Abstract:
A mixing system has a parallel droplet dispenser capable of making droplets of a first working material in the range of 10 nanometers to 10 micrometers, a pump to deliver fluid for the droplets of the first working material and to produce a first emulsion, a compact mixer having low inter-voxel mixing to receive the emulsion and produce a homogenous material, and a dispensing system. A method of dispensing a graded material includes generating droplets of a first working material, the droplets having a size in the range of 10 nanometers to 10 micrometers, adding the droplets of the first working material into a fluid to create a first emulsion, wherein addition of the droplets of the first working material is controlled to create gradient in the emulsion, mixing the first emulsion to create a homogenous, graded mixture, and dispensing the homogenous, graded mixture onto a surface.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to rapid mixing of highly viscous materials, more particularly to rapid mixing of highly viscous materials in small volumes such as those used in digital printers. 
       BACKGROUND 
       [0002]    Mixing is a common problem and there are many different approaches. However very few of them are appropriate for use in a high-resolution digital functional gradient manufacturing system. A digital functional gradient system is one that deposits materials having varying concentrations of different substances. The materials used usually have high viscosity. For example, typical UV curable three-dimensional printing resins have viscosities roughly 10 to 1000 times higher than water. Thermoset resins, also typically have viscosities much higher than that of water and the viscosity can be increased even higher with the addition of reinforcement agents such short fibers. In order to achieve high resolution, the absolute size of a bead or drop of material is small. The combination of high viscosity and small geometry means that the Reynolds number is much less than 1. The Reynolds number is the ratio of inertial forces to viscous forces and consequently quantifies the relative importance of these two types of forces for given flow conditions. 
         [0003]    One goal behind mixing is to create the shortest possible diffusion path length, or in the case of immiscible polymer blending, to achieve nano-scale emulsion, by repeated cutting, folding and stretching operations. This is typically referred to as distributive mixing. At some very small diffusion path length, typically in the microns to nanometer scale, the material becomes sufficiently distributive such that diffusive mixing occurs. 
         [0004]    In a digital functional gradient manufacturing system, the ratio of the mix changes as the system deposits the material. In order to be able to control this gradient of mixture, the mixing must be achieved in a way that the components of a deposited material are mixed within a voxel (or volumetric pixel), but that nearby voxels are not mixed together substantially. The discussion here refers to mixing between voxels as upstream and downstream of flow, while mixing within voxels is characterized by cross flow mixing. 
         [0005]    Current approaches include static line mixers, twin-screw mixers and microfluidic mixers, but none of them solves these particular problems. In-line static mixers achieve multiple stretching and folding operations that reduce diffusion path length, but have high dispersion upstream and downstream. Twin-screw mixers typically achieve polymer compounding with multiple stretching and folding operations for extremely viscous fluids but have extremely high upstream and downstream dispersion. 
         [0006]    Microfluidic mixers use droplet fission, flow instabilities, or valves to achieve mixing, but these are typically demonstrated in aqueous media with significantly lower viscosities. Active mixers are highly complex concepts. These approaches use small chambers with stir bars, acoustics or electrical fields, but these do not work well with high viscosities and the small scales of interest in digital function material printing. 
       SUMMARY 
       [0007]    One embodiment is a mixing system has a parallel droplet dispenser capable of making droplets of a first working material in the range of 10 nanometers to 10 micrometers, a pump to deliver fluid for the droplets of the first working material and to produce a first emulsion, a compact mixer having low inter-voxel mixing to receive the emulsion and produce a homogenous material, and a dispensing system. 
         [0008]    Another embodiment is a method of dispensing a graded material includes generating droplets of a first working material, the droplets having a size in the range of 10 nanometers to 10 micrometers, adding the droplets of the first working material into a fluid to create a first emulsion, wherein addition of the droplets of the first working material is controlled to create gradient in the emulsion, mixing the first emulsion to create a homogenous, graded mixture, and dispensing the homogenous, graded mixture onto a surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows a prior art embodiment of mixing. 
           [0010]      FIG. 2  shows a system diagram of a mixer. 
           [0011]      FIGS. 3-5  show alternative embodiments of a mixing system. 
           [0012]      FIG. 6  shows embodiments of a deposition nozzle. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0013]      FIG. 1  shows a prior art process  10  for mixing. The majority of operations in static or twin mixers create surface area, referred to here as distributive mixing. As shown in  FIG. 1 , the original material  12 , most likely a high viscosity fluid undergoes stretching between  12  and  14 . The mixer then cuts and stacks the material to create surface area as in  16  or folded as in  18 , or more than likely a combination of both. Another method available for mixing is dispersive mixing. The process typically stretches the material at  20  and the material breaks into droplets at  22 . Droplets break up as in  22  or coalesce as in  24 , in single or multiple steps. Repetition and combinations of these steps generally leads to better dispersion. For example, one might start by taking a mixture of fluids through distributive mixing and then doing dispersive mixing. Multiple mixing steps that lead to good cross flow dispersion, often also lead to significant upstream and downstream dispersion. 
         [0014]    It is possible to skip multiple steps in the process by creating an emulsion of small droplets directly and varying the composition of the emulsion. This allows a relatively short mixing section with limited upstream and downstream dispersion to create a homogenous graded material with controlled and sharp changes in composition. The changes in composition provided digitally graded material for deposition on a substrate. 
         [0015]    The dispersion of the mixed material can be defined as a measure of the degree of variation in composition across a volume. Various statistical measures can be made to quantify the degree of dispersion across a fluid domain. This can be measured across an entire fluid domain or a subset of that domain. For example, one can measure the dispersion of a mixed voxel, or between several voxels. 
         [0016]    In the following discussion, “low dispersion” means a dispersion of a first fluid into a second fluid having a coefficient of dispersion less than 1. A “compact mixer” is one that has a size that has a working volume close to the size of a single voxel or less. A “voxel” is a volumetric pixel. The compact mixers under discussion here have ‘low’ inter-voxel mixing, defined as less than 10% cross-voxel mixing. Much like pixels, or picture elements, voxels have a resolution depending upon their size. For a droplet based deposition system, such as a printer, the resolution generally correlates to the size of the deposited droplet. For a continuous extrusion process such as a digital functional gradient manufacturing machine, the size of a voxel relates to the cross section of the extrusion and the rate at which the composition of the extrudate can be changed. 
         [0017]    One issue that arises in creating an emulsion lies in the dispensing of one material into another with sufficiently small droplet size to create an emulsion with little or no required additional mixing. However, it is possible. Several calculations support the viability of the embodiments here. For a voxel size of 100 microns, one assumes a hemisphere having a diameter of 100 microns to give a volume of roughly 250 Pico liters. For a deposition speed of 1 cm/s or 10,000 microns/sec, the rate of deposition for 100-micron voxels is 100 voxels/sec. In the most extreme case, when a voxel is mixed on demand and then immediately deposited, a mixer must be able to mix the voxel in less than 0.01 s. However, it is possible to mix multiple voxels simultaneously. Mixing multiple voxels increase the complexity and size of the deposition system, but decreases the performance requirement on the mixer. 
         [0018]    To make a material with 50% volume fraction, it is necessary to inject 125 Pico liters/sec of each material. Each material may be injected directly into the other fluid or a neutral carrier fluid that is not part of the final mixture. In order to estimate the size of droplets needed, one must calculate the diffusion rate. As an example, the diffusivity of fluorescein dye in 70,000 MW 8 g/dl dextran has been estimated to be approximately 4×10 −10  m 2 /s. If one assumes the mixing materials have diffusivities of 2 orders of magnitude slower, given a mixing time target of 0.01 s, a reasonable diffusion length estimate is 0.2 microns. 
         [0019]    Under these conditions, it would be ideal to produce droplets with a diameter of 0.2 microns or less. In order to achieve the deposition rates required for droplets produced on demand, 250,000 droplets must be produced every 0.01 seconds or 2.5 million droplets/s. 
         [0020]    It is worth noting that unlike a large number of small droplet dispensing techniques the distribution of droplet sizes is not of primary importance. These droplets are primarily used to create surface area between the fluids being mixed and will eventually be all mixed to together, so it is only important that the total dispensed volume is accurate. Even a system with a high coefficient of variation can achieve this by dispensing a large number of droplets. 
         [0021]    While 2.5 million droplets could be created per second using a high speed, such as a 100 kHz piezo actuator and thousands of nozzles, the mixing operations for a voxel do not necessarily need to be done within the deposition rate for one voxel. The system may generate droplets upstream at a reduced rate that is easily accessible using other established means. Alternatively, it is possible to create larger drops that achieve the correct volume fraction and use downstream mixing to complete the creation of the required surface area. 
         [0022]    A number of methods exist to produce small droplets of less than 1 micron. Using flow focusing architectures, such as extensional flows or elongational flows, it is possible to make drops using simple to fabricate geometries that can produce droplets on the order of a micron or less. References exist that show parallel nozzles for emulsion generation that can produce droplets on the order of 5 microns in a controllable, stable and robust manner. Spray techniques such as electrospray or filament extension atomization can produce large quantities of smaller particles, though another apparatus is required to modulate the quantity of droplets. 
         [0023]    The above discussion demonstrates that it is possible to achieve rapid and low axial dispersion mixing for the digital deposition of graded materials. U.S. patent application Ser. No. 14/578,044, “SYSTEM FOR DIGITAL FABRICATION OF GRADED, HIERARCHICAL MATERIAL STRUCTURES,” filed Dec. 29, 2014 teaches an embodiment of this type of system. These systems typically employ some kind of mixer, such as shown in  FIG. 2 .  FIG. 2  shows an embodiment of a droplet mixer for a digital functional graded material (DFGM) print head. 
         [0024]    The droplet mixer  30  has a first source of a carrier fluid  32 . The source may be a tank, reservoir or other container  31 . As will be discussed in more detail later with reference to  FIGS. 3-5 , the carrier fluid may also consist of a second polymer material. The reservoir  34  allows the first polymer material  36  to be injected into the carrier fluid  32  in the form of small droplets. The injection of the droplets will typically be under the control of some sort of positive placement system such as a pump or other injector  37 . 
         [0025]    The reservoir  34  represents the portion of the system creating the emulsion that would otherwise require a distributive mixing system, the replacement of which makes the system smaller and faster than using multiple mixing steps. By creating significant surface area directly, there is no need for distributive steps. After the creation of the emulsion, the emulsion may undergo further downstream mixing at  38  and then is dispensed at printhead  39 . 
         [0026]    The embodiments of this system may take one of many forms.  FIGS. 3-5  show some embodiments of different configurations, with the understanding that these are merely examples and should not limit in anyway the scope of the claims.  FIG. 3  shows an embodiment in which the droplets of two different materials disperse into a same immiscible carrier liquid, one that cannot undergo mixing or blending. The first material  36  pre-disperses into the carrier fluid at reservoir  41  and the second material  32  is pre-dispersed into the carrier fluid at reservoir  43 . Pumps  42  and  44 , which may consist of gear pumps, then pump the material out of the reservoirs into a common channel  48 . The ratio of the first and second material mixed is controlled by the relative flow rate of the pumps. A selective membrane  46  before the mixing and dispensing at  50  allows the carrier fluid to be removed. The carrier fluid can be recycled and reused. 
         [0027]    This system has the advantage of having easily controlled dispensing systems at a much larger scale than small droplet dispensing systems and a consistent, upstream process that may lack variation can create the small droplets. Examples include electrosprays or filament extensional atomizers. This allows droplet creation systems that may otherwise be difficult to engineer into the system 
         [0028]    In the embodiment  40  of  FIG. 4 , a second fluid  32  and the first fluid  36  disperse into the common channel  48 . The pump  42  enables the injection into the second fluid. The pump  44  pulls the liquid from the common channel and sends it to the downstream mixing and dispensing  50 . The ratio of the first fluid and second fluid change by altering the rate of droplet dispensing of the first fluid in relation to the gear pump  42 . The flow rate of the second optional gear pump  44  must equal the flow rate of the first pump  42  plus the flow rate of the droplet dispenser. In some embodiments, one of the fluids may contain reinforcement agents, such as carbon fiber. In order to avoid making small droplets of reinforced material, the preferred embodiment is to dispense the material without reinforcement into the fluid with reinforcement. 
         [0029]      FIG. 5  shows an embodiment in which the system dispenses the droplets of both materials into a carrier fluid, which later separates from the entire mixture before mixing and dispensing. In  FIG. 5 , the system dispenses the two polymers  32  and  36  into the common channel  48 . The pump  42  provides positive displacement to inject the fluids and moves the mixed fluid downstream to the mixing and dispensing  50  through the separator, in this embodiment the separator consists of membrane  46 . 
         [0030]    One of the controlling factors in drop wise dispensing accuracy and precision is the pinch off behavior of the droplet. Both the surfaces of the dispensing apparatus and the fluid into which the droplet is being dispensed impacts this behavior. Controlling the fluid into which the droplet is dispensed reduces the systematic errors and non-linearity in the system due to change in the fluid into which the droplet is being dispensed. 
         [0031]    For higher viscosity fluids, such as those of interest here, one method of injection the droplets into another liquid uses a positive displacement system. A positive displacement system applies some sort of positive force on the fluid injected into the other fluid. One possibility consists of a capillary and small piston. The piston actuates in the capillary to dispense a droplet of the fluid.  FIG. 6  shows an example of such a system. 
         [0032]    The dispenser  60  has a channel  64  in which a piston  62  moves. A single droplet system converts to a continuous droplet system with the addition of a one-way valve  66  that controls the flow of the fluid  68 . The piston starts in the neutral position and then moves in a downward direction to touch the surface  70 . This may increase the accuracy and decrease the droplet size. When dispensing into another fluid, the surface  70  may be the bottom of the common channel  48  from the previous figures, or the reservoirs for the fluids. After dispensing the drop  74  on the surface, the piston moves upwards in the channel  62  as shown by the arrow. The system draws new fluid  68  through the one-way valve  66 . The piston may have a seal  72  that prevents the fluid from moving in the wrong direction in the dispenser. 
         [0033]    In this manner, a faster and smaller mixer can provide graded materials as part of a digital deposition system or printhead. By controlling the flow of the two materials relative to each other, one can finely control the concentrations of the different polymers to have sharp changes in composition, creating the gradations desired. The use of a droplet generator to create the emulsion eliminates the need for distributive mixing and allows the mixer to be smaller and mix fluids faster to reduce the overall volume in the system. 
         [0034]    It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art, which are also intended to be encompassed by the following claims.