FIG. 6, in the flow paths of 
separation collectors 138 and 140, respectively. As valves 139 and 141 are 
opened, it will be seen that the residence time will be decreased and when 
valves 139 and 141 are closed some what, the opposite effect will be seen. 
Adjusting the positions of valves 139 and 141 has an effect similar to 
moving the orientation of separation surface 142. Thus, the percentage of 
separation may be altered by adjusting valves 139 and 141. 
Furthermore, it will be appreciated that the use of separation vessel 126, 
which prevents contact between the medium and the propagation material, 
not only assists with the aggregation and separation process, as will be 
explained later, but also facilitates the convenient use of the embodiment 
in many applications. 
For example, as will be discussed later, the illustrated embodiment has 
valuable application for separation of blood cells from plasma. Since 
separation vessel 126 may be constructed so as to be easily replaced, the 
need for sterilizing the complete embodiment is avoided since separation 
vessel 126 may be disposable. 
Reference will now be made to FIG. 7 in order to further explain the 
operation of the embodiment shown in FIG. 6. FIG. 7 is an elevated 
cross-sectional view of the embodiment illustrated in FIG. 6. 
The pseudo-standing wave referred to in connection with FIGS. 3 and 4 
above, may be created in the embodiment shown in FIG. 7, as explained 
below. Variable frequency generator 118 is operated so as to create a wave 
form whose frequency increases as time passes. The ramp wave form, 
generally designated 144 in FIG. 7, generally indicates the function used 
by variable frequency generator 118 to increase the frequency over time 
between two values and then to return rapidly to the lowest frequency. 
Alternatively, a variable frequency generator may be structured so as to 
create a wave form whose frequency decreases with time. The case in which 
the frequency increases with time will be used to explain the operation of 
the embodiment illustrated in the figures; however, it will be understood 
that if the frequency is decreased over time, the effect of the pressure 
waves within propagation chamber 102 and separation vessel 126 will be the 
same with only the direction of movement being reversed. Furthermore, it 
is to be understood that ramp wave form 144 is merely representative of 
the wave forms which may be used. For example, the wave form could be the 
linear wave form shown, an exponential wave form, or could follow some 
other function depending on the particular application of the embodiment. 
With variable frequency generator 118 creating a wave form which increases 
with frequency as time passes, transducer 110 will create a corresponding 
wave form in the propagation material. Thus, as the wave form propagates 
through propagation chamber 102, the wavelengths of the pressure waves 
found nearest reflector 124 will be longer than the wavelength of the 
pressure waves nearest transducer 110. 
This is shown schematically by the pressure gradient wave form labeled 146 
in FIG. 7, which is exaggerated to show the effect of "ramping" the 
frequency generator 118. Since the frequency found at the face of 
transducer 110 has been "ramping" upward, the wave reflected from 
reflector 124, represented by the pressure gradient wave form marked 148, 
will be decreasing in frequency, relative to incident wave 146 as it 
approaches transducer 110. 
Since propagation chamber 102 may be only several wavelengths long, and 
frequency generator is ramping upward in frequency only a small amount, 
for example as little as about 0.0001%, during the time it takes pressure 
waves to traverse propagation chamber 102 twice, the difference in 
frequency between the acoustic pressure waves at any particular two points 
within propagation chamber 102 will only be slight and will create a beat 
wave whose envelope is of low frequency, or in other words, a 
"pseudo-standing wave". It should be noted that the pressure gradient wave 
forms shown in FIG. 7, representing incident wave 146 and reflected wave 
148, are not to scale and have been exaggerated to demonstrate the concept 
that the frequencies of the two waves differ. 
The frequency difference between the incident wave 146 and the reflected 
wave 148 will be zero at the surface of reflector 124 and will be greatest 
at the face of transducer 110. The reflected wave 148 superimposed upon 
the incident wave 146 creates the pseudo-standing wave. 
The pseudo-standing wave which has been created as described above moves 
through space and time within propagation chamber as explained in 
connection with FIG. 5 above. The velocity of the pressure gradients the 
pseudo-standing wave is termed the group velocity. Thus, the pressure 
gradients of the pseudo-standing wave move as described in connection with 
the pseudo-standing wave illustrated in FIG. 4. The particles contained in 
the medium are carried along with the moving pressure gradients in the 
direction of arrow 152. 
However, due to the fact that the movement of particles is restricted by 
separation vessel wall 130, the particles are aggregated adjacent to 
separation vessel wall 130. The creation of pseudo-standing waves, whose 
movement is described by the group velocity equation to be provided later, 
cause the segregation of particles and medium within separation vessel 126 
as shown in detail in FIG. 7A. 
FIG. 7A is an elevated cross-sectional view of separation chamber 126 along 
a section through which the vessel traverses propagation chamber 102. In 
FIG. 7A, blood cells, represented by the objects marked 154, are shown 
being fed into separation vessel 126 in the direction indicated by arrow 
130. 
As blood cells 154 and plasma mixture enter separation vessel 126, blood 
cells 154 are uniformly dispersed throughout the plasma. As blood cells 
154 continue to travel through separation vessel 126, they are acted upon 
by pressure gradients 156, moving in the direction shown by arrow 152. 
The action of pressure gradients 156 cause blood cells 154 to aggregate in 
the regions of pressure minima. Since the pressure gradients 156 are 
moving, the blood cells aggregate along separation vessel wall 130. With a 
high percentage of blood cells 154 aggregated along wall 130, blood cells 
154 may be separated from the plasma by directing blood cells 154 through 
separation collector 138. 
As will be appreciated, the embodiment may be used to separate many 
different materials having different physical properties, not just blood 
cells from plasma. It should also be appreciated that altering the feed 
flow rate into separation vessel 126, the length of separation vessel 126, 
the position of separation surface 142, and the orientation of valves 139 
and 141, all will affect the percentage of separation. Furthermore, the 
particular frequencies used, the rate of change of the frequencies (i.e., 
the ramp rate), the width of the separation chamber, and additional 
factors, must be considered when using the embodiments illustrated in the 
figures. 
The creation of a pseudo-standing wave and its associated group velocity 
(i.e., the velocity of the pressure gradients of the pseudo-standing 
wave), may be described mathematically. As explained above, the difference 
in frequency between the incident and reflected acoustic pressure waves 
will be at a minimum at the surface of reflector 124 and at a maximum at 
the point at which the acoustic pressure wave is transmitted into the 
propagation material, i.e., the surface of impedance matching section 114, 
as shown in FIG. 7A. This result is caused by the fact that the reflected 
wave will have traveled the maximum distance possible once it has reached 
the first end of propagation chamber as well as the fact that the 
reflected wave will have the maximum difference in frequency from the 
acoustic pressure wave currently being propagated by transducer 110 due to 
the ramp function of variable frequency generator 110. 
Choosing a locus ("z") somewhere within the propagation chamber with the 
reflector surface being defined as the starting point where z=0 and where 
L equals the length of the propagation chamber, the group velocity 
(sometimes also referred to as the beat velocity) may be determined by 
defining the ramp rate. The ramp rate is the rate of change of the 
transducer frequency and is expressed by Equations C.sub.1 and C.sub.2 : 
##EQU3## 
Where: .omega..sub.o =instantaneous frequency of transducer 
R=ramp rate 
.omega..sub.inc &gt;.omega..sub.ref at z, ie., a positive or upward ramp 
Having defined the ramp rate, the group velocity may be determined by 
Equation D: 
##EQU4## 
Where: V.sub.g =group velocity 
The ramp rate, R, may be either positive or negative. As generally 
explained earlier, the group velocity describes the movement of the nodes 
and antinodes, and thus the pressure gradients, of the pseudo-standing 
wave. Thus, the group velocity also describes the general movement of the 
particles which have been aggregated at pressure minima by the action of 
the moving pressure gradients. The particles will, in many cases, move 
with their corresponding band. 
It will be appreciated that in practice not all of the particles will 
acquire the same velocity as the group velocity. In the present invention, 
each particle may be acted upon by a plurality of pressure gradients, 
since, at the frequencies used, many pressure gradients may have moved 
past each particle during its time in residence. Furthermore, it is not 
necessary that the particles migrate to a node or antinode, but only that 
the particles be acted upon by the moving pressure gradients sufficiently 
to overcome the drag forces present and cause the particles to move a very 
short distance. The maximum lateral distance each particle is required to 
move is equal to the distance between separation vessel walls 130 and 132. 
Another embodiment using the variable frequency generator is shown in FIG. 
7B. FIG. 7B is an elevated cross-section of the retro-reflector embodiment 
of the present invention illustrating the pressure amplitude of a single 
wave transmitted by the transducer and reflected by the retro-reflector 
surface. It is to be understood that the pressure wave is transmitted as a 
plane wave across the entire transducer surface. Similar to the variable 
frequency embodiment shown in FIG. 7, the retro-reflector embodiment uses 
a variable frequency generator 118, power amplifier 120, and transducer 
110 which is coupled to the propagation material contained in propagation 
chamber 102 by quarter-wave impedance matching section 114. 
The retro-reflector embodiment shown in FIG. 7B has two significant 
differences from the variable frequency embodiment shown in FIG. 7, as 
will be pointed out. First, reflector surface 160 is preferably formed in 
a conical shape having a 90.degree. apex. The conical shape causes 
acoustic pressure waves to be reflected laterally across the axis of the 
chamber and then reflected in the opposite direction. Second, the 
retro-reflector embodiment does not require a separation vessel since the 
particle and medium mixture is introduced directly into the propagation 
chamber 102. 
Shapes other than the conical shape represented in FIG. 7B may be used for 
reflector surface 160; the important factor is that the reflector surfaces 
are oriented at 90.degree. in relation to the opposite reflecting surface. 
For example, a pyramid shape may be used if its apex forms a 90.degree. 
angle. 
The particle and medium mixture is introduced into propagation chamber 102 
through feed passage 162 in the direction indicated by arrow 164. As the 
particles enter propagation chamber 102 they are acted upon by the 
pressure gradients of the beat wave formed by superposition of the 
reflected waves upon the incident waves from the transducer. This results 
in pseudo-standing waves possessing a group velocity as explained earlier. 
Due to the group velocity of the pseudo-standing waves, the particles are 
urged to move in the direction indicated by arrow 166. The incident wave 
pressure gradients are marked 165 while the reflected wave pressure 
gradients are marked 167. 
The number of segregated particles per unit volume increases as reflector 
surface 160 is approached. Due to the 90.degree. reflection at the 
retro-reflector, the particles will be urged into annular pressure minima 
surrounding the axis of the cone. The particles are eventually aggregated 
into a central rod, which subsequently exits the device through a first 
discharge passage 168 in the direction indicated by arrow 170. The 
particle free medium leaves propagation chamber 102 through a second 
discharge passage 172, in the direction of arrow 174, as additional 
particle and medium mixture is forced into propagation chamber 102 through 
feed passage 162. 
Below are given several examples showing the effectiveness of the variable 
frequency embodiment illustrated in FIGS. 6, 7 and 7B. 
EXAMPLE 1 
An embodiment within the scope of the present invention and substantially 
similar to that shown in FIG. 7 above was used in order to separate red 
blood cells from plasma. The dimensions of the structures used in this 
example have the following values: 
Dimensions of the propagation chamber: 
Height=2.5 centimeters (cm) 
Width=2.5 cm 
Length=7.5 cm 
Dimensions of the separation vessel: 
Width=6 millimeters (mm) 
Residence length=2.5 cm 
Distance from propagation chamber ends=3.25 cm 
Separation surface orientation: centered 
Separation vessel material: urethane rubber 
The frequency generator used in this example was manufactured by Exact 
Electronics, Inc., model no. 528; the power amplifier was manufactured by 
Electronic Navigation Industries, Inc., model no. 240L RF Power Amplifier; 
the transducer material was composed of lead zirconate titanate (PZT-4); 
and, the reflector material was aluminum. 
The method of the present invention was operated utilizing the following 
parameters: 
Transducer frequency=2.2 mHz 
Ramp rate=10 kHz/second 
Ramp direction: Upward 
Cycle time for ramp=10 seconds 
Input power=50 watts peak 
Output power=5 watts/cm.sup.2 
Input feed flow=2 milliliters (ml)/minute (min) 
The original blood cell percentage in the plasma was about 26% in the 
continuous flow stream. However, analysis of the products separated 
according to the procedures of this example showed that there was less 
than about 0.5% of the blood cells remaining in the plasma. 
EXAMPLE 2 
A process within the scope of the present invention was conducted in order 
to separate red blood cells from plasma. The conditions, structures, 
parameters, and process were the same as in Example 1, except that the 
transducer frequency was 3.2 mHz, the ramp rate was 8 kHz/second, and the 
power input was 40 watts peak. 
The original red blood cell percentage in the plasma was about 26% in the 
continuous fluid stream. However, analysis of the products separated 
according to the procedures of this example showed that there was less 
than about 1% of the red blood cells remaining in the plasma. 
EXAMPLE 3 
A process within the scope of the present invention was conducted in order 
to separate red blood cells from plasma. The conditions, structures, 
parameters, and process were the same as in Example 1, except that the 
transducer frequency was 2.0 mHz, the input power was 60 watts peak, and 
the output power was 6 watts/cm.sup.2. 
The original percentage of red blood cells in the plasma was about 35% in 
the continuous fluid stream. However, analysis of the products separated 
according to the procedures of this example showed that there was less 
than about 6% of the red blood cells remaining in the plasma. 
EXAMPLE 4 
A process within the scope of the present invention is conducted using an 
embodiment substantially similar to that shown in FIG. 7B above for the 
purpose of separating gas bubbles from water. The bubbles are dispersed 
into the water to provide bubbles ranging in size from about 10 to about 
100 microns. The dimensions of the structures for this example have the 
following values: 
Dimensions of the propagation chamber: 
Height=2.5 cm 
Width=2.5 cm 
Length=2.5 cm 
The frequency generator used in this example is manufactured by Exact 
Electronics, Inc., model no. 528; the power amplifier is manufactured by 
Electronic Navigation Industries, Inc., model no. 240L RF Power Amplifier; 
the transducer material is composed of lead zirconate titanate (PZT-4); 
the impedance matching section is made of boron nitride; and, the 
reflector material is aluminum formed in a 90.degree. cone. 
The method of the present invention is operated utilizing the following 
parameters: 
Transducer frequency=2.2 mHz 
Ramp rate=10 kHz 
Ramp direction: upward 
Cycle time for ramp=10 seconds 
Input power=50 watts peak 
Output power=5 watts/cm.sup.2 
Input feed flow=6 ml/min 
Utilizing the procedures of to this example, good separation of the bubbles 
from the water is visually observable. 
EXAMPLE 5 
An embodiment within the scope of the present invention and substantially 
similar to that shown in FIG. 7B above was used for the purpose of 
separating a mixture of two immiscible liquids (oil and water). The oil 
was dispersed into the water to provide droplets ranging in size from 
about 1 to about 100 microns. The dimensions of the structures used in 
this example have the following values: 
Dimensions of the propagation chamber: 
Height=2.5 cm 
Width=2.5 cm 
Length=2.5 cm 
The frequency generator used in this example was manufactured by Exact 
Electronics, Inc., model no. 528; the power amplifier was manufactured by 
Electronic Navigation Industries, Inc., model no. 240L RF Power Amplifier; 
the transducer material was composed of lead zirconate titanate (PZT-4); 
the impedance matching section was made of boron nitride; and, the 
reflector material was aluminum formed in a 90.degree. cone. 
The method of the present invention was operated utilizing the following 
parameters: 
Transducer frequency=2.2 mHz 
Ramp rate=10 kHz 
Ramp direction: upward 
Cycle time for ramp=10 seconds 
Input power=50 watts peak 
Output power=5 watts/cm.sup.2 
Input feed flow=6 ml/min 
Output flow=1.2 ml/min for oil 
Output flow=4.8 ml/min for water 
The original oil percentage in the water was about 22% in the continuous 
flow stream. However, analysis of the products separated according to the 
procedures this example showed that there was less than about 5% of the 
oil remaining in the water after passing through the separation chamber. 
EXAMPLE 6 
A process within the scope of the present invention was conducted in order 
to separate oil from water. The conditions, structures, parameters, and 
process were the same as in Example 5, except that the original oil 
concentration in the water was 5% with droplets ranging in size from about 
1 to about 10 microns, the input feed flow was 4 ml/min, and the output 
flow was 0.5 ml/min for oil and 3.5 ml/min for water. 
The original percentage in the water was about 5% in the continuous fluid 
stream. However, analysis of the products separated according to the 
procedures of this example showed that there was less than about 0.1% of 
the oil remaining in the water after through the separation chamber. 
EXAMPLE 7 
A process within the scope of the present invention was conducted in order 
to separate two immiscible liquids (Puritan.RTM. brand salad oil and 
water). The conditions, structures, parameters, and process were the same 
as in Example 5, except that no impedance matching section was used. 
The original oil percentage in the water was about 22% in the continuous 
fluid stream. However, analysis of the products separated according to the 
procedures of this example showed that there was less than about 1% of the 
oil remaining in the water after passing through the separation chamber. 
EXAMPLE 8 
An embodiment within the scope of the present invention and substantially 
similar to that shown in FIG. 7 above was used in order to separate 
polystyrene microspheres (having an average diameter of 30 microns) from 
water. The dimensions of the structures used in this example had the 
following values: 
Dimensions of the propagation chamber: 
Height=2.5 cm 
Width=2.5 cm 
Length=7.5 cm 
Dimensions of the separation vessel: 
Width=6 mn 
Residence length=2.5 cm 
Distance from propagation chamber ends=3.25 cm 
Separation surface orientation: centered 
Separation vessel material: urethane rubber 
The frequency generator used in this example was manufactured by Exact 
Electronics, Inc., model no. 528; the power amplifier was manufactured by 
Electronic Navigation Industries, Inc., model no. 240L RF Power Amplifier; 
the transducer material was lead zirconate titanate (PZT-4); and, the 
reflector material was aluminum. 
The method of the present invention was operated utilizing the following 
parameters: 
Transducer frequency=2 mHz 
Ramp rate=10 kHz/sec 
Ramp direction: Upward 
Cycle time for ramp=10 sec 
Input power=20 watts peak 
Output power=2 watts/cm.sup.2 
Input feed flow=1.5 ml/min 
The original volume of microspheres in the water was about 1% in the 
continuous flow stream. However, analysis of the products separated 
according to the procedures of this example showed that there was less 
than about 0.1% of the microspheres remaining in the water after passing 
through the separation chamber. 
EXAMPLE 9 
A process within the scope of the present invention was conducted in order 
to separate polystyrene microspheres from water. The conditions, 
structures, parameters, and process were the same as in Example 8, except 
that the microspheres were 5 microns in diameter. 
The original microsphere percentage in the water was about 1% in the 
continuous fluid stream. However, analysis of the products separated in 
this example showed that there was less than about 0.1% of the 
microspheres remaining in the water after passing through the separation 
chamber. 
EXAMPLE 10 
An embodiment within the scope of the present invention substantially 
similar to that shown in FIG. 7B above was used according to the present 
invention in order to separate polystyrene microspheres 34 microns in 
diameter from water. The dimensions of the structures used in this example 
have the following values: 
Dimensions of the propagation chamber: 
Height=2.5 cm 
Width=2.5 cm 
Length=7.5 cm 
The frequency generator used in this example was manufactured by Exact 
Electronics, Inc., model no. 528; the power amplifier was manufactured by 
Electronic Navigation Industries, Inc., model no. 240L RF Power Amplifier; 
the transducer material was lead zirconate titanate (PZT-4); and, the 
reflector material was aluminum. 
The method of the present invention was operated utilizing the following 
parameters: 
Transducer frequency=2 mHz 
Ramp rate=10 kHz/sec 
Ramp direction: Upward 
Cycle time for ramp=10 sec 
Input power=20 watts peak 
Output power=2 watts/cm.sup.2 
Input feed flow=1.5 ml/min 
The original volume of microspheres in the water was about 1% in the 
continuous flow stream. However, analysis of the products separated 
according to the procedures of this example showed that there was less 
than about 0.1% of the microspheres remaining in the water after passing 
through the separation chamber. 
D. Moving Reflector Embodiment 
FIG. 8 is a cross-sectional view showing the structure of the moving 
reflector embodiment of the present invention. The structure of the moving 
reflector embodiment is similar to the structure of the variable frequency 
embodiment shown in FIGS. 6 and 7 with two significant alterations. 
First, variable frequency generator 118, shown in FIGS. 6 and 7, has been 
replaced by a fixed frequency generator 202. Second, reflector 124, shown 
in FIGS. 6 and 7, has been replaced by a moving reflector 204 which is 
driven by a motor 206 and interconnecting linkage 208, such that the 
reflector is driven in a reciprocating piston-like fashion. In some 
applications it may be advantageous to incorporate a conical-shaped 
reflective surface, as described in connection with the retro-reflector 
embodiment shown in FIG. 7B, into moving reflector 204. 
As is the case with the variable frequency transducer embodiment, shown in 
FIGS. 6, 7, and 7B, the objective of the embodiment shown in FIG. 8 is to 
produce pseudo-standing waves whose moving pressure gradients may be 
utilized to move particles to one side of separation vessel 126. In order 
to create a pseudo-standing wave having moving pressure gradients, it is 
necessary to propagate a reflected acoustic pressure wave processing a 
different frequency than the incident acoustic pressure wave propagated by 
transducer 110. 
With frequency generator 202 being fixed at a single frequency, moving 
reflector 204 allows the reflected wave to increase or decrease in 
frequency, as compared to the incident pressure wave, due to the 
well-known Doppler effect. The shift in frequency due to the Doppler 
effect, hereinafter called the "Dopper shift," causes the reflected wave 
to be shifted in frequency by an amount which is directly related to the 
velocity of moving reflector 204. 
As shown in FIG. 8, incident wave 210 has a wavelength designated by 212, 
while the reflected wave 214 has a wavelength designated by 216. The 
reflected wavelength 216 is shorter than the incident wavelength 212 when 
moving reflector 204 travels in the direction marked by the arrow labeled 
218. 
Alternatively, when moving reflector 204 travels in the direction shown by 
arrow 222, the Doppler-shifted reflected wave 214 will have a shorter 
wavelength than incident wave 210. The Doppler-shifted reflected wave 210 
superimposed upon the incident wave 214 creates a pseudo-standing wave. 
Furthermore, the group velocity (i.e., the velocity of the travel of the 
pressure gradients of the pseudo-standing wave) is linearly related to the 
velocity of moving reflector 204. 
Thus, by incorporating a moving reflector 204 into the embodiment, 
pseudo-standing waves having moving pressure gradients may be created in 
propagation chamber 102 and transmitted through separation vessel 126, 
thereby causing the aggregation and separation of particles from the 
medium as shown in FIG. 7A. As is the case with the variable frequency 
embodiment shown in FIGS. 6 and 7 above, the particles may be caused to 
aggregate along either separation vessel wall 130 or separation vessel 
wall 132. 
The travel of moving reflector 204 in the direction indicated by arrow 218 
will cause the pressure gradients of the pseudo-standing waves to move in 
the direction indicated by arrow 218, thereby causing the particles to 
move in the same direction. When the direction of moving reflector 204 is 
reversed, as in the direction of arrow 222, the particles will similarly 
travel in the opposite direction. In order to avoid having a zero net 
movement of particles when using the moving reflector embodiment, it is 
necessary to prevent the movement of the particles while moving reflector 
is traveling to its initial position. 
A first method of preventing movement of particles during one direction of 
moving reflector travel is to cause moving reflector 204 to move at two 
velocities: a first velocity termed a "Doppler velocity", and a second 
velocity termed a "return velocity." The Doppler velocity is the correct 
velocity which moving reflector 204 should travel in order to cause the 
correct Doppler shift in the reflected pressure wave 214. 
The return velocity must be much greater than the Doppler velocity. The 
criteria for selecting the return velocity should be that the return 
velocity must be high enough that any movement in the pseudo-standing wave 
pressure gradients would be too rapid to be followed by the particles with 
their relatively high inertia and viscous drag. 
A preferred method of eliminating the effect of moving reflector 204 
traveling in both directions is to switch frequency generator 202 off 
during the period that moving reflector 204 is moving in the direction 
opposite to that desired for segregating particles. By switching off 
frequency generator 202 during the proper interval, motor 206 and linkage 
208 may be allowed to operate continuously to provide the reciprocating 
motion. 
As stated above, the movement of the pressure gradients of the 
pseudo-standing wave, and thus the movement of the particles, are linearly 
related to the movement of moving reflector 204. The linear relationship 
may be expressed by the equations as set forth below. 
The Doppler shift introduced by moving reflector is expressed by Equation 
E: 
##EQU5## 
Where: .mu.=Velocity of the moving reflector The equation giving the 
Doppler shift of the reflected wave may be substituted into the group 
velocity equation, given above, to obtain Equation F: 
##EQU6## 
By properly choosing and controlling the frequencies generated by frequency 
generator 202 and appropriately controlling the velocity of moving 
reflector 204, as shown in FIG. 8, the particles and medium may be 
separated from one another as described in connection with FIGS. 6, 7, and 
7A, above. Examples of the moving reflector embodiment shown in FIG. 8 
being used to separate various materials are given below. 
EXAMPLE 11 
An embodiment within the scope of the present invention and substantially 
similar to that shown in FIG. 8 above was used in order to separate blood 
cells from plasma. The dimensions of the structures used in this example 
have the following values: 
Dimensions of the propagation chamber: 
Height=2.5 cm 
Width=2.5 cm 
Length=5 cm 
Dimensions of the separation vessel: 
Width=6 mm 
Residence length=2.5 cm 
Distance from propagation chamber ends=2.5 cm 
Separation surface orientation: centered 
Separation vessel material: acrylic 
The frequency generator used in this example was manufactured by Exact 
Electronics, Inc., model no. 528; the power amplifier was manufactured by 
Electronic Navigation Industries, Inc., model no. 240L RF Power Amplifier; 
the transducer material was lead zirconate titanate (PZT-8); the impedance 
matching section was made of glass; and, the reflector material was 
aluminum. 
The method of the present invention was operated utilizing the following 
parameters: 
Transducer frequency=5 mHz 
Reflector velocity=0.1 mm/sec 
Cycle time for reflector=10 sec 
Input power=10 watts peak 
Output power=10 watts/cm.sup.2 
Input feed flow=6 ml/min 
The original blood cell percentage in the plasma was about 26% in the 
continuous flow stream. However, analysis of the products separated 
according to the procedures of this example showed that there was less 
than about 10% of the blood cells remaining in the plasma. 
EXAMPLE 12 
A process within the scope of the present invention is conducted in order 
to separate blood cells from plasma. The conditions, structure, 
parameters, and process are the same as in Example 11, except that the 
reflector velocity is 0.07 mm/sec, and the input feed flow is 4 ml/min. 
According to the procedures of this example, excellent separation of the 
blood cells from the plasma is achieved. 
EXAMPLE 13 
A process within the scope of the present invention was conducted in order 
to separate blood cells from plasma. The conditions, structures, 
parameters, and process were the same as in Example 11, except that the 
transducer frequency was 2.2 mHz. 
The original blood cell percentage in the plasma was about 26% in the 
continuous fluid stream. However, analysis of the products separated 
according to the procedures of this example showed that there was less 
than about 3% of the blood cells remaining in the plasma. 
EXAMPLE 14 
A process within the scope of the present invention is conducted using an 
embodiment substantially similar to that shown in FIG. 8 above in order to 
separate gas bubbles ranging in size from about 10 to about 100 microns 
from water. The dimensions of the structures used in this example have the 
following values: 
Dimensions of the propagation chamber: 
Height=2.5 cm 
Width=2.5 cm 
Length=5 cm 
Dimensions of the separation vessel: 
Width=6 mm 
Residence length=2.5 cm 
Distance from propagation chamber ends=2.5 cm 
Separation surface orientation: centered 
Separation vessel material: acrylic 
The frequency generator used in this example is manufactured by Exact 
Electronics, Inc., model no. 528; the power amplifier is manufactured by 
Electronic Navigation Industries, Inc., model no. 240L RF Power Amplifier; 
the transducer material is PZT-4; the impedance matching section is made 
of boron nitride; and, the reflector material is aluminum. 
The method of the present invention is operated utilizing the following 
parameters: 
Transducer frequency=2.5 mHz 
Reflector velocity=0.1 mm/sec 
Cycle time for reflector=10 sec 
Input power=50 watts peak 
Output power=5 watts/cm.sup.2 
Input feed flow=4 ml/min 
According to the procedures of this example, adequate separation of the 
bubbles from the water is achieved. 
EXAMPLE 15 
A process within the scope of the present invention is conducted using an 
embodiment substantially similar to that shown in FIG. 8 above for the 
purpose of separating crude oil dispersed in droplets ranging in size from 
about 1 to about 100 microns from water. The dimensions of the structures 
used for this example have the following values: 
Dimensions of the propagation chamber: 
Height=2.5 cm 
Width=2.5 cm 
Length=5 cm 
Dimensions of the separation vessel: 
Width=6 mm 
Residence length=2.5 cm 
Distance from propagation chamber ends=2.5 cm 
Separation surface orientation: centered 
Separation vessel material: acrylic 
The frequency generator for this example is manufacturd by Exact 
Electronics, Inc., model no. 528; and power amplifier is manufactured by 
Electronic Navigation Industries, Inc., model no. 240L RF Power Amplifier; 
the transducer material is PZT-4; the impedance matching section is made 
of boron nitride; and, the reflector material is aluminum. 
The method of the present invention is operated utilizing the following 
parameters: 
Transducer frequency=2.5 mHz 
Reflector velocity=0.1 mm/sec 
Cycle time for reflector=10 sec 
Input power=50 watts peak 
Output power=5 watts/cm.sup.2 
Input feed flow=4 ml/min 
According to the procedures of this example, excellent separation of the 
oil from the water is achieved. 
EXAMPLE 16 
A process within the scope of the present invention is conducted for the 
purpose of separating oil from water. The conditions, structures, 
parameters, and process are the same as in Example 15, except that the 
transducer frequency is 2 mHz, the reflector velocity is 0.09 mm/min, the 
power input is 60 watts peak, the power output is 6 watts/cm.sup.2, and 
the input flow rate is 3 ml/min. 
According to the procedures of this example, excellent separation of the 
oil from the water is achieved. 
EXAMPLE 17 
An embodiment within the scope of the present invention and substantially 
similar to that shown in FIG. 8 above was used in order to separate 
polystyrene microspheres 34 microns in diameter from water. The dimensions 
of the structures used in this example have the following values: 
Dimensions of the propagation chamber: 
Height=2.5 cm 
Width=2.5 cm 
Length=5 cm 
Dimensions of the separation vessel: 
Width=6 mm 
Residence length=2.5 cm 
Distance from propagation chamber ends=2.5 cm 
Separation surface orientation: centered 
Separation vessel material: acrylic 
The frequency generator used in this example was manufactured by Exact 
Electronics, Inc., model no. 528; the power amplifier was manufactured by 
Electronic Navigation Industries, Inc., model no. 240L RF Power Amplifier; 
the transducer material was lead zirconate titanate (PZT-4); and, the 
reflector material was zinc. 
The method of the present invention was operated utilizing the following 
parameters: 
Transducer frequency=2 mHz 
Reflector velocity=0.2 mm/sec 
Cycle time for reflector=10 sec 
Input power=50 watts peak 
Output power=5 watts/cm.sup.2 
Input feed flow=3 ml/min 
The original microsphere percentage in the water was about 5% in the 
continuous flow stream. However, analysis of the products separated 
according to the procedures of this example showed that there was less 
than about 0.5% of the microspheres remaining in the water. 
EXAMPLE 18 
A process within the scope of the present invention is conducted for the 
purpose of separating microspheres from water. The conditions, structures, 
parameters, and process are the same as in Example 17, except that the 
reflector velocity is 0.1 mm/sec. 
According to the procedures of this example, excellent separation of the 
microspheres from the water is visually observable. 
EXAMPLE 19 
A process within the scope of the present invention is conducted in order 
to separate microspheres from water. The conditions, structures, 
parameters, and process are the same as in Example 17, except that the 
input feed flow is 1 ml/min. 
According to the procedures this example, excellent separation of the 
microspheres from the water is achieved. 
E. Double Transducer Embodiment 
Pseudo-standing waves may also be formed by providing a transducer at each 
end of propagation chamber 102 and operating each transducer at a slightly 
different frequency. FIG. 9 shows an elevated cross-section view of the 
structure of the double transducer embodiment of the present invention. 
Similar to the embodiments discussed previously, the double transducer 
embodiment includes propagation chamber 102 and separation vessel 126 
which are constructed considering the same factors as considered in 
connection with the embodiments shown in FIGS. 7 and 8 above. However, in 
the double transducer embodiment, transducers 110A and 110B are provided 
at each end of propagation chamber 102. Each transducer 110A or 110B may 
be provided with a quarter wave matching section 114A or 114B, 
respectively, to effect an impedance match between transducer 110A or 110B 
and the propagation material contained within propagation chamber 102. 
Since the embodiment illustrated in FIG. 9 is particularly susceptible to 
the creation of "true" standing waves some additional structures may be 
provided to diminish their creation. This is due to the fact that 
transducers 110A and 110B will reflect some of the acoustic pressure waves 
incident upon them. Since standing waves are detrimental to the operation 
of the embodiment a structure may be provided for each transducer, 110A or 
110B, which will dampen any acoustic pressure wave impinging upon 
transducer 110A or 110B. These structures 205A or 205B, which in the 
present embodiment are composed of absorbent rubber and often termed 
"absorbers," are coupled to the reverse side of transducers 110A or 110B 
by second impedance matching sections 252A or 252B, respectively. 
Each transducer 110A or 110B is driven by a separate frequency generator 
202A or 202B, and power amplifier 120A or 120B, respectively. Both 
transducers are operated simultaneously--each at a slightly different 
frequency. The difference in frequencies is chosen as explained later in 
this section. With both transducers operating, the acoustic pressure wave 
represented by the wave marked 256A, which is at the same frequency as 
transducer 110A is operating, propagates in the direction indicated by 
arrow 254A. 
Similarly, the pressure wave marked 256B, which is at the same frequency as 
transducer 110B is operating, moves in the direction of arrow 254B. 
Superimposed, these two opposing waves will provide a pseudo-standing wave 
with moving pressure gradients. The double transducer embodiment has the 
advantage of allowing transducers 110A and 110B which are operated over a 
very narrow range of frequencies, and thus exhibit a high "Q", to be used. 
Such transducers are characteristically more efficient than variable 
frequency transducers operable over a broad range of frequencies. 
With transducer 110A operating at a frequency slightly above the frequency 
at which transducer 110B is operating, the pressure gradients of the 
pseudo-standing wave will move in the direction indicated by arrow 254B. 
The movement of the pseudo-standing wave is expressed mathematically 
below. 
The pressure of the two wave fronts is expressed by equations G.sub.1 and 
G.sub.2 : 
EQU P.sub.1 =P.sub.A sin (.omega..sub.1 t-k.sub.1 z) (G.sub.1) 
EQU P.sub.2 =P.sub.A sin (.omega..sub.2 f+k.sub.2 z) (G.sub.2) 
Where: 
P=acoustic pressure amplitude of waves 1 and 2 
.omega..sub.1 =frequency of first transducer 
.omega..sub.2 =frequency of second transducer 
t=time 
k=wave number of waves 1 and 2 
z=distance from second transducer 
The superposition of the pressures upon one another gives a new pressure 
equation. The resultant wave may be expressed by Equation H: 
##EQU7## 
The frequency of the quasi-standing wave can be expressed as set forth in 
Equation I: 
##EQU8## 
Where: .omega..sub.psw =frequency of the pseudo-standing wave 
In addition, the group velocity may be expressed by the same equation as 
mentioned above and again set for below as Equation J: 
##EQU9## 
Examples of the double transducer embodiment shown in FIG. 9 being used to 
separate various materials are given below. 
EXAMPLE 20 
An embodiment within the scope of the present invention and substantially 
similar to that shown in FIG. 9 above was used in order to separate blood 
cells from plasma. The dimensions of the structures used in this example 
have the following values: 
Dimensions of the propagation chamber: 
Height=2.5 cm 
Width=2.5 cm 
Length=3.0 cm 
Dimensions of the separation vessel: 
Width=6 mm 
Residence length=2.5 cm 
Distance from propagation chamber ends=15 cm 
Separation surface orientation: centered 
Separation vessel material: urethane rubber 
The frequency generators used in this example were commercially available 
radio frequency treanceivers; the transducer material was PZT-4; the 
impedance matching section was fabricated from magnesium. 
The method of the present invention was operated utilizing the following 
parameters: 
Transducer frequencies=2.000001 mHz and 2.0 mHz 
Input power=10 watts peak 
Output power=2 watts/cm.sup.2 
Input feed flow=0.5 ml/min 
The original blood cell percentage in the plasma was about 26% in the 
continuous flow stream. However, analysis of the products separated 
according to the procedures of this example showed that there was less 
than about 10% of the blood cells remaining in the plasma. 
EXAMPLE 21 
A process within the scope of the present invention is conducted for the 
purpose of separating blood cells from plasma. The conditions, structures, 
parameters, and process are the same as in Example 20, except that the 
input power is 50 watts peak, the output power is 5 watts/cm.sup.2, and 
the input feed flow is 3 ml/min. 
According to the procedures of this example, good separation of the blood 
cells from the plasma is achieved. 
EXAMPLE 22 
A process within the scope of the present invention is conducted using an 
embodiment substantially similar to that shown in FIG. 9 for the purpose 
of separating Phillips crude oil TK-126 dispersed droplets ranging in size 
from about 1 micron to about 100 microns from water. The dimensions of the 
structures used for this example have the following values: 
Dimensions of the propagation chamber: 
Height=2.5 cm 
Width=2.5 cm 
Length=5 cm 
Dimensions of the separation vessel: 
Width=6 mm 
Residence length=2.5 cm 
Distance from propagation chamber ends=2.5 cm 
Separation surface orientation: centered 
Separation vessel material: acrylic 
The frequency generators for in this example are commercially available 
radio frequency transceivers; the transducer material is PZT-4; the 
impedance matching section is fabricated from magnesium. 
The method of the present invention is operated utilizing the following 
parameters: 
Transducer frequencies=2.000001 mHz and 2.0 mHz 
Input power=50 watts peak 
Output power=5 watts/cm.sup.2 
Input feed flow=3 ml/min 
According to the procedures for this example, good separation of the oil 
from the water is achieved. 
EXAMPLE 23 
A process within the scope of the present invention is conducted for the 
purpose of separating oil from water. The conditions, structures, 
parameters, and process are the same as in Example 22, except that the 
input power is 60 watts peak, the output power is 6 watts/cm.sup.2, and 
the input feed flow is 2 ml/min. 
According to the procedures of this example, excellent separation of the 
oil from the water is achieved. 
EXAMPLE 24 
An embodiment within the scope of the present invention and substantially 
similar to that shown in FIG. 9 above was used in order to separate 
polystyrene microspheres 34 microns in diameter from water. The dimensions 
of the structures used in this example have the following values: 
Dimensions of the propagation chamber: 
Height=2.5 cm 
Width=2.5 cm 
Length=30 cm 
Dimensions of the separation vessel: 
Width=6 mm 
Residence length=7.5 cm 
Distance from propagation chamber ends=15 cm 
Separation surface orientation: centered 
Separation vessel material: urethane 
The frequency generators used in this example were commercially available 
radio frequency transceivers; the transducer material was PZT-4; the 
impedance matching section was fabricated from magnesium. 
The method of the present invention was operated utilizing the following 
parameters: 
Transducer frequencies=1.990 mHz and 1.990001 mHz 
Input power=10 watts peak 
Output power=1 watt/cm.sup.2 
Input feed flow=1 ml/min 
The original microsphere percentage in the water was about 1% in the 
continuous flow stream. However, analysis of the products separated 
according to the procedures of this example showed that there was less 
than about 0.1% of the microspheres remaining in the water. 
EXAMPLE 25 
A process within the scope of the present invention is conducted for the 
purpose of separating microspheres from water. The conditions, structures, 
parameters, and process are the same as in Example 24, except that the 
polystyrene microspheres are 10 microns in diameter and the transducer 
frequencies are 1.900000 mHz and 1.900001 mHz. 
According to the procedures of this example, excellent separation of the 
microspheres from the water is achieved. 
EXAMPLE 26 
A process within the scope of the present invention is conducted for the 
purpose of separating microspheres from water. The conditions, structures, 
parameters, and process are the same as in Example 24, except that the 
transducer frequencies are 10 mHz and 10.000003 mHz, the input power is 10 
watts peak, and the output power is 1 watt/cm.sup.2. 
According to the procedures of this example, excellent separation of the 
microspheres from the water is achieved. 
F. Synchronized Moving Reflector/Variable Frequency Embodiment 
Analysis of the operation of the variable frequency embodiment shown in 
FIGS. 6 and 7, and the moving reflector embodiment shown in FIG. 8, will 
reveal that the power required by transducer 110 to maintain a constant 
acoustic output power to the propagation material will vary since the 
frequency will vary. This is due to the fact that the acoustic impedance 
of the system "seen" by transducer 110 will change as the frequency 
changes and the frequency of the pressure wave transmitted by transducer 
110 sweeps through the resonant nodes of propagation chamber 102. 
In order to avoid the inefficient use of power input to the transducer that 
is observed in the variable frequency and moving reflector embodiments the 
synchronized moving reflector/variable frequency embodiment, shown in FIG. 
10, may be used. 
As shown in FIG. 10, the synchronized reflector/transducer embodiment is 
very similar in structure to the moving reflector embodiment, shown in 
FIG. 8 above. However, fixed frequency generator 202 shown in FIG. 8 has 
been replaced by a variable frequency generator 118 similar to that used 
in the variable frequency embodiment shown in FIG. 7. Additionally, a 
control circuit 300 has been added so as to synchronize the change of 
frequency by variable frequency generator 118 and the motion of moving 
reflector 204. 
It will be appreciated that it is desirable to maintain the acoustic 
impedance presented to transducer 110 at a constant value. However, in 
either the variable frequency or moving reflector embodiments, the 
acoustic impedance seen by transducer 110 will vary since either the 
wavelength of the acoustic pressure wave is constantly varying as is the 
case in the variable frequency embodiment, or the length of the 
propagation chamber is constantly varying as is the case in the moving 
reflector embodiment. Combining these two features, and properly 
synchronizing them, allows the transducer to "see" a constant acoustic 
impedance which allows the transducer output power to be constantly 
maintained with a constant input power level. 
The operation of the synchronized reflector/frequency embodiment will be 
explained by reference to FIG. 10. As shown in FIG. 10, a pressure wave 
308, generated by transducer 110, travels through propagation chamber 
towards reflector 204. The wave propagated by transducer 110 ramps upward 
in frequency as explained above in connection with the variable frequency 
transducer embodiment. Furthermore, for example, as the frequency 
transmitted by transducer 110 increases, moving reflector 204 moves toward 
transducer 110 such that moving reflector 204 is always the same number of 
wavelengths from transducer 110. 
In FIG. 10, this effect has been indicated by showing the position of 
reflector 204 at a second position, designated 204A. While moving 
reflector 204 is traveling to the position indicated at 204A, the 
frequency of the wave being propagated by transducer 110 is increasing. 
The frequency of the wave propagated by transducer 110 at the time that 
moving reflector 204 reaches the position indicated at 204A has increased 
as indicated by the wave form designated 308A. As can be observed in FIG. 
10, the number of wavelengths in wave forms 308 and 308A are the same. It 
should be appreciated that the frequency, and thus the wavelength, of wave 
forms 308 and 308A are actually constantly changing. However, for clarity, 
each wave form is shown to possess a constant wavelength. 
As will be appreciated by examining FIG. 10, any particles contained within 
separation vessel 126 will be acted upon by the pressure gradients of 
acoustic pressure wave 308 so as to be moved toward, and aggregated 
adjacent to, separation vessel wall 130. When moving piston 204 has 
reached its point of maximum travel, as indicated at 204A, frequency 
generator 118 is turned off and moving reflector 204 is returned to its 
original position. Moving reflector 204 is preferably constructed so as to 
allow the propagating material to flow through or around moving reflector 
204, while moving reflector is opaque to the acoustic pressure waves 
propagated through the propagating material. This is easily accomplished 
by using porous reflecting surfaces known in the art. 
The control circuit 300 synchronizes the movement of moving reflector 204 
and the frequency of variable frequency generator 118 such that the 
distance between moving reflector 204 and transducer 110, as expressed in 
wavelengths of the acoustic pressure wave, is kept constant. Control lines 
304 and 306 interconnect control circuit 300 and motor 206 and variable 
frequency generator 118, respectively. In one embodiment, the power 
reflected by transducer 110 is monitored as the frequency is altered and 
the movement of reflector 204 is effected by control circuit 300 in order 
to maintain minimum reflected power. Line 310 represents the sensing 
components used to monitor the power input to transducer 110. 
By maintaining the distance between moving reflector 204 and transducer 110 
a constant number of wavelengths, the acoustic impedance "seen" by 
transducer 110 is maintained at a constant value. Thus, constant power 
input to transducer 110 results in a constant output power from transducer 
110. In this way, the embodiment shown in FIG. 10 allows a single 
transducer 110 to be used with a moving reflector 204 to efficiently 
separate materials possessing different physical properties while still 
retaining the advantage of allowing the transducer 110 to "see" an 
acoustic impedance which does not vary. 
Examples of the operation and results obtained with use of the synchronized 
moving reflector/variable frequency transducer embodiment are given below. 
EXAMPLE 27 
A process within the scope of the present invention is conducted using an 
embodiment substantially similar to that shown in FIG. 10 above for the 
purpose of separating blood cells from plasma. The dimensions of the 
structures for this example have the following values: 
Dimensions of the propagation chamber: 
Height=2.5 cm 
Width=2.5 cm 
Length=2.5 cm 
Dimensions of the separation vessel: 
Width=6 mm 
Residence length=2.5 cm 
Distance from propagation chamber ends=2.5 cm 
Separation surface orientation: centered 
Separation vessel material: acrylic 
The frequency generator for this example is manufactured by Exact 
Electronics, Inc., model no. 528; the power amplifier is manufactured by 
Electronic Navigation Industries, Inc., model no. 240L RF Power Amplifier; 
the transducer material is lead zirconate titanate (PZT-4); the impedance 
matching section is a half wave transformer and is fabricated from glass; 
and, the reflector material is zinc. 
The method of the present invention is operated utilizing the following 
parameters: 
Transducer frequency=5 MHz 
Ramp rate=10 kHz/sec 
Ramp direction: upward 
Cycle time for ramp=10 sec 
Input power=50 watts peak 
Output power=20 watts/cm.sup.2 
Input feed flow=6 ml/min 
According to the procedures of this example, excellent separation of the 
blood cells from the plasma is achieved. 
EXAMPLE 28 
A process within the scope of the present invention is conducted for the 
purpose of separating blood cells from plasma. The conditions, structures, 
parameters, and process are the same as in Example 27, except that the 
ramp rate is 8 kHz/sec and the input feed flow is 4 ml/min. 
According to the procedures of this example, excellent separation of the 
blood cells from the plasma is achieved. 
EXAMPLE 29 
A process within the scope of the present invention is conducted for the 
purpose of separating blood cells from plasma. The conditions, structures, 
parameters, and process are the same as in Example 27, except that the 
transducer frequency is 2.2 mHz, and the input feed flow is 4 ml/min. 
According to the procedures of this example, good separation of blood cells 
from the plasma is achieved. 
EXAMPLE 30 
A process within the scope of the present invention is conducted using an 
embodiment substantially similar to that shown in FIG. 10 above for the 
purpose of separating crude oil dispersed into droplets from water. The 
dimensions of the structures for this example have the following values: 
Dimensions of the propagation chamber: 
Height=2.5 cm 
Width=2.5 cm 
Length=5 cm 
Dimensions of the separation vessel: 
Width=6 mm 
Residence length=2.5 cm 
Distance from propagation chamber ends=2.5 cm 
Separation surface orientation: centered 
Separation vessel material: urethane 
The frequency generator for this example is manufactured by Exact 
Electronics, Inc., model no. 528; the power amplifier is manufactured by 
Electronic Navigation Industries, Inc., model no. 240L RF Power Amplifier; 
the transducer material is lead zirconate titanate (PZT-4); the impedance 
matching section is fabricated from boron nitride; and, the reflector 
material is aluminum. 
The method of the present invention is operated utilizing the following 
parameters: 
Transducer frequency=2.5 MHz 
Ramp rate=10 KHz/sec 
Ramp direction: upward 
Cycle time for ramp=10 sec 
Input power=40 watts peak 
Output power=5 watts/cm.sup.2 
Input feed flow=4 ml/min 
According to the procedure of this example, good separation of the blood 
cells from the plasma is achieved. 
EXAMPLE 31 
A process within the scope of the present invention is conducted for the 
purpose of separating oil from water. The conditions, structures, 
parameters, and process are the same as in Example 30, except that the 
transducer frequency is 2 mHz, the ramp direction is downward, the input 
power is 50 watts peak, the output power is 5 watts/cm.sup.2, and the 
input feed flow was 3 ml/min. 
According to the procedures of this example, excellent separation of the 
oil from the water is achieved. 
EXAMPLE 32 
A process within the scope of the present invention is conducted using an 
embodiment substantially similar to that shown in FIG. 10 above for the 
purpose of separating polystyrene microspheres 34 microns in diameter from 
water. The dimensions of the structures for this example have the 
following values: 
Dimensions of the propagation chamber: 
Height=2.5 cm 
Width=2.5 cm 
Length=5 cm 
Dimensions of the separation vessel: 
Width=6 mm 
Residence length=2.5 cm 
Distance from propagation chamber ends=2.5 cm 
Separation surface orientation: centered 
Separation vessel material: acrylic 
The frequency generator for in this example is manufactured by Exact 
Electronics, Inc., model no. 528 power amplifier is manufactured by 
Electronic Navigation Industries, Inc., model no. 240L RF Power Amplifier; 
the transducer material is lead zirconate titanate (PZT-4); and, the 
reflector material is zinc. 
The method of the present invention is operated utilizing the following 
parameters: 
Transducer frequency=7 mHz 
Ramp rate=10 kHz/sec 
Ramp direction: upward 
Cycle time for ramp=10 sec 
Input power=40 watts peak 
Output power=6 watts/cm.sup.2 
Input feed flow=3 ml/min 
According to the procedures of this example, excellent separation of the 
microspheres from the water is achieved. 
EXAMPLE 33 
A process within the scope of the present invention is conducted for the 
purpose of separating microspheres from water. The conditions, structures, 
parameters, and process are the same as in Example 32, except that the 
input feed flow is 1 ml/min. 
According to the procedures of this example, excellent separation of the 
microspheres from the water is achieved. 
EXAMPLE 34 
A process within the scope of the present invention is conducted for the 
purpose of separating microspheres from water. The conditions, structures, 
parameters, and process are the same as in Example 32, except that the 
ramp rate is 5 kHz/sec. 
According to the procedures of this example, excellent separation of the 
microspheres from the water is achieved. 
G. Summary 
As will be appreciated from the explanation of the invention and 
description of several embodiments, the present invention provides methods 
and apparatus for controlling the movement of materials having different 
physical properties in a fluid. The methods and apparatus of the present 
invention are significantly more efficient than those methods and 
apparatus available in the prior art. The present invention allows 
materials contained within a fluid, whether the materials be particles, 
immiscible liquids, or undissolved gases, to be controllably moved to a 
predetermined location. The ability to controllably move materials allows 
for the efficient separation of those materials from a liquid in which 
they are suspended. 
The present invention, in contrast to the techniques shown in the prior 
art, may be used to produce either movement or separation of the material 
without relying on flocculation, sedimentation, centrifugation, the 
buoyancy of the material, or any other gravity dependent characteristic. 
The present invention is also able to make use of differing physical 
properties of the materials in order to effect movement or separation of 
the materials. These physical properties include properties such as 
acoustical properties, the densities of the materials, the volume which 
the material occupies, and other properties. 
Furthermore, the present invention is well-suited for use with either batch 
processing or continuous flow processing operations. Since the invention 
is well-suited for continuous flow operations, nearly any desired 
throughput may be achieved, using the embodiments described herein, by 
connecting the embodiments in a parallel fashion. Still further, the 
flexibility of the invention is such that if a high degree of separation 
is desired, several embodiments may be serially joined so as to allow the 
continuous flow of the medium to be processed by several of the 
embodiments. 
The present invention also provides that the materials which are being 
subjected to the process will not be harmed by the process. This 
attribute, as well as those mentioned above, make the present invention 
particularly well-suited for processing biological materials, such as 
separating blood cells from plasma. Still further, the invention is very 
useful for separating immiscible liquids from one another or undissolved 
gases from a liquid. 
It will be appreciated that the methods and apparatus of the present 
invention are capable of being incorporated in the form of a variety of 
embodiments, only a few of which have been illustrated and described 
above. The invention may be embodied in other specific forms without 
departing from its spirit or essential characteristics. The described 
embodiments are to be considered in all respects only as illustrative, and 
not restrictive, and the scope of the invention is, therefore, indicated 
by the appended claims rather than by the foregoing description. All 
changes which come within the meaning and range of equivalency of the 
claims are to be embraced within their scope.