Friday, January 16, 2015

Spinodal decomposition

A metal alloy is formed by heating two metals to a high temperature homogeneous mixture, which is then quenched (rapidly cooled). As it cools, phase separation sets in at the microscopic level. That is, if you look on the atomic level, the metals are no longer evenly mixed, and their mixture is an interesting pattern, something like a spot pattern on a leopard (but in three dimensions). The type of pattern -- or phase -- depends on the composition of the alloy and the quenching speed. One such quench is spinodal decomposition. This pattern is modeled by a fourth order partial differential equation called the Cahn-Hilliard equation. The model above is a spinodal decomposition on a very small box of alloy, where the plastic represents one of the two metals, and the air represents the other metal.

The model was made by simulating the Cahn-Hilliard equation, using Paraview to get a level set from the three-dimensional data, printing this figure with support given by a dissolvable filament. So when the print came off the printer, the box was completely filled. Then I put the print in an acid called D-Limonene. The acid dissolves the filament, leaving only the desired shape.

As a side note on the acid experience, D-Limonene is made out of highly concentrated orange rind and  is an ingredient in some air fresheners. Even though I left the container with the acid sealed in the garage, for several days the entire house smelled like someone had over freshened the air. Even after several soaks in water, the print still has a pleasant orangey tang.

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