741 research outputs found

    Surface-charge-induced freezing of colloidal suspensions

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    Using grand-canonical Monte Carlo simulations we investigate the impact of charged walls on the crystallization properties of charged colloidal suspensions confined between these walls. The investigations are based on an effective model focussing on the colloids alone. Our results demonstrate that the fluid-wall interaction stemming from charged walls has a crucial impact on the fluid's high-density behavior as compared to the case of uncharged walls. In particular, based on an analysis of in-plane bond order parameters we find surface-charge-induced freezing and melting transitions

    Feedback-controlled transport in an interacting colloidal system

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    Based on dynamical density functional theory (DDFT) we consider a non-equilibrium system of interacting colloidal particles driven by a constant tilting force through a periodic, symmetric "washboard" potential. We demonstrate that, despite of pronounced spatio-temporal correlations, the particle current can be reversed by adding suitable feedback control terms to the DDFT equation of motion. We explore two distinct control protocols with time delay, focussing on either the particle positions or the density profile. Our study shows that the DDFT is an appropriate framework to implement time-delayed feedback control strategies widely used in other fields of nonlinear physicsComment: 6 pages, 5 figure

    Theory of repulsive charged colloids in slit-pores

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    9 pags., 8 figs., 1 tab.Using classical density functional theory (DFT) we analyze the structure of the density profiles and solvation pressures of negatively charged colloids confined in slit pores. The considered model, which was already successfully employed to study a real colloidal (silica) suspension [S. H. L. Klapp, Phys. Rev. Lett. 100, 118303 (2008)10.1103/PhysRevLett.100.118303], involves only the macroions which interact via the effective Derjaguin-Landau-Verwey-Overbeek (DLVO) potential supplemented by a hard core interaction. The solvent enters implicitly via the screening length of the DLVO interaction. The free energy functional describing the colloidal suspension consists of a hard sphere contribution obtained from fundamental measure theory and a long range contribution which is treated using two types of approximations. One of them is the mean field approximation (MFA) and the remaining is based on Rosenfelds perturbative method for constructing the Helmholtz energy functional. These theoretical calculations are carried out at different bulk densities and wall separations to compare finally to grand canonical Monte Carlo simulations. We also consider the impact of charged walls. Our results show that the perturbative DFT method yields generally qualitatively consistent and, for some systems, also quantitatively reliable results. In MFA, on the other hand, the neglect of charge-induced correlations leads to a breakdown of this approach in a broad range of densities. © 2012 American Institute of Physics.A.G. and N.G.A. acknowledge financial support from the Dirección General de Investigación Científica y Técnica under Grant No. FIS2010-15502, and from the Dirección General de Universidades e Investigación de la Comunidad de Madrid under Grant No. S2009/ESP-1691 and Program MODELICO-CM. S.G. and S.H.L.K. would like to thank the German Research Foundation (DFG) for financial support within the Collaborative Research Center (CRC) 951 “Hybrid Inorganic/organic systems for Optoelectronics” and the International Graduate School (IRTG) 1524 “Selfassembled soft-matter nanostructures at interfaces.

    Non-equilibrium condensation and coarsening of field-driven dipolar colloids

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    In colloidal suspensions, self-organization processes can be easily fueled by external fields. One particularly interesting class of phenomena occurs in monolayers of dipolar particles that are driven by rotating external fields. Here we report results from a computer simulation study of such systems focusing on the clustering behavior also observed in recent experiments. The key result of this paper is a novel interpretation of this pattern formation phenomenon: We show the clustering to be a by-product of a vapor-liquid first order phase transition. In fact, the observed dynamic coarsening process corresponds to the spindodal demixing that occurs during such a transitionComment: 6 pages, 5 figure

    Shear-stress controlled dynamics of nematic complex fluids

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    Based on a mesoscopic theory we investigate the non-equilibrium dynamics of a sheared nematic liquid, with the control parameter being the shear stress σxy\sigma_{\mathrm{xy}} (rather than the usual shear rate, γ˙\dot\gamma). To this end we supplement the equations of motion for the orientational order parameters by an equation for γ˙\dot\gamma, which then becomes time-dependent. Shearing the system from an isotropic state, the stress- controlled flow properties turn out to be essentially identical to those at fixed γ˙\dot\gamma. Pronounced differences when the equilibrium state is nematic. Here, shearing at controlled γ˙\dot\gamma yields several non-equilibrium transitions between different dynamic states, including chaotic regimes. The corresponding stress-controlled system has only one transition from a regular periodic into a stationary (shear-aligned) state. The position of this transition in the σxy\sigma_{\mathrm{xy}}-γ˙\dot\gamma plane turns out to be tunable by the delay time entering our control scheme for σxy\sigma_{\mathrm{xy}}. Moreover, a sudden change of the control method can {\it stabilize} the chaotic states appearing at fixed γ˙\dot\gamma.Comment: 10 pages, 11 figure

    Phase separation dynamics in a two-dimensional magnetic mixture

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    Based on classical density functional theory (DFT), we investigate the demixing phase transition of a two-dimensional, binary Heisenberg fluid mixture. The particles in the mixture are modeled as Gaussian soft spheres, where one component is characterized by an additional classical spin-spin interaction of Heisenberg type. Within the DFT we treat the particle interactions using a mean-field approximation. For certain magnetic coupling strengths we calculate phase diagrams in the density-concentration plane. For sufficiently large coupling strengths and densities, we find a demixing phase transition driven by the ferromagnetic interactions of the magnetic species. We also provide a microscopic description (i.e., density profiles) of the resulting non-magnetic/magnetic fluid-fluid interface. Finally, we investigate the phase separation using dynamical density functional theory (DDFT), considering both nucleation processes and spinodal demixing.Comment: 15 pages, 10 figure