Biomedical subjects

H Löwen

Publications and source records attributed to H Löwen.

At least 19 recordsLinked to original sources

PubMed2002

Decoration lattices of colloids adsorbed on stripe-patterned substrates.

The equilibrium structure of decoration lattices composed of colloidal particles adsorbed on periodic stripe-patterned substrates is calculated as a function of the stripe width and separation and for different interparticle interactions. Due to a competition of length scales, a wealth of different stable decoration lattices occurs such as triangular, quadratic, rhombic, kitelike, and sheared honeycomb lattices, triangular slices as well as triangle superlattices. This is of relevance for constructing templates that enforce crystal growth of unusual solid structures.

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PubMed2002

Triple-point wetting on rough substrates.

The influence of substrate roughness on the wetting scenario of adsorbed van der Waals films is investigated by theory and experiment. Calculating the bending free energy penalty of a solid sheet picking up the substrate roughness, we show that a finite roughness always leads to triple-point wetting reducing the widths of the adsorbed solid films considerably as compared to that of smooth substrates. Testing the theory against our experimental data for molecular hydrogen adsorbed on gold, we find quantitative agreement.

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PubMed2002

Lane formation in colloidal mixtures driven by an external field.

The influence of an external field on a binary colloidal mixture performing Brownian dynamics in a solvent is investigated by nonequilibrium computer simulations and simple theory. In our model, one half of the particles are pushed into the field direction while the other half of them are pulled into the opposite direction. For increasing field strength, we show that the system undergoes a nonequilibrium phase transition from a disordered state to a state characterized by lane formation parallel to the field direction. The lanes are formed by the same kind of particles moving collectively with the field. Lane formation accelerates particle transport parallel to the field direction but suppresses massively transport perpendicular to the field. We further show that lane formation also occurs in a time-dependent oscillatory field. If the frequency of the external field exceeds a critical value, however, the system exhibits a transition back to the disordered state. Our results can be experimentally verified in binary colloidal suspensions exposed to external fields under nonequilibrium conditions.

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PubMed2001

Conformations and interactions of star-branched polyelectrolytes.

Combining monomer-resolved molecular dynamics simulations with a theory based on a variational free energy, we calculate the conformational properties and the effective interactions of star-branched polyelectrolytes for a large variety of arm numbers, degrees of polymerization, and charge fractions, with and without added salt. We find quantitative agreement between theory and simulation and put forward analytical expressions that allow the calculation of the interaction between such macromolecules.

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PubMed2001

Phase separation in star-polymer-colloid mixtures.

We examine the demixing transition in star-polymer-colloid mixtures for star arm numbers f=2,6,16,32 and different star-polymer-colloid size ratios 0.18< or =q< or =0.50. Theoretically, we solve the thermodynamically self-consistent Rogers-Young integral equations for binary mixtures using three effective pair potentials obtained from direct molecular computer simulations. The numerical results show a spinodal instability. The demixing binodals are approximately calculated and found to be consistent with experimental observations.

Journal Article↗

PubMed2001

Sedimentation profiles of systems with reentrant melting behavior.

We examine sedimentation density profiles of star polymer solutions as an example of colloidal systems in sedimentation equilibrium that exhibit reentrant melting in their bulk phase diagram. Phase transitions between a fluid and a fluid with an intercalated solid are observed below a critical gravitational strength alpha*. Characteristics of the two fluid-solid interfaces in the density profiles occurring in Monte Carlo simulations for alpha<alpha* are in agreement with scaling laws put forth in the framework of a phenomenological theory. Furthermore, we detect density oscillations at the fluid-gas interface at high altitudes for high-gravitational fields, which are verified with density-functional theory and should be observable in surface scattering experiments.

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PubMed2001

Colloids confined to a flexible container.

A model of hard spheres trapped inside a container of fluctuating shape is proposed to describe colloidal particles in a vesicle or in an emulsion droplet. The container is assumed to be the convex hull of the particles and is described by an integral geometric approach including volume and surface terms. In the limit of large volume coupling, the model reduces to the well-known geometric problem of natural bin packing. Using computer simulations and cell theory, we calculate equilibrium properties for various finite numbers of confined particles in conformations ranging from clusters to planar and linear structures and identify transitions between these different conformations.

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PubMed2001

Influence of solvent granularity on the effective interaction between charged colloidal suspensions.

We study the effect of solvent granularity on the effective force between two charged colloidal particles by computer simulations of the primitive model of strongly asymmetric electrolytes with an explicitly added hard-sphere solvent. Apart from molecular oscillating forces for nearly touching colloids that arise from solvent and counterion layering, the counterions are attracted towards the colloidal surfaces by solvent depletion providing a simple statistical description of hydration. This, in turn, has an important influence on the effective forces for larger distances which are considerably reduced as compared to the prediction based on the primitive model. When these forces are repulsive, the long-distance behavior can be described by an effective Yukawa pair potential with a solvent-renormalized charge. As a function of colloidal volume fraction and added salt concentration, this solvent-renormalized charge behaves qualitatively similar to that obtained via the Poisson-Boltzmann cell model, but there are quantitative differences. For divalent counterions and nanosized colloids, on the other hand, the hydration may lead to overscreened colloids with mutual attraction while the primitive model yields repulsive forces. All these new effects can be accounted for through a solvent-averaged primitive model (SPM) which is obtained from the full model by integrating out the solvent degrees of freedom. The SPM was used to access larger colloidal particles without simulating the solvent explicitly.

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PubMed2001

Criterion for determining clustering versus reentrant melting behavior for bounded interaction potentials.

We examine in full generality the phase behavior of systems whose constituent particles interact by means of potentials that do not diverge at the origin, are free of attractive parts, and decay fast enough to zero as the interparticle separation r goes to infinity. By employing a mean field-density functional theory which is shown to become exact at high temperatures and/or densities, we establish a criterion that determines whether a given system will freeze at all temperatures or it will display reentrant melting and an upper freezing temperature.

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PubMed2000

Effective interaction between helical biomolecules.

The effective interaction between two parallel strands of helical biomolecules, such as deoxyribose nucleic acids (DNA), is calculated using computer simulations of the "primitive" model of electrolytes. In particular we study a simple model for B-DNA incorporating explicitly its charge pattern as a double-helix structure. The effective force and the effective torque exerted onto the molecules depend on the central distance and on the relative orientation. The contributions of nonlinear screening by monovalent counterions to these forces and torques are analyzed and calculated for different salt concentrations. As a result, we find that the sign of the force depends sensitively on the relative orientation. For intermolecular distances smaller than 6 A it can be both attractive and repulsive. Furthermore, we report a nonmonotonic behavior of the effective force for increasing salt concentration. Both features cannot be described within linear screening theories. For large distances, on the other hand, the results agree with linear screening theories provided the charge of the biomolecules is suitably renormalized.

Computer Simulation↗

PubMed1999

Freezing and glass transition of hard spheres in cavities.

Dynamical and static properties of N=13-4000 hard spheres in spherical cavities with smooth and rough walls have been calculated by molecular-dynamics computer simulations. We use a dynamical criterion to distinguish between fluidlike and solidlike states. The associated crossover densities show a strong dependence both on the system size and on the surface roughness. For large N, these crossover densities tend to the bulk glass transition density for rough walls and to the bulk crystallization density for smooth walls. The crossover densities for finite N are found to be significantly smaller than the corresponding bulk densities. A detailed examination of the layer-resolved radial- and tangential mean-square displacements reveals qualitatively different dynamics for smooth and rough cavities.

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PubMed1999

Melting of polydisperse colloidal crystals in nonequilibrium.

The influence of a time-dependent oscillatory external field on the melting transition of a polydisperse colloidal crystal is examined by theory and computer simulation. In a monodisperse crystal the field just induces an overall dynamical mode which does not affect the melting line. For a polydisperse sample, on the other hand, the field shifts the melting line towards smaller temperatures. Combining a solid cell approach and a Lindemann criterion in nonequilibrium, a simple theory is presented showing that the temperature shift scales with the square of the relative polydispersity. The theory is in reasonable agreement with nonequilibrium Brownian dynamics computer simulations.

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PubMed1999

Effect of geometrical confinement on the interaction between charged colloidal suspensions.

The effective interaction between charged colloidal particles confined between two planar like-charged walls is investigated using computer simulations of the primitive model describing asymmetric electrolytes. In detail, we calculate the effective force acting onto a single macroion and onto a macroion pair in the presence of slitlike confinement. For moderate Coulomb coupling, we find that this force is repulsive. Under strong-coupling conditions, however, the sign of the force depends on the distance to the plates and on the interparticle distance. In particular, the particle-plate interaction becomes strongly attractive for small distances which may explain the occurrence of colloidal crystalline layers near the plates observed in recent experiments.

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PubMed1999

Interfacial free energy of hard-sphere fluids and solids near a hard wall.

A hard-sphere system near a planar structureless hard wall is considered in thermodynamic equilibrium. The associated interfacial free energies are calculated both for a bulk fluid and a bulk face-centered-cubic crystal along (111), (110), and (100) orientation. Combining Monte Carlo simulations and thermodynamic integration, we obtain the wall-fluid and the wall-solid interfacial free energy over the whole range of possible bulk densities. The "exact" computer simulation data are compared to theoretical approximations. For moderate bulk densities, the wall-fluid interfacial free energies compare reasonably well with scaled-particle theory and density functional results. For the wall-crystal interface, we propose a simple analytical cell theory which yields good agreement with our simulation data over the whole range of bulk crystal densities.

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PubMed1996