Abstract:
A switchable element is described, which comprises a fluid chamber ( 111 ) containing a polar ( 101 ) and a non-polar ( 102 ) liquid, which are immiscible. The element is further provided with two electrodes ( 103,104 ), which are arranged to control the spatial distribution of the liquids, by means of an applied voltage across two the two electrodes. By the addition of surfactant to one or both of the liquids, the actuation voltage of the element is lowered. The element can work for example as an optical device or a motor.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to switchable elements, and to devices including switchable elements.  
       TECHNICAL BACKGROUND  
       [0002]     Electro-wetting is essentially the phenomenon whereby an electric field modifies the wetting behavior of a polar liquid in contact with a hydrophobic surface. By applying an electric field, a surface energy gradient is created in the polar liquid, which can be used to manipulate the liquid. In common applications, water is used as the polar liquid.  
         [0003]     Recently, switchable elements have been proposed and demonstrated to operate based on the principle of electro-wetting. Such switchable elements typically consist of a closed cell that is filled with one part of water and one part of oil, although other liquids might also be used. The important characteristics are that the liquids are immiscible and that one of them is polar and or electrically conducting (e.g. water), while the other one is non-polar (e.g. oil). Other liquid parameters of importance, when the device is used as an optical element, are for example refractive index, melting point, transmission and density.  
         [0004]     Since the liquids are immiscible, a well-defined interface will always be present between them. The inner surface of the cell generally comprises two separate surfaces, one that is hydrophobic and one that is non-hydrophobic. The hydrophobic surface will by nature reject the water, and by configuring the surfaces properly the spatial relationship between the liquids can be predetermined, i.e. the water is forced to a predetermined location opposite the hydrophobic surface. Consequently, also the interface between the two liquids can be predetermined.  
         [0005]     Furthermore two electrodes are arranged in the cell, one address electrode arranged behind the hydrophobic and electrically insulating coating and one counter electrode in direct contact with or capacitively coupled to the conducting liquid. By applying a potential between the electrodes, an electric field is created across the insulating coating. The electric field gives rise to an electrostatic force that overrides the force of attraction exercised by the molecules in the conducting liquid and thus modifies the spatial relation between the liquids and consequently also the shape and position of the liquid interface. In effect, the liquid interface can be controlled by means of controlling the applied potential.  
         [0006]     There are several known principles exploiting this mechanism and with which a cell can be controlled. According to a first principle the liquids are chosen to have different transmission properties. By changing the spatial distribution of the liquids, the transmission of the component is varied. According to a second principle, the liquids are chosen to have different indices of refraction. This turns the meniscus between the liquids into a lens having refractive properties that can be controlled by means of the electrode potential. Typically, this lens can be changed between a convex, light focusing state and a concave, light defocusing state. In this document an OFF-state refers to a condition, wherein an applied voltage between the electrodes is substantially zero. Further, and an ON-state refers to a condition, wherein an applied voltage causes a substantial change in spatial distribution of the liquids, compared to the OFF-state.  
         [0007]     Electro wetting cells are further described in WO2002/099527, WO2003/069380 and WO2004/027489.  
         [0008]     These electro-wetting elements can be arranged to work as different optical and other components, for example as motor, variable focus lenses, variable diaphragms, variable filters, gratings, beam deflectors, mechanical actuators and electro-wetting based displays. When these electro-wetting elements are used in portable devices, the power consumption and the actuation voltages of the device are of particular importance. If the actuation voltage is too high it might be necessary to include additional electronics to drive the device. Such additional electronics has a number of drawbacks, one being increased development and manufacturing costs of the device. Another drawback related to a high power consumption might be that batteries of the portable device need to be recharged so frequently, that use of the portable device is substantially restricted.  
       SUMMARY OF THE INVENTION  
       [0009]     It is an object of the present invention to eliminate, or at least alleviate, the above described problems related to power consumption and actuation voltage in electro-wetting devices.  
         [0010]     This object is achieved by a method and a device in accordance with the appended claims  1  and  10 , respectively. Preferred embodiments are defined in the dependent claims.  
         [0011]     The invention is based on a realization of the inventors, that by affecting the surface tension of the liquids in an optical element, one can lower the voltage needed to control the optical cell without substantially altering its optical performance.  
         [0012]     Equation (1) shows how the voltage can be influenced by changing the surface tensions between a first non-polar liquid NPL and a second polar liquid PL, which are comprised in an electro wetting cell.  
                 γ     NPL   /   PL       ⁢   cos   ⁢           ⁢   θ     =       γ     NPL   /   wall       -     γ     PL   /   wall       +           ɛ   0     ⁢     ɛ   r         2   ⁢   d       ⁢     V   2                 (   1   )             
 
 γ are the various interfacial tensions, θ is the contact angle between the meniscus and the wall of the fluid chamber, measured through the conducting liquid. Apart from the correct interfacial tension the liquids must fulfill also a series of other requirements, like having appropriate refractive indices, melting points, transmissions, densities, viscosities etc. Therefore, depending on the choice of the liquids, only a limited choice of discrete values for the three interfacial tensions is possible. 
 
         [0013]     Thus, according to a first aspect thereof, the present invention provides a switchable element. The element comprises a fluid chamber, which comprises a first and a second body of fluid. The first fluid is a non-conducting liquid, and the second fluid is a polar and/or electrically conducting liquid. Said element further comprises a first and a second electrode, which are arranged to control the spatial distribution of said liquids. Finally, at least one of said first and second fluids comprises a surfactant  
         [0014]     According to a second aspect thereof, the invention provides a device, which comprises said switchable element.  
         [0015]     One of the advantages of changing the surface tension by dissolving surfactants in the liquids, is that one can affect the surface tension, and by this the actuation voltage, without substantially changing other characteristic parameters of the liquids. This is due to the fact that the surfactants predominantly influence interfaces. Therefore they are only needed in small amounts, and thereby they hardly influence the bulk properties of the liquids. The man skilled in the art could by way of experiments, and by the aid of the further explanations below, determine the amount of surfactants that is needed in a specific case.  
         [0016]     A switchable device, wherein the surface tension between said first and second liquids has been affected in the way that is defined in claim  2 , advantageously influences the only surface tension that affects the sensitivity of the contact angle with respect to the applied voltage. A desired change in contact angle between the meniscus and the wall of the fluid chamber, can be achieved by adjusting the voltage applied to the electrodes. As can be seen in equation 1, the only surface tension affecting the amount of voltage needed to achieve a certain change in contact angle, is the surface tension between the two liquids. In other words, a lower γ NPL/PL  lowers the voltage needed to effect a certain change in contact angle.  
         [0017]     A switchable device, wherein the surface tension between the cell wall and said first or second liquids, as defined in claim  3  or  4 , has the advantage of enabling an adjustment of the contact angle in the OFF-state, which does not influence the sensitivity of the contact angle with respect to the applied voltage.  
         [0018]     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  schematically shows a side view of an embodiment of the present invention, wherein the surfactant affects the interfacial tension between a polar and a non-polar liquid.  
         [0020]      FIG. 2  schematically shows a side view of an embodiment of the present invention, wherein the surfactant affects the interfacial tensions between a non-polar liquid and a wall of the cell.  
         [0021]      FIG. 3  schematically shows a side view of an embodiment of the present invention, wherein the surfactant affects the interfacial tensions between a polar liquid and a wall of the cell.  
         [0022]      FIGS. 4   a  and  4   b  shows, in a cross-sectional view, an activated electrowetting motor at two different moments in time. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     In this description, identical or corresponding parts have identical or corresponding reference numerals. The invention will now be described in further detail, with reference to  FIG. 1 . Although  FIG. 1  illustrates a switchable element, which is used as an variable focus lens, it is understood that the invention applies equally well to other kinds of electro-wetting elements, such as motors, variable diaphragms, filters, gratings, beam deflectors, motors and electro-wetting based displays.  
         [0024]      FIG. 1  schematically shows a side view of a switchable optical element  100 . The switchable optical element  100  comprises a closed cell or fluid chamber  111 , which contains a first  101  and a second  102  body of fluid. Said fluids  101 ,  102  are substantially immiscible. Said first fluid  101  preferably is water, which is a polar and electrically conducting fluid, or liquid, and said second fluid  102  preferably is oil, which is a non-polar liquid. When a polar and a non-polar liquid are in contact with each other, a meniscus is formed between them, i.e. between said first  101  and second  102  liquids. One way of measuring the shape of the meniscus is to measure an angle θ  112  between a wall of the cell  111  and the common surface between said first and second liquids  101 ,  102 .  
         [0025]     Further, said switchable optical element comprises a first electrode  103  and a second electrode  104 , wherein said second electrode  104  is in contact with said first liquid  101 . Between the first electrode  103  and the liquids  101 ,  102 , there is an hydrophobic insulator  109 , such that said first  103  electrode is not in contact with said liquids  101  and  102 . Moreover, said switchable optical element is arranged such that the electrical voltage  110  between said two electrodes  103 ,  104  can be varied. As can be seen from Equation 1, the angle θ  112  is dependent, among other parameters, on the interfacial surface tension γ NPL/wall    108  between the oil  102  and a wall of the cell  111 , as well as on the interfacial surface tension γ PL/wall    107  between the water  101  and a wall of the cell  111 . By increasing the difference in interfacial surface tension between these two parameters  107 ,  108 , the angle  112  is increased, without affecting the sensitivity of the contact angle with respect to the applied voltage, and by decreasing it the angle  112  is decreased, still without affecting said sensitivity.  
         [0026]     Another, more dynamic, way of changing the angle  112 , is to change the applied voltage  110 . The amount with which the angle is changed, in response to a certain change in applied voltage, can be derived from Equation 1. The equation gives that by lowering the interfacial surface tension  106  between the water and the oil, the voltage level required to change the angle  112  by a certain amount is decreased.  
         [0027]     A preferred way of lowering the interfacial surface tensions  106 ,  107 ,  108  is to add surfactant  105  to said first liquid  101  or said second liquid  102 . An example of a surfactant  105  that is able to influence the interfacial surface tension between oil and water, is an alcohol, for example decanol. If the wall consists of fluorocarbon (e.g. Teflon™ AF1600 produced by DuPont™) in combination with a hydrocarbon oil, the surface tension between the oil and the wall  108  can be influenced, for example, by molecules with a hydrocarbon part and a fluorocarbon part. With the same wall the surface tension between the water and the wall can be influenced by molecules with a polar head and a fluorocarbon tail, for example a fluorinated alcohol. Surfactants can decrease surface tensions significantly already in very low concentrations, and thus driving voltages can decrease significantly by the use of surfactants, while not affecting bulk properties of the liquids. Examples of surfactants for the wall/water interface are 2,2,3,4,4,4-hexafluoro-1-butanol and 2,2,3,3,4,4,4-heptafluoro-1-butanol. Surfactants that can be used for the wall/oil interface are pentafluorophenyltrimethylsilane, trifluoromethyltrimethylsilane and trifluoromethyltriethylsilane.  
         [0028]      FIGS. 2 and 3  illustrate the fact that there are surfactants that influence also the interfacial surface tension between said liquids and the wall surrounding the liquids, as stated above. It is understood that many surfactants influence not only one of these interfacial surface tensions, but two if not all three of them, to various extent.  
         [0029]     As to the amount of surfactant that should be added to the liquid the concentration is dependent of a number of factors. One factor is the ratio between the area of the interfacial surface and the liquid volume, where a higher concentration is required for a larger ratio than for a smaller ratio. Thus, generally, a larger liquid body requires a smaller concentration of the surfactant for obtaining the same effect as in a smaller liquid body. Further, it is desired to keep the concentration as low as possible while achieving a desired influence on the interfacial surface tension. On the other hand, increasing the concentration means increasing the influence, but only to a certain extent. When the interfacial surface is saturated with surfactant(s) the full effect is obtained. If there are still surfactant molecules left within the liquid  101 ,  102  these will, or at least may, cause undesired negative effects on the properties of the switchable element. A state that is often desired to obtain is where the interfacial surface, and in particular the meniscus, is covered with at least one monolayer of surfactant. The strongest effects of adding a surfactant can be obtained at the interfacial surface between water and oil and between water and the wall of the cell  111 .  
         [0030]     The invention may also be used in an electrowetting motor wherein use is made of the fact that the shape of the interface can be changed by means of an electric force, on the basis of the wetting technique, for manipulating a volume of a fluid along a predetermined path.  FIGS. 4A and 4B  show a cross-sectional view of an embodiment of such a motor  30 , in particular a rotary motor, at different time moments. The motor comprises a substantially cylindrical first body  33  and a substantially cylindrical second body  35 , which is concentrically positioned within the first body  33 . The first and second body  33 ,  35  enclose between their respective inner and outer surface a substantially cylindrical chamber  34 , which is filled with a non-polar and/or non-conductive first fluid  36 , such as an oil, and volumes  37   a - d  of a polar and/or conductive second fluid  37 , in this example an aqueous solution, for instance (salted) water. The fluids  36 ,  37  are immiscible.  
         [0031]     The first body  33  is provided with means for varying the wettability of its inner surface, namely twelve electrodes  40  extending in axial direction of the first body  33 , spaced at substantially regular radial intervals along the circumference. The inner surface of the first body  33  is covered with a layer  42  of electrically insulating, hydrophobic material or more generally, a material having a wettability by the second fluid  37  which is lower than the wettability by the first fluid  36 . Examples of such material are for instance Teflon-like materials like the amorphous fluoropolymer AF1600 provided by Dupont or parylene or a combination thereof, in case where the first fluid  36  is an oil or air and the second fluid is (salted) water. The electrodes  40  are connected to a voltage supply (not shown).  
         [0032]     The second body  35  is of solid design but could be hollow, if so desired, and is mounted movably, in particular rotatably, in the first body  33  by one or more suitable bearings. The or each bearing could for instance be an oil bearing, configured by providing the first and/or second body  33 ,  35  with an annular groove, in which upon rotation of the second body  35 , pressure will build up, centering the second body  35  in the first body  33 .  
         [0033]     The second body  35  is provided at its outer surface with coupling means in the form of four hydrophilic areas  44 , said number corresponding to the number of volumes  37   a - d.  These areas  44  could for instance be made of or covered by a material having a wettability by the second fluid  37  that is higher than the wettability by the first fluid  36 , which material could for instance be glass. The areas  44  are separated from each other in radial direction by areas  45 , made of or covered by hydrophobic material, which could be a selection from any of the materials mentioned before. Additionally or alternatively, the hydrophilic areas  44  may be recessed to enhance the coupling force with the volumes. Furthermore, two or more of the volumes  37   a - d  could be interconnected via at least one suitable conduit  39  in second body  35 , as illustrated in broken lines in  FIGS. 4A and 4B . The areas of high and low wettability  44 ,  45  may be omitted, but can also be maintained, to increase the maximum force of the motor may exert.  
         [0034]     A motor as described above operates as follows. In  FIG. 4A  the electrodes  40  marked with Roman numerals I (that is the upper, lower, left and right electrodes) are supplied with a voltage. Consequently, the hydrophobic layer  42  covering said electrodes I will become locally hydrophilic. The four volumes  37   a - d  will therefore contact the first body  33  at the four electrodes I. They furthermore contact the second body  35  at the coupling means, that is the hydrophilic areas  44  and the conduits  39 . If subsequently the voltage supply is shifted to second electrodes II, situated next tot the former electrodes I, the layer above said second electrodes II will become hydrophilic, whereas the layer above the first electrodes I will switch back to hydrophobic. This gives rise to electrowetting forces which draw the volumes  37   a - d  towards the hydrophilic areas II as shown in  FIG. 4B . During this movement the volumes  37   a - d  will move along the hydrophilic area  44  of the second body  35  up to the edge of the hydrophobic area  45 . Further movement along the second body  35  will be blocked by the combined action of the hydrophobic area  45  and the first fluid  36 , enabling the volumes  37   a - d  to exert a wetting force on the second body  35 , which will cause the body  35  to rotate. Hence by sequentially activating successive electrodes  40  I, II with a suitable voltage, the second body  35  can be rotated continuously. Preferably, the electrodes  40  are positioned relatively close to each other or even overlap through a “tooth” structure. Also, the radial dimensions of the electrodes  40  are preferably equal to or smaller than the radial dimensions of the volumes  37   a - d.  Such positioning and/or dimensioning of the electrodes  40  will ensure that the volumes  37   a - d  can “sense” a newly supplied voltage to a succeeding electrode  40  II.  
         [0035]     In the given example the rotation is clockwise. It will be appreciated that this direction can be readily reversed by reversing the order in which the electrodes  10  I, II are activated. Obviously, the frequency of rotation will depend on the activation frequency of successive electrodes  40  I, II. It is noted that although in the illustrated example four volumes  37   a - d  of conductive fluid are used, any number of volumes can be used. The volumes  37   a - d  may be line-shaped in axial direction or consist of a series of axially spaced droplets. It is further noted that with the embodiment of  FIGS. 4A and 4B , it is also possible to have the first body  33  rotate instead of the second body  35 , provided that the first body  33  is rotatable mounted and the second body  35  is fixed. In that case, upon switching the voltage from the first I to the second electrodes II, the volumes  37   a - d  would move towards the second electrodes II (featuring the higher wettability) up till the edge of the hydrophilic area  44 . Subsequently, the second electrodes II due to wetting forces would be drawn to the volumes  37   a - d,  causing the first body  33  to rotate anti-clockwise. From this discussion it is also immediately clear that for the operation of the motor  30  it is irrelevant whether the electrodes  40  are positioned on the static body or the movable body. Therefore, although in practice the electrodes  40  will usually be placed on the static body to avoid wiring problems, the presented embodiment should in no way be seen as limiting.  
         [0036]     Consequently, as described above, the present invention presents a way to lower the driving voltage in a switchable devices. It is to be noted, that for the purposes of this application, and in particular with regard to the appended claims, the word “comprising” does not exclude other elements or steps, that the word “a” or “an”, does not exclude a plurality, and that at least some of the means can be implemented in either hardware or software, which per se will be apparent to a person skilled in the art.