Search

Publisher version: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.110.138303 5 pages; International audience; The drag force experienced by a horizontal cylinder rotating around the vertical axis in a granular medium under gravity is experimentally investigated. We show that for deeply buried objects, the drag force dramatically drops after half a turn, as soon as the cylinder crosses its own wake. Whereas the drag during the first half turn increases linearly with the depth, the drag after several rotations appears to be independent of depth, in contradiction with the classical frictional picture stipulating that the drag is proportional to the hydrostatic pressure. We systematically study how the saturated drag force scales with the control parameters and show that this effect may be used to drill deeply in a granular medium without developing high torques.

International audience; We describe a new spreading regime during the drop impact of model yield-stress fluids (Carbopol microgel solutions) on rough hydrophobic surfaces, in a range of parameters where classical Newtonian drops usually splash. For large surface roughness and high impact velocity, we observe that the maximal inertial spreading diameter of the drops can be as much as twice larger than on smooth surfaces in the same conditions, corresponding to apparent basal friction reductions of more than 80%. We interpret this large drag reduction using a simple energy balance model and a dynamic slip length that depends on both the surface roughness and the drop's dynamics.

International audience; Single-bubble sonoluminescence is characterized by a great concentration of energy during the collapse of a gas bubble, which leads to the generation of photons from low-frequency ultrasound. The narrow stability domain of sonoluminescence has limited previous attempts to reinforce this inertial confinement in order to generate photons of higher energy or to ignite a nuclear fusion reaction. We present a new experimental approach where an ultrasonic pulse of high frequency is adaptively focused on the bubble during the collapse. Using an array of eight transmitters, a pressure pulse of 0.7 MPa doubles the flash intensity; this technique can easily be extended to higher pressure.