How an e-ink screen holds an image without power

HostI was looking at my e-reader today and noticed something strange. The battery died a few days ago, but the screen is still showing the cover of the book I was reading. It's like a frozen image that refuses to go away. How can a screen keep showing a picture when there's no power at all?

GuestIt's wild when you first see it because we're so used to our phones and TVs. A phone screen is a bit like a window with a very bright light behind it. If you cut the power, the light goes off and the window goes dark. But an e-ink screen works more like a physical object, like a piece of paper or a whiteboard. Think of the screen as a very thin sandwich. Inside that sandwich, there are millions of tiny clear bubbles. They're so small that you could fit several of them across the width of a single human hair. Inside each bubble, there's a clear, oily liquid. Floating in that liquid are thousands of tiny bits of ink. Some bits are white and some are black.

HostSo instead of tiny light bulbs, I'm looking at actual bits of ink moving around inside the glass?

GuestThat's exactly what's happening. The white bits have a plus charge and the black bits have a minus charge. Under the screen, there's a grid of wires. When you want to turn a page, the device sends a quick burst of power through those wires to create a pull. If a wire sends a minus charge to a certain spot, it pulls all the plus-charged white bits to the front of the glass. At the same time, the minus-charged black bits get pushed to the back where you can't see them. When you look at that spot, it looks white to you. To make a black letter, the wires just flip the charge and pull the black bits to the front.

HostBut okay, I get how they move, but why do they stay there? If I turn off a magnet, the paperclip falls. Why don't the black and white bits just mix back together once the power is gone?

GuestThat's the clever part. The oily liquid inside those bubbles is very thick, almost like honey. It takes a push of power to move the ink bits through that honey. But once the power stops, the honey is thick enough to hold the bits right where they are. There's enough friction in the liquid to keep them from drifting around. You could take the battery out of the device and throw it in a drawer, and those ink bits would stay in their spots for years. The only time the device uses any battery is during that one second when it pushes the bits to a new spot. Once the page is set, it costs zero energy to keep it there.

HostThat sounds like it would be a huge win for every screen we use. I would love a phone that lasts a month. But there must be a catch. If I try to scroll on my e-reader, it looks messy and slow.

GuestIt's very slow. Think about the physics of what's happening. On a phone screen, you're just switching a tiny light on or off. That happens almost instantly. But in an e-ink screen, you're asking physical bits of matter to swim through a thick liquid. It takes time for those bits to push through the oil. This is why you can't really watch a movie on one of these screens. The ink bits can't move fast enough to keep up with the action. If you try to move things too fast, the bits get stuck halfway, which is why you see those faint shadows of old words on the screen. We call that ghosting. To fix it, the screen has to do a full reset where it flashes every bubble black and then white to clear the path.

HostI also noticed that the screen is much easier to read when I'm outside in the sun. My phone just turns into a mirror when I go outdoors, but the e-ink looks better the brighter it gets.

GuestThat goes back to the lack of a backlight. Since there are no lights behind the ink bits, you're reading by the light in the room, just like you would with a normal book. The light from the sun or a lamp hits the white ink bits and bounces back to your eye. A phone screen is trying to outshine the sun, which is a fight it usually loses. Because the e-ink is just bouncing light instead of beaming it directly into your eyes, it also tends to be much easier on your vision. It doesn't have that harsh blue glow that keeps people awake at night.

HostIt seems like we're stuck with black and white for now, though. Most of the e-ink screens I see are just shades of gray. Is it just too hard to move different colors through that oil?

GuestIt's a lot harder. To get color, you either have to put a colored filter on top of the bubbles, which makes the screen look a bit dim, or you have to put more types of ink bits inside the bubbles. Imagine trying to sort red, green, blue, and black bits all in the same tiny ball of oil using just plus and minus charges. You have to use different levels of power to pull the red bits to the top while keeping the others down. People are working on it, and there are color screens out there now, but they still look a bit like a faded comic book rather than a bright TV.

GuestThe big goal now is finding a way to make those tiny ink bits swim through the oil fast enough to show a movie without needing a full black flash every few seconds.

HostMy old e-reader is happy to just sit there with that same frozen book cover, perfectly still, even with the battery long gone.