At breakfast this morning, I was looking at a thin film of milk on the spoon resting in my bowl, and noticed a very rapid sparkling. It looked like some sort of white noise - very fine, and very fast. If you've ever looked at a moving laser spot while squinting, it was kind of like that.

I have a couple theories. I know milk is a colloidal suspension, and the fat particles might be small enough to exhibit Brownian motion. Now if the particles are reflective, that could make them sparkle. Or maybe there's some diffraction going on. But I'm not really sure.

Anybody know what actually causes this?

  • 1
    $\begingroup$ This sounds like sunlight speckle. $\endgroup$ – user137289 Jul 22 '18 at 0:09

It's the Tyndall effect and coherent backscattering.

Because colloids have a suspension of small particles, (from 1 – 1000 nanometres in size which can scatter light falling on them), and milk is white. Milk is not completely opaque and light scatters not only from the surface but from the subsurface too.

The solution is brightly illuminated and has multiple random reflective points.

Video: "Scattering of light" and "True Solutions, Colloidal Solutions and Suspensions


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