Heterogeneous crystallization near structured walls in quenched two-dimensional binary colloidal suspensions

Abstract

Crystallization near structured substrates is explored by extensive Brownian dynamics computer simulations of two-dimensional equimolar binary colloidal suspensions. The particles are interacting via repulsive dipole–dipole forces. The two species have different (big and small) dipole moments which are tunable by an external magnetic field. A quench is realized by a sudden drastic increase of the magnetic field which formally corresponds to a quench to zero temperature. The structured walls are modelled by fixed particles on an alternating binary equimolar square lattice which is cut along the (10) direction. Two situations are studied and compared where the outermost layer of fixed particles either consists of big or of small dipolar particles. Both of these structures favour local crystallites which pick up the square symmetry of the substrate. After the quench the equilibrium state is an alternating square lattice which coincides exactly with that imposed by the external wall. The relaxation behaviour following the quench is explored. The amount of local crystallites with triangular and square symmetry is monitored as a function of time for different initial temperatures. It is found that the number and structure of crystallites near the walls strongly depend on the wall pattern. Even though local square structures are favoured energetically and the equilibrium state is an alternating square lattice, the number of triangular crystallites close to the wall which has outermost fixed small particles is significantly higher than in the unconfined case. The actual number of triangular clusters depends on the depth of the quench. This effect is not contained in classical nucleation theory.