Period tripling causes rotating spirals in agitated wet granular layers

Internal and surface waves in vibrofluidized granular materials: Role of cohesion

Wave phenomena in vibrofluidized dry and partially wet granular materials confined in a quasi-two-dimensional geometry are investigated with numerical simulations considering individual particles as hard spheres. Short-ranged cohesive interactions arising from the formation of liquid bridges between adjacent particles are modeled by changing the velocity-dependent coefficient of restitution. Such a change effectively suppresses the formation of surface waves, in agreement with previous experimental observations. The difference in pattern creation arises from the suppressed momentum transfer due to wetting and it can be quantitatively understood from an analysis of binary impacts.

Density-wave fronts on the brink of wet granular condensation

Density-wave fronts in a vibrofluidized wet granular layer undergoing a gas-liquid-like transition are investigated experimentally. The threshold of the instability is governed by the amplitude of the vertical vibrations. Fronts, which are curved into a spiral shape, propagate coherently along the circular rim of the container with leading edges. They are stable beyond a critical distance from the container center. Based on an analysis of the emerging time and length scales, we propose a model for the pattern formation by considering the competition between the time scale for the condensation of wet granular particles from a gas-like state and that of the energy injection resisting this process.

Radar for tracer particles

We introduce a radar system capable of tracking a 5 mm spherical target continuously in three dimensions. The 10 GHz (X-band) radar system has a transmission power of 1 W and operates in the near field of the horn antennae. By comparing the phase shift of the electromagnetic wave traveling through the free space with an IQ-mixer, we obtain the relative movement of the target with respect to the antennae. From the azimuth and inclination angles of the receiving antennae obtained in the calibration, we reconstruct the target trajectory in a three-dimensional Cartesian system. Finally, we test the tracking algorithm with target moving in circular as well as in pendulum motions and discuss the capability of the radar system.

Influence of the liquid film thickness on the coefficient of restitution for wet particles

The normal coefficient of restitution (COR) for a spherical particle bouncing on a wet plane is investigated experimentally and compared with a model characterizing the energy loss at impact. For fixed ratios of liquid film thickness δ to particle diameter D, the wet COR is always found to decay linearly with St^{-1}, where St, the Stokes number, measures the particle inertia with respect to the viscous force of the liquid. Such a dependence suggests a convenient way of predicting the wet COR with two fit parameters: a critical COR at infinitely large St and a critical St at zero COR. We characterize the dependence of the two parameters on δ/D and compare it with a model considering the energy loss from the inertia and the viscosity of the wetting liquid. This investigation suggests an analytical prediction of the COR for wet particles.

Subharmonic instability of a self-organized granular jet

Downhill flows of granular matter colliding in the lowest point of a valley, may induce a self-organized jet. By means of a quasi two-dimensional experiment where fine grained sand flows in a vertically sinusoidally agitated cylinder, we show that the emergent jet, that is, a sheet of ejecta, does not follow the frequency of agitation but reveals subharmonic response. The order of the subharmonics is a complex function of the parameters of driving.

Pattern formation in wet granular matter under vertical vibrations

Experiments on a thin layer of cohesive wet granular matter under vertical vibrations reveal kink-separated domains that collide with the container at different phases. Due to the strong cohesion arising from the formation of liquid bridges between adjacent particles, the domains move collectively upon vibrations. Depending on the periodicity of this collective motion, the kink fronts may propagate, couple with each other, and form rotating spiral patterns in the case of period tripling or stay as standing wave patterns in the case of period doubling. Moreover, both patterns may coexist with granular "gas bubbles"-phase separation into a liquidlike and a gaslike state. Stability diagrams for the instabilities measured with various granular layer mass m and container height H are presented. The onsets for both types of patterns and their dependency on m and H can be quantitatively captured with a model considering the granular layer as a single particle colliding completely inelastically with the container.

Formation of spiral patterns in dielectric-barrier discharge investigated by an intensified-charge coupled device

The formation of the spiral patterns in dielectric-barrier discharge is investigated through instantaneous pictures at successive driving half cycles taken by an intensified-charge coupled device (ICCD). The filaments discretely distributed in each instantaneous picture corresponding to one discharge pulse indicate that they are the basic unit of the spiral patterns. An individual filament can undergo two movement states within continuous discharges: pinning at the same locations and shifting along the arm with a small displacement (less than the filament radius), which results in the formation of smooth arms. The averaged correlation time between the instantaneous patterns is found to be about two driving half-cycles by the cross-correlation calculations.