Re: Why do larger particles move up when a mixture is shaken?

Date: Tue Oct 12 00:24:44 1999
Posted By: Martin Thomas, Post-doc/Fellow, Phyiscal Chemistry, Quantachrome Corporation
Area of science: Physics
ID: 939606751.Ph

Message:

Good observation and question. Here's the answer.

In fact, it's not really a matter of the large particles moving up, but rather the small particles moving down. Shaking the mixture allows the particles to move around enough to allow the small particles to fall through the gaps between the large particles. The large particles move up as the small particles pack in underneath them. You could call this effect "auto-sieving" (auto = self or self caused, from the Greek autos).

This effect can be quite dramatically demonstrated even when the large particles are much more dense than the small particles... a steel ball bearing buried in some sand will gradually rise to the surface when the whole mass is vibrated!

A couple of everyday examples:

As every farmer, landscaper and backyard gardner knows, pebbles, stones and rockes "mysteriously" appear even when the ground has previously been cleared of large surface items. Same reason.

Less dramatic, but as you know the whole breakfast cereal is at the top of the box, and at the bottom of the box are crumbs and dust.

Thanks for your question and good luck with your particle experiments.

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