Phase diagram and dynamics of Yukawa systems

Abstract

The phase diagram and dynamical properties of systems of particles interacting through a repulsive screened Coulomb (Yukawa) potential have been calculated using molecular and lattice dynamics techniques. The phase diagram contains both a melting transition and a transition from fcc to bcc crystalline phases. These phase transitions have been studied as a function of potential shape (screening length) and compared to phenomenological criteria for transition temperatures such as those of Lindemann and of Hansen and Verlet. The transition from fcc to bcc with increasing temperature is shown to result from a higher entropy in the bcc phase because of its softer shear modes. Even when the stable solid phase below the melting temperature is fcc, bcc-like local order is found in the liquid phase. This may substantially slow crystallization. The calculated phase diagram and shear modulus are in good agreement with experiments on colloidal suspensions of polystyrene spheres. The single particle dynamics of Yukawa systems show several unusual features. There is a pronounced subdiffusive regime in liquids near and below the melting temperature. This regime reflects the existence of two time scales: a typical phonon period, and the time for a particle to feel a new environment. The second time scale becomes longer as the temperature is lowered or the range of interaction (screening length) increases.

Brownian dynamics simulation of the crystallization dynamics of charged colloidal particles

“…The reduced cut-off distance at large j as for long-ranged interactions was taken to be 3.045 in units of the relevant length scale in this study. This value is, to some extent, larger than that suggested in some previous studies [10], as the weekly screened system considered in this study requires a larger cutoff radius [18].…”

“…The MC [7,8] and MD [9,10] methods have already been used to study the phase behavior of colloidal systems with Yukawa potentials, in which the two-phase coexistence lines are determined utilizing the methods of Helmholtz free energy calculations and the Kofke integration [22]. The results indicate that BCC structure is thermodynamically stable for relatively small j, but the dynamics of the crystallization were not discussed.…”

“…On the other hand, experimental studies on colloidal particles with repulsive interaction have also shown that the possibility of the formation of FCC crystal structures [3,22]. Actually, MC and MD simulations have already verified that the FCC is a thermodynamically stable state for the systems featured with the Yukawa potential with large j [7][8][9][10]. As is well known, however, the thermodynamically favored structures are not necessarily dynamically directly reachable.…”

“…However, MC method is not suitable for providing the information of dynamic behavior, including the nucleation pathways. Molecular dynamics (MD) simulation has also been adopted to study the colloidal crystallization [9,10]. But it is not adaptable to simulate both colloidal particles and solvent particles motion, because the timescale characterizing the motions of the colloidal particles and the solvent particles can differ by several orders of magnitude.…”

“…But it is not adaptable to simulate both colloidal particles and solvent particles motion, because the timescale characterizing the motions of the colloidal particles and the solvent particles can differ by several orders of magnitude. To solve this problem, the colloidal particles are taken as MD particles, and the force of solvent particles is ignored for such MD simulation [10], in which the system is quenched and super-cooled and the temperature rescaling method is applied. On the other hand, the Brownian dynamics (BD) technique, as a mesoscopic method, is more suitable for characterizing the motion of colloidal particles because of its ability to average out these fast modes of the solvent allows one to simulate much larger time scales.…”