Acoustic probing of the particle concentration in turbulent granular suspensions in air.

Dilute gas-particle suspensions in which the particles are carried by the fluid are found in various industrial and geophysical contexts. One fundamental issue that limits our understanding of such systems is the difficulty to obtain information on the particle concentration inside these often optically opaque suspensions. To overcome this difficulty, we develop ultrasonic spectroscopy to monitor the local particle concentration [Formula: see text] of glass particles (with diameters [Formula: see text] 77 [Formula: see text]m or 155 [Formula: see text]m) suspended in air. First, we determine the minimal air velocity, [Formula: see text], necessary to suspend the particles from the maximum decrease in the transmitted wave amplitude and velocity of ultrasound propagating through the suspension. Next, setting the air velocity at [Formula: see text], we increase the mass of particles and monitor acoustically the local solid volume fraction, [Formula: see text], by measuring the ultrasound wave attenuation coefficient and phase velocity as a function of frequency on the basis of classical scattering and hydrodynamic models. For the frequency ranges and suspensions considered here, the viscous dissipation dominates over scattering and thermal conduction losses. We show that, for a characteristic air velocity [Formula: see text], the locally measured [Formula: see text] reaches a critical value, in agreement with a recent study on turbulent gas-particle mixtures. Moreover, we find that this critical [Formula: see text] increases with the size of the particles. Finally, analysis of the temporal fluctuations of the locally measured solid volume fraction, suggests that high density regions (clusters) are present even in suspensions with concentrations below the critical concentration. This differs from the current hypothesis according to which the critical concentration coincides with the onset of cluster formation.

Ultrasonic tracking of a sinking ball in a vibrated dense granular suspension.

Observing and understanding the motion of an intruder through opaque dense suspensions such as quicksand remains a practical and conceptual challenge. Here we use an ultrasonic probe to monitor the sinking dynamics of a steel ball in a dense glass bead packing (3D) saturated by water. We show that the frictional model developed for dry granular media can be used to describe the ball motion induced by horizontal vibration. From this rheology, we infer the static friction coefficient and effective viscosity that decrease when increasing the vibration intensity. Our main finding is that the vibration-induced reduction of the yield stress and increase of the sinking depth are presumably due to micro-slips induced at the grain contacts but without visible plastic deformation due to macroscopic rearrangements, in contrast to dry granular packings. To explain these results, we propose a mechanism of acoustic lubrication that reduces the inter-particle friction and leads to a decrease of the yield stress. This scenario is different from the mechanism of liquefaction usually invoked in loosely packed quicksands where the vibration-induced compaction increases the pore pressure and decreases the confining pressure on the solid skeleton, thus reducing the granular resistance to external load.

Pumping and probing vibrational modulated coupled electronic coherence in HCN using short UV fs laser pulses: a 2D quantum nuclear dynamical study.

The coupled electronic-nuclear coherent dynamics induced by a short strong VUV fs pulse in the low excited electronic states of HCN is probed by transient absorption spectroscopy with a second weaker fs UV pulse. The nuclear time-dependent Schrodinger equation is solved on a 2D nuclear grid with several electronic states with a Hamiltonian including the dipole coupling to the pump and the probe electric fields. The two internal nuclear coordinates describe the motion of the light H atom. There is a band of several excited electronic states at about 8 eV above the ground state (GS) that is transiently accessed by the pump pulse. We tailored the pump so as to selectively populate the lowest 1A'' electronic state thereby the pulse creates an electronic coherence with the GS. Our simulations show that this electronic coherence is modulated by the nuclear motion and persists all the way to dissociation on the 1A'' state. Transient absorption spectra computed as a function of the delay time between the pump and the probe pulses provide a detailed probe of the electronic amplitude and its phase, as well as of the modulation of the electronic coherence by the nuclear motion, both bound and dissociative.