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The small white and black pigment spheres inside E-ink are positively and negatively charged. In the page-turning process, an electric field is applied to the E-ink. The pigment spheres will either be drawn toward or away from the charge, which forms the image on the screen. However, when the electric field is not present (no page-turn in progress) then the pigment spheres are merely held in place by the viscosity of the suspension fluid. Furthermore, if you recall the white and black pigment spheres are oppositely charged. Thus, over time I suspect that they will eventually drift toward each other.
Take a look at the attached eink.jpg diagram.
These are cross-sections of the E-ink capsules. On the left, the capsule is being subjected to an electric field (a page change). The colored arrows represent the forces on the pigment spheres due to the electric field. Note that all the white pigment spheres (titanium dioxide, I think) are forced against the surface-side of the capsule. On the right, there is no electric field. The pigment spheres, being oppositely charged, slowly begin to migrate toward the center. This manifests as a loss in contrast.
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