Dynamic Simulation Of Formation Of Soft Particle Beam Crystals

Under different conditions,soft spherical colloids can be in liquid,solid or liquid-solid coexistence states.By changing density and temperature,the soft colloid particles can go through a first order phase transition from liquid to solid,or remelt to form a new solid structure.At low temperature,the phase transition of solid to solid couldhappen.We call it as isomorphic phase transition,inwhicha new solid structure will be formed.There is a crystal structure in which each lattice position is occupied by several overlapping particles.Even if the particles show unusual softening property,the degree of overlap of the particles is limited.This structure is called the cluster crystal.We study the dynamic formation process of Multiply Occupied Crystal by the Generalized Exponential Model of index 4 model by using computer simulations.We use this model to study the influence of dynamic paths in which the soft colloid particle system mayform the different phase structures.Moreover,we give insight to the law of the influence of the dynamic path on the structures.First,we study the dynamic behaviors of the?r?=?exp?-?r/??~n?(n=4)model particles in the NVT ensemble.Weexamine the process of forming soft particle cluster crystalsafter the colloid particles beingcooled or compressed.Due to that the microdynamic processesare quit difficult to be obtained by the experiments,the molecular dynamics simulations are employed to have the detailed dynamic information.By our study,the mechanism of the formation of the soft particle cluster crystal can be revealed.For this purpose,we build a research platform including molecular dynamics simulationand data analysis tools andoptimize the simulation conditions.The Gauss potential particles witha high degree of softening is easy to overlap.So it is very important to distinguish which particles belong to the same cluster.We choose different cut-off radius R to determine the spacial range of particle cluster.The radial distribution function g(R)is calculated,and it is found that the choice of the first truncated radius R=0.4 of the first wave valley in the g(R)is the best.We use Nose-Hoover thermostat which is developedbased on statistical mechanics to control temperature.Through a series of testing,we have established a reasonable simulation scheme for cooling or compression.Finally,we study the dynamic process of the formation of cluster crystal on isothermal compression and isodensitycooling pathways.Through isothermal compression,the structure changes in the sequence of"disorder?FCC1?FCC1 and BCC hybrid?FCC2?BCC2".Meanwhile,the particle morphology changes from single particle to dimer cluster.Compared with Monte Carlo simulations,the structures obtained by molecular dynamics simulation have obvious hysteresis with the density changes.In order to fully understand the phenomenon of the reentrant melting and isomorphic transition of colloid particles,we simulate the isodensity cooling and find that the structures of the system go through"disordered clusters?FCC2?FCC3".There is a crucial value to determine the particle morphology,over which the number of dimers is predominant.From the two different thermodynamic pathways of isothermal compression and isodensity cooling,we find that the results obtained by different paths are not same.At a same density and temperature,the structure obtained by isothermal compression is BCC2,whle obtained by isodensitycooling or Monte Carlo simulation is FCC3.Therefore,the dynamic process will have a great influence on the simulation results,and even give different structures.We think that the results of molecular dynamics simulations which are different from the ones of Monte Carlo simulations should be due to the real dynamic process included in the molecular dynamics simulations.