Turn that bottle of body wash upside down, for example. It doesn’t flow out as water would
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Today I learned from the Wall Street Journal about brushing hair. This is true. I’ll return to that.
In reading on from there, I ran across a geological phenomenon that has fascinated me for years. This is columnar rhyolitic lava-rock formations known for their shape. You can find them on a certain beach in Karnataka, for example. As I wrote in this space last year, they are “hexagonal columns with flat tops, joined together, rising out of the sand and water". (She sells hexagonal rocks by the seashore, 8 April 2021, bit.ly/3LRdU3o).
I like to think this is the first time hair-brushing and those hexagonal rocks have both been mentioned in one nondescript sentence.
The hexagons are formed by volcanic lava that, as it cools, solidifies at the surface first and then inside. This “differential cooling" produces the hexagons that you can see in a few spots around the world—like in Karnataka, like on the coast of Northern Ireland. There, they form the “Giant’s Causeway"; it’s this Causeway that the hair-brushing-prompted reading led me to.
I’m referring to the study of so-called “soft materials". These “have a number of emergent properties that arise from the combination of geometry and softness, and encompass everyday materials such as the polymers, gels, powders, colloids, suspensions etc., that are in us, on us, and surround us". (See bit.ly/3FgiYf6). Note: Geometry and softness, hexagons and lava. And sure enough, this Harvard University research programme offers up a theory that “explains the differential cooling-driven hexagonal patterns in geophysical formations such as Giant’s Causeway, with predictions that have been confirmed experimentally". No, I don’t understand the theory. But I am fascinated by everything this research covers. Here’s a flavour of that. (Wait for the carpet.)
The theory that explains hexagonal rock formations came from the study of “soft fluid-infiltrated gels". That phrase probably puts in mind materials like ubiquitous body washes, toothpaste, or, indeed, lava. These straddle a middle ground somewhere between solid and liquid. Turn that bottle of body wash upside down, for example. The stuff doesn’t flow out as water would. It’s much more viscous and slow-moving. But of course, it isn’t like a cake of soap either. How do we explain its behaviour? Toothpaste is closer to being solid yet isn’t really there either. Lava is essentially stone that is heated to a temperature so high that it melts. What happens as it emerges out of the furnace that is a volcano and comes into contact with much cooler temperatures on earth’s surface? Hexagonal rocks are one result. Or check a remote corner of Alaska, where lava from the Great Sitkin volcano has formed a shape uncannily like a giant spider (bit.ly/37ibxrf).
Then there’s a material that lubricates. That suggests oil but think too of the pieces of cartilage that separate bones in our bodies. They are the reason our knees, elbows, ankles, and wrists all move and flex so easily. That is, until injury or wear damages the cartilage, and we feel the pain of a suddenly unlubricated joint. The Harvard researchers have proposed a theory for this “cartilaginous joint lubrication". Intriguingly, they found they can generalize this theory to explain the motion of fish of the family Rajidae—the broadly square animals with long tails that we know as rays or even stingrays. This motion, we learn, can also help us understand “how carpets may fly close to a wall". (I told you to wait.)
The Royal Society of Chemistry publishes a journal called Soft Matter that covers plenty of such research. Again, most of it is esoteric beyond my ability to understand. But even a quick look at the titles of some recent papers in the journal gives a taste of the range of subjects in this field. There’s Effect of network topology and crosslinker reactivity on microgel structure and ordering at liquid-liquid interface: certainly esoteric, but note that there’s geometry and softness there. Or take a moment to ponder this one: Differences in cell death and division rules can alter tissue rigidity and fluidization. According to the authors, such rigidity and fluidity are suggestive of “health and disease in various biological processes, including cancer". Enough reason to investigate, right there. Then there’s Edible mechanical metamaterials with designed fracture for mouthfeel control. There’s every reason to believe the authors of this paper had plenty of fun with their research. That’s because they attempted to “control mouthfeel sensory experience" by—wait for it again—“using chocolate as a model material". There’s even a charming diagram of a man stuffing what looks like three slabs of chocolate into his wide-open mouth.
As you can tell, the study of soft materials covers all manner of subjects. If it takes in cells, it also examines how certain animals tend to flock (see my column Collective complexity, bit.ly/3KMovLm). If some researchers try to persuade us that they are studying chocolate instead of eating it, others have looked into “how sperm cooperate in a competitive environment".
If you think about it, this wide range nicely reflects a world in which there’s plenty of middle ground between solid and liquid. Sure there’s concrete, sure there’s water —but is a leaf solid? What about a pile of leaves? What about a snake? What about cotton candy before and after it melts in your mouth?
What, come to think of it, about hair? What about a head full of hair that needs brushing? Why is this sometimes easy but sometimes so hard that it causes pain in the owner of the hair?
The Wall Street Journal article quotes Prof L. Mahadevan, who leads the Harvard soft matter programme: “At least half of humanity combs their hair every day, and yet almost no one pauses to think deeply about it." (The other half probably doesn’t think deeply about it either.) When he put his mind to it, though, he realized that fundamentally, this was a problem of the “tangle". Individual hairs winding themselves into a tangle are, in essence, no different from strands of microscopic structures —the helix of DNA, for example—winding around each other. The more the tangles, the harder it is to separate the strands. The harder it is, the more likely that brushing will lead to anguished yelps.
Mahadevan’s team recently published a paper in Soft Matter (Combing a double helix, 22 March 2022, rsc.li/3KLaoGj). They report that most of the tangles in the hair are between pairs of strands. So detangling is a matter of pulling a brush between them, leaving “two untangled filaments in its wake". How easy this is, depends on what the authors call “link density", a measure of how knotted the hairs are. Naturally then, every head of tangled hair has an “optimal combing strategy". As a scientist at MIT told the Wall Street Journal, “You start at the bottom of the hair with a short-stroke... As you work your way up, the strokes become longer and longer."
Maybe you knew that. What about the connection to hexagonal rocks?
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.