Dynamic simulations of colloids by core-modified dissipative particle dynamics

Abstract

We develop a core-modified dissipative particle dynamics model of colloidal systems which includes an extra term to counteract depletion forces. Results are presented covering the full range of volume fractions. Radial distribution functions for the suspending fluid are shown to change significantly as the volume fraction of colloid increases. Equilibrium results for the long-time diffusion coefficient behave as expected, but the short-time coefficient is anomalous. The form of the equilibrium stress correlation functions is discussed and the derived Green-Kubo viscosities are compared with expected semiempirical forms. For nonequilibrium shear-field simulations we find that the system temperature is not adequately controlled by the dissipative particle dynamics (DPD) thermostat alone. Results using three alternative auxiliary thermostats are compared; a naive choice leading to a string phase at high shear rate. Using a thermostat based on relative particle velocities, the model reproduced the four classical regions of colloid rheology: a first Newtonian plateau, a shear-thinning region, a second Newtonian plateau, and finally a shear-thickening region at high strain rate. The most unexpected result of this exercise is that the core-modified DPD model without auxiliary thermostat almost exactly follows the same curve despite recording a temperature increase of a factor approximately 2.5 over the range.

Strongly overdamped Dissipative Particle Dynamics for fluid-solid systems

“…A suspended particle of spherical/circular shape can be modelled using a single DPD particle [15][16][17][18][19]8] or a set of frozen DPD particles [20][21][22][23]. The single particle model involves three groups of DPD parameters: the first group associated with the interactions between solventsolvent particles, the second with solvent-colloidal particles and the third with colloidalcolloidal particles.…”

Bounded soft-sphere fluids by molecular dynamics and integral equation techniques

“…Assuming a>0, equation (3) has the useful feature that it goes over smoothly to the soft-sphere system as a→0+; it is also longer ranged than the Gaussian potential. We refer to this potential as the Bounded Soft-Sphere or BSS potential.…”

“…Energy is expressed in terms of ε and distance in σ. A convenient (non-dimensional) measure of the density of the system is the nominal volume or packing fraction, ζ, defined as 3 6 ρσ π ς = (2) where ρ is the number of particles per unit volume. Both ρ and ζ can be used as measures of the particle number density.…”

“…Such potentials can be used as coarse-grained representations of certain soft matter systems, notably polymeric molecules in solution [1,2], and are useful as interaction spreading kernels in various mesoscale dynamical (fluid mechanics-related) modelling techniques such as Dissipative Particle Dynamics [3] and Smooth Particle Hydrodynamics [4]. The 'bounded' nature of the potential introduces some new features into the phase diagram which are not seen in potential systems whose interaction potentials diverge at the origin.…”

Investigation of particles size effects in Dissipative Particle Dynamics (DPD) modelling of colloidal suspensions

“…The suspended phase has been modelled with either the single particle models [9,10,11,12,13] or the frozen particle models [14,15,16]. Recently, a new suspended-particle model has been proposed using just a few basic DPD particles (referred to here as "constituent particles") connected to reference sites by stiff linear springs [17]; the reference sites move as a rigid body motion.…”