Rotational and translational motions in a homogeneously cooling granular gas.

A granular gas composed of monodisperse spherical particles was studied in microgravity experiments in a drop tower. Translations and rotations of the particles were extracted from optical video data. Equipartition is violated, the rotational degrees of freedom were excited only to roughly 2/3 of the translational ones.

Platonic solids bouncing on a vibrating plate.

The energy transfer between bouncing particles and rigid boundaries during impacts is crucially influenced not only by restitution coefficients of the material but also by particle shapes. This is particularly important when such particles are mechanically agitated with vibrating plates. Inertial measurement units are able to measure all acceleration and rotational velocity components of an object and store these data for subsequent analysis.

Coarsening of Quasi Two-Dimensional Emulsions Formed by Islands in Free-Standing Smectic Films.

We study the coarsening behavior of assemblies of islands on smectic A freely suspended films in ISS microgravity experiments. The islands can be regarded as liquid inclusions in a two-dimensional fluid in analogy to liquid droplets of the discontinuous phase of an emulsion. The coarsening is effectuated by two processes, predominantly by island coalescence, but to some extend also by Ostwald ripening, whereby large islands grow at the expense of surrounding smaller ones.

Cooling of a granular gas mixture in microgravity.

Granular gases are fascinating non-equilibrium systems with interesting features such as spontaneous clustering and non-Gaussian velocity distributions. Mixtures of different components represent a much more natural composition than monodisperse ensembles but attracted comparably little attention so far. We present the observation and characterization of a mixture of rod-like particles with different sizes and masses in a drop tower experiment.

Coalescence of biphasic droplets embedded in free standing smectic A films.

We investigate micrometer-sized flat droplets consisting of an isotropic core surrounded by a nematic rim in freely suspended smectic A liquid-crystal films. In contrast to purely isotropic droplets which are characterized by a sharp edge and no long-range interactions, the nematic fringe introduces a continuous film thickness change resulting in long-range mutual attraction of droplets. The coalescence scenario is divided in two phases.

Secondary flow in ensembles of nonconvex granular particles under shear.

Studies of granular materials, both theoretical and experimental, are often restricted to convex grain shapes. We demonstrate that a nonconvex grain shape can lead to a qualitatively novel macroscopic dynamics. Spatial crosses (hexapods) are continuously sheared in a split-bottom container.

Shape instabilities of islands in smectic films under lateral compression.

Smectic liquid crystals are fluids, and in most rheological situations they behave as such. Nevertheless, when thin freely floating films of smectic A or smectic C materials are compressed quickly in-plane, they resist such stress by buckling similar to solid membranes under lateral stress. We report experimental observations of wrinkling and bulging of finite domains within the films, so-called islands, and give a qualitative explanation of different observed patterns.

Smectic free-standing films under fast lateral compression.

Smectic freely-suspended films can wrinkle like solid sheets. This has been demonstrated earlier with shape-fluctuating smectic bubbles. Here, we exploit the collapse of smectic catenoid films with a central equatorial film to expose the latter to rapid lateral compression.

Transient hexagonal structures in sheared emulsions of isotropic inclusions on smectic bubbles in microgravity conditions.

We describe the collective behavior of isotropic droplets dispersed over a spherical smectic bubble, observed under microgravity conditions on the International Space Station (ISS). We find that droplets can form two-dimensional hexagonal structures changing with time. Our analysis indicates the possibility of spatial and temporal periodicity of such structures of droplets.

Visual analysis of density and velocity profiles in dense 3D granular gases.

Granular multiparticle ensembles are of interest from fundamental statistical viewpoints as well as for the understanding of collective processes in industry and in nature. Extraction of physical data from optical observations of three-dimensional (3D) granular ensembles poses considerable problems. Particle-based tracking is possible only at low volume fractions, not in clusters.

Silo discharge of mixtures of soft and rigid grains.

We study the outflow dynamics and clogging phenomena of mixtures of soft, elastic low-friction spherical grains and hard frictional spheres of similar size in a quasi-two-dimensional (2D) silo with narrow orifice at the bottom. Previous work has demonstrated the crucial influence of elasticity and friction on silo discharge. We show that the addition of small amounts, even as low as 5%, of hard grains to an ensemble of soft, low-friction grains already has significant consequences.

Self similarity of liquid droplet coalescence in a quasi-2D free-standing liquid-crystal film.

Coalescence of droplets is an ubiquitous phenomenon in chemical, physical and biological systems. The process of merging of liquid objects has been studied during the past years experimentally and theoretically in different geometries. We introduce a unique system that allows a quasi two-dimensional description of the coalescence process: Micrometer-sized flat droplets in freely suspended smectic liquid-crystal films.

Frequency-dependent conversion of the torque of a rotating magnetic field on a ferrofluid confined in a spherical cavity.

The dynamics of magnetic nanoparticles in rotating magnetic fields is studied both experimentally and theoretically. The experimental investigation is focused on the conversion of the magnetic forces to a mechanical torque acting on a ferrofluid confined in a spherical cavity in a rotating magnetic field. Polydispersity usually present in diluted ferrofluids is shown to play a crucial role in the torque conversion.

Structure and dynamics of a two-dimensional colloid of liquid droplets.

Droplet arrays in thin, freely suspended liquid-crystalline smectic A films can form two-dimensional (2D) colloids. The droplets interact repulsively, arranging locally in a more or less hexagonal arrangement with only short-range spatial and orientational correlations and local lattice cell parameters that depend on droplet size. In contrast to quasi-2D colloids described earlier, there is no 3D bulk liquid subphase that affects the hydrodynamics.

Marangoni Flow in Freely Suspended Liquid Films.

We demonstrate controlled material transport driven by temperature differences in thin freely suspended smectic films. Films with submicrometer thicknesses and lateral extensions of several millimeters were studied in microgravity during suborbital rocket flights. In-plane temperature differences cause two specific Marangoni effects, directed flow and convection patterns.

Free Cooling of a Granular Gas of Rodlike Particles in Microgravity.

Granular gases as dilute ensembles of particles in random motion are at the basis of elementary structure-forming processes in the Universe, involved in many industrial and natural phenomena, and also excellent models to study fundamental statistical dynamics. The essential difference to molecular gases is the energy dissipation in particle collisions. Its most striking manifestation is the so-called granular cooling, the gradual loss of mechanical energy E(t) in the absence of external excitation.

Mechanical excitation of rodlike particles by a vibrating plate.

The experimental realization and investigation of granular gases usually require an initial or permanent excitation of ensembles of particles, either mechanically or electromagnetically. One typical method is the energy supply by a vibrating plate or container wall. We study the efficiency of such an excitation of cylindrical particles by a sinusoidally oscillating wall and characterize the distribution of kinetic energies of excited particles over their degrees of freedom.

Smectic C to smectic A transition induced mechanically by the rupture of freely suspended liquid crystal films.

The tilt angle of smectic C phases can be controlled by external forces of very different nature. In particular near a smectic A-smectic C transition, it is susceptible to temperature changes. It can be influenced with electric fields (electroclinic effect), and even mechanically by intra-layer stresses in elastomers.

Outflow and clogging of shape-anisotropic grains in hoppers with small apertures.

Outflow of granular material through a small orifice is a fundamental process in many industrial fields, for example in silo discharge, and in everyday's life. Most experimental studies of the dynamics have been performed so far with monodisperse disks in two-dimensional (2D) hoppers or spherical grains in 3D. We investigate this process for shape-anisotropic grains in 3D hoppers and discuss the role of size and shape parameters on avalanche statistics, clogging states, and mean flow velocities.

Freely floating smectic films.

We have investigated the dynamics of freely floating smectic bubbles using high-speed optical imaging. Bubbles in the size range from a few hundred micrometers to several centimeters were prepared from collapsing catenoids. They represent ideal model systems for the study of thin-film fluid dynamics under well-controlled conditions.

Granular gases of rod-shaped grains in microgravity.

Granular gases are convenient model systems to investigate the statistical physics of nonequilibrium systems. In the literature, one finds numerous theoretical predictions, but only few experiments. We study a weakly excited dilute gas of rods, confined in a cuboid container in microgravity during a suborbital rocket flight.

Tension of freely suspended fluid filaments.

Stable fluid filaments with diameters of several micrometers and slenderness ratios well above 1000 are unique objects formed by some liquid crystalline phases of bent-core mesogens. We present a technique to determine filament tensions from their deflection under defined loads. A strong temperature dependence is observed, with a minimum near the clearing temperature.

Smectic foams.

Because of their layered structure, thermotropic smectic mesogens can form stable foams. In this study, two-dimensional foams of 8CB are prepared in the smectic A phase. We determine the structures of the foam cells and study the aging dynamics.