Abstract

An immersed soft particle or oil droplet is severely deformed when engulfed into an advancing ice front. This deformation strongly depends on the engulfment velocity, even forming pointy-tip shapes for low velocities. We found that such singular deformations are mediated by interfacial flows in nanometric thin liquid films separating the nonsolidifying dispersed soft particles or droplets and the solidifying bulk. The competing forces in the thin film originate from the disjoining pressure and the surface tension gradient (Marangoni forces). We analytically modelled the fluid flow in these intervening thin films, using a lubrication approximation in the boundary layers. In an exact analytical calculation and with a formal analogy to a nonlinear pendulum, we then related the fluid flow to the deformation sustained by the dispersed droplet. We find it astounding that the nanoscopic interaction (van der Waals forces, disjoining pressure) determines the shape of the macroscopic immersed soft particle or droplet.

We then extended this line of research to the interaction of several immersed soft particles or droplets over which a solidification front is passing. This time it is the relative thermal conductivity of the soft particles and the liquid which determines whether the two soft particles repel or attract. We call the effect the frozen Cheerios effect.

Finally, we identified a freezing-induced topological transition of a double-emulsion, i.e., an oil droplet with an immersed water droplet inside, and as a whole immersed in water, passing through a freezing front. Whether the water droplet inside the oil droplet survives or whether it literally bursts due to pressure forces emerging at solidification depends on the control parameters, in particular the freezing front velocity.