What is this “de-mixing” and what causes it?

Q: We use a tumble blender. I’ve heard that if you mix too long, you can undo the mixing results. What is this “de-mixing” and what causes it?

A: Unlike most liquid blends, in which mutually soluble liquids form a stable blend that doesn’t degrade over time, dry mixtures have a tendency to segregate. This is because particles have different sizes, shapes, and densities. When the particles are in motion (as in mixing), the differences cause the particles to segregate.

In a tumble blender, the ingredients are loaded in batches, often forming stratified layers in the vessel. Then the unit begins tumbling, and its symmetrical rotation creates a nonrandom mixing pattern. This pattern, when combined with the various particles’ different trajectories, can demix or segregate the particles in a pattern different from that formed by the layers during loading. To avoid this segregation in the later mixing stages, it’s best to use a tumble blender for relatively short mixing times. A tumble blender can also be designed to provide asymmetrical rotation to minimize demixing caused by trajectory segregation. For instance, a V-blender can have one extended leg and a double cone mixer can have offset cones; by using such designs, you can shorten mixing times, improve mixing quality, and limit further segregation.

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