Molecular dynamics simulations on the dynamics of two-dimensional rounded squares

The collective motion of rounded squares with different corner-roundness ζ is studied by molecular dynamics(MD)simulation in this work. Three types of translational collective motion pattern are observed, including gliding, hopping and a mixture of gliding and hopping. Quantitatively, the dynamics of each observed ordered phase is characterized by both mean square displacement and van Hove functions for both translation and rotation. The effect of corner-roundness on the dynamics is further studied by comparing the dynamics of the rhombic crystal phases formed by different corner-rounded particles at a same surface fraction. The results show that as ζ increases from 0.286 to 0.667, the translational collective motion of particles changes from a gliding-dominant pattern to a hopping-dominant pattern, whereas the rotational motion pattern is hopping-like and does not change in its type, but the rotational hopping becomes much more frequent as ζincreases(i.e., as particles become more rounded). A simple geometrical model is proposed to explain the trend of gliding motion observed in MD simulations.

Curvature-induced lipid segregation

We investigate how an externally imposed curvature influences lipid segregation on two-phase-coexistent membranes.We show that the bending-modulus contrast of the two phases and the curvature act together to yield a reduced effective line tension.On largely curved membranes,a state of multiple domains(or rafts) forms due to a mechanism analogous to that causing magnetic-vortex formation in type-II superconductors.We determine the criterion for such a multi-domain state to occur;we then calculate respectively the size of the domains formed on cylindrically and spherically curved membranes.