When Shear Meets Slip

A molecular dynamics simulation showing the orientation of a nanoplatelet in a shear flow.

One of the classic concepts students learn early in their fluids education is the no-slip condition. In essence, this idea says that friction between a solid object — say, a wall — and the fluid immediately next to it is such that no movement is possible where they meet. The fluid cannot “slip” along the surface, hence “no-slip”. It’s a simple concept, but one that can create a lot of complexity in practice.

Imagine, for example, a fluid sandwiched between two surfaces: one stationary and one moving at a constant speed. This movement creates a shear flow, in which the velocity of the fluid varies from the speed of the moving plate all the way down to zero, the speed of the stationary plate. If we placed a little platelet in the middle of this flow, we’d expect it to rotate because of the faster flow on one side.

But a new paper finds something rather different, at least when considering an extremely small nanoplatelet. With a tiny enough plate, individual molecules can slip along the surface, and when that happens, instead of rotating, the nanoplatelet aligns itself with the flow. That alignment means the added particle would disturb the flow less, creating a lower viscosity and better flowability. (Image and research credit: C. Kamal et al.; submitted by Simon G.)

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