In the simplest case of a black and white display
the top substrate is covered with a single transparent electrode,
while the bottom substrate contains a complex pattern of line-electrodes.
Using active matrix driving, a single pixel can be addressed, meaning that the bottom electrode can be made either positive or negative compared to the top-electrode.
The electrophoretic ink between these electrodes is a mixture of transparent liquid and microscopic charged pigment particles.
The usual choice is negatively charged black particles (carbon black) and positively charged white particles (TiO2).