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

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This disclosure is a continuation of and claims priority to U.S. patent application Ser. No. 11/816,716, filed Aug. 21, 2007, which is a 35 U.S.C. 371 national stage application of International Application No. PCT/IB2006/050528, filed Feb. 17, 2006, which claims priority to European Patent Application No. 05101497.5, filed Feb. 28, 2005, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The invention relates to a display device comprising picture elements, each picture element having at least one first fluid and a second fluid immiscible with each other within a space between a first support plate and a second support plate, the second fluid being electroconductive or polar. 
         [0003]    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. 
         [0004]    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. 
         [0005]    More generally 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 support plate and a second support plate, the second fluid being electroconductive or polar. 
         [0006]    Optical switches may be used in shutter applications, diaphragms, but also in switchable color filters in e.g. display applications. 
       SUMMARY 
       [0007]    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. 
         [0008]    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. When a (diffuse) reflector is positioned below the oil in or under the picture element a reflective display is realized. 
         [0009]    Display devices based on this principle have been described in PCT-Application WO 03/00196 (PH-NL 02.0129). In all embodiments shown in this Application picture elements are described in which the picture element walls do not extend across the whole picture element thickness and in the second state the other fluid substantially completely adjoins the first support plate. Embodiments, not shown in this Application, where a picture element (pixel) can correspond to a defined space (e.g., a substantially closed space) are also possible. 
         [0010]    Said display devices have low power consumption due to the low leakage current in the state when the voltage is applied. However, in some applications, such as an e-book, ultra-low power consumption is desired. Also an increasing need exists for reflective displays in these applications having intermediate transmission or reflection values (gray-values). 
         [0011]    It is one of the objects of the present invention to overcome at least partly the above-mentioned problems. 
         [0012]    To this end, according to the invention, the device is provided on a first support plate, within a picture area, with barriers separating the first fluid to define sub-picture elements, the first support plate comprising an electrode for each sub-picture element. If a barrier is incorporated within a picture element (pixel) that prevents oil motion without the application of a (local) voltage hi-stability is incorporated in the system, i.e. the optical state of the system is unchanged, even when the voltage is removed. In this way power consumption is reduced. 
         [0013]    In a preferred embodiment the barriers comprise more wettable surfaces provided on walls. 
         [0014]    One of the further advantages of bi-stability, next to the use of reduced power, is the possibility of obtaining gray-values by dividing the area of the picture element into different areas, be it that said method for obtaining gray-values in bi-stable display devices per se is known in the art. 
         [0015]    A further preferred embodiment of the invention however offers the possibility of obtaining analogue gray-scales by applying to one of the electrodes pulsed high voltage, the pulse being shorter than the time required for the oil to move across the barrier entirely. 
         [0016]    These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    In the drawings: 
           [0018]      FIGS. 1A and 1B  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, 
           [0019]      FIG. 2  is a plan view of a part of a display device according to the invention, while 
           [0020]      FIGS. 3-7  are diagrammatic cross-sections along line III-III in  FIG. 2  at various driving conditions and 
           [0021]      FIGS. 8-11  are plan view of parts of other display devices according to the invention. 
       
    
    
       [0022]    The figures are diagrammatic and not drawn to scale. Corresponding elements are generally denoted by the same reference numerals. 
       DETAILED DESCRIPTION 
       [0023]      FIGS. 1A and 1B  show diagrammatic cross-sections of a part of a display device  1 , e.g. one picture element, which shows the principle on which a display device according to the invention is based. Between two transparent substrates or support plates  3 ,  4 , and between pixel walls  2 , 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). By means of a reflecting layer  10  positioned below the oil in this example below the transparent electrode  7 , a reflective display picture element is realized. 
         [0024]    In a first state, when no external voltage is applied ( FIG. 1A ) 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  the 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). Due to the presence of the oil over the whole area of the picture element appears black (arrows  11 ). 
         [0025]    When a voltage is applied (voltage source  9 ) via interconnections  20 ,  21  the layer  5  moves aside or breaks up into small droplets ( FIG. 1B ). This occurs when the electrostatic energy gain is larger than the surface energy loss due to the creation of curved surfaces. In this case light is reflected at the main area of the picture element, while only a small part appears black (arrows  12 ). 
         [0026]      FIG. 2  shows a plan view of a part of a display device according to the invention with the intermediate layer  8  removed for clarity, while  FIGS. 3-7  show diagrammatic cross-sections along line III-III in  FIGS. 2 . A further hydrophilic barrier or wall  13  is incorporated in a picture element that prevents oil motion in certain driving conditions. 
         [0027]    The pixel walls  2  do not need to extend across the whole pixel thickness. The walls  2 ,  13  may be obtained by photolithography, offset printing or other printing techniques known in the art. The other reference numerals have the same meaning as those in  FIGS. 1A and 1B . 
         [0028]    The wall  13  in this example divides the picture element in two segments having equal size. In addition, the picture element comprises two electrodes  7 ,  7 ′ of equal size that can be addressed individually. For said wall  13  (which is chosen to be as narrow as possible) a material is chosen such that it is hydrophilic, which implies that the oil  5  is prevented from wetting the barrier, particular from wetting the top of said barrier. The latter can be achieved by providing the top of the barrier or wall  13  with a more wettable surface  13 ′. At the same time it is chosen to be less hydrophilic than the material of the pixel walls  2 . This has the effect that applying a voltage to either or both of the electrodes  7 ,  7 ′ will cause the oil to move to one of the neighboring sub-picture elements. All voltages in the examples discussed below are chosen with respect to a ground potential applied to the second fluid  6 . 
         [0029]    In  FIG. 3  the oil  5  is distributed evenly over both picture element parts. This is achieved when V 1  (applied to electrode  7 ) and V 2  (applied to electrode  7 ′) are equal and both have a small magnitude. 
         [0030]    Fixing the oil on the sides of the wall  13  is sufficiently strong to ensure separation of the two oil volumes. However, when either V 1  or V 2  is increased, at some point, the oil will want to wet the barrier, since at the area of the electrode having a higher voltage, the intermediate layer  8  becomes more hydrophilic than the wall  13  itself. As a result, the oil will move across the wall  13  to the other side, resulting in the situation sketched in  FIG. 4  (V 1 &gt;&gt;V 2 ) or  FIG. 5  (V 1 &lt;&lt;V 2 ). 
         [0031]    To reset the picture element in the fully dark state (the state where the oil is distributed evenly as shown in  FIG. 6 ) a high voltage is applied to both sub-picture elements, such that the surface underlying the picture element becomes more hydrophilic than the wall  13 . As a result, the oil  5  will move over the wall  13 , and re-distribute itself over the entire picture element as in  FIG. 6 . Removing the voltage will split the oil layer at the wall  13 , since this is now more hydrophilic than the surface underlying the picture element. The electrode configuration chosen here is merely an example. By choosing equal areas for the electrodes  7 ,  7 ′ the situations of both  FIG. 5  and  FIG. 6  in fact have similar reflection (or transmission) properties, viz. mid-gray. 
         [0032]      FIG. 7  shows how stable gray values in an analogue gray-scale are obtained by using timing. If in the situation shown in  FIG. 3  a pulsed high voltage is applied to one of the electrodes  7 ,  7 ′, the pulse being shorter than the time required for the oil to move across the wall  13  entirely, at the end state the oil film is thicker in one part of the picture element than in the other. Since the nominal thickness of the film is such that about 1% of the light is reflected, the reflectivity cannot be reduced much further in the part where the film has become thicker. However, the reflectivity of the part where the oil film has become thinner will be increased significantly, resulting in an overall increase of reflectivity of the picture element. Depending on the pulse length compared to the time it takes for the oil to move, stable gray-values in an analogue gray-scale are obtained in this way. 
         [0033]    The electrode configuration chosen here is merely an example. Other electrode configurations can be chosen, such as a circular geometry. The area of the two pixel parts can be different, resulting in multiple gray values. 
         [0034]      FIG. 8  shows an electrode configuration in which the picture element area has been divided into four parts  7 ,  7 ,′  7 ″,  7 ′″ having a certain area ratio and are driven by voltages V 1 , V 2 , V 3 , V 4 .  FIG. 9  shows an electrode configuration in which the picture element area has been divided into three parts  7 ,  7 ,′  7 ″ having another area ratio and are driven by voltages V 1 , V 2  and V 3 .  FIG. 10  shows an electrode configuration having a central picture element area  7  being enclosed by successive picture element area parts  7 ′,  7 ″,  7 ′″. The parts do not necessarily have to be square or rectangular, as shown in  FIG. 11 .  FIGS. 8-11  have the intermediate layer  8  removed for clarity. 
         [0035]    Several variations to the principle are possible. Although a reflective device has been described, the display may be made transmissive. Also the height of the walls  13  may be varied and said wall may have for instance a curved surface. Modifying the height will modify the curvature of the oil in the fixed states, so it will influence the required voltages and the stability of the states. If the material of the barriers  13  is the same as the material used for the pixel walls  2  they are equally hydrophilic as the pixel wall, and now the height of the barrier  13  (which is always lower than the pixel wall and may even be zero, which possibly requires a rather strong hydrophilicity at the position of dividing sub-picture elements, possibly even stronger than the one of the pixel wall  2 ) is the parameter determining the picture element dynamics. In this case, the volume of the oil compared to the picture element size will play an important role. There should be sufficient oil to make the oil fill the whole picture element, i.e. cross the lower barrier in case both electrodes are switched to a high voltage. 
         [0036]    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.