Field of the invention

The present invention relates to an electro-osmotic display. These displays typically comprise a first transpar- ent substrate and a second substrate, wherein a transparent common electrode is arranged on the first substrate and a pixel electrode is arranged on the second substrate. A liquid is arranged in between the first and second substrates, con ^¬ tacting the pixel and common electrodes. Charged pigment par- tides are dispersed in the liquid, thereby forming an elec- trophoretic dispersion.

Background of the invention

The electro-osmotic display comprises a plurality of pixels. For each pixel, a potential for the pixel electrode can be set individually, whereas a potential for the common electrode is set for a plurality of pixels at the same time. The pixels may or may not be identical.

The operation of the known display will be explained by referring to cross sectional views of a pixel as illus- trated in figures 1A and IB. Here, two parts of the same pix ^¬ el electrode 1 are arranged laterally offset with respect to a common electrode 2. Negatively charged black pigment parti ^¬ cles 3 are suspended in the liquid along with, inter alia, negatively and positively charged non-pigment ions and/or molecules 4 ^~ , 4 ⁺ . Common electrode 2 is arranged on a first transparent substrate 5 and pixel electrode 1 is arranged on a second substrate 6.

When a positive voltage is applied between common electrode 2 and pixel electrode 1, i.e. common electrode 2 is at a higher potential, negatively charged black pigment par ^¬ ticles 3 and negatively charged non-pigment ions and/or mole ^¬ cules 4 ^~ will be attracted towards common electrode 2 and pos ^¬ itively charged non-pigment ions and/or molecules 4 ⁺ will be attracted towards pixel electrode 1. Charged non-pigment ions and/or molecules 4 ^~ , 4 ⁺ are typically smaller and more mobile than negatively charged black pigment particles 3. Conse ^¬ quently, near common electrode 2 a thin layer of negatively charged non-pigment ions and/or molecules 4 ^~ is formed that is relatively fixed with respect to common electrode 2. This layer is illustrated in figures 1A and IB. Adjacent to this layer, in a direction away from common electrode 2, a depletion layer having a net positive charge is formed due to the surplus of positively charged non-pigment ions and/or mole ^¬ cules 4 ⁺ . At pixel electrode 1, the opposite holds. There, a thin layer of positively charged non-pigment ions and/or mol ^¬ ecules 4 ^~ is formed, adjacent to which a depletion layer hav ^¬ ing a net negative charge is formed. Unlike the thin layers near the surface mentioned before, the depletion layers are not fixed to the electrodes.

It is known from the art that the depletion layers near electrodes 1,2 will experience a net force parallel to the electrodes 1, 2 as a result of the applied voltage be- tween electrodes 1, 2. More in particular, in figure 1A, the depletion layer near common electrode 2 will be attracted towards pixel electrode 1, and the depletion layer near pixel electrode 1 will be attracted towards common electrode 2 due to the electric field that is directed from electrode 2 to electrode 1. Consequently, a flow as indicated by arrows 7 will be induced in the liquid that is responsible for the transport of negatively charged pigment particles 3. Herein ^¬ after, this flow will be referred to as the primary electro- osmotic flow.

By inspecting figure IB, which illustrates the situ ^¬ ation wherein common electrode 2 has a lower potential, it can be seen that the flow in the liquid has the same direc ^¬ tion. This is because the change in direction of the electric field between electrodes 1, 2 is compensated by the net charge of the depletion layers, which is opposite to the sit ^¬ uation depicted in figure 1A.

The visual appearance of a pixel is determined by the distribution of the pigment particles in the pixel. Typi ^¬ cally, an area of pixel electrode 1 is much smaller than an area of common electrode 2. Light entering the pixel will propagate through the liquid and is reflected by a diffusive reflector 8. Consequently, when all pigment particles 3 are accumulated near pixel electrode 1, the pixel will be largely transparent. Using diffusive reflector 8 will in this situa ^¬ tion provide for a white appearance. Conversely, in figure IB, light will encounter pigment particles 3 and the pixel will therefore appear black.

Figure 2 illustrates a top view of a known pixel electrode and common electrode layout. Figure 2 only illus ^¬ trates half of the pixel layout as the layout is symmetric with respect to (a virtual) line of symmetry 9.

Pixel electrode 1 comprises a base 10 from which two fingers 11 extend. Similarly, common electrode 2 comprises a base 12 from which a finger 13 extends in between two fingers 11 of pixel electrode 1. Side parts 14 of common electrode 2 could also be designated as fingers.

Arrows 7 indicate the direction of the primary elec- tro-osmotic flow. However, a secondary electro-osmotic flow, indicated by arrows 15 will be generated due to asymmetry of the electrode layout. This secondary electro-osmotic flow is also known in the art.

In view of background art documents reference can be made to WO2009/108187, US 2011/013259 Al, WO2007/092253A2 , and WO2011/036433 Al .

The applicant has found that the known electrode layout is not without drawbacks. More in particular, the ap ^¬ plicant has found that during operation charged pigment par- tides will accumulate in region A indicated in figure 2.

These particles will not respond adequately to the applica ^¬ tion of a voltage difference between pixel electrode 1 and common electrode 2. In some cases, the particles are essen ^¬ tially stuck in region A. As such, a pixel cannot achieve the proper white values and/or contrast it is designed for.

It is an object of the present invention to provide a solution to the abovementioned problem.

Summary of the invention

This object has been achieved by an electro-osmotic display as defined in claim 1. According to the invention, the display comprises a transparent common electrode arranged on a first transparent substrate, a preferably transparent pixel electrode arranged on a second preferably transparent substrate, and a liquid in between the first and second sub ^¬ strates in which charged pigment particles are dispersed. The common electrode comprises a base and at least two fingers extending from that base and the pixel electrode comprises a base and at least one finger extending from that base. Fur ^¬ thermore, the fingers of the common electrode and the pixel electrode are arranged in an interdigitated manner.

The display is characterized in that the fingers of the pixel electrode and the common electrode taper outwardly in a direction away from the base of that electrode.

Not being bound by theory, it is stipulated that the tapering improves the secondary electro-osmotic flow allowing pigment particles previously stuck in region A indicated in figure 2 to be transported away. It is further stipulated that the underlying physical mechanism by which the charged particles are stuck in region A is a lack of secondary elec ^¬ tro-osmotic flow and a lack of primary electro-osmotic flow, the latter being related to the symmetry of the electrode layout. For instance, region A in finger 13 lies in between two fingers 11 of pixel electrode 1. In case common electrode 2 is made negative with respect to pixel electrode 1, the negatively charged pigment particles will be pushed away from common electrode 2. An electric field having a component par ^¬ allel to common electrode 2 will be generated in the direc- tion from fingers 11 to finger 13. However, due to symmetry, no net electric field parallel to common electrode 2 will be present near (virtual) axis of symmetry S in finger 13. In case no suitable secondary electro-osmotic flow is present in the pixel, charged pigment particles will become trapped in region A as indicated in figure 2.

The applicant has found that the use of an outwardly tapering finger for both the pixel electrode and the common electrode alleviates the abovementioned problem considerably. It is stipulated that the use of tapering improves the sec- ondary electro-osmotic flow allowing charged pigment parti ^¬ cles previously trapped to be transported away efficiently.

The fingers of the common electrode and the at least one finger of the pixel electrode can be adjacently arranged and the at least one finger of the pixel electrode may extend towards the base of the common electrode and the fingers of the common electrode may extend towards the base of the pixel electrode .

A distance between the fingers of the common elec ^¬ trode and the at least one finger of the pixel electrode may be constant along a longitudinal direction of the fingers

An angle of tapering may lie in a range between 1 and 10 degrees, and preferably between 3 and 7 degrees such as 5 degrees. Increasing the angle of tapering increases the space occupied by the pixel electrode. The skilled person therefore understands that the angle of tapering cannot be chosen too large.

A smallest width of the fingers of the common elec- trode may be larger than a largest width of the at least one finger of the pixel electrode. Additionally or alternatively, a largest width of the fingers of the common electrode may be at least three times larger than a largest width of the at least one finger of the pixel electrode.

It is noted that the pixel electrode functions as an accumulation electrode in the sense that a white or transpar ^¬ ent appearance of a pixel corresponds to the pigment parti ^¬ cles being completely or largely accumulated near the pixel electrode. The total size of this electrode must therefore be kept as small as possible. This being said, enough pixel electrode area must be available to generate the primary and secondary electro-osmotic flows.

Detailed description of the invention

Next, the invention will be described in more detail referring to the appended figures, wherein:

Figure 1A and IB illustrate cross sections of a pix ^¬ el of a known electro-osmotic display;

Figure 2 illustrates a top view of the electrode layout of the pixel of figure 1 ;

Figures 3A-3F illustrate electrode layouts of a pix ^¬ el of an electro-osmotic display in accordance with the in ^¬ vention . The abovementioned problem of the trapping of charged pigment particles at particular positions in the electrode layout has not been observed, or at least to a lesser extent, using the electrode layouts depicted in fig- ures 3A-3F.

The pixel electrode is for example driven using an active matrix. Typically, the distance between the first and second substrates is in the order of 10-20 micrometer, such as 12-15 μπι. The distance between the pixel electrode and common electrode near the fingers of these electrodes is preferably less than two times the distance between the sub ^¬ strates .

In an example the present pixel further comprises a diffusive reflector 8 will in this situation provide for a white appearance.

In an example the present pixel comprises a virtual line of symmetry 9.

In an example the present pixel comprises a virtual axis of symmetry.

In an example the present pixel comprises at least one pixel wall.

The invention is not limited to the embodiments shown in the figures. For instance, it is not limited to the particular flow direction of the primary and secondary elec- tro-osmotic flows shown in the figures. More in particular, the flow direction may be reversed without departing from the principles of the present invention. The number, polarity and type of the charged pigment particles can be varied. For in ^¬ stance, two oppositely charged pigment particles may be used, such as red and green.

It should therefore be obvious to the skilled person that various modifications can be made without departing from the scope of the present invention which is defined in the appended claims.