Corn starch with a little water will be fun. You may have seen a lot of experimental dynamic pictures, and this account has been posted before. But while playing and watching others play, the explanation for this phenomenon always seems to be very simple, often just to say that it belongs to a shear-thickened non-Newtonian fluid. So, here I would like to add a little bit about the explanation of starch non-Newtonian fluid phenomenon. Of course, this is still a rather simplified version.

the main point of this experiment is to add a little bit of water to more starch (for example, the volume ratio of starch: water = 3:1) to mix evenly, and then experience the completely different reaction of this fluid under different forces: gently stir and pour as a smooth fluid, while stirring and hammering violently will feel a sharp increase in resistance, even feel like a solid.

A video I shot earlier:

Why corn starch?

Why does the mixture of starch and water reflect this property, and why not sand? Among them, the chemical properties of starch are not important, what is important is the particle size-the diameter of starch particles is just small enough, about 1-10 microns. However, only when the particles are small enough, it is easy to observe that the resistance (viscosity) increases sharply with the applied force (shear force).

this phenomenon of shear thickening occurs not only in corn starch, but also in other systems in which fine particles are dispersed in liquid with high density. In fact, when studying this phenomenon, scientists generally do not directly use starch to do experiments, because the particle size of starch is uneven and the amount of variation is difficult to control. They choose things like colloidal silica and polymethyl methacrylate particles with controllable diameters.

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Why is it easy to stir gently?

because there is a liquid between the small particles as a lubrication layer, there is no direct contact, so the friction is small. These small particles such as starch are not easy to gather and contact spontaneously because they are mutually exclusive to a certain extent, for example, because they have the same electric charge.

Why is it so difficult to stir hard?

when the shear force increases, the applied force overcomes the original repulsion between the particles, making them directly contact and friction with each other, resulting in a sharp increase in resistance. In fact, there are two theories about the source of resistance, one is the direct contact and friction between small particles, and the other is that the resistance comes from the process of extruding liquid from small particles. The brief explanation of

is roughly like this, and the real quantitative research is actually quite complicated. Recently, there happens to be a relatively perfect model of this system [2], which can predict its response under different conditions. I can also read the paper if I am interested. I don't understand it very well.

finally, it is easy to look at the picture

Source

[1] http://cohengroup.lassp.cornell.edu/research/projects/shear-thickening-suspensions

[2]http://news.mit.edu/2019/oobleck-behavior-predict-cornstarch-1006