Department of Polymer Engineering
This video shows three single crystals nucleate and shoot away from a growing crystal interface. We understand this self propelled motion as a new crystal nucleation instability in the context of the Marangoni effect. As the irregular crystal interface grows from the bottom right corner it pulls in solute from the solution, creating a depleted zone. As a single crystal nucleates in this zone, its surface tension becomes dependent on the concentration gradient. This is a manifestation of the Marangoni effect, or in terms of a moving particle, Marangoni propulsion.
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Depth of the depletion zone depends on both solute diffusion towards the interface and the rate solute is incorporated into the interface, or crystal growth rate. The growth rate increases as a solution is undercooled further below its melting temperature, seen as the liquidus line in the phase diagram below. Our experiments confirm that solutions quench depth from the isotropic region deeper into crystal + liquid region (below the liquidus) controls crystal shooting speed. In the figure below, the crystal + liquid region is bound by the simulated liquidus and solidus lines; this simulation was confirmed by DSC investigation of the melting temperatures, shown with open circles. The video above shows crystal shooting of a 39wt.% solution of azobenzene in a photo-curable solvent trimethylolpropane triacrylate quenched from the isotropic state to room temperature.