Abstract:
An optical switch, e.g., a display device based on layer break up or layer displacement having at least two different states, in which one of the fluids, e.g., oil in a first state, adjoins at least a first support plate and, in the second state, the other fluid at least partly adjoins the first support plate, in which picture elements are separated by areas having a hydrophilic surface.

Description:
BACKGROUND 
     1. Technical Field 
     The invention relates to an optical switch comprising at least one first fluid and a second fluid immiscible with each other within a space between a first transparent support plate and a second support plate, the second fluid being electroconductive or polar. 
     In particular the invention relates to a display device comprising picture elements (pixels) having at least one first fluid and a second fluid immiscible with each other within a space between a first transparent support plate and a second support plate, the second fluid being electroconductive or polar. 
     2. Related Art 
     Optical switches may be used in shutter applications, diaphragms, but also in switchable color filters in e.g. display applications. 
     Display devices like TFT-LCDs are used in laptop computers and in organizers, but also find an increasingly wider application in GSM telephones. Instead of LCDs, for example, (polymer) LED display devices are also being used. 
     Apart from these display effects that are well established by now other display techniques are evolving like electrophoretic displays, which are suitable for paper white applications. 
     The invention is based on a principle called electro-wetting. The invention provides new ways of using this principle in which one of the fluids in a first state adjoins a greater part of the first support plate and in the second state the other fluid at least partly adjoins the first support plate. 
     If for instance a (first) fluid is a (colored) oil and the second (the other) fluid is water (due to interfacial tensions) a two layer system is provided which comprises a water layer and an oil layer. However, if a voltage is applied between the water and an electrode on the first support plate the oil layer moves aside or breaks up due to electrostatic forces. Since parts of the water now penetrate the oil layer the picture element becomes partly transparent. 
     Display devices based on this principle have been described in PCT-Application WO 03/071346. In most of the embodiments shown in this Application, a picture element corresponds to a defined space (e.g., a substantially closed space) and, in the second state, the other fluid substantially completely adjoins the first support plate. In one embodiment shown in this Application, picture elements are described in which the pixel walls do not extend across the whole pixel thickness. 
     In practice however these pixel walls have a certain wall height of about 80 μm or more which has severe implications for the optical performance of small pixels in particular. The presence of such pixel walls results in an effectively reduced pixel area, since the area close to the walls does not contribute to the pixel brightness at the higher incoming angles. As a result, the total pixel will become less bright. This reduction of brightness will be more severe for smaller pixels. At a wall height of about 80 μm, the loss of brightness is about 10-15% for a 500 μm×500 μm pixel. 
     Moreover the walls are generally glued to the underlying surface. This underlying surface is often a fluoropolymer surface so the bonding is rather weak mechanically due to the fact that the fluoropolymer is very hydrophobic. Also the glue in between the walls and the fluoropolymer surface tends to move into the pixel area. In this case, the oil will not cover this part of the pixel, since the oil is repelled from the glue surface. These areas cause a considerable increase in reflectivity in the “black” off-state, thereby reducing the optical contrast. 
     SUMMARY 
     It is one of the objects of the present invention to overcome at least partly the above mentioned problems. 
     To this end an optical device according to the invention comprises at one support plate within said space a surface area less wettable for the second fluid, the less wettable surface area being surrounded by areas having a more wettable surface area. 
     Although not strictly necessary the less wettable surface area is preferably separated by more wettable surfaces provided on walls. 
     By lowering the walls to 20 μm thickness, or less, the device can be operating in a range where the oil height upon contraction is in fact larger than the actual wall height. The inventors have experimentally confirmed that the oil remains within the pixel, due to the pinning of the oil/water interface at the bottom of the wall. In display applications the possibility of oil in adjacent pixels mixing when both pixels are switched can be avoided by incorporating a controlled oil motion e.g. by electromagnetic forces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. 
       In the drawings: 
         FIGS. 1   a  and  1   b  are diagrammatic cross-sections of a part of a display device, to show the principle on which a display device according to the invention is based, 
         FIG. 2  is another diagrammatic cross-section of a part of a display device according to an exemplary embodiment of the invention, 
         FIGS. 3   a  and  3   b  are diagrammatic cross-sections of a part of a display device according to an exemplary embodiment of the invention, while 
         FIG. 4  is a plan view of a part of a further display device according to an exemplary embodiment of the invention, and 
         FIG. 5  is a diagrammatic cross-section along line V-V in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIGS. 1   a  and  1   b  show a diagrammatic cross-section of a part of a display device  1  which shows the principle on which a display device according to the invention is based. Between two transparent substrates or support plates  3 ,  4  a first fluid  5  and a second fluid  6  are provided, which are immiscible with each other. The first fluid  5  is for instance an alkane-like hexadecane or as in this example a (silicone) oil. The second fluid  6  is electroconductive or polar, for instance water or a salt solution (e.g., a solution of KCl in a mixture of water and ethyl alcohol). 
     In a first state, when no external voltage is applied ( FIG. 1   a ) the fluids  5 ,  6  adjoin the first and second transparent support plates  3 ,  4  of e.g. glass or plastic. On the first support plate  3  a transparent electrode  7 , for example indium (tin) oxide is provided and an intermediate less wettable (hydrophobic) layer  8 , in this example an amorphous fluoropolymer (AF1600). 
     When a voltage is applied (voltage source  9 ) via interconnections  20 ,  21  the layer  5  moves aside or breaks up into small droplets ( FIG. 1   b ). This occurs when the electrostatic energy gain is larger than the surface energy loss due to the creation of curved surfaces. As a very important aspect it was found that reversible switching between a continuous film  5  covering the support plate  3  and a film adjoining the wall  2  is achieved by means of the electrical switching means (voltage source  9 ). 
       FIG. 2  shows an embodiment of a display device according to the invention, in which walls between separate picture elements have been omitted for the greater part for the sake of clarity. In this embodiment the pixel walls  13  do not extend across the whole pixel thickness. Such walls may be obtained by offset printing or other printing techniques known in the art. It appears that the oil film  5  is very stable, which is enhanced even further as the pixel size decreases. So during switching the oil remains confined in each area. The other reference numerals have the same meaning as those in  FIG. 1 . 
     The layer  13  in this example is approximately 20 μm thick or less and consists of a lower part  13   b , more wettable to the first fluid (hydrophobic) and an upper less wettable (hydrophilic) part  13   a  (see  FIGS. 3   a  and  3   b ). In this way the wetting of the lower part of the pixel wall by the oil ensures a homogeneous optical off-state. Then when the pixel is activated the reversibility of oil movement due to the less wettable (hydrophilic) upper part is also maintained. Especially the less wettable (hydrophilic) part (or a surface part) has been found to be essential for good switching. 
     The layer thickness of approximately 20 μm or less is a significant improvement with respect to the 80 μm layer, leading to an improved brightness and speed. Furthermore, due to the absence of glue, the oil covers the entire pixel area, improving the optical contrast. The layer  13  or layer parts  13   a ,  13   b  in this example are printed. A printing process is very simple and flexible, thereby facilitating the fabrication of electrowetting displays at high resolutions significantly. In such a printing process it may be advantageous to use a pre-treatment of the substrate, such as a reactive ion-etching step using O 2  to make it more hydrophilic. 
     In the display of  FIGS. 4 ,  5  the separating area between pixel areas has been integrated into the substrate by making interpixel areas  13 ′ of the substrate less wettable to the first fluid (hydrophilic) for instance by means of laser radiation, a UV-ozone treatment or an oxygen-plasma treatment. The further reference numerals in  FIGS. 4 ,  5  have the same meaning as those in the other examples. The possibility of oil in adjacent pixels mixing when both pixels are switched to the transparent state is avoided by incorporating a controlled oil motion by electromagnetic control, in this case by leaving apertures  17  in the electrodes  7 . 
     Several variations to the principle are possible. Although a transmissive device has been described, the display may be made reflective by adding a reflector as shown in element  14  in  FIGS. 3   a  and  3   b . The electrode configuration chosen here is merely an example. Other electrode configurations can be chosen, such as a circular geometry. Such a circular geometry is used in e.g., shutter applications and diaphragms. 
     The invention resides in each and every novel characteristic feature and each and every combination of characteristic features. Reference numerals in the claims do not limit their protective scope. Use of the verb “to comprise” and its conjugations does not exclude the presence of elements other than those stated in the claims. Use of the article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.