Multi-Particle Collision Dynamics -- a Particle-Based Mesoscale Simulation Approach to the Hydrodynamics of Complex Fluids

In this review, we describe and analyze a mesoscale simulation method for fluid flow, which was introduced by Malevanets and Kapral in 1999, and is now called multi-particle collision dynamics (MPC) or stochastic rotation dynamics (SRD). The method consists of alternating streaming and collision steps in an ensemble of point particles. The multi-particle collisions are performed by grouping particles in collision cells, and mass, momentum, and energy are locally conserved. This simulation technique captures both full hydrodynamic interactions and thermal fluctuations. The first part of the review begins with a description of several widely used MPC algorithms and then discusses important features of the original SRD algorithm and frequently used variations. Two complementary approaches for deriving the hydrodynamic equations and evaluating the transport coefficients are reviewed. It is then shown how MPC algorithms can be generalized to model non-ideal fluids, and binary mixtures with a consolute point. The importance of angular-momentum conservation for systems like phase-separated liquids with different viscosities is discussed. The second part of the review describes a number of recent applications of MPC algorithms to study colloid and polymer dynamics, the behavior of vesicles and cells in hydrodynamic flows, and the dynamics of viscoelastic fluids

Dynamic density functional theory for drying colloidal suspensions: Comparison of hard-sphere free-energy functionals

Dynamic density functional theory (DDFT) is a promising approach for predicting the structural evolution of a drying suspension containing one or more types of colloidal particles. The assumed free-energy functional is a key component of DDFT that dictates the thermodynamics of the model and, in turn, the density flux due to a concentration gradient. In this work, we compare several commonly used free-energy functionals for drying hard-sphere suspensions including local-density approximations based on the ideal-gas, virial, and Boubl\'{i}k-Mansoori-Carnahan-Starling-Leland (BMCSL) equations of state as well as a weighted-density approximation based on fundamental measure theory (FMT). To determine the accuracy of each functional, we model one- and two-component hard-sphere suspensions in a drying film with varied initial heights and compositions, and we compare the DDFT-predicted volume-fraction profiles to particle-based Brownian dynamics (BD) simulations. FMT accurately predicts the structure of the one-component suspensions even at high concentrations and when significant density gradients develop, but the virial and BMCSL equations of state provide reasonable approximations for smaller concentrations at a reduced computational cost. In the two-component suspensions, FMT and BMCSL are similar to each other but modestly overpredict the extent of stratification by size compared to BD simulations. This work provides helpful guidance for selecting thermodynamic models for soft materials in nonequilibrium processes such as solvent drying, solvent freezing, and sedimentation

How to derive and parameterize effective potentials in colloid-polymer mixtures

Polymer chains in colloid-polymer mixtures can be coarse-grained by replacing them with single soft particles interacting via effective polymer-polymer and polymer-colloid pair potentials. Here we describe in detail how Ornstein-Zernike inversion techniques, originally developed for atomic and molecular fluids, can be generalized to complex fluids and used to derive effective potentials from computer simulations on a microscopic level. In particular, we consider polymer solutions for which we derive effective potentials between the centers of mass, and also between mid-points or end-points from simulations of self-avoiding walk polymers. In addition, we derive effective potentials for polymers near a hard wall or a hard sphere. We emphasize the importance of including both structural and thermodynamic information (through sum-rules) from the underlying simulations. In addition we develop a simple numerical scheme to optimize the parameterization of the density dependent polymer-polymer, polymer-wall and polymer-sphere potentials for dilute and semi-dilute polymer densities, thus opening up the possibility of performing large-scale simulations of colloid-polymer mixtures. The methods developed here should be applicable to a much wider range effective potentials in complex fluids.Comment: uses revtex4.cls; submitted for archival purpose

Charged dendrimers revisited: Effective charge and surface potential of dendritic polyglycerol sulfate

We investigate key electrostatic features of charged dendrimers at hand of the biomedically important dendritic polyglycerol sulfate (dPGS) macromolecule using multi-scale computer simulations and Zetasizer experiments. In our simulation study, we first develop an effective mesoscale Hamiltonian specific to dPGS based on input from all-atom, explicit-water simulations of dPGS of low generation. Employing this in coarse-grained, implicit-solvent/explicit-salt Langevin dynamics simulations, we then study dPGS structural and electrostatic properties up to the sixth generation. By systematically mapping then the calculated electrostatic potential onto the Debye-H\"uckel form -- that serves as a basic defining equation for the effective charge -- we determine well-defined effective net charges and corresponding radii, surface charge densities, and surface potentials of dPGS. The latter are found to be up to one order of magnitude smaller than the bare values and consistent with previously derived theories on charge renormalization and weak saturation for high dendrimer generations (charges). Finally, we find that the surface potential of the dendrimers estimated from the simulations compare very well with our new electrophoretic experiments

STRUCTURE, DYNAMICS AND RHEOLOGY OF SURFACTANT MICELLES AND MICELLE-NANOPARTICLE SOLUTIONS: A MOLECULAR DYNAMICS STUDY

Surfactant micelles are widely used in a number of industrial, commercial and household products and processes. Understanding flow-microstructure coupling in micellar systems can benefit applications ranging from targeted drug delivery and detergency to enhanced oil recovery and hydrofracking. Amongst micellar fluids, wormlike micelles (WLMs) are extremely interesting due to their structural similarity to polymers and their ability to constantly undergo scission and recombination at equilibrium. More recently, much has been generated in studying the effect of adding colloidal particles to WLMs. Colloidal particles can not only add functionality to the fluid but also act as viscosity modifiers. Such solutions can be used to design active nanomaterials for applications in energy harvesting and sensing. While several theories and continuum-level computational models have been developed to study the dynamics and rheology of WLMs, molecular-level explorations of the flow-structure coupling in such solutions is lacking. Further, in the case of mixtures of colloidal particles and WLMs, there are only a handful of attempts to develop theoretical/computational frameworks capable of describing their thermodynamics, self-assembly and phase behavior. The goals of this thesis are to uncover mechanisms by which WLMs interact with colloidal particles and to determine how these interactions affect the macroscopic properties of mixtures of model WLMs and colloidal nanoparticles (NPs) using molecular dynamics (MD) simulations. Coarse-grained (CG) molecular models and corresponding force-fields are employed to describe the NP, cationic cetyltrimethylammonium chloride (CTAC) surfactant, hydrotropic sodium salicylate (NaSal) salt, solvent and the underlying physico-chemical interactions. Results are first presented for the dynamics of a single self-assembled rodlike micellar aggregate under shear flow. The effect of shear rate on the configurational dynamics, e.g. orientation distribution of the end-to-end vector and tumbling frequency are presented and compared to experimental observations as well as predictions from stochastic simulations and mesoscopic theories. Further, a relationship between micelle length and stretching force is presented and compared with experimental estimates of similar forces in biological systems. Finally, a shear rate-independent energy barrier for micelle scission is identified for relatively large shear rates. We also show that the addition of NPs to surfactant solutions can result in the formation of NP-surfactant complexes (NPSCs). The effect of NP charge and surface chemistry on the nature of the self-assembly is discussed. Further, such NPSCs can further interact with WLMs, in the presence of NaSal salt, to form electrostatically stabilized micelle-NP junctions via an end cap attachment mechanism. The dynamics, energetics and stability of such junction formation is also described in detail. These junctions can give rise to unique rheological modifications of WLMs such as significant buildup in viscosity and viscoelasticity. Large-scale equilibrium and non-equilibrium MD simulations consisting of several NPs and WLMs are performed to study the flow-microstructure coupling in such systems. The relationship between the zero-shear viscosity, NP volume fraction and salt concentration at a fixed surfactant concentration is presented. Shear thinning behavior is observed for all of the systems studied. Shear thinning is accompanied by flow-alignment and shear-induced isotropic-to-nematic transitions in micellar systems. Further, the evolution of the first normal stress difference, N1, is presented as a function of time and shear rate, and compared with experimental observations for similar systems. The results of this work provides insight into the mechanisms of self-assembly in WLMs and colloidal NPs and demonstrate that rheological properties of WLMs can be uniquely controlled by the addition of NPs

Computational studies of biomembrane systems: Theoretical considerations, simulation models, and applications

This chapter summarizes several approaches combining theory, simulation and experiment that aim for a better understanding of phenomena in lipid bilayers and membrane protein systems, covering topics such as lipid rafts, membrane mediated interactions, attraction between transmembrane proteins, and aggregation in biomembranes leading to large superstructures such as the light harvesting complex of green plants. After a general overview of theoretical considerations and continuum theory of lipid membranes we introduce different options for simulations of biomembrane systems, addressing questions such as: What can be learned from generic models? When is it expedient to go beyond them? And what are the merits and challenges for systematic coarse graining and quasi-atomistic coarse grained models that ensure a certain chemical specificity

Phase-field-crystal models for condensed matter dynamics on atomic length and diffusive time scales: an overview

Here, we review the basic concepts and applications of the phase-field-crystal (PFC) method, which is one of the latest simulation methodologies in materials science for problems, where atomic- and microscales are tightly coupled. The PFC method operates on atomic length and diffusive time scales, and thus constitutes a computationally efficient alternative to molecular simulation methods. Its intense development in materials science started fairly recently following the work by Elder et al. [Phys. Rev. Lett. 88 (2002), p. 245701]. Since these initial studies, dynamical density functional theory and thermodynamic concepts have been linked to the PFC approach to serve as further theoretical fundaments for the latter. In this review, we summarize these methodological development steps as well as the most important applications of the PFC method with a special focus on the interaction of development steps taken in hard and soft matter physics, respectively. Doing so, we hope to present today's state of the art in PFC modelling as well as the potential, which might still arise from this method in physics and materials science in the nearby future.Comment: 95 pages, 48 figure