Droplets spontaneously move when they are placed at the tip of a cone surface. Using three dimensionally printed structures, we experimentally explore a large panel of configurations regarding the aperture angle of the cone. We evidence a change of the droplet geometry while moving along the conical fiber. This transition is a switch of configuration from barrel to clamshell shape. The consequence is a change in the droplet dynamics. We estimate the position of this geometrical transition and we propose two models to describe the motion of the barrel and the clamshell droplets. While both shapes are driven by capillary forces, the dissipation is dependent on the geometrical configuration. For barrel shape droplets the main dissipation appears to be in the liquid wedge. For clamshell shape droplets the dissipation occurs mainly in the liquid film close to the conical fiber.

During a typical silo discharge, the material flow rate is determined by the contact forces between the grains. Here, we report an original study concerning the discharge of a two-dimensional silo filled with repelling magnetic grains. This non-contact interaction leads to a different dynamics from the one observed with conventional granular materials. We found that, although the flow rate dependence on the aperture size follows roughly the power-law with an exponent 3/2 found in non-repulsive systems, the density and velocity profiles during the discharge are totally different. New phenomena must be taken into account. Despite the absence of contacts, clogging and intermittence were also observed for apertures smaller than a critical size determined by the effective radius of the repulsive grains. Received: 10 July 2015, Accepted: 7 August 2015; Edited by: L. A. Pugnaloni; Reviewed by: J. R. Darias, Universidad Simón o Bolívar, Caracas, Venezuela; DOI: http://dx.doi.org/10.4279/PIP.070013Cite as: G Lumay, J Schockmel, D Henández-Enríquez, S Dorbolo, N Vandewalle, F Pacheco-Vázquez, Papers in Physics 7, 070013 (2015)This paper, by G Lumay, J Schockmel, D Henández-Enríquez, S Dorbolo, N Vandewalle, F Pacheco-Vázquez, is licensed under the Creative Commons Attribution License 3.0.

The Leidenfrost effect, classically associated with drops levitating on their own vapor over hot solid surfaces, can also be observed over hot baths of nonvolatile liquids. In view of substrate fluidity, heat transfer through the bath to the drop should most certainly be dominated by convection and not by only conduction as in the solids, which may be instrumental for an efficient heat supply to the drop given typically poor thermal conductivity of the liquids. Here, we undertake an experimental and numerical study of the flow in a bath of silicone oil V20 induced by an overlying Leidenfrost drop, highlighting that a toroidal vortex is formed underneath the drop whose direction of circulation turns out to be different for drops of different liquids. We show that this is due to a shift in a delicate interplay between three mechanisms pulling in different directions: (i) shear stresses exerted by the vapor escaping from the gap between the bath and the drop, as well as (ii) buoyancy action and (iii) thermocapillary (Marangoni) stresses, both due to local evaporative cooling of the bath by the drop. Whatever the structure of this locally induced convection, its crucial heat transfer enhancing efficiency is readily confirmed in numerical simulations as favoring levitation.

We investigate the stability of a granular monolayer composed of spherical grains on an inclined plate. When the tilt angle $\alpha$ increases, some reorganizations are observed throughout the pile. The packing fraction $\rho$ of the packing evolves by successive jumps. Those discontinuous events precede the collapse of the pile at a critical angle ${\alpha }_c$. The occurrence of precursors before avalanches is modeled by stop-and-go motions of blocks due to the competition between sliding friction and the Janssen effect [J. Durand, Sands, Powders, and Grains: An Introduction to the Physics of Granular Materials (Springer-Verlag, New York, 2000)].

We studied the behavior of a simple apparatus that allows charging droplets by induction. We imposed the following constraints: (i) any liquid should be charged (more precisely whatever its surface tension and therefore whatever its wettability, capillary length, etc.); and (ii) the system is to remain as simple as possible. The design of the device results of a compromise regarding both constraints. The device developed is based on a planar capacitor in which the droplets are generated. The influence of the nature of the liquid (i.e., presence of ions in solution, polarity, surface tension, and conductivity) on the charge induced was measured and was found to be independent of the liquid properties (except for the silicone oil). We deduced that, in the considered configuration, fluids behave like a “perfect conductor” as soon as conductive relaxation time is smaller than the droplet ejection time. Under this condition, the charge has the time to move in the droplet to reach the electrostatic regime.