Abstract:
Embodiments generally relate to systems and methods for separating an emulsion in a cavity of a device such as a liquid lens device. In one embodiment, the method comprises at least one of: applying a bias voltage to electrodes in the device, causing at least one of droplet migration, flattening of large droplets, and reduced droplet surface tension; applying an oscillating actuation voltage waveform comprising an actuation frequency to the electrodes, such that fluid pumping and turbulence is created within the device cavity; and applying an oscillating excitation voltage waveform comprising an excitation frequency to the electrodes, such that the varying electric field created by the oscillating voltage causes small droplets of the first liquid to coalesce.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Patent Application Ser. No. 62/099,097, entitled “Method to remove emulsion in a liquid lens”, filed on Dec. 31, 2015, which is hereby incorporated by reference as if set forth in full in this application for all purposes. 
     
    
     BACKGROUND 
       [0002]    An emulsion is defined as a fine dispersion of minute droplets of a first liquid in a second liquid in which the first liquid is not soluble or miscible. The emulsion can occur by vigorous mixing or shaking of the liquid mixture. A liquid lens (such as those produced by Optilux of Santa Barbara, Calif.) is created by enclosing two fluids with an engineered index of refraction difference within a housing. The housing incorporates electrodes which manipulate the shape of the lens based on the principles of electro-wetting. In the case of a liquid lens, an emulsion of the fluids in undesirable as it will negatively impact the optical performance of the lens. This can occur if the lens is subject to shock loads, such as caused by an impact due to dropping the device. 
         [0003]    Methods by which an emulsion of two fluids may be “demulsified” or separated back into its two fluid constituents are known in fields such as the oil/gas industry. For example, a centrifuge can be used to separate the two fluids based on differences in their density. Another prior art method to separate an emulsion is to use chemicals that affect the surface tension of the fluids. Yet another method involves using electric fields to induce coalescence of the droplets of one fluid. However, there is no known prior art that specifically addresses the problem of an emulsion in a liquid lens cavity, and in particular an emulsion caused by severe shock loading of a liquid lens device including such a cavity. 
         [0004]    The need therefore exists for methods and systems specifically tailored to clear or remove an emulsion from the field of view of a liquid lens. Ideally the clearing would occur very quickly (in less than 10 seconds, for example) without the need for deliberate user input or the involvement of devices external to the liquid lens system. The clearing could occur automatically during power up, or may be initiated in response to a signal from a sensor. 
       SUMMARY 
       [0005]    The present invention includes a method for separating an emulsion comprising droplets of a first liquid suspended in a second liquid within a cavity of a liquid lens device. The method comprises at least one of: applying a bias voltage to electrodes in the liquid lens device, causing at least one of droplet migration, flattening of large droplets, and reduced droplet surface tension; applying an oscillating actuation voltage waveform comprising an actuation frequency to the electrodes, such that fluid pumping and turbulence is created within the liquid lens cavity; and applying an oscillating excitation voltage waveform comprising an excitation frequency to the electrodes, such that the varying electric field created by the oscillating voltage causes small droplets of the first liquid to coalesce. 
         [0006]    In one aspect, the bias voltage applied to the electrodes encourages migration of the first liquid towards one side of the liquid lens cavity and migration of the second liquid towards an oppositely situated side of the liquid lens. 
         [0007]    In one aspect, the excitation and oscillating actuation waveforms are applied, and the actuation frequency is lower than the excitation frequency. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic cross sectional view of a prior art liquid lens that may be operated on according to one embodiment. 
           [0009]      FIG. 2  is a flowchart of a method for removing an emulsion in a liquid lens according to one embodiment. 
           [0010]      FIG. 3  illustrates sequential top-down views of a liquid lens in which an emulsion, clearly visible in the top left view, is gradually separated to provide a clear liquid lens in the bottom right view, according to one embodiment. 
           [0011]      FIG. 4A  illustrates side view images of the early stages of droplet coalescence according to one embodiment. 
           [0012]      FIG. 4B  illustrates side view images showing coalescence of two out of three droplets according to one embodiment. 
           [0013]      FIG. 5  illustrates excitation and actuation voltage waveforms according to one embodiment. 
           [0014]      FIG. 6  illustrates a swept sine excitation voltage waveform and a liquid lens response according to one embodiment. 
           [0015]      FIG. 7  illustrates four possible combinations of excitation and actuation voltage waveforms according to some embodiments. 
           [0016]      FIG. 8  illustrates an excitation voltage waveform including a dwell time according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The manner in which the present invention provides its advantages can be more easily understood with reference to  FIGS. 1 through 8 . 
         [0018]      FIG. 1  is a schematic cross sectional view of a prior art liquid lens  100  that may benefit by being operated on according to embodiments of the present invention. Liquid lens cavity  101  contains polar liquid  114  and non-polar liquid  116 . Liquids  114  and  116  are chosen to be non-miscible and of different refractive indices, but of similar specific gravity, as required for liquid lens functionality. They may, for example, comprise an aqueous component and an oil component respectively. The base and top plates bounding lens cavity  101  are transparent, at least in their central regions directly underlying and overlying the cavity, at the wavelength of intended operation of the liquid lens. A layer of insulating material  112  lies on the side walls of cavity  101  on top of electrodes  106 . 
         [0019]      FIG. 2  is a flowchart illustrating one embodiment of a method  200  for carrying out the present invention. The illustrated embodiment shows three steps,  202 ,  204 ,  206 , but it should be noted that first, any one or two of the steps may be omitted, and second, the order in which the steps are shown is arbitrary. In some embodiments, any two or all three steps may be carried out simultaneously. 
         [0020]    At step  202 , a bias voltage is applied to electrodes of the liquid lens. Typically, this will be a simple DC voltage applied across the same electrodes  106  used to actuate the liquid lens in its normal, focusing applications, independent of any shock event. At step  204 , an oscillating actuation voltage waveform is applied to liquid lens actuation electrodes  106 . At step  206 , an excitation voltage waveform is applied to liquid lens electrodes. Again, this will typically be the same electrodes  106  used to actuate the liquid lens in its normal, focusing applications. 
         [0021]    Consider some of the cases of interest to the present invention, where shock has caused an emulsion to form within cavity  101 , meaning that droplets of one of liquids  114  and  116  are suspended in the other liquid, and then one or more of the three voltage waveforms is applied. 
         [0022]    The application of a bias voltage may induce or facilitate migration of the droplets by virtue of its effect on polar liquid  114 , which in turn affects the spatial positioning of liquid  116 . Such migration may in itself clear the useful optical path of the liquid lens, even if the emulsion is not completely removed. In addition, the application of the bias voltage creates a DC electric field that may cause larger droplets to flatten and/or reduce their surface tension, This in turn encourages their merging or coalescence into even larger drops, which is very beneficial to the goal of demulsification, as described below. 
         [0023]    Independently of whether a bias voltage is applied, the application of an oscillating actuation voltage waveform will cause the shape of the meniscus to change in a periodic manner, via normal electro-wetting processes, physically moving the droplets in a pumping manner. Collisions of these moving droplets with each other encourages coalescence. Pumping can create turbulence and motivate larger drop mixing as well as release of droplets adhered to the wall of the lens cavity, which in turn can coalesce. Collisions with the cavity walls can also encourage coalescence and migration. 
         [0024]    Independently of whether either the bias voltage or the actuation voltage waveforms are applied, the application of an oscillating excitation voltage waveform will cause the droplets to combine based on the principles of electro-coalescence and electro-wetting. Under the force of the electric field the suspended droplets merge together, or coalesce, to form larger droplets. The process continues as larger and larger droplets are formed, until ultimately there is preferably a single predominant droplet (i.e. complete separation of the fluids). 
         [0025]    In some embodiments, all three of the voltage types described above are applied at substantially the same time, to achieve the goal of combining all droplets as quickly and efficiently as possible. 
         [0026]    In some embodiments, droplet migration can be aided by affinity between the liquid in the droplets to the surfaces of the lens cavity. The surfaces of the lens cavity can be hydrophobic or hydrophilic, either naturally, by virtue of the bulk material from which the liquid lens is fabricated, or by addition of a coating, which would attract or repel the polar fluid. Conversely, the surface can be oliophobic or oliophilic, which would attract or repel the non-polar fluid respectively. These surface properties can be used to aid in separation of the emulsion, as the liquid lens is subject to the oscillating voltage for example by using an hydrophilic coating on the surface near one electrode and an oliophilic coating on the surface near an opposing electrode. In some embodiments, the affinity coating may be the same electrically insulating coating  112  overlying an electrode  106 . In some embodiments, the affinity coating may be an additional coating layer deposited over coating  112 . 
         [0027]      FIG. 3  shows a sequence of seven top-down views of an actual experimental liquid lens in which an emulsion, clearly visible in the top left view, is gradually separated (viewing images from left to right in the top row and then in the bottom row) to provide a clear liquid lens in the bottom right view, according to one embodiment. The first image, the top left view, shows a large number of droplets, seen as dark circles against a lighter background. The optical performance of the lens would obviously be very badly impacted by the presence of these droplets. However, applying a method of the present invention to the lens results in a dramatic reduction in the number of droplets, especially the smaller droplets, over time, until in the final view, no droplets are seen at all. The time taken to achieve such clearing of an emulsion can be as short as a few seconds or a few minutes at most, as the liquids respond to applied voltages within 10s of milliseconds. 
         [0028]      FIGS. 4A and 4B  illustrate coalescence events in two different cases. In  FIG. 4A , the smaller drop is shown (above) approaching the much larger drop, only a portion of whose surface is shown, and then (below) as it is about merge into that larger drop. In  FIG. 4B , from left to right, two larger drops are shown distorting towards the smaller drop positioned between them, until the lower larger drop coalesces completely with that smaller drop, although remaining separate from the other large drop. It should be noted that the relatively low surface tension of larger versus smaller drops leads to the tendency of the former to distort and merge with the latter. When two smaller drops come into contact, the requirement that the relatively high surface tension of either one be overcome makes coalescence events less likely. 
         [0029]    In the present invention, the application of a DC field may be helpful in directly or indirectly encouraging the flattening or other distortion of droplets, making coalescence events more likely. 
         [0030]    A typical frequency for an actuation waveform is 50 Hz, while a typical waveform for an excitation waveform is 100 Hz, but the values chosen for any given device may be determined according to the size of the lens cavity and the particular liquids that are used.  FIG. 5  illustrates sinusoidal actuation voltage waveform  502  and excitation voltage waveform  504  for one exemplary device, where the frequency of the latter is approximately 10× the frequency of the former. 
         [0031]    In general, the frequency or frequency band of the oscillating excitation waveform, their constancy or variability, and the resulting detailed shape are tuned or tailored to stimulate electro-coalescence according to the droplet size of the emulsification and the specific two-liquid combination used in the liquid lens. The voltage excitation signal may be a sine, square, triangle, or saw-tooth shape or intermixed combinations thereof. The frequency may be constant, and preferably in the range 1 Hz to 1,000 Hz. In other cases, the frequency can vary, for example following a swept sine function  604 , as shown in  FIG. 6 . As the frequency of the swept sine function increases, the liquid lens acts as a filter as in a LRC circuit, illustrated by curve  608 . The liquid lens thus has a decaying amplitude response as shown by the envelope  610 , which can result in a DC voltage if the frequency is specified high enough. In the case shown, where the sine function is centered on zero volts, the final DC voltage would actually be zero. 
         [0032]    In some embodiments, the actuation waveform applied to the liquid lens cycles from minimum voltage (Vmin) to maximum voltage (Vmax) at a period required for the lens to settle near each end of its operating range. This waveform moves the lens meniscus through a desired range of focus and/or through a desired range of fluidic tilt, thereby physically moving the liquids in the lens cavity and encouraging drop-to-drop contact and coalescence. The frequency of this actuation waveform is typically in the range 20 to 200 Hz, corresponding to a settling time of 50 to 5 ms for focus actuation. The higher frequency excitation waveform superimposed on the actuation voltage waveform operates too quickly to cause meniscus movement, operating instead on the droplets of the emulsion, stimulating smaller droplets within the emulsion to agglomerate or move toward the bulk fluid interface. The frequency of this higher frequency signal may be in a range of 2× to 100× that of the lens actuation signal, and preferably sweep through a range of frequencies. Four options for combining or superimposing the actuation and excitation signals are shown in  FIG. 7 . 
         [0033]    In some embodiments, the liquid lens can be actuated in tilt. Electrodes positioned at opposite sides of the lens cavity are actuated at differing voltages to cause the meniscus to tilt. The tilt actuation can be a back and forth tilt, along the positive and negative direction of any axis or it may oscillate around the vertical axis of symmetry through the lens center, like the rotation of a clock hand around the 360 degree azimuth of the lens. This tilting actuation can be used independently or in addition to the focus actuation to stimulate coalescence in the liquid lens. 
         [0034]    A further aspect of the novel method proposed here for a liquid lens is the option of incorporating a dwell time at prescribed intervals during the voltage excitation process. This dwell time, during which the excitation voltage is held at a constant value, enhances the coalescence process by allowing time for the droplets to build electrical charge and change shape before being subjected to the next cycle. In one embodiment taking advantage of this aspect, every set of 5 cycles of the actuation waveform plus swept sine function excitation waveform is followed by a dwell time of 10 ms at Vmax, before the next set of 5 cycles is applied.  FIG. 8  shows another example of an electrical excitation waveform  804  incorporating a dwell time  820  after each cycle of the sweep. 
         [0035]    Advantages of the present invention over demulsification approaches described in prior art include the ability to achieve rapid separation with much lower electric fields (35 V/m has been found to work well, in comparison to the several hundred V/mm reported in the literature) avoidance of shorting issues (because liquid lenses are designed to include a dielectric coating covering the electrodes), and the absence of particles or other debris that often hamper electro-coalescence in other applications (because pure liquids are used in liquid lenses). 
         [0036]    A typical design choice for electrowetting lenses is to match the densities of the two liquids well within the anticipated use or storage range to eliminate any effects of gravity or accelerations on the optical performance of the lens. However, this actually counters the ability of an emulsified lens to coalesce on its own over time by the natural settling effect of gravity. A further aspect of this invention involves matching liquid densities at the higher end of the use or storage temperature range of the product to prevent emulsification and aid the coalescing effect of the applied voltage waveforms. Elevated temperature reduces the viscosity thereby increasing the height of the capillary waves that can cause break off of droplets leading to an emulsion. By matching the densities at elevated temperatures, the motivation for the fluids to emulsify is greatly reduced as predicted by the Bond number. At lower temperatures, where the viscosity is higher, the densities can diverge a bit more because electro-coalescence is more efficient when the densities diverge. 
         [0037]    The methods of demulsification discussed above may be applied when needed, after emulsification has occurred, but may also be applied automatically, without regard to any specific event causing emulsification. They may, for example, be carried out as a routine process during powering up of the liquid lens device. 
         [0038]    In some embodiments, the methods of demulsification discussed above may be applied during a shock event or in anticipation of such an event. In either case, an acceleration sensor may be used to anticipate an impending shock force, for example a fall, and the voltage would then be applied in response to the sensor signal. This is analogous to shock protection used in the hard disk drive industry to prevent damage to the disk head or magnetic platens. 
         [0039]    A further aspect of the present invention is the possibility of preventing an emulsion from forming by applying a voltage determined by a control algorithm to an un-emulsified liquid lens. In this case, a voltage may be applied to the liquid lens to apply a force to the liquids that reduces their tendency to separate. For example, a specific voltage may be used to minimize the length of the meniscus and thereby lower the characteristic length parameter in the Bond number. 
         [0040]    The present invention is also applicable to Lab-On-Chip (LOC) applications. Electro-coalescence is known to be a useful method of achieving mixing in microfluidic channels. The principles of electro-coalescence and electro-wetting can be applied in combination as disclosed above, and used to demulsify fluids or combine droplets in LOC applications. In particular, liquid beads on a plate can be submersed in water or other suitable fluid and then physically moved using the principles of electro-wetting via on-chip conductive electrodes. The motion in coordination with the varying electric signals as described here can be used to enhance droplet coalescence. 
         [0041]    Embodiments described herein provide various benefits. In particular, embodiments provide for the rapid and efficient separation of an emulsion in a liquid lens device into its component liquids, restoring the optical clarity of the lens. Some embodiments carry out the separation during or in anticipation of a shock event tending to cause emulsification. Some embodiments aim to prevent the formation of emulsions. 
         [0042]    The above-described embodiments should be considered as examples of the present invention, rather than as limiting the scope of the invention. Various modifications of the above-described embodiments of the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.