The Flow Behavior Of Colloids In Viscoelastic Solvents Studied By Multiparticle Collision Dynamic Simulation

The dispersions that colloids are dispersed in a complex fluid such as polymers have wide applications in daily life,industry and the preparation of functional materials.It was found in such systems that colloids exhibit complex flow behaviors such as flow induced migration,flow induced assembly,etc.,which have not been observed in Newtonian solvents.To better tune such systems’ properties,there is a great demand to understand the flow behavior of colloids in viscoelastic solvents and further adjust the spatial structure of colloids.However,it is still a challenging task for experiments and theories due to the complex and multiscale structures of the colloid/polymer systems.Computer simulation is a promising research method as which can study one system’s structure and property on different scales simultaneously.In this work,an efficient viscoelastic solvent model has been constructed with multiparticle collision dynamics(MPC)simulation method,which was further applied to study the flow behaviors of colloids in viscoelastic and Newtonian solvents under different flow fields.The main contents are as follows:Firstly,MPC simulation method was introduced in Chapter II.MPC simulation is a mesoscopic particle based simulation method with high computational efficiencies,which has been widely used to study complex fluids such as polymers,colloids.The primary principles,the popular algorithms and corresponding simulation details of MPC simulation are discussed.We compared various existing viscoelastic solvent models and colloidal models in MPC simulations respectively.The validity of our algorithm was examined by studying the flow properties of MPC fluid under simple shear flow and Poiseuille flow.In Chapter Ⅲ,We have developed an efficient shear-thinning viscoelastic solvent model,which is a key step to study the flow behavior of colloids in viscoelastic solvents.Previous studies have proved that dumbbell model can be used as complex solvents in MPC simulation:harmonic dumbbells are simulation efficient but show a constant viscosity;FENE dumbbells exhibit a shear thinning behavior but with less simulation efficiency.Since shear thinning is required to observe the flow induced assembly,we have proposed an improved FENE dumbbell solvent,which combine the advantages of both harmonic and FENE dumbbell models.The simulation results proved that the modification of the algorithm doesn’t alter the flow behavior of FENE dumbbells while increases the computational efficiency obviously.Chapter Ⅳ examined the flow behavior of colloids,mainly the relative motions of pair colloids,under simple shear flow.The relative trajectories are symmetric in an inertialess Newtonian fluid,while this symmetry disappears due to the presence of inertia and reverse trajectories can be observed.Besides the disappearance of the symmetry,two colloids rotate around each other with a proper initial position in the shear thinning viscoelastic solvent,the so called kissing-tumbling-tumbling motion.The simulation results prove that the relative pair trajectory is similar to the streamlines around an isolated colloid.The rotation speed of the colloid in viscoelastic fluid compared with the Newtonian fluid is slow down.In our simulations,only unstable aggregation structures are found while multiple colloids are under shear flow.In Chapter V,the sedimentation of colloids in different solvents were examined.Results suggest that the settlement procedure of colloids is affected by the rheological properties of the solvent and the presence of the wall.There is an equilibrium position during the sedimentation of one single colloid,which is always the midline of system in a viscoelastic solution while varies with the solvent Reynolds number in a Newtonian solvent.The rheological properties of solvents show significantly influence on the sedimentation of two colloids:two colloids tends to form an integral and settle together even when they are released apart.But in the Newtonian solution,they will separate from each other and sink along the wall even when they are released together.