arXiv subjects

Hartmut Löwen

Publications and source records attributed to Hartmut Löwen.

306 records · Page 17Linked to original sources

arXiv2009

Ultra-fast quenching of binary colloidal suspensions in an external magnetic field

An ultra-fast quench is applied to binary mixtures of superparamagnetic colloidal particles confined at a two-dimensional water-air interface by a sudden increase of an external magnetic field. This quench realizes a virtually instantaneous cooling which is impossible in molecular systems. Using real-space experiments, the relaxation behavior after the quench is explored. Local crystallites with triangular and square symmetry are formed on different time scales and the correlation peak amplitude of the small particles evolves nonmonotonically in time in agreement with Brownian dynamics computer simulations.

cond-mat.soft↗

arXiv2008

Oscillatory driven colloidal binary mixtures: axial segregation versus laning

Using Brownian dynamics computer simulations we show that binary mixtures of colloids driven in opposite directions by an oscillating external field exhibit axial segregation in sheets perpendicular to the drive direction. The segregation effect is stable only in a finite window of oscillation frequencies and driving strengths and is taken over by lane formation in direction of the driving field if the driving force is increased. In the crossover regime, bands tilted relative to the drive direction are observed. Possible experiments to verify the axial segregation are discussed.

cond-mat.soft↗

arXiv2008

Lane formation in driven colloidal mixtures: is it continuous or discontinuous?

Binary mixtures of oppositely charged colloids driven by an electric field are shown to exhibit a nonequilibrium transition towards lane formation if the driving force is increased. Brownian dynamics computer simulations and real-space experiments are employed to study hysteresis effects in an order parameter measuring the extent of lane formation upon increasing and decreasing the driving force. Both from simulation and experiment, we find that lane formation due to electrical fields is continuous. However, simulations show a discontinuous transition if the driving force is gravity.

cond-mat.soft↗

arXiv2008

Non-equilibrium melting of colloidal crystals in confinement

We report on a novel and flexible experiment to investigate the non-equilibrium melting behaviour of model crystals made from charged colloidal spheres. In a slit geometry polycrystalline material formed in a low salt region is driven by hydrostatic pressure up an evolving gradient in salt concentration and melts at large salt concentration. Depending on particle and initial salt concentration, driving velocity and the local salt concentration complex morphologic evolution is observed. Crystal-melt interface positions and the melting velocity are obtained quantitatively from time resolved Bragg- and polarization microscopic measurements. A simple theoretical model predicts the interface to first advance, then for balanced drift and melting velocities to become stationary at a salt concentration larger than the equilibrium melting concentration. It also describes the relaxation of the interface to its equilibrium position in a stationary gradient after stopping the drive in different manners. We further discuss the influence of the gradient strength on the resulting interface morphology and a shear induced morphologic transition from polycrystalline to oriented single crystalline material before melting.

cond-mat.soft↗

arXiv2008

Direct observation of hydrodynamic instabilities in driven non-uniform colloidal dispersions

A Rayleigh-Taylor-like instability of a dense colloidal layer under gravity in a capillary of microfluidic dimensions is considered. We access all relevant lengthscales with particle-level microscopy and computer simulations which incorporate long-range hydrodynamic interactions between the particles. By tuning the gravitational driving force, we reveal a mechanism whose growth is connected to the fluctuations of specific wavelengths, non-linear pattern formation and subsequent diffusion-dominated relaxation. Our linear stability theory captures the initial regime and thus predicts mixing conditions, with important implications for fields ranging from biology to nanotechnology.

cond-mat.soft↗

arXiv2008

Dynamics of a Brownian circle swimmer

Self-propelled particles move along circles rather than along a straight line when their driving force does not coincide with their propagation direction. Examples include confined bacteria and spermatozoa, catalytically driven nanorods, active, anisotropic colloidal particles and vibrated granulates. Using a non-Hamiltonian rate theory and computer simulations, we study the motion of a Brownian "circle swimmer" in a confining channel. A sliding mode close to the wall leads to a huge acceleration as compared to the bulk motion, which can further be enhanced by an optimal effective torque-to-force ratio.

cond-mat.soft↗

arXiv2008

Crystallization of magnetic dipolar monolayers: a density functional approach

We employ density functional theory to study in detail the crystallization of super-paramagnetic particles in two dimensions under the influence of an external magnetic field that lies perpendicular to the confining plane. The field induces non-fluctuating magnetic dipoles on the particles, resulting into an interparticle interaction that scales as the inverse cube of the distance separating them. In line with previous findings for long-range interactions in three spatial dimensions, we find that explicit inclusion of liquid-state structural information on the {\it triplet} correlations is crucial to yield theoretical predictions that agree quantitatively with experiment. A non-perturbative treatment is superior to the oft-employed functional Taylor expansions, truncated at second or third order. We go beyond the usual Gaussian parametrization of the density site-orbitals by performing free minimizations with respect to both the shape and the normalization of the profiles, allowing for finite defect concentrations.

cond-mat.soft↗

arXiv2008

Colloidal crystal growth at externally imposed nucleation clusters

We study the conditions under which and how an imposed cluster of fixed colloidal particles at prescribed positions triggers crystal nucleation from a metastable colloidal fluid. Dynamical density functional theory of freezing and Brownian dynamics simulations are applied to a two-dimensional colloidal system with dipolar interactions. The externally imposed nucleation clusters involve colloidal particles either on a rhombic lattice or along two linear arrays separated by a gap. Crystal growth occurs after the peaks of the nucleation cluster have first relaxed to a cutout of the stable bulk crystal.

cond-mat.soft↗

arXiv2007

Influence of hydrodynamic interactions on lane formation in oppositely charged driven colloids

The influence of hydrodynamic interactions on lane formation of oppositely charged driven colloidal suspensions is investigated using Brownian dynamics computer simulations performed on the Rotne-Prager level of the mobility tensor. Two cases are considered, namely sedimentation and electrophoresis. In the latter case the Oseen contribution to the mobility tensor is screened due to the opposite motion of counterions. The simulation results are compared to that resulting from simple Brownian dynamics where hydrodynamic interactions are neglected. For sedimentation, we find that hydrodynamic interactions strongly disfavor laning. In the steady-state of lanes, a macroscopic phase separation of lanes is observed. This is in marked contrast to the simple Brownian case where a finite size of lanes was obtained in the steady-state. For strong Coulomb interactions between the colloidal particles a lateral square lattice of oppositely driven lanes is stable similar to the simple Brownian dynamics. In an electric field, on the other hand, the behavior is found in qualitative and quantitative accordance with the case of neglected hydrodynamics.

cond-mat.soft↗

arXiv2007

Stable crystalline lattices in two-dimensional binary mixtures of dipolar particles

The phase diagram of binary mixtures of particles interacting via a pair potential of parallel dipoles is computed at zero temperature as a function of composition and the ratio of their magnetic susceptibilities. Using lattice sums, a rich variety of different stable crystalline structures is identified including $A_mB_n$ structures. [$A$ $(B)$ particles correspond to large (small) dipolar moments.] Their elementary cells consist of triangular, square, rectangular or rhombic lattices of the $A$ particles with a basis comprising various structures of $A$ and $B$ particles. For small (dipolar) asymmetry there are intermediate $AB_2$ and $A_2B$ crystals besides the pure $A$ and $B$ triangular crystals. These structures are detectable in experiments on granular and colloidal matter.

cond-mat.soft↗

arXiv2006

Density functional theory of freezing for soft interactions in two dimensions

A density functional theory of two-dimensional freezing is presented for a soft interaction potential that scales as inverse cube of particle distance. This repulsive potential between parallel, induced dipoles is realized for paramagnetic colloids on an interface, which are additionally exposed to an external magnetic field. An extended modified weighted density approximation which includes correct triplet correlations in the liquid state is used. The theoretical prediction of the freezing transition is in good agreement with experimental and simulation data.

cond-mat.soft↗

arXiv2006

Partial Clustering in Binary Two-Dimensional Colloidal Suspensions

Strongly interacting binary mixtures of superparamagnetic colloidal particles confined to a two-dimensional water-air interface are examined by theory, computer simulation and experiment. The mixture exhibits a partial clustering in equilibrium: in the voids of the matrix of unclustered big particles, the small particles form subclusters with a sponge-like topology which is accompanied by a characteristic small-wave vector peak in the small-small structure factor. This partial clustering is a general phenomenon occurring for strongly coupled negatively non-additive mixtures.

cond-mat.soft↗

arXiv2005

Relaxation back to equilibrium after cessation of shear for confined colloidal bilayers

Crystalline bilayers of charged colloidal suspensions which are confined between two parallel plates and sheared via a relative motion of the two plates are studied by extensive Brownian dynamics computer simulations. The charge-stabilized suspension is modeled by a Yukawa pair potential. The unsheared equilibrium configuration are two crystalline layers with a nested quadratic in-plane structure. For increasing shear rates $\dot γ$, we find the following steady states: first, there is a static solid which is elastically sheared until a yield-stress limit is reached. Then there are two crystalline layers sliding on top of each other with a registration procedure. Higher shear rates melt the crystalline bilayers and even higher shear rates lead to a reentrant solid stratified in the shear direction. This qualitative scenario is similar to that found in previous bulk simulations. We have then studied the relaxation of the sheared steady state back to equilibrium after an instantaneous cessation of shear and found a nonmonotonic behavior of the typical relaxation time as a function of the shear rate $\dot γ$. In particular, application of high shear rates accelerates the relaxation back to equilibrium since shear-ordering facilitates the growth of the equilibrium crystal. This mechanism can be used to grow defect-free colloidal crystals from strongly sheared suspensions. Our theoretical predictions can be verified in real-space experiments of strongly confined charged suspensions.

cond-mat.soft↗

arXiv2004

Equilibrium properties of highly asymmetric star-polymer mixtures

We employ effective interaction potentials to study the equilibrium structure and phase behavior of highly asymmetric mixtures of star polymers. We consider in particular the influence of the addition of a component with a small number of arms and a small size on a concentrated solution of large stars with a high functionality. By employing liquid integral equation theories we examine the evolution of the correlation functions of the big stars upon addition of the small ones, finding a loss of structure that can be attributed to a weakening of the repulsions between the large stars due to the presence of the small ones. We analyze this phenomenon be means of a generalized depletion mechanism which is supported by computer simulations. By applying thermodynamic perturbation theory we draw the phase diagram of the asymmetric mixture, finding that the addition of small stars melts the crystal formed by the big ones. A systematic comparison between the two- and effective one-component descriptions of the mixture that corroborates the reliability of the generalized depletion picture is also carried out.

cond-mat.soft↗

arXiv2004

Soft effective interactions between weakly charged polyelectrolyte chains

We apply extensive Molecular Dynamics simulations and analytical considerations in order to study the conformations and the effective interactions between weakly charged, flexible polyelectrolyte chains in salt-free conditions. We focus on charging fractions lying below 20%, for which case there is no Manning condensation of counterions and the latter can be thus partitioned in two states: those that are trapped within the region of the flexible chain and the ones that are free in the solution. We examine the partition of counterions in these two states, the chain sizes and the monomer distributions for various chain lengths, finding that the monomer density follows a Gaussian shape. We calculate the effective interaction between the centers of mass of two interacting chains, under the assumption that the chains can be modeled as two overlapping Gaussian charge profiles. The analytical calculations are compared with measurements from Molecular Dynamics simulations. Good quantitative agreement is found for charging fractions below 10%, where the chains assume coil-like configurations, whereas deviations develop for charge fraction of 20%, in which case a conformational transition of the chain towards a rodlike configuration starts to take place.

cond-mat.soft↗

arXiv2004

Unwrapping of DNA-protein complexes under external stretching

A DNA-protein complex modelled by a semiflexible chain and an attractive spherical core is studied in the situation when an external stretching force is acting on one end monomer of the chain while the other end monomer is kept fixed in space. Without stretching force, the chain is wrapped around the core. By applying an external stretching force, unwrapping of the complex is induced. We study the statics and the dynamics of the unwrapping process by computer simulation and simple phenomenological theory. We find two different scenarios depending on the chain stiffness: For a flexible chain, the extension of the complex scales linearly with the external force applied. The sphere-chain complex is disordered, i.e. there is no clear winding of the chain around the sphere. For a stiff chain, on the other hand, the complex structure is ordered, which is reminiscent to nucleosome. There is a clear winding number and the unwrapping process under external stretching is discontinuous with jumps of the distance-force curve. This is associated to discrete unwinding processes of the complex. Our predictions are of relevance for experiments, which measure force-extension curves of DNA-protein complexes, such as nucleosome, using optical tweezers.

cond-mat.soft↗

arXiv2004

Colloidal brazil nut effect in sediments of binary charged suspensions

Equilibrium sedimentation density profiles of charged binary colloidal suspensions are calculated by computer simulations and density functional theory. For deionized samples, we predict a colloidal ``brazil nut'' effect: heavy colloidal particles sediment on top of the lighter ones provided that their mass per charge is smaller than that of the lighter ones. This effect is verifiable in settling experiments.

cond-mat.soft↗

arXiv2003

Capillary Condensation and Interface Structure of a Model Colloid-Polymer Mixture in a Porous Medium

We consider the Asakura-Oosawa model of hard sphere colloids and ideal polymers in contact with a porous matrix modeled by immobilized configurations of hard spheres. For this ternary mixture a fundamental measure density functional theory is employed, where the matrix particles are quenched and the colloids and polymers are annealed, i.e. allowed to equilibrate. We study capillary condensation of the mixture in a tiny sample of matrix as well as demixing and the fluid-fluid interface inside a bulk matrix. Density profiles normal to the interface and surface tensions are calculated and compared to the case without matrix. Two kinds of matrices are considered: (i) colloid-sized matrix particles at low packing fractions and (ii) large matrix particles at high packing fractions. These two cases show fundamentally different behavior and should both be experimentally realizable. Furthermore, we argue that capillary condensation of a colloidal suspension could be experimentally accessible. We find that in case (ii), even at high packing fractions, the main effect of the matrix is to exclude volume and, to high accuracy, the results can be mapped onto those of the same system without matrix via a simple rescaling.

cond-mat.soft↗