Interfacial Crystallization within Janus Saline Marbles

We report interfacial crystallization in Janus saline marbles built of two hemispheres, one coated with hydrophobic [polytetrafluoroethylene (PTFE)] and the other with hydrophilic powder (carbon black particles). Evaporation of the Janus saline marbles yielded NaCl crystals formed within the hemisphere, coated with carbon black. This observation is predicted by the theory of heterogeneous nucleation occurring on the solid surfaces. The energetic barrier of the heterogeneous nucleation for the saline/carbon black hemisphere is much smaller than the same barrier for the saline/PTFE hemisphere. We also studied the evaporation of half-coated Janus marbles filled with saline. The half-coated saline marbles were built of two hemispheres, one of which was coated by powder and the second was formed by the naked saline/vapor interface. In this case, crystallization started from the equatorial contact line separating the powder-coated and naked saline/vapor hemispheres for all kinds of the studied powders.