Solidification in soft-core fluids: disordered solids from fast solidification fronts

TL;DR

This study uses dynamical density functional theory to analyze how solidification fronts propagate in soft-core fluids, revealing different mechanisms depending on quench depth and resulting in varying degrees of disorder in the formed solids.

Contribution

It identifies two distinct mechanisms of front propagation in soft-core fluids and links the resulting disorder to the dynamics of the solidification process.

Findings

Front speed can be determined by linear or nonlinear mechanisms depending on quench depth.

Density modulations behind the front differ from equilibrium crystal spacing, causing disorder.

Binary mixtures exhibit persistent disorder due to inability to rearrange after solidification.

Abstract

Using dynamical density functional theory we calculate the speed of solidification fronts advancing into a quenched two-dimensional model fluid of soft-core particles. We find that solidification fronts can advance via two different mechanisms, depending on the depth of the quench. For shallow quenches, the front propagation is via a nonlinear mechanism. For deep quenches, front propagation is governed by a linear mechanism and in this regime we are able to determine the front speed via a marginal stability analysis. We find that the density modulations generated behind the advancing front have a characteristic scale that differs from the wavelength of the density modulation in thermodynamic equilibrium, i.e., the spacing between the crystal planes in an equilibrium crystal. This leads to the subsequent development of disorder in the solids that are formed. For the one-component fluid, the particles are able to rearrange to form a well-ordered crystal, with few defects. However, solidification fronts in a binary mixture exhibiting crystalline phases with square and hexagonal ordering generate solids that are unable to rearrange after the passage of the solidification front and a significant amount of disorder remains in the system.