Abstract:
The invention relates to a variable-focus lens comprising two opposing walls ( 21, 22 ) which are which define a space containing an isolating liquid ( 31 ) on the first wall ( 22 ) and a conductive liquid ( 32 ) which covers the isolating liquid and which comes into contact with the second wall ( 21 ). The surface ( 24 ) of the first wall ( 22 ) has high wettability for the isolating liquid and low wettability for the conductive liquid, while the surface ( 23 ) of the second wall ( 21 ) has high wettability for the conductive liquid and low wettability for the isolating liquid.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to variable-focus lenses and more particularly to such lenses employing the deformation of a drop of liquid by the phenomenon of electrowetting.  
       PRIOR ART  
       [0002]     Various embodiments of variable-focus lenses are described in the Applicant&#39;s European patent 1166157.  FIG. 1  of the present application substantially reproduces  FIG. 12  of that patent. A cell is defined by two insulating plates  1  and  2  and sidewalls (not shown). The lower plate  2 , which is not plane, includes an indentation or recess  3  that receives a drop of an insulating liquid  4 . The rest of the cell is filled with an electrically conducting liquid  5 , which is immiscible with the insulating liquid, has a different refractive index but substantially the same density. An annular electrode  7 , open facing the recess, is placed on the rear face of the lower plate  2 . Another electrode  8  is in contact with a conducting liquid  5 . Through the phenomenon of electrowetting, it is possible, depending on the voltage V applied between the electrodes  7  and  8 , to modify the curvature of the contact surface between the two liquids, which for example passes from the concave initial shape denoted by the reference  4  to the convex shape shown by the dotted line and denoted by the reference  9 . Thus, a beam of light passing through the cell orthogonally to the plates  1  and  2  in the region of the drop  4  will be focused at a longer or shorter distance depending on the applied voltage.  
         [0003]     Other embodiments of variable-focus lenses are described in document WO-A-03/069380.  FIG. 2  of the present application substantially reproduces  FIG. 1  of that document. A cell is defined by two insulating plates  11  and  12  and a cylindrical sidewall  13 . The lower part  2  of the cell receives a drop  14  of an insulating liquid. The rest of the cell is filled with a conducting liquid  15 , which is immiscible with the insulating liquid and has a different refractive index, but substantially the same density. The cylindrical sidewall  13  contains an annular electrode  17 . Another electrode  18  formed on the internal face of the upper plate  11  is in contact with the conducting liquid  15 . It is possible, depending on the voltage V applied between the electrodes  17  and  18 , to modify the curvature of the contact surface between the two liquids, which for example passes from the concave initial shape denoted by the reference  14  to the convex shape, shown by the dotted line and denoted by the reference  19 . Thus, a light beam passing through the cell orthogonally to the plates  11  and  12  in the region of the drop  14  will be focused at a longer or shorter distance depending on the applied voltage.  
         [0004]     Although these solutions are satisfactory, they do have drawbacks in their implantation. This is because in practice the conducting liquid is generally an aqueous liquid and the drop, generally of an oily liquid, is positioned when the cell has been totally immersed beforehand in the aqueous liquid. The oily drop is injected via the bottom of the cell, but it may be difficult to expel the water from the bottom of the cell. The result may be that the drop is poorly positioned or that a drop of water is trapped beneath the drop of oil, introducing unacceptable optical perturbations.  
         [0005]     Thus, it is necessary to take certain operating precautions in order to position the drop correctly, this drop being liable to divide and adhere partly to each of the plates  1  and  2  or  11  and  12 . In the case of  FIG. 1 , the drop may also end up partly above the plane upper part of the lower plate  2 .  
         [0006]     These drawbacks also occur when the lens is used in an environment subject to shocks hence, under the effect of an acceleration, the drop may have a tendency to deform or be displaced, and to depart from the housing provided for this purpose. It may then be very tricky, if not impossible, to reposition it correctly.  
       SUMMARY OF THE INVENTION  
       [0007]     One object of the present invention is to mitigate these drawbacks, so as to make it easier to carry out the drop implantation operations, and to prevent excessively large displacements of the liquids resulting in the drop being out of position or divided.  
         [0008]     Another object of the present invention is to provide a particular variable-focus lens cell structure enabling its parameters to be optimized.  
         [0009]     To achieve these objects, and others, the present invention provides a variable-focus lens comprising, in a space defined between two facing walls orthogonal to the axis of the lens, an insulating liquid on a first of the walls and, covering the insulating liquid and coming into contact with the second wall, a conducting liquid. The surface of the first wall has high wettability for the insulating liquid and low wettability for the conducting liquid and the surface of the second wall has high wettability for the conducting liquid and low wettability for the insulating liquid.  
         [0010]     According to one embodiment of the present invention, the surface of at least one of the walls is coated with a layer giving it the desired wettability characteristics.  
         [0011]     According to one embodiment of the present invention, the wettability of the surface of the first wall is such that the contact angle of a drop of the insulating liquid in the conducting liquid for this surface is less than 75°.  
         [0012]     According to one embodiment of the present invention, the wettability of the surface of the second wall is such that the contact angle of a drop of the conducting liquid in the insulating liquid for this surface is less than 75°.  
         [0013]     According to one embodiment of the present invention, the wall bearing the insulating liquid comprises a plate of insulating material coated with a material having high wettability for the insulating liquid and low wettability for the conducting liquid, a conducting plate with a central opening, in which the insulating liquid is placed, being deposited on said insulating plate, this conducting plate being coated, on the inside of the cell, with one or more materials suitable for providing insulation and having optimum wetting properties for the operation of the lens.  
         [0014]     Another subject of the present invention is a variable-focus lens assembly comprising, between an upper transparent plate and a lower transparent plate, a pair of liquids having an interface, the profile of which can be modified by varying a voltage applied between one of the liquids, a conducting one, and an electrode close to the other liquid, the insulating one, this assembly including a first conducting ring, in electrical contact with the conducting liquid and bearing on the upper plate, and a second conducting ring, in electrical contact with said electrode and bearing on the lower plate, each of these rings having a cylindrical region, the opposed faces of these cylindrical regions being threaded, and a third ring, which is insulating, having internal and external threads, being interposed between said two cylindrical parts. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The objects, features and advantages, together with others, of the present invention will be explained in detail in the following description of particular embodiments, given as nonlimiting examples, in relation to the appended figures in which:  
         [0016]      FIGS. 1 and 2 , described above, are sectional views of examples of variable-focus lenses according to the prior art, which illustrate problems that the invention seeks to solve;  
         [0017]      FIG. 3  is a sectional view of one embodiment of a lens according to the invention; and  
         [0018]      FIG. 4  is a sectional view of an example of a lens assembly for the lens of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION  
       [0019]      FIG. 3  shows one embodiment of a variable-focus lens cell according to the present invention. The cell is bounded by two plates  21  and  22  made of transparent insulating materials, which are orthogonal to the optical axis of the lens. According to a fundamental aspect of the present invention, which also applies to cells of the types illustrated in  FIGS. 1 and 2 , these plates consist of, or are coated on their internal walls with, materials having specific wettability properties, namely the internal wall of the upper plate  21  that has to be in contact with the conducting liquid has high wettability for this conducting liquid and low wettability for the insulating liquid and the internal wall of the lower plate  22  that has to be in contact with the insulating liquid has wettability characteristics the reverse of the above.  
         [0020]     In the standard case in which the conducting liquid is an aqueous liquid and the insulating liquid is an oily liquid, the material or coating of the upper plate will be highly hydrophilic, while that of the lower plate will be highly hydrophobic.  
         [0021]     In  FIG. 3 , a hydrophilic coating has been denoted by the reference  23  and a hydrophobic coating by the reference  24 .  
         [0022]     In practice, various materials with hydrophilic properties may be used. Good results have been obtained using, as hydrophilic surface, a mineral glass or a silicon-oxide-coated polycarbonate.  
         [0023]     Among hydrophobic materials, perfluoro-ethylene-propylene (FEP) and perfluoroalkoxy (PFA) may be especially mentioned. These materials, which are transparent, may constitute the wall itself. Other materials may also be used, but only as coating layer. Polymers such as soluble fluoropolymers may especially be mentioned, for example the products sold under the reference AF 1600 by DuPont de Nemours, or the products sold by Cytonix, or else layers of silanes grafted onto the surface of the materials, which give the latter desirable wettability properties.  
         [0024]     In practice, the wettability of the surface of a wall is measured by the angle that the contact surface between the two fluids makes to the surface of the wall. The contact angle is defined as the angle between the contact surface between two fluids and the solid wall on which this contact surface bears. This angle depends only on the nature of the fluids in question and on the wall, and not the shape of the wall. If the two fluids are two immiscible liquids, identical wettability for the two liquids means that the contact surface is substantially perpendicular to the surface of the wall. If the surface of the wall exhibits greater wettability for one of these liquids, this means that the contact surface between the two liquids makes an acute angle on the side with the liquid in question.  
         [0025]     Thus, a surface may be termed hydrophobic if the peripheral surface of the drop makes an obtuse angle to the surface of the wall on the drop side. Conversely, if this angle is an acute angle, the surface is considered to be hydrophilic. As a general rule, the wettability of a surface is measured although two different liquids are present, the interface surface between the two liquids forming the periphery of the drop.  
         [0026]     Given below is a table summarizing the representative wettability contact angles of certain materials used as regards hydrophobic and hydrophilic surfaces, in combination with the liquid forming the characteristic drop and the surrounding fluid (gas or liquid). These tests were carried out using, as water, deionized ultrapure water, and, as oil, an oil sold by Exxon Mobil Chemical under the name ISOPAR V (CAS No. 64742-46-47). The glass used as hydrophilic surface was an optical glass, such as that denoted by the acronym BK7.  
                                                                                     Liquid   Surrounding   Contact           Coating   drop   liquid   angle                                    Hydrophobic   AF1601   Water   Air   105°       surface   (DuPont)   Oil   Air   57°               Oil   Water   &lt;10°           801-A   Water   Air   104°           (Cytonix)   Oil   Air   65°               Oil   Water   45°       Hydrophilic   Glass   Water   Air   &lt;10°       surface                  
 
         [0027]     In practice, good results are obtained when the wettability of the surface  24  of the plate receiving the oil drop is such that the contact angle of the oil in the water is less than 750 and the wettability of the surface  23  of the plate in contact with the aqueous liquid is such that the contact angle of the water in the oil is less than 750.  
         [0028]     In the exemplary embodiment shown in  FIG. 3 , the lower part of the cell comprises the abovementioned insulating plate  22 , the upper surface (or a coating)  24  of which has the desired wettability characteristics. A plate  26  of a conducting material coated with an insulating coating  27  is laid on, preferably bonded to the insulating plate. The insulating coating  27  may be a multilayer coating. Its external surface is such that its wettability characteristics are optimized for obtaining the desired meniscus shape at rest between the insulating liquid  31  and the conducting liquid  32 . Thus, according to one advantage of this embodiment, the wettability properties of the coating of the upper part  24  of the insulating plate  22  can be dissociated from the wettability properties of the external face of the insulating coating  27 . The properties of the layer  23  are mainly intended to optimize the adhesion of the drop to its support, and the properties of the layer  27  are mainly intended to optimize the characteristics of the electrowettability function.  
         [0029]      FIG. 4  illustrates an example of an assembly of the various plates of the lens of  FIG. 3 . A first metal ring  40  has a lower part in the form of a plate  41  that supports the lower insulating and conducting plates  22 ,  26  and ensures that there is electrical conductivity with the conducting material of the plate  26 , and a cylindrical peripheral part  42  that surrounds the structure and is provided with an internal thread  44 . A second metal ring  50  has an upper part in the form of a plate  51 , which bears on the upper insulating plate  21  and ensures that there is electrical contact with the conducting liquid  32 , preferably via conductive coating  53  provided on the periphery of the plate  21 . The second metal ring  50  thus has a cylindrical peripheral part  52  provided with an external thread  54 . The cylindrical peripheral part  52  is internal to the cylindrical peripheral part  42 . A first cylindrical insulating ring  60  is placed between the external periphery of the lens and the internal periphery of the second conducting ring  50 . A second cylindrical insulating ring  70  is placed between the cylinders  42  and  52  and is provided with internal and external threads that are respectively intended for coupling with the external thread  54  of the cylinder  52  and the internal thread  44  of the cylinder  42 . An O-ring seal  80  keeps the plate  26  separated from the plate  21 . The height is also maintained by the length of the ring  60 .  
         [0030]     This structure is assembled in the following manner. The cell  21 - 27  is fitted with the seal  80 , the oil  31  and the water  32  being in place. Next, the following are fitted in succession: the ring  40 , the ring  70  screwed into this ring  40 , the ring  60 , and the ring  50  screwed into the ring  70  so as to clamp the seal  80  and to maintain sealing. The conducting rings  40  and  50  may be provided with threads or drillholes for the insertion of connection elements and/or for mounting the lens on the device to which it has to be fitted.  
         [0031]     Of course, this merely shows one possible example of an assembly, and various alternative embodiments will be apparent to those skilled in the art. In particular, the various rings may be provided with locking shoulders. The seal  80  and the insulating ring  60  may be placed differently or even combined provided that the function of isolating the rings  40  and  50 , the sealing function and the function of keeping the upper and lower plates of the lens spaced apart are ensured. In addition, this type of assembly may be suitable for variable-focus lenses other than that described in relation to  FIG. 3 .  
         [0032]     As indicated above, the present invention also applies to the exemplary embodiments shown in  FIGS. 1 and 2 . In these embodiments, the materials of the plates  1  and  11 , or of the internal coatings on these plates, will be such that the internal walls of these plates have strongly hydrophilic properties. In the case of  FIG. 2 , the material of the lower plate  12 , or a coating on this plate, will have highly hydrophobic properties. In the case of  FIG. 1 , a highly hydrophobic coating will preferably be provided at the bottom of the recess  3 , a coating optimized for the operation of the lens on the oblique walls of the recess, and a hydrophilic coating on the upper plane peripheral part of the plate  2 , in order to prevent the oil drop from adhering thereto.  
         [0033]     Moreover,  FIGS. 1 and 3  show regions for positioning the insulating liquid that are in the form of a cup with plane sidewalls inclined at  450 . Various other shapes could be used, for example cylindrical or toric shapes.  
         [0034]     It follows from the structure according to the present invention that the insulating drop will naturally tend to be positioned in the highly hydrophobic region of the bottom of the recess in the case of  FIGS. 1 and 3 , and of the lower plate  12  in the case of  FIG. 2 , and that, even if the drop accidentally comes into contact with another surface, such as the surface of the upper plate, it will not adhere to this surface and will naturally be repositioned on the lower face side.  
         [0035]     To give an example, in the case of  FIG. 3 , the lower part of the opening in the plate  26  may have a diameter of 3 to 5 mm, this plate having a thickness of a few tenths of an mm, the distance between the plate  26  and the plate  21  also being a few tenths of an mm, and the upper and lower plates being glass plates with a thickness of the order of 1 mm.