We’ve used cork for centuries, for everything from the stoppers in wine bottles to the bulletin boards in coffee shops.

It’s an unassuming material  that’s easy to take for granted. But if you pin up all the things it can do, it’s not a conspiracy to call it, it’s a physics miracle.

Cork is waterproof, fire-resistant,  insulating, mechanically strong, and degrades slowly. And it comes from trees! Given that it’s such an  awesome and versatile material, it’s no wonder that we’ve tried to make  our own so we can churn out lots of it. but we still can't figure out how to replicate mother nature's secret formula for cork We’ve cracked the code on cork, but concocting a copy is another conundrum.

Try saying that five times fast. And the funny thing is, we already know what the special sauce is. [intro music]

Cork can actually be found in just about any tree. It forms a thin layer right under the bark.

But one kind of tree, Quercus suber, has a particularly hefty cork layer  that’s several centimeters thick, so that’s the species that  supplies almost all of our cork. To harvest it, people just peel back the bark and  cut out the cork layer underneath. It sounds like that would kill the tree, but it’s surprisingly okay with it.

After 9 to 12 years, the cork and bark regenerate and  are ready to be harvested again, meaning cork is a renewable resource. Trees make cork in the first place  to protect against the elements, which is exactly why it has that  incredible list of superpower properties. So when humans discovered  this amazing natural material, they put it straight to work, like for shoe soles or fishing net floats.

By the seventh century, people were already using  cork to seal wine bottles, since it was flexible enough to fit in the bottle, but rigid enough to form a tight seal. Cork keeps air and liquid out so that  icky stuff doesn’t grow in the wine, allowing the wine to be stored for  longer while staying drinkable! The buzz is much nicer  without the risk of botulism.

Cork also serves as insulation  in walls and ceilings. It’s good at keeping heat  in and keeping sounds out. It’s even been used in planes and spacecraft!

In these cases it reduces vibrations  and insulates the rockets or engines from overheating other parts of the craft. These amazing properties are all  possible thanks to cork’s cell structure. In 1665, Robert Hooke took a first look at it.

He sliced up a bit of cork,  put it under a microscope, and saw those cells. But this wasn’t just the first  time someone saw cork cells, this was the first time humanity  had ever seen A cell, period. Cork cells were how we discovered cells!!

Which I guess led to some of the most  important developments in all of biology. But more importantly, taught us a bit about cork. Cork’s cells are densely  packed with almost no gaps.

Each cell is about 14 microns  wide by 35 microns tall, about a quarter of the size  of a strand of human hair. The walls of these tightly-packed cells  are covered in fatty acids and waxes, which give cork its waterproofing  and fire-resistance. The structure and chemical composition  also make cork impermeable, meaning gas and liquid can’t  easily flow through it— helpful for wine stoppers!

The cells themselves are mostly full of air, making up about 85-90% of cork’s volume, creating a super low density  material that easily floats. And because air is a great insulator, cork is too. We’ll talk more about cork’s amazing  properties after this quick break.

THEY WERE THE FIRST CELLS WE SAW, I STILL CAN’T BELIEVE THAT. This SciShow video is supported by Brilliant, the online learning platform  for students in math, science, and engineering classes. And now, Brilliant has a tutor, Koji, made for students in grade 5 through undergrad.

Many students today don’t think  about answers to their questions. They type out the question and let  AI come up with answers for them. Koji is the opposite.

Brilliant courses are full  of interactive questions. And Koji never gives away the answer. Instead, Koji will ask questions and guide  students to find the answer themselves.

To get started with Brilliant’s tutor for  free, click the link below or scan the QR code. You can upgrade to Premium to unlock all courses. And right now, SciShow viewers can save 20% off an annual subscription at brilliant.org/scishow Now, if we were to look at  a tree’s cork layer head on and zoom into this radial view, we would see that the cells are  arranged in a honeycomb-like structure with just a touch of chaos.

Many of the cells are hexagons,  that classic honeycomb shape. But pentagons and heptagons also appear. All cork cells have anywhere  from four to nine sides, but 5 to 7 sides are the most common.

What’s more, there’s no pattern for how all these different shapes are arranged, creating an amorphous cell structure. Now, if we were to rotate our view and look at cells from the  side for a profile view, we would see a different cell structure  with distinct horizontal layers, almost like a brick wall. Except the cell walls aren’t straight, ut rather wavy, or corrugated.

Together, the corrugation and the  amorphous honeycomb-like structure create unique responses to compression. Normally, if you squish a  material in one direction, it bulges out in another direction, like what happens when you squish a stress ball. But if you squish cork in the radial direction, so that you’re pressing down  flat on the honeycomb faces, the corrugated cell walls act like an accordion so that the walls collapse  without bulging outward.

We can actually quantify just how  much a material bulges outward when squished, using a number called Poisson’s ratio. For example, clay has a large  positive Poisson’s ratio— near the maximum value— because it bulges up and down a  lot when squished in the middle. On the other hand, cork barely budges when compressed in the radial direction, making its Poisson’s ratio basically zero.

So cork can be crazy resilient to large  compressive forces in this direction. Due to cork’s asymmetric cell structure, its Poisson’s ratio is actually different  when compressed along other directions. It’s still a strong and insulating  material in those directions, though.

Having a near-zero Poisson’s  ratio is really cool and useful, but finding this property in nature  is hard. Really only cork does this. Scientists have been trying to make  these types of materials themselves, with only partial success.

For example, in 2015 a team  of researchers published work on a new amorphous sponge  material made from sheets of graphene. At the nanoscale, graphene is pretty orderly. It’s made of a single layer of carbon atoms arranged in a perfect honeycomb structure.

Then, to add that signature touch of chaos, the graphene sheets are scrunched up randomly. They chemically bond to form  a sponge of carbon atoms with an overall amorphous mesh cell structure. Each mesh “cell” is on the  order of tens of micrometers, similar to the cell sizes of cork, giving  the sponge a near-zero Poisson’s ratio.

But unlike cork, the graphene sponge has the same type of structure throughout, enabling it to achieve near-zero  Poisson’s ratio along all dimensions, not just the radial direction. Which sounds promising as a cork replacement! Except it’s hard to make a lot of  graphene cheaply and consistently, so graphene sponge is  unfortunately out of the running for widespread commercial use, at least for now.

Any cyberpunk writers you can use  that graphene wine cork idea though. Over the years, researchers have also tried to pinpoint why certain honeycomb structures can  achieve that zero Poisson’s ratio. To make the math and analysis manageable, they only looked at perfect honeycomb patterns with a repeating set of polygon-shaped cells.

What they found was that concave polygons with inverted vertices can yield  super small Poisson’s ratio. Which means that theoretically, we can design a small Poisson’s  ratio material out of any material, as long as we can build a  structure with the right geometry. But these methods are still in the research phase, and not ready for mass manufacturing yet.

Now, people have also tried to  make synthetic cork using plastic. To do this, they usually either  inject the plastic into a mold or extrude the plastic and chop it up. The key step is injecting the plastic  with carbon dioxide to create a foam, mimicking the airiness of cork.

But synthetic cork is not as mechanically  strong or resilient as the real thing. That’s because cork’s cell structure, with its amorphous pattern of  different-sided squiggly walls, is just so unique. Our plastic foams are still just an approximation of the perfect amount of chaos  that nature gave to cork.

So for the time being, we still  depend on trees for our cork. Which is kind of beautiful, in a way! Despite everything humans have learned, we can’t always out-engineer nature. [ outro music ]