Crystallisation of soft matter under confinement at interfaces and in wedges

2016-03-04T12:22:56Z (GMT) by Andrew Archer Alexandr Malijevsky
The surface freezing and surface melting transitions that are exhibited by a model two-dimensional soft matter system is studied. The behaviour when con ned within a wedge is also considered. The system consists of particles interacting via a soft purely repulsive pair potential. Density functional theory (DFT) is used to calculate density pro les and thermodynamic quantities. The external potential due to the con ning walls is modelled via a hard-wall with an additional repulsive Yukawa potential. The surface phase behaviour depends on the range and strength of this repulsion: When the repulsion strength is weak, the wall promotes freezing at the surface of the wall. The thickness of this frozen layer grows logarithmically as the bulk liquid-solid phase coexistence is approached. Our mean- eld DFT predicts that this crystalline layer at the wall must be nucleated (i.e. there is a free energy barrier) and its formation is necessarily a rst-order transition, referred to as `prefreezing', by analogy with the prewetting transition. However, in contrast to the latter, prefreezing cannot terminate in a critical point, since the phase transition involves a change in symmetry. If the wall- uid interaction is su ciently long ranged and the repulsion is strong enough, surface melting can instead occur. Then the interface between the wall and the bulk crystalline solid becomes wet by the liquid phase as the chemical potential is decreased towards the value at liquid-solid coexistence. It is observed that the nite thickness uid lm at the wall has a broken translational symmetry due to its proximity to the bulk crystal and so the nucleation of the wetting lm can be either rst-order or continuous. Our mean- eld theory predicts that for certain wall potentials there is a premelting critical point analogous to the surface critical point for the prewetting transition. When the uid is con ned within a linear wedge, this can strongly promote freezing when the opening angle of the wedge is commensurate with the crystal lattice.