Marisol Ripoll

Dynamics of polymers in a particle-based mesoscopic solvent

We study the dynamics of flexible polymer chains in solution by combining multiparticle-collision dynamics (MPCD), a mesoscale simulation method, and molecular-dynamics simulations. Polymers with and without excluded-volume interactions are considered. With an appropriate choice of the collision time step for the MPCD solvent, hydrodynamic interactions build up properly. For the center-of-mass diffusion coefficient, scaling with respect to polymer length is found to hold already for rather short chains. The center-of-mass velocity autocorrelation function displays a long-time tail which decays algebraically as (Dt)(-3/2) as a function of time t, where D is the diffusion coefficient. The analysis of the intramolecular dynamics in terms of Rouse modes yields excellent agreement between simulation data and results of the Zimm model for the mode-number dependence of the mode-amplitude correlation functions.

Rod-like colloids and polymers in shear flow: a multi-particle-collision dynamics study

The effect of the hydrodynamic interaction on the dynamics of flexible and rod-like polymers in solution is investigated. The solvent is simulated by the multi-particle-collision dynamics (MPCD) algorithm, a mesoscale simulation technique. The dynamics of the solvent is studied and the self-diffusion coefficient is calculated as a function of the mean free path of a particle. At small mean free paths, the hydrodynamic interaction strongly influences the dynamics of the fluid particles. This solvent model is then coupled to a molecular dynamics simulation algorithm. We obtain excellent agreement between our simulation results for a flexible polymer and the predictions of Zimm theory. The study of the translational diffusion coefficient of rod-like polymers confirms the predicted chain-length dependence. In addition, we study the influence of shear on the structural properties of rod-like polymers. For shear rates exceeding the rotational relaxation time, the rod-like molecule aligns with the shear flow, leading to an orientational symmetry breaking transverse to the flow direction. The comparison of the obtained shear rate dependencies with theoretical predictions exhibits significant deviations. The properties of the orientational tensor and the rotational velocity are discussed in detail as a function of shear rate.

A self-propelled thermophoretic microgear

An asymmetric microgear will spontaneously and unidirectionally rotate if it is heated in a cool surrounding solvent. The resulting temperature gradient along the edges of the gear teeth translates in a directed thermophoretic force, which will exert a net torque on the gear. By means of computer simulations, the validity of this scenario is proved. The rotational direction and speed are dependent on gear–solvent interactions, and can be analytically related to system parameters like the thermal diffusion factor, the solvent viscosity, or the temperature difference. This microgear provides a simple way to extract net work from non-isothermal solutions, and can become a valuable tool in microfluids.

Thermophoretically induced flow field around a colloidal particle

A colloidal particle suspended in a fluid solvent with a non-homogeneous temperature undergoes a thermophoretic force. This force may translate into a directed drift of the particle and a source-dipole-like flow field around it. Alternatively, if the colloid is fixed in space, the accompanying flow is long-ranged. In this work, we provide a first simulation study of the thermophoretic force-induced flow fields by a particle-based mesoscopic method. The simulation results are quantitatively consistent with theoretical predictions obtained by solving hydrodynamic equations. Based on these results, we propose a single-particle microfluidic pump without movable parts, in which the flow direction can be reversed. Furthermore, we quantify the long-range hydrodynamic attraction between two suspended particles near the boundary wall induced by the thermophoretic flow field.

Thermophoresis of colloids by mesoscale simulations.

The motion of a colloid induced by a temperature gradient is simulated by means of multiparticle collision dynamics, a mesoscale simulation technique. Two algorithms to quantify the thermophoretic behavior are employed and contrasted. The validity of the methods is verified as a function of the temperature gradient, system size, and algorithm parameters. The variation of the solvent-colloid interaction from attractive to purely repulsive interestingly results in the change of the colloid behavior from thermophobic to thermophilic.

Motility-sorting of self-propelled particles in microchannels

Spontaneous segregation of run-and-tumble particles with different velocities in microchannels is investigated by numerical simulations. Self-propelled particles are known to accumulate in the proximity of walls. Here we show how fast particles expel slower ones from the wall leading to a segregated state. The mechanism is understood as a function of particle velocities, particle density, or channel width. In the presence of an external fluid flow, particles with two different velocities segregate due to their different particle fluxes. Promising applications can be found in the development of microfluidic lab-on-a-chip devices for sorting of particles with different motilities.

Simulation of complex fluids by multi-particle-collision dynamics

The particle-based mesoscale simulation technique called Multi-Particle-Collision Dynamics (MPCD) (also denoted as Stochastic Rotation Dynamics (SDR)) is introduced. The algorithm is outlined and applications to complex fluids are described. Results for the dynamics of a Gaussian polymer and a rodlike colloid are discussed. Moreover, the density dependence of the diffusion coefficient of a colloidal fluid is presented. Our investigations show that the MPCD algorithm accounts for hydrodynamic interactions.

Driving forces and polymer hydrodynamics in the Soret effect

A temperature gradient induces different driving forces on the components of a mixture which translates into their segregation. We show that these driving forces constitute the physical picture behind the thermodiffusion effect, and provide an alternative expression of the Soret coefficient which can be applied to both colloidal suspensions and molecular mixtures. To verify the validity of the formalism, we quantify the related forces in an Eulerian reference frame by non-equilibrium molecular simulations. Furthermore, we present an analytical argument to show that the hydrodynamic interactions need to be accounted for to obtain the proper scaling of the thermophoretic force. This result combined with the presented expression satisfactorily explains the experimentally known size dependence of the thermodiffusion coefficient in dilute polymer solutions.

Temperature inhomogeneities simulated with multiparticle-collision dynamics

The mesoscopic simulation technique known as multiparticle collision dynamics is presented as a very appropriate method to simulate complex systems in the presence of temperature inhomogeneities. Three different methods to impose the temperature gradient are compared and characterized in the parameter landscape. Two methods include the interaction of the system with confining walls. The third method considers open boundary conditions by imposing energy fluxes. The transport of energy characterizing the thermal diffusivity is also investigated. The dependence of this transport coefficient on the method parameters and the accuracy of existing analytical theories is discussed.

Mesoscale hydrodynamics simulations of attractive rod-like colloids in shear flow

Suspensions of rod-like colloids show in equilibrium an isotropic–nematic coexistence region, which depends on the strength of an attractive interaction between the rods. We study the behavior of this system in shear flow for various interaction strengths. A hybrid simulation approach is employed, which consists of a mesoscale particle-based hydrodynamics technique (multi-particle collision dynamics) for the solvent and molecular dynamics simulations for the colloidal rods. The shear flow induces alignment in the initially isotropic phase, which generated an additional free volume around each rod and causes the densification of the isotropic phase at the expense of an erosion of the initially nematic phase. Furthermore, the nematic phase exhibits a collective rotational motion. The associated rotational time decreases linearly in with increasing shear rate , and increases with increasing attraction strength between the rods. The density difference between these two regions at different shear rates allows us to determine the binodal line of the phase diagram. For large applied shear rates, the difference between the phases disappears in favor of a homogeneous flow-aligned state.