Liquid Rules

I hadn’t given much thought to how glues work until reading “Sticky”, the fourth chapter of Mark Miodownik’s Liquid Rules. But it’s not just glues that stick. Water sticks too. It sticks to glass. It sticks to towels (by wicking, so that it sticks less to your skin). I like Miodownik’s description of paint – essentially you’re trying to get colored substances to stick to paper, canvas, concrete, wood, your face, or the walls of a cave.

 

What makes substances sticky? It has to do with a change of state. Miodownik writes: “Glues start off as liquid and then, generally speaking, turn into a solid, creating a permanent bond.” He then describes the myriad ways this can take place with many interesting examples. Here’s what I learned:

 

·      The 5000-year old ice-man Otzi had an axe that used birch-bark resin as glue. The major component is a methoxyphenol. Once the liquid turpentine in the mixture evaporates, the remaining methoxyphenol polymerizes. Tree resins are a great source of glue; some smell fragrant such as frankincense!

·      Animal glues come from collagen. Separate the collagen in hot water and you can turn it into gelatin. Apparently, the Egyptians were pioneers in constructing plywood by crisscrossing the grains of thin pieces of wood and glue-stacking them. Thankfully Egypt is dry, because gelatin falls apart in hot and humid environments. Woodcrafters took advantage of unsticking animal glue with steam to cleanly repair and restore everything from furniture to violins. Knowing how to stick and how to unstick a glue makes it versatile!

·      Rubber is a glue of sorts. Its ability to be stretched and molded is what helps it retain a “grip” be it a bicycle’s handlebars or a car’s tires. Post-it notes use rubber, and that’s why they don’t damage the surface when you peel them off.

·      Phenol and formaldehyde make a polymer that is a strong glue. (I’ve published a number of research papers related to formaldehyde, but never with phenol.)

·      Superglue is cyanoacrylate; oily by itself but super-sticky when it comes into contact with water because of polymerization.

 

In between a liquid and a solid, you can have liquid crystals. They’re fascinating substances! Miodownik discusses how they alter the polarization of light, and how you change a display by changing the electric field. It’s how my cheap handheld calculator works. I also learned that unlike watercolor paints which stick quickly once the water evaporates, oil paints adhere much more slowly through oxidation reactions. Famous paintings in museums are mostly oils and famous painters layered these oils to “create complex visual effects… [by] controlling color, luminosity and texture”. The same effect can be realized by tiny dots placed close to each other. Inkjet printers just need four primary colors to do this: cyan, magenta, yellow, and black. Thus, CMYK on your ink cartridge.

 

Reading Liquid Rules reminded me of two things. First, I should be leveraging a much wider variety of interesting examples in my G-Chem classes; there is so much fascinating chemistry in the world around us and instead students get bogged down in abstraction (which is partly my fault). It’s another reason to consider ditching the textbook. Second, in P-Chem I’ve mostly avoided the mathematical modeling of liquids because they’re hard to deal with; it’s much easier to describe solids and gases with “clean” mathematical equations. But the dynamic liquid space is where much of the interesting chemistry happens; Liquids Rule! I should consider what changes I can make beyond the simple modeling of “dilute” solutions where I currently spend a small sliver of time. Any book that gets me thinking about improving my teaching makes reading it worth my while. Liquid Rules certainly qualifies. (I also recommend his previous book, Stuff Matters. Here is my post on aerogels.)