Abstract:
An internal field activated display sheet is disclosed which comprises a medial plane disposed between reservoirs in the display sheet. The reservoirs communicate with each other through apertures in the medial plane, which includes a plurality of conductors. At least one of the reservoirs is filled with a liquid means, which is responsive to a peristaltic internal field developed within the medial plane. Applying a field across selected apertures in the medial plane causes the liquid means to be electrically pumped from one reservoir into the other, thereby displaying an image.

Description:
INCORPORATIONS BY REFERENCE  
       [0001]    The following patent application is hereby incorporated by reference into this application: U.S. Pat. application Ser. No. 09/216,829 by Biegelsen et al. titled “Ferrofluidic Electric Paper”. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates generally to internal field activated display sheets and more particularly concerns an internal field activated display sheet which utilizes liquid in a plurality of reservoirs in which the liquid can be moved from each reservoir into an open space and can be moved back into the reservoir by applying an electric field to the liquid.  
           [0003]    Typically, a display device, in sheet form, comprises a thin sheet, which has many attributes of a paper document. It looks like paper, has ambient light valve behavior like paper (i.e. the brighter the ambient light, the more easily it may be seen), is flexible like paper, can be carried around like paper, can be written on like paper, can be copied like paper, and has nearly the archival memory of paper.  
           [0004]    There have been different approaches to making a field induced display sheet such as U.S. Pat. No. 5,956,005 titled “Electrocapillary Display Sheet which Utilizes an Applied Electric Field to Move a Liquid Inside the Display Sheet”, in which the display sheet utilizes three transparent parallel sheets spaced from each other. The medial plane has a plurality of reservoirs, which are filled with a dyed or pigmented ink. Each of the reservoirs has an individually addressable voltage source to create an individual electric field. Ink from a reservoir flows into the space between the medial plane and one of the other two sheets with the application or removal of an electric field.  
           [0005]    An alternate approach was disclosed in U.S. Pat. No. 5,717,283 titled “Display Sheet with a Plurality of Hourglass Shaped Capsules Containing Marking Means Responsive to External Fields”, in which the display sheet contains a plurality of hourglass shaped capsules for each pixel of an image. Each hourglass shaped capsule contains ink in one of its chambers. With the application of an external electric field, ink is moved from one chamber to the other in each hourglass shaped capsule to display an image. Visibility of the ink is otherwise blocked by an opaque medial plane.  
           [0006]    Although these approaches, utilizing a standard vertical electric field, are useful, it is desirable to improve on their performance. Accordingly, it is an object of this invention to provide a means for more effectively moving material within electric paper pixels than is possible with a standard vertical electric field.  
         SUMMARY OF THE INVENTION  
         [0007]    Briefly stated, and in accordance with one aspect of the present invention, there is provided an internal activated display sheet including a medial plane disposed between a first and second reservoir. Apertures in the medial plane permit communication between the first and second reservoirs. At least one of the reservoirs is filled with a liquid responsive to an internal peristaltic field developed within the medial plane, which includes a plurality of conductors. Applying an internal field across selected apertures in the medial plane causes liquid to move from one reservoir to the other.  
           [0008]    In an alternate aspect of the invention there is provided a method for activating a display sheet having a first non-conductive sheet, a plurality of first reservoirs, a plurality of second reservoirs located beneath the first reservoirs, a medial plane containing conductive means interposed between the first and second reservoirs. Apertures in the medial plane permit communication between the first and second reservoirs. At least one of the reservoirs is filled with a liquid means. A peristaltically driven internal field within the medial plane pumps the liquid means from at lease one of the filled reservoirs into one of the reservoirs not containing the liquid means. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The foregoing and other features of the instant invention will be apparent and easily understood from a further reading of the specification, claims and by reference to the accompanying drawings in which:  
         [0010]    [0010]FIG. 1 shows a pixel wide cross sectional view of one embodiment of the electric display sheet of this invention;  
         [0011]    [0011]FIG. 2 shows a multi-pixel cross sectional view of the medial plane of one embodiment of the electric display sheet of this invention;  
         [0012]    [0012]FIG. 3A shows a portion of the top view of the medial plane of this invention;  
         [0013]    [0013]FIG. 3B shows a perspective view of the three conductive layers of the medial plane;  
         [0014]    [0014]FIG. 3C shows a perspective view of an alternate embodiment for the three conductive layers of the medial plane;  
         [0015]    [0015]FIG. 4A shows an approximately sinusoidal phased waveform that is applied to the conductive layers of the medial plane;  
         [0016]    [0016]FIG. 4B shows the waveform of FIG. 4A with the phasing reversed and applied to the conductive layers of the medial plane;  
         [0017]    [0017]FIG. 4C shows an approximately sawtooth phased waveform that is applied to the conductive layers of the medial plane;  
         [0018]    [0018]FIG. 4D shows an interrupted application of a waveform to the conductive layers of the medial plane;  
         [0019]    [0019]FIG. 5 shows a pixel wide cross sectional view of the electric display sheet in operation;  
         [0020]    [0020]FIG. 6 shows an alternate embodiment of the medial plane;  
         [0021]    [0021]FIG. 7 shows a pixel wide cross sectional view of an alternate embodiment of the medial plane with a frustoconical aperture;  
         [0022]    [0022]FIG. 8 shows a portion of the top view of the medial plane of an alternate embodiment of this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    Referring to FIG. 1, there is shown a pixel wide cross sectional view of an embodiment of the display sheet  10  of this invention. The display sheet  10  comprises a transparent and insulating sheet  12 , such as glass or Mylar, an opaque sheet  16 , a medial plane  14 , and intermediate layers  18  and  20 . Transparent as used herein shall mean “having low optical absorption so that objects may be easily seen on the other side”. Intermediate layer  20  forms walls for a hidden lower reservoir  26 , medial plane  14  forms a top layer for the lower reservoir  26  and the bottom of a viewable upper reservoir  30 , and intermediate layer  18  forms walls for the upper viewable reservoir  30 . The layers may be thermo-compressively bonded together, using adhesive layers (wherein the patterned adhesive may suffice for layers  18  and  20 ), or the layers could be deposited one on top of the other by any means known in the art. Apertures  22  and  24  are defined in medial plane  14 . Apertures  22  provide vapor or liquid return vents through medial plane  14 . Aperture  24  forms a passageway which controls passage of a coloring fluid  28  between the upper reservoir  30  and the lower reservoir  26 . In one embodiment, the coloring fluid  28  is provided in the lower reservoir  26 . The coloring fluid  28  has a color that contrasts with the color of sheet  14  and intermediate layer  18 . In this first exemplary embodiment, the coloring fluid  28  is low surface energy, non-transparent and non-white in color, such as black. Surfaces of apertures  24  are created or treated to be wetting to coloring fluid  28 . Coloring fluid  28 , which may be transparent and be colored by neutrally buoyant pigment or dye, may include dyed or pigmented non-polar liquids such as Dow Corning  200  Series silicone oil, Exxon Isopar or 3M Fluorinert and mixtures of these and other suitable liquids. (Polar, e.g. water based, fluids can be used if the conductivity is sufficiently low and if fields are low enough to avoid hydrolysis.) Alternatively, coloring fluid  28  may comprise a conducting fluid, for example an organic dielectric liquid such as isopar, with charge directors to add charge pairs (positive and negative) to the fluid. As described above, the conducting fluid may be non-transparent and non-white, such as black, or it may be a transparent fluid colored by neutrally buoyant pigments or dyes. In this embodiment coloring particles or pigments carried by the liquid are not necessarily charged.  
         [0024]    For purposes of simplicity hereinafter, the following discussion will describe embodiments in which coloring fluid  28  comprises a conducting fluid carrying neutrally buoyant, uncharged pigment particles. However, one skilled in the art will appreciate that the display sheet disclosed herein would also operate beneficially with insulating fluids containing charged pigmented particles.  
         [0025]    The reservoir  30  and apertures  22  can be filled with a liquid  32  such as water, alcohol, ethylene glycol and mixtures of these and other suitable liquids. The two liquids  28  and  32  are immiscible. The liquid  32  may be clear, dyed or pigmented with a contrasting color to liquid  28 . The reservoir  30  and apertures  22  may also be filled with a gas such as air. Alternatively, a single transparent (and not dyed) conducting fluid can uniformly fill both lower and upper reservoirs. Then pigment is pumped with the fluid between reservoirs but is not allowed to recirculate along with the liquid through fluid return vents  22 .  
         [0026]    In any of the above cases the display is bistable. That is, after writing into the hidden or revealed state the image is non-volatile due to surface tension constraints (arising from curvature forces and/or surface coatings.) The bottom sheet  16  forms a carrier layer that may be opaque, or white. The bottom layer  16 , the medial plane  14 , and the intermediate layer  20  define the boundaries and dimensions of the hidden lower reservoir  26 . The upper reservoir  30  is formed by the intermediate layer  18  and the transparent cover layer  12 . The volume of the lower reservoir  26  needs to be at least as large as the volume of the upper reservoir  30 . The volume of the lower reservoir is primarily controlled by the thickness of the intermediate layer  20 . The distance d 1  between sheets  12  and  14  and the distance d 2  between sheets  14  and  16  both are in the range between 0.0001 and 0.05 inches.  
         [0027]    Referring to FIG. 2, there is shown a cross sectional view of medial plane  14  of one embodiment of this invention. In this embodiment, medial plane  14  comprises seven layers  210 ,  212 ,  214 ,  216 ,  218 ,  220  and  222 . Layers  210 ,  212 ,  214  and  216  are each a thin, flexible, white (layer  210  only), opaque and highly reflective material such as TiO 2 -filled polymer membrane, Mylar®, Lexan®, Plexiglas®, ceramic, etc. Electric field generating elements  218 ,  220  and  222  are interposed (e.g. by deposition on the top surfaces of  212 ,  214 , and  216  respectively, or on the bottom surfaces of  210 ,  212 , and  214  respectively) to form a stacked electrode structure. Electric field generating elements  218 ,  220  and  222  are comprised of any conductive material such as aluminum. In FIG. 2, apertures  24  have the same properties and serve the same purpose as in FIG. 1. Apertures  22  have been omitted for clarity.  
         [0028]    Referring to FIG. 3A, there is shown a top view of medial plane  14 . Conductive strips  218  are patterned on layer  212  and form lines (columns) parallel to the edge  310  of medial plane  14 . Conductive strips  220  are patterned on layer  214  and form lines (rows) parallel to the edge  320  of medial plane  14 . As indicated in FIG. 3A, conductive unpatterned layer  222  is placed on layer  216  and all points within the plane are set at the same voltage. It should be clear that patterns chosen for metal layers  218 ,  220 , and  222  can be interchanged, and that the unpatterned plane, as shown in perspective view in FIG. 3B, can in fact be patterned. For example, if layers  218  and  220  are patterned into column and row strips, respectively, as above and layer  222  is patterned into column strips vertically displaced below columns in layer  218  (FIG. 3C), then separate phases can be applied to the columns of layer  222  to extend the set of operations that can be achieved. In the following description, layer  222  will initially be taken to be patterned as in FIG. 3C. The conductive elements  218 ,  220  and  222  are all fabricated by well-known methods of depositing and patterning a conductive material such as metal or polysilicon and may be encapsulated. For example, polyester sheets can be aluminized uniformly by sputtering and patterned into aluminum stripes by laser ablation in a roll to roll process. Apertures  24  have the same properties and serve the same purpose as in FIG. 1. Apertures  22  have been omitted from FIG. 3C for clarity. The crossing points of conductive strips  218  and  220  align with at least one corresponding aperture  24 . The crossing points can be larger than the apertures  24  in order to each activate more than one aperture  24 . When the elements  218 ,  220  and  222  are activated with mutually phased waveforms P 1 , P 2 , and P 3 , a moving electric field wave, a peristaltic wave, is created which causes the fluid in the corresponding reservoir to move from one reservoir to another, as described in more detail hereinbelow. The peristaltic fields separate charge pairs locally, but transport both signs of charge in the same direction. The fields drive the charged species, which in turn viscously drag the fluid, which in turn drags the pigment particles (if pigment particles are used). Conductive layers  218 ,  220  and  222  are connected to control logic, not shown, from the edges of sheet  10  by any well-known means such as edge connectors.  
         [0029]    Referring to FIGS.  4 A-D, there is shown three different phased waveforms P 1 , P 2 , and P 3  that are applied to conductive layers  218 ,  220  and  222 , respectively, to write a particular pixel, that is, to move pigment from below to above the medial plane. The waveforms may be either digital voltage signals or analog voltage signals. For simplicity in the discussion, digital phased waveforms will be described in this embodiment. Referring to FIGS.  4 A-D and  5 , in operation, medial plane layer  216  is adjacent to reservoir  26  holding conductive liquid  28 . The liquid  28  may be a conductive fluid containing charge directors, as described above.  
         [0030]    The phased digital waveforms P 1 , P 2 , and P 3  are applied to the conductive layers  218 ,  220 , and  222  through a known control logic. The control logic has a selection architecture such as multiplexers or programmable logic array (PLA) to select a given electrode at a given time. As shown in FIGS. 4A and B, an approximately sinusoidal voltage wave is applied to the three electrodes  218 ,  220 , and  222 . A positive (negative) voltage here can be thought of as corresponding to an accumulation of positive (negative) charge in the electrode. This charge attracts oppositely charged species in the liquid and repels similarly charged species. The net neutral liquid is thus locally polarized, but no long distance charge separation is induced. The conductivity of the resultant fluid is adjusted to be low enough that field screening does not occur and approximately all charges in the liquid between the electrodes are separated. Field strengths and charged species mobilities are chosen so that drift times are comparable with switching times and are much shorter than diffusion times.  
         [0031]    In order to support an applied field, the concentration of mobile ions in the liquid must be sufficiently low. For voltages V and spacings d, the simple application of Poisson&#39;s equation to the electrode region shows that the concentration of ions must be less than  
         ε·ε 0   ·V /( q   0   ·d   2 ),  
         [0032]    where ε is the relative permittivity (usually 2-4 for organic liquids) and ε 0  is the permittivity of free space (8.85e-12F/m), V is the typical voltage on the electrodes (10V for this example), q 0  is the ion charge (1.6e-19 C for most ions) and d is the electrode spacing (10 microns for this example). This gives a typical ion concentration of 1.1e19 m −3 . Mobilities for ions in these liquids are found in the literature to be typically in a range of from 1e-9 to 1e-8 m 2 /V −s , so that the corresponding fluid conductivity (the product of ion concentration and mobility) is somewhere around 2 to 20 nS/m.  
         [0033]    Once the voltage and spacings have been selected, the wave speed must be determined. In order to drag the ions effectively, it is necessary that the wave speed be somewhat less than the drift speed of the fastest ions. In that manner, the ions of each sign will stay separated from their opposites, and follow the potential profile as it travels along the length of the aperture. There will be no ion “slippage” and hence mixing of positive and negative ions, which would tend to reduce the effectiveness of the pumping action. The highest drift speed one would expect would be determined from the product of mobility and the highest electric field in the system, that is, μV/d, or, with the above numbers, 1e-2 m/s. The traveling speed of the wave would necessarily be a bit less than this value. The frequency of the phased voltages is then determined by the wave speed and the wavelength. The wavelength is determined by the electrode spacing, and for a 3-phase system is 3·d. The frequency is then the wave speed divided by the wavelength, or for this example, about 30 Hz.  
         [0034]    The resultant force on the liquid is determined by the force exerted on the fluid by that quantity of ions being dragged at the wave speed. The resulting force per unit area and per unit length is given by  
           nq   0   v   t /μ,  
         [0035]    where n is the ion concentration (1.1e19 m −3 ), v t  is the wave speed (1e-2 m/s), and μ is the ion mobility. For the present example, this is approximately 2e7 N/m 3 , or the equivalent to a pressure gradient of about 200 bar/m. This can then be used to determine the flow rate through the aperture.  
         [0036]    While these numbers are chosen for particular voltages and spacings, it should be understood that higher voltages, smaller spacings, and higher concentrations could be used. By combination of the above relations, it can be shown that a scaling law for the fluid force is limited according to the relation  
         ε·ε 0   ·V   2   /d   3 ,  
         [0037]    so that, as long as breakdown is avoided, the force scales with voltage squared. Organic liquids can typically withstand field strengths of several megavolts per meter, so that for 10 micron spacings, fluid forces of 1.6e7 N/m 3 , or 160 bar/m are possible, although at voltages of 30V. Spacings smaller than 10 microns help to reduce the necessary voltage and, at the same time, increase the force on the fluid.  
         [0038]    As seen in FIG. 4A for a sinusoidal-like sequence, and FIG. 4C for a sawtooth-like sequence, the voltages are changed and a wave-like voltage pattern is shifted across the medial plane. Note that both positive and negative charged species are transported in the same direction, so called ‘ambipolar’ transport. The moving charged species drag the fluid and its contents along. In FIG. 4 a  charges are driven peristaltically upward in time, thereby pumping fluid and suspended pigment upward through the orifice  24 . In the indicated configuration, this is a ‘write’ operation. As seen in FIG. 4 b  reversing the phasing applied to the electrodes reverses the wave direction, which in turn reverses the fluid transport. This represents an ‘erase’ operation.  
         [0039]    In a display system each pixel must be put into its own state. The present system is able to support passive matrix addressing wherein a single row at a time is selected, and pixels at the intersection with each column within the row are selectively and simultaneously driven in a single selected direction. Thus, as shown in FIG. 3 c , phase P 2  is applied to a given row in plane  220 , P 1  is applied to each column in layer  218  and P 3  is applied to each column in layer  222  which is being addressed for writing. Peristaltically phasing P 1 , P 2 , P 3  drives fluid from the bottom reservoir to the top reservoir (writing). The voltages are switched synchronously through their cycles many times until all the pigment has been transferred. Simultaneously, in the case that layer  222  is also patterned into columns as in FIG. 3C, or in another time interval if layer  222  is unpatterned as in FIG. 3 b , other pixels in the same row can be driven as P 3 , P 2 , P 1  (on layers  218 ,  220  and  222 , respectively) to drive fluid in the opposite direction (erasing.) In the case of FIG. 3C all pixel setting is completed after addressing each row once with no frame erase required. In the case of FIG. 3B each row can be addressed only once if preceded by a frame erase. Alternatively each row can be addressed twice: once to write selected column crossings and once again to erase the remaining column crossings. Each row is similarly addressed in turn until the entire display has been set.  
         [0040]    As described above proper phasing of voltages on the electrodes at a pixel are required to transport fluid. Referring to FIG. 4D it can be seen that, conversely, breaking the peristaltic pattern halts the flow. This effect is used to provide non-switching states for the pixels in non-addressed rows residing in the same columns as the desirably addressed pixels. Thus, if P 1 , P 2  and P 3  are ordered and cycled so as to transport fluid at desired column intersections for selected row, i, for example, then applying P 1 , P 1 , P 3  or P 1 , P 3 , P 3  does not transport fluid at intersection pixels in all other rows, j, where no switching is desired. Thus, by addressing rows and columns, transport will occur only at intersections where proper phasing is provided.  
         [0041]    As a result, the wave forms P 1 , P 2 , and P 3  cause the coloring fluid to move from reservoir  26 , through the opening  24 , past conductive layers  222 ,  220  and  218 , and into reservoir  30 . Referring to FIG. 5, by applying the waveforms P 1 , P 2 , and P 3 , to the conductive layers  222 ,  220  and  218  in the reverse order, coloring fluid  28  can be moved from reservoir  30  and down the opening  24  back into reservoir  26 . Gas or fluid originally contained in reservoir  30  flows into reservoir  26  through apertures  22  to maintain intra-pixel equilibration. In the case that gas fills half the pixel (one reservoir) and apertures  22  are made to be non-wetting to fluid  28 , then a dyed, non-pigmented fluid  28  can be used. In another embodiment, gas or fluid originally contained in reservoir  30  may also move through apertures  230  to maintain inter-pixel equilibration. Apertures  22  and/or  230  are at least half the diameter of the smallest pigment particles. For the case in which dyed, unpigmented fluids are used, then apertures  22  and/or  230  are either treated to be non-wetting or are made sufficiently small so that one fluid is not able to pass, but any gas or second fluid may pass.  
         [0042]    Furthermore, the number of conductive layers can be modified to be more than three. For example, four conductive layers  420 ,  422 ,  424 , and  426  can be used as shown in FIG. 6. Extra conductive layers can be used to increase the suppression of transport in non-selected pixels, and to enhance pumping in selected pixels. In this embodiment, conductive layers  420  and  426  are unpatterned and conductive layers  422  and  424  are patterned in the same manner as layers  218  and  220  respectively, as shown in FIG. 3B.  
         [0043]    Many different variations, combinations, and arrangements of this invention can be implemented to move the coloring fluid. For example, referring to FIG. 7, the shape of apertures  24  may be varied such that the sides are angled, rather than parallel. In the same manner, the patterned conductive layers may have different shapes or be configured differently, as shown in FIG. 8. In FIG. 8, conductive strips  340  are arranged in such a manner that they form parallel lines which are diagonal with respect to conductive strips  220 ′. In FIG. 8, those elements which are the same as those disclosed in the description of FIG. 3A, are designated by the same reference numerals with a prime “″” affixed thereto and have the same properties and serve the same purpose as their counterparts. In further variations apertures  24  and  22  can be hidden from viewing by masking layers (not shown).  
         [0044]    The sheet of the present invention may be produced in a continuous process from webs of material. Webs are fed from rolls, to die cutting stage to partial lamination stage to inking stage to final lamination stage to roll, as described in U.S. application Ser. No. 09/216,829, cited above. The orifices may be cut with a laser drilling apparatus, but alternatively could be die punched. A separate operation would cut the sheet to size.  
         [0045]    It is therefore apparent that there has been provided, in accordance with the present invention, a display sheet with a stacked electrode structure. The advantage of the display sheet disclosed in this invention over prior display sheets using a standard electric field is the more efficient movement of material from one reservoir to another and the minimization of space charge creation and field screening, and the resultant long time constants for charge re-equilibration. Accordingly, the present invention is intended to embrace all such alternatives, modifications, and variations which may fall within the spirit and scope of the following claims.