Please Why

Shear thinning came up in a conversation the other day. This is a property of many polymer solutions that decrease in viscosity with an applied shear force. You see this in ketchup: it’s really thick and tough to get started flowing out of the bottle, but once the flow starts, there is a high shear rate from the walls of the bottle as the ketchup flows past it, so it decreases in viscosity and starts to flow out really quick.

The reason this happens is because the high shear rate causes the polymers to line up, which allows them to slide past each other more easily.

Anyhow, that reminded me of this video:

A viscous heap of soap forms, and as it gets taller, eventually the incoming stream hits the heap slightly off to the side, so instead of plopping itself down on the top of the heap, it hits the steep side, causing a high shear rate between the side of the heap and the fast moving stream that “scrapes” the side of the heap on its way down.

So then, the soap right on the edge between the heap and the stream decreases in viscosity dramatically, which allows the incoming stream to “slip” right down the side of the heap. The weight of the stream then forms a dimple on the side of the heap, which serves essentially as a ramp for the stream to launch itself off of. Leaping shampoo!