Theoretical Study On Effects Of Hydrodynamic Interaction In Non-Equilibrium Soft Matter Systems

As the frontier of scientific research moves towards the living and nanoscale systems, mesoscopic soft matter systems, especially complex fluids such as colloidal suspensions and polymer solutions, have received a lot of attention. On the one hand, the components of these systems are so intricate that their dynamic behaviors are abundant, such as crystallization and melting of colloidal systems, shear thinning of polymer solutions. It is hard to use simple analytical theory to study them, so high performance computer simulation may play an important role in soft matter systems. On the other hand, the dynamic processes of such systems are commonly at mesoscopic scale, from several nanometers to hundreds of micrometers. Macroscopic continuous hydrodynamic equation which ignoring the fluctuations of environment cannot describe the dynamic behaviors accurately, and microscopic molecular dynamics which taking many microscopic details into account are too expensive to simulate. It is very necessary to seek a simple but high-effective simulation method. Actually, the soft matter systems are often immersed in a complex fluid, they not only subject to Brownian motion due to thermal fluctuation, but also are affected by long-range hydrodynamic interactions (HI) among them. Recent years, it is shown that HI can play a subtle role in dynamic process of soft matter systems. Thus, effects of long-ranged HI are new interdiscipline research hotspots in physical chemistry and non-equilibrium statistical physics.In order to investigate the HI effects in these complex soft matter systems, the present paper takes colloidal systems and polymer solutions as example systems to study. We will use multiparticle collision dynamics to study the effects of HI on dynamic behaviors of colloidal systems and polymer solutions. The detailed summary is described below.1. Effects of hydrodynamic interactions on crystallization of two dimensional active and passive soft colloidal systemsIn this part, we have studied crystallization of a two-dimensional suspension of colloidal particles interacting via soft Yukawa potential by using a hybrid molecular dynamics and multiparticle collision dynamics simulation method which facilitates us to investigate the specific effects of long range HI. We have compared the results for passive systems and active systems, where in the latter case the colloidal particles are driven by an external force that changes direction randomly in a tumbling manner. Interestingly, we found that the role of HI is quite different in an active system from that in a passive system. Firstly, HI can significantly shift the freezing density for an active system to much higher value, while in contrast, it slightly shifts the freezing density to a smaller value for a passive system. In addition, HI does not change much the equilibrium structure of a passive system, while it may lead to more pronounced structural heterogeneities in an active system. Furthermore, the effects of HI become more remarkable with increasing driving force, and freezing transition becomes less sharply when HI takes effect. We also found that the relaxation process may be accelerated by HI for both active and passive systems.2. Diffusion of Nanoparticles in Semi-dilute Polymer SolutionsIn the present part, we used a hybrid multiparticle collision dynamics simulation method to investigate the diffusion dynamics of NPs in semi-dilute polymer solutions. Firstly, we demonstrated that the method can well reproduce the scaling relations between the radius of gyration and the concentration of polymer solution, which serves as a validation of the simulation approach. Then we turn to study the mobility of nanoparticles in polymer solutions. Extensive simulations were performed to calculate the long time diffusion coefficient of the nanoparticle, particularly, as functions of the solution concentration. Interestingly, we found that the dependence of diffusion coefficients on polymer concentration can be fitted very well by Phillies equation which is a stretched exponential function. In addition, our method makes it convenient to investigate the very role of hydrodynamic interaction by performing simulations with turn it off. It was shown that hydrodynamic interaction can play a significant role for the diffusion dynamics of nanoparticles. On the one hand, it can enhance considerably the long time diffusion constant and shorten the transient time between the ballistic motion and normal diffusion. On the other hand, the scaling relation between diffusion coefficient and polymer concentration changes remarkably if hydrodynamic interaction is off and Phillies fitting exponent may become unreasonable. Our simulation results are in good agreements with recent experimental observations for nanoparticle diffusion in semidilute polymer solutions.