Why the crease is the hardest part of a foldable phone

HostThere's something about folding a piece of paper that just feels right, like you're finishing a thought or putting something away for later. We have spent our whole lives folding books and maps, so it feels like a dream to finally do that with a high tech screen. But when you actually hold one of these new phones, your eye immediately goes to that tiny dip right in the middle where it bends. Why is that one little line so much harder to get rid of than the rest of the phone was to build?

GuestIt's basically a fight against the way things work in the real world. When you take a piece of paper and fold it, you're actually breaking the tiny bits that hold the paper together. Once those are snapped, they don't just go back. Now, think about a phone screen. It's not just one thing. It's a stack of many very thin layers. You have the part that shows the colors, the part that feels your touch, and then a clear top layer to keep it safe. To make a phone fold, that top layer has to be strong enough to not scratch when you touch it, but soft enough to bend thousands of times without snapping. For a long time, we used plastic, but plastic gets marks and dents easily. Lately, companies have started using glass that's so thin it's almost like a human hair. But even glass, when you bend it over and over, starts to get tired. That crease you see is the spot where all that stress gathers every single time you open and close the device.

HostBut we have plenty of things that bend without leaving a mark, like a piece of rubber or a thick cloth. If the glass is the problem, why not just move away from it and find a new kind of clear stuff that can take the hit?

GuestWell, the problem is that we want two things that don't usually go together. We want a screen that feels hard and smooth like a normal phone, but we want it to act like a piece of skin that can stretch and fold. If you use something too soft, your fingernail would leave a permanent dent the first time you swiped to check your mail. If you use something too hard, it'll just shatter like a window. So we're stuck in the middle. Most of these screens use a tiny layer of glass with a bit of plastic on top. The crease happens because when you fold that stack, the stuff on the inside of the bend gets squished and the stuff on the outside gets stretched. There's no way to move that much material without it having to go somewhere. It's like when you fold a thick rug; it has to bunch up at the hinge. That bunching and stretching over time creates a little valley that never quite goes flat again.

HostOkay, I see the physical side of it, but what about the hinge itself? I have seen some phones where the screen doesn't fold tight like a book, but more like a loop. Does giving the screen more room to breathe actually help hide the line?

GuestIt helps a lot, but it adds its own mess of problems. Most of the newer phones use what people call a teardrop hinge. Instead of folding the screen flat like a piece of paper in a wallet, the hinge lets the screen tuck back into the body of the phone in a round shape, kind of like the bottom of a water drop. This means the bend isn't as sharp, so the screen doesn't get as stressed out. But even then, you have to support that screen. A screen is very floppy on its own. It needs a solid floor underneath it so it doesn't feel like you're pressing on a piece of plastic wrap. When the phone opens, these little metal plates have to slide into place to give the screen that floor. The crease we see is often the tiny gap between those plates or the spot where the screen isn't being pushed up quite as much as the rest.

HostSo it's not just a dip in the screen, but also a change in what's behind the screen? That sounds like we're fighting a losing battle. If it's about the light hitting a gap or a curve, will we ever have a phone that looks totally flat?

GuestWe're getting closer, but your eyes are very good at spotting even the smallest change in light. Even if a screen is nearly flat, if there's a tiny slope of just a fraction of a millimeter, the light will hit it differently than the rest of the screen. It creates a shadow or a bright spot that catches your eye. It's like looking at a calm lake; you can see the tiniest ripple from a mile away. To fix it, we would need a material that can be compressed to zero thickness when it folds and then grow back to its original shape perfectly when it opens. We just don't have anything in nature that does that yet. We're also fighting dust and bits of grit. If one tiny grain of sand gets behind that screen or inside that hinge, it can push up and create a new bump that looks just like a crease.

HostIt's wild to think that after all the work to make a screen that can actually show pictures while it's bent in half, the biggest hurdle is just how a shadow falls on a tiny piece of glass.

GuestThe real trick might not be making the crease go away, but making our brains stop looking for it, though for now, the physics of stretching and squishing a solid object means that little valley is here to stay.

HostIt turns out that trying to fold a screen is a lot like trying to fold that same piece of paper; the mark it leaves behind is just the price we pay for the bend.