Search PubMed⌕ Search

Biomedical subjects

Hartmut Löwen

Publications and source records attributed to Hartmut Löwen.

14 recordsLinked to original sources
PubMed2006

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 spongelike 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 nonadditive mixtures.

Colloids↗
PubMed2006

Collapse of telechelic star polymers to watermelon structures.

Conformational properties of star-shaped polymer aggregates that carry attractive end groups, called telechelic star polymers, are investigated by simulation and analytical variational theory. We focus on the case of low telechelic star polymer functionalities, f < or = 5, a condition which allows aggregation of all attractive monomers on one site. We establish the functionality- and polymerization-number dependence of the transition temperature from the "star burst" to the "watermelon" macroparticle structure. Extensions to telechelic stars featuring partially collapsed configurations are also discussed.

Journal Article↗
PubMed2006

Confined colloidal bilayers under shear: steady state and relaxation back to equilibrium.

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 is two crystalline layers with a nested quadratic in-plane structure. For increasing shear rates (.)gamma, we find the following steady states: First, up to a threshold of the shear rate, there is a static solid which is elastically sheared. Above the threshold, 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 (.)gamma. 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.

Journal Article↗
PubMed2005

Wetting, drying, and layering of colloid-polymer mixtures at porous interfaces.

The influence of interface porosity on the wetting properties of colloid-polymer mixtures is studied within density functional theory for the Asakura-Oosawa-Vrij model at the surface of a quenched hard-sphere matrix. While the porosity hardly changes the location of the transition from partial to complete wetting at colloidal bulk gas-liquid coexistence, the onset of wetting, as signaled by the first discontinuous layering transition, can be efficiently controlled by tailoring the porosity. We furthermore find that the penetrability of the porous interface induces complete drying into the matrix upon approaching capillary coexistence.

Journal Article↗
PubMed2004

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 by 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.

Journal Article↗
PubMed2004

Structure and phase behavior of polyelectrolyte star solutions.

Using the recently developed effective interaction potentials between polyelectrolyte stars, we examine the structure and phase behavior of solutions of the same. The effective interaction is ultrasoft and density dependent, owing to the integration of the counterionic degrees of freedom. The latter contribute extensive volume terms that must be taken into account in drawing the phase diagram of the system. The structural behavior of the uniform fluid is characterized by anomalous structure factors, akin to those found previously for solutions of uncharged star polymers. The phase diagram of the system is very rich, featuring a fluid phase at low arm numbers of the stars, two reentrant melting regions, as well as a variety of crystal structures with unusual symmetry. The physical origin of these features can be traced back to the ultrasoft nature of the effective interaction potential.

Journal Article↗
PubMed2004

Unwrapping of DNA-protein complexes under external stretching.

A DNA-protein complex modeled 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 a 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 dynamics of the unwrapping process by computer simulations 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 of 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 with 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.

Biophysics↗
PubMed2004

Crystal nucleation of colloidal suspensions under shear.

We use Brownian dynamics simulations in combination with the umbrella sampling technique to study the effect of shear flow on homogeneous crystal nucleation. We find that a homogeneous shear rate leads to a significant suppression of the crystal nucleation rate and to an increase of the size of the critical nucleus. A simple, phenomenological extension of classical nucleation theory accounts for these observations. The orientation of the crystal nucleus is tilted with respect to the shear direction.

Biophysical Phenomena↗
PubMed2003

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 small 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.

Biophysics↗
PubMed2003

Reentrant transitions in colloidal or dusty plasma bilayers.

The phase diagram of crystalline bilayers of particles interacting via a Yukawa potential is calculated for arbitrary screening lengths and particle densities. Staggered rectangular, square, rhombic, and triangular structures are found to be stable including a first-order transition between two different rhombic structures. For varied screening length at fixed density, one of these rhombic phases exhibits both a single and even a double reentrant transition. Our predictions can be verified experimentally in strongly confined charged colloidal suspensions or dusty plasma bilayers.

Journal Article↗
PubMed2003

Nonequilibrium pattern formation in strongly interacting driven colloids.

Dynamical instabilities are discussed for strongly interacting colloidal suspensions which are driven into nonequilibrium by an external field in the limit where hydrodynamic interactions can be neglected. Brownian dynamics computer simulations indicate that stripe-like patterns of particles driven alike are spontaneously formed if the external drive exceeds a critical strength. Recent previous studies of stripe formation obtained for symmetric equimolar mixtures in the steady state are reviewed. These results are then extended in two directions: first, we show that stripe-like segregation occurs also in asymmetric mixtures and observe an additional compression/expansion effect in the stripes composed of the small/large particles. Second, we study the relaxation into the stripe-patterned steady state starting from a uniform demixed state and show that different transient processes such as jamming, anisotropic coarsening and void formation are relevant on the route into the stratified steady state.

Journal Article↗
PubMed2002

Do liquids exhibit local fivefold symmetry at interfaces?

We calculate the layer-resolved local fivefold symmetry of liquids near interfaces using computer simulation of hard sphere fluids on structured substrates. In the first few adjacent liquid layers, the presence of a surface suppresses local icosahedral packing while the magnitude of local fivefold symmetry in the next layers depends on details of the particle-substrate interaction. For a strong particle-substrate attraction, the local fivefold symmetry in these layers is higher than in the bulk liquid.

Journal Article↗
PubMed2001

Freezing transition of hard hyperspheres.

We investigate the system of D-dimensional hard spheres in D-dimensional space, where D>3. For the fluid phase of these hyperspheres, we generalize scaled-particle theory to arbitrary D and furthermore use the virial expansion and the Percus-Yevick integral equation. For the crystalline phase, we adopt cell theory based on elementary geometrical assumptions about close-packed lattices. Regardless of the approximation applied, and for dimensions as high as D=50, we find a first-order freezing transition, which preempts the Kirkwood second-order instability of the fluid. The relative density jump increases with D, and a generalized Lindemann rule of melting holds. We have also used ideas from fundamental-measure theory to obtain a free energy density functional for hard hyperspheres. Finally, we have calculated the surface tension of a hypersphere fluid near a hard smooth (hyper-)wall within scaled-particle theory.

Journal Article↗