I saw that a mixture of starch in water, according to one Youtuber, of 1.5:1.0 (any starch? exactly this ratio?) produces this spooky dancing liquid and the straight-up walking on “water” (this second video has some explanation that I can’t read on my monitor). Wiki, in its helpfulness to the common reader, goes absolutely batshit with (to me) high-level math.
Also, why is this stuff called “oobleck”?
Can someone explain this in language suitable for the mythical “scientific layman”?
Fluids like the one you describe in the OP are “Non-Newtonian” - that is, in this case (and many others) the viscosity of the fluid depends on the current shear forces acting on it.
Water is a pretty good example of a Newtonian fluid. It doesn’t matter what forces are acting on it, viscosity at a given temp and pressure is pretty much constant.
Laypeople interact with a lot of thixotropic (i.e., non-Newtonian shear-thinning) fluids. The idea with these is that when a shear force is applied, the viscosity drops (“the fluid flows better” is a reasonable layperson description). A good example is paint - when you apply a shear force (a brushstroke, or forcing it through a spray nozzle) the paint has a lower viscosity. It flows easily. Once it’s on the surface of the part being painted, the shear force is gone (except for gravity pulling on it.) The fluid is then much more viscous. This is desirable for something like paint - it’s easy (thin, low viscosity) to apply; once applied (thick, high viscosity) it will “stick” or “hang” to a vertical surface (i.e., a wall) and not just run off to the ground. Fumed Silica is a common thixing agent - it’s used in both food, industrial and commercial applications. Fumed silica, under low shear forces, forms a network of weak hydrogen bonds throughout the fluid, making it thick and viscous. Under high shear forces, the hydrogen bonds are broken and the fluid becomes much more thin and low viscosity.
The OP asks about shear-thickening fluids. These fluids see a significant increase in viscosity under high shear forces. Wiki has a good explanation:
That’s wrong. But even textbooks sometimes make this error, so you should probably be excused. Wikipedia’s got it right, though.
Thixotropy is a time-dependent phenomenon. Viscosity decreases with time under shear, regardless of whether that shear is constant or if it increases. Water-based latex paints are typical examples of thixotropic fluids: They have an almost gel-like behavior when you open the bucket, but after you’ve stirred the paint for a couple of minutes, the paint flows easily and is easy to spread. Left standing, the paint thickens again after some time. Ketchup is slightly thixotropic, because it pours more easily just after you’ve shaken the (glass) bottle.
Shear thinning (pseudoplasticity) is a time-independent phenomenon. A shear-thinning liquid flows more easily (i.e. the viscosity is lower) at higher shear rates (flow rates), but the thinning is instantaneous. Also the thickening after you remove the shear is instantaneous. A solution of a polymer is a typical shear-thinning liquid. Ketchup is shear thinning (as well as thixotropic), because it flows easily out of the spout, but it doesn’t flow when it’s on the burger.
The two phenomena sometimes occur simultaneously, but they’re not the same.
Related question: where can I get a speaker that looks like the one used in the video? If I bought a speaker from, say, Goodwill, how can I easily set it up to play a tone at a certain frequency? I’m tempted to play around with some of this stuff on my own.
Some internet searching tells me that we are arguing about definitions about similar and closely related phenomena, though.
In my industry (unsaturated polyester and vinyl ester polymers), Thixotropy is understood as shear dependent. Maybe there’s a time component, but the time component is negligible because the “rebound viscosity” time is at most a few seconds. Which doesn’t matter to anyone spraying paint or fiberglass.
…and for what it’s worth, ketchup is not time-dependent shear-thinning - at least as I can measure it - at human scale time intervals. Yes, one afternoon when I was bored I used our viscometer to measure the viscosity at different shear rates of some ketchup. Viscosity at any shear rate stabilized after a second or two. So you are probably correct in calling it a pseudoplastic material, but on typical everyday time scales it’s thixotropic.
Similar? Yes, it takes a bit of careful experimental planning to separate time-dependent and shear-dependent phenomena, because when you vary shear rate, you also vary time from starting shearing. That’s probably why so many people confuse thixotropy and pseudoplasticity.
I’ve heard quite a number of misconceptions which are regarded as “correct” within certain groups, even groups of educated people. Redefining a term doesn’t necessarily make you right
Bottom line: Thixotropy and pseudoplasticity are different phenomena, no matter how many Internet scribblings you find where those two are confused.
ETA: Upon reading your last post, I guess we agree on the terms after all
