Biomedical subjects

Hartmut Löwen

Publications and source records attributed to Hartmut Löwen.

5 recordsLinked to original sources

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↗