# Non-Newtonian fluids and walking on water: what gives? **URL:** \ **Category:** Factual Questions\ **Created:** [December 3, 2010, 1:50am UTC](https://boards.straightdope.com/t/non-newtonian-fluids-and-walking-on-water-what-gives/562746 "2010-12-03T01:50:32Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [December 3, 2010, 1:50am UTC](https://boards.straightdope.com/t/non-newtonian-fluids-and-walking-on-water-what-gives/562746/1 "2010-12-03T01:50:32Z")
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](http://www.youtube.com/watch?v=3zoTKXXNQIU) and the straight-up [walking on “water”](http://www.youtube.com/watch?v=IRlBuZsScho&NR=1) (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”? Thanks, Leo ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [December 3, 2010, 1:54am UTC](https://boards.straightdope.com/t/non-newtonian-fluids-and-walking-on-water-what-gives/562746/2 "2010-12-03T01:54:49Z")
The mix will flow if given an easy push, or left alone, but if you hit it hard all the starch bumps into itself and it acts like a solid. Oobleck is from a Dr. Seuss story, which is an unusual weather condition where this phlegm falls from teh sky. ---
**Author:** ![spenczar](https://avatars.discourse-cdn.com/v4/letter/s/858c86/32.png) [@spenczar](https://boards.straightdope.com/u/spenczar)\ **Post date:** [December 3, 2010, 2:18am UTC](https://boards.straightdope.com/t/non-newtonian-fluids-and-walking-on-water-what-gives/562746/3 "2010-12-03T02:18:30Z")
It divided by zero. Now you know why your math teachers have always been so insistent. ---
**Author:** ![GameHat](https://avatars.discourse-cdn.com/v4/letter/g/ed655f/32.png) [@GameHat](https://boards.straightdope.com/u/GameHat)\ **Post date:** [December 3, 2010, 2:36am UTC](https://boards.straightdope.com/t/non-newtonian-fluids-and-walking-on-water-what-gives/562746/4 "2010-12-03T02:36:17Z")
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](http://en.wikipedia.org/wiki/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: > [@Wikipedia](#): > > A dilatant (also termed shear thickening) material is one in which viscosity increases with the rate of shear. Such a shear thickening fluid, also known by the acronym STF, is an example of a non-Newtonian fluid. > > The dilatant effect occurs when closely packed particles are combined with enough liquid to fill the gaps between them. At low velocities, the liquid acts as a lubricant, and the dilatant flows easily. At higher velocities, the liquid is unable to fill the gaps created between particles, and friction greatly increases, causing an increase in viscosity.[1] This can readily be seen with a mixture of cornstarch and water[2] (sometimes called oobleck), which acts in counterintuitive ways when struck or thrown against a surface. ---
**Author:** ![Norse](https://avatars.discourse-cdn.com/v4/letter/n/a9adbd/32.png) [@Norse](https://boards.straightdope.com/u/Norse)\ **Post date:** [December 3, 2010, 6:50am UTC](https://boards.straightdope.com/t/non-newtonian-fluids-and-walking-on-water-what-gives/562746/5 "2010-12-03T06:50:15Z")
> [@GameHat](#): > > thixotropic (i.e., non-Newtonian shear-thinning) fluids That’s wrong. But even textbooks sometimes make this error, so you should probably be excused. [Wikipedia’s got it right](http://en.wikipedia.org/wiki/Shear_thinning), [though](http://en.wikipedia.org/wiki/Thixotropic). 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. ---
**Author:** ![Astral\_Rejection](https://avatars.discourse-cdn.com/v4/letter/a/258eb7/32.png) [@Astral\_Rejection](https://boards.straightdope.com/u/Astral_Rejection)\ **Post date:** [December 3, 2010, 7:00am UTC](https://boards.straightdope.com/t/non-newtonian-fluids-and-walking-on-water-what-gives/562746/6 "2010-12-03T07:00:00Z")
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. ---
**Author:** ![GameHat](https://avatars.discourse-cdn.com/v4/letter/g/ed655f/32.png) [@GameHat](https://boards.straightdope.com/u/GameHat)\ **Post date:** [December 3, 2010, 7:58am UTC](https://boards.straightdope.com/t/non-newtonian-fluids-and-walking-on-water-what-gives/562746/7 "2010-12-03T07:58:20Z")
> [@Norse](#): > > That’s wrong. But even textbooks sometimes make this error, so you should probably be excused. [Wikipedia’s got it right](http://en.wikipedia.org/wiki/Shear_thinning), [though](http://en.wikipedia.org/wiki/Thixotropic). I was very close to posting, “NO UR WRONG!” 😃 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. 😃 ---
**Author:** ![GameHat](https://avatars.discourse-cdn.com/v4/letter/g/ed655f/32.png) [@GameHat](https://boards.straightdope.com/u/GameHat)\ **Post date:** [December 3, 2010, 8:11am UTC](https://boards.straightdope.com/t/non-newtonian-fluids-and-walking-on-water-what-gives/562746/8 "2010-12-03T08:11:38Z")
> [@GameHat](#): > Shit, now I’m mixing up terms. Sorry for the confusion. I consider unsat polymers and ketchup to be shear-thinning, time-independent measured against norman human time scales. That’s my assertion. …sorry I can’t organize my thoughts tonight, heh. :rolleyes: ---
**Author:** ![Norse](https://avatars.discourse-cdn.com/v4/letter/n/a9adbd/32.png) [@Norse](https://boards.straightdope.com/u/Norse)\ **Post date:** [December 3, 2010, 8:15am UTC](https://boards.straightdope.com/t/non-newtonian-fluids-and-walking-on-water-what-gives/562746/9 "2010-12-03T08:15:07Z")
> [@GameHat](#): > > Some internet searching tells me that we are arguing about definitions about similar and closely related phenomena, though. 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. > [@GameHat](#): > > In my industry (unsaturated polyester and vinyl ester polymers), Thixotropy is understood as shear dependent 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 🆒 ---
**Author:** ![GameHat](https://avatars.discourse-cdn.com/v4/letter/g/ed655f/32.png) [@GameHat](https://boards.straightdope.com/u/GameHat)\ **Post date:** [December 3, 2010, 8:24am UTC](https://boards.straightdope.com/t/non-newtonian-fluids-and-walking-on-water-what-gives/562746/10 "2010-12-03T08:24:36Z")
> [@Norse](#): > > Bottom line: Thixotropy and pseudoplasticity are different phenomena, no matter how many Internet scribblings you find where those two are confused. It’s not internet scribblings, it’s an industry standard. Just saying. Academia and Industry don’t always agree. ---
**Author:** ![Norse](https://avatars.discourse-cdn.com/v4/letter/n/a9adbd/32.png) [@Norse](https://boards.straightdope.com/u/Norse)\ **Post date:** [December 3, 2010, 8:38am UTC](https://boards.straightdope.com/t/non-newtonian-fluids-and-walking-on-water-what-gives/562746/11 "2010-12-03T08:38:31Z")
> [@GameHat](#): > > It’s not internet scribblings, it’s an industry standard. But it’s still factually wrong 🆒 ---
**Author:** ![GameHat](https://avatars.discourse-cdn.com/v4/letter/g/ed655f/32.png) [@GameHat](https://boards.straightdope.com/u/GameHat)\ **Post date:** [December 3, 2010, 8:46am UTC](https://boards.straightdope.com/t/non-newtonian-fluids-and-walking-on-water-what-gives/562746/12 "2010-12-03T08:46:39Z")
> [@Norse](#): > > But it’s still factually wrong 🆒 Unless you actually work with the material 🆒 LOL, ok, let’s call a truce. I’ll accept your definitions. There is certainly a difference between time-dependent and shear-dependent viscosity. …of course, at my job tomorrow morning, I’ll be back to calling it thixotropy 😃 ---
**Author:** ![Norse](https://avatars.discourse-cdn.com/v4/letter/n/a9adbd/32.png) [@Norse](https://boards.straightdope.com/u/Norse)\ **Post date:** [December 3, 2010, 9:04am UTC](https://boards.straightdope.com/t/non-newtonian-fluids-and-walking-on-water-what-gives/562746/13 "2010-12-03T09:04:49Z")
> [@GameHat](#): > > …of course, at my job tomorrow morning, I’ll be back to calling it thixotropy 😃 😃 Just make sure that you do it when I’m not around, to prevent me from starting another rant 😉