In an E-Ink display, a clear fluid is used to suspend millions of tiny capsules filled with black and white pigment. The black pigment is negatively charged while the white pigment is positively charged, and the fluid layer is sandwiched between two electrode layers which are divided up into regions. Each region is one “pixel” in the display.

This whole process is called electrophoresis. Depending on how the electrode layers are charged, the ratio of pigment in each region changes, and that ratio is what products varying degrees of grayscale on the screen.

So when the bottom electrode creates a positive electric field, the positively-charged white pigment is pushed to the top of the fluid layer, thus obscuring the negatively-charged black pigment that moves to the bottom of the fluid layer. Together, all of the white pigment appears as a white pixel.

Conversely, when the bottom electrode creates a negative electric field, the negatively-charged black pigment is pushed to the surface of the capsule, thus obscuring the white pigment. This results in a black pixel on the display.

But when the bottom electrode creates both positive and negative electric fields, a mixture of black and white pigment is pushed to the surface of the capsule, resulting in a shade of gray that can be darker or lighter depending on how much white and black is on display for that pixel.

Unlike an LCD display, which requires constant power to keep the contents of the display on screen, E-Ink only requires power to change the polarity of electrodes on a per-region basis. This means that your e-reader only uses power when it turns pages, and that’s how an e-reader can last for up to one month on a single charge.

E-Ink devices can reduce power consumption even more by only changing the pixels that need to be changed per page turn. In other words, if a particular pixel stays black from one page to the next, nothing needs to be changed and no power needs to be expended.

However, over time, some pixels may become stuck and refuse to change even with a new polarity, and this results in an imprint of text which stays even after the page has been turned. This phenomenon is known as “ghosting” and is usually fixed by a full-page refresh. That’s why every so often the screen flashes completely black, then white, then back to the page.