Computer Simulation Of The Behavior Of Colloidal Particles In Polymer Solutions

Soft matter,like polymer solution and melt,exhibits complex rheological behaviors,such as shear thinning,extrusion expansion and so on.The system of particles dispersed in polymer solutions is widely applied to coatings,painting,nanocomposites,microfluidics and other fields.On one hand,the addition of colloid particles makes the complex rheological behavior of polymer solutions more complicated.On the other hand,colloids exhibit unique behaviors in viscoelastic fluid,known as flow-induced migration and flow-induced assembly,which is absent in Newtonian fluids.The properties of colloids/polymer composites can be superior than those of either colloids or polymers,which are determined by the nature of the polymer solution,relative motion between particles,interaction between polymer and colloid,hydrodynamics interaction,and so on.Researching the colloid/polymer composite is helpful for a better design and production process of functional materials.This paper mainly focuses on the effect of viscoelastic solvents on the behavior of colloidal particles.It has been reported that cell separation,enrichment and preparation of flow-induced assembly structures can be achieved by using the unique rheological behavior of colloidal particles in viscoelastic fluid.It has been proved that shear thinning and nonzero normal stress difference have significant influences on the behavior of colloidal particles,but it is far from a thoroughly understand at molecular level.Traditional simulation methods,such as molecular dynamics simulation and Monte Carlo simulation,usually are not suitable to study the colloid/polymer composite systems due to the spared size and relaxation time spectrum and presence of hydrodynamic interaction.We adopted a mesoscopic particle simulation method--Multiple Particles Collision Dynamics(MPC)which inherently includes thermal fluctuation and hydrodynamic interaction and widely applied in the study of soft matter.A viscoelastic solvent model is constructed in MPC simulation,and then applied to study the motion of colloidal particles in viscoelastic solvent.It should be pointed that the influence of colloidal particles on the properties of polymers and the overall properties of composite systems have not been considered in this theses.The main contents of this paper are as follows:MPC simulation method is mainly described in Sec.II.The principles of MPC simulation and the two common algorithms to describe particles’ collision--Random rotation dynamics(Stochastic rotation dynamic,SRD)and Anderson heat bath(Anderson thermal state,AT)--are introduced sequentially.Then we explained how to embed particles in MPC simulation by coupling with molecular dynamics simulations.Some technical details of MPC simulation are also discussed,such as the boundary conditions and the introduction of the flow field.In Sec.Ⅲ,the rheological behavior of viscoelastic solvents is studied.Connecting two MPC particles with a certain potential,a simplest viscoelastic solvent model--dumbbell--is obtained.We studied the rheological behavior of harmonic dumbbell and FENE dumbbell respectively.Properties like elongation,orientation and viscosity have been examined.The results show that harmonic dumbbells exhibit no shear thinning behavior while FENE dumbbells do,thus FENE dumbbell will used as viscoelastic solvent model in future studies.The results of particles behavior under simple shear field are described in Sec.IV.We use FENE dumbbell as viscoelastic solvent and bead-spring-chain model as colloidal particle.Then we mainly studied the rotational velocity of colloid particles,and found that the rotational velocity of a colloidal particle was half of the shear rate in the Newtonian fluid,while it is slow down in the viscoelastic solvent.In Sec.Ⅴ,the results of colloidal particle moving under temperature field are discussed.Firstly,the temperature field is introduced under both periodic boundary and hard wall conditions.Then we investigate the behaviors of the colloid particles in Newtonian fluid and viscoelastic fluid respectively and the effect of temperature on viscoelastic solvent under hard wall boundary condition.The simulation results show that temperature positively affects the length of the dumbbell,colloidal particles move toward lower temperature region with Brownian motion,and viscoelasticity decreases the thermophoretic mobility.