International audienceLike many natural objects, raindrops are distributed in size. By extension of what is known to occur inside the clouds, where small droplets grow by accretion of vapour and coalescence, raindrops in the falling rain at the ground level are believed to result from a complex mutual interaction with their neighbours. We show that the raindrops' polydispersity, generically represented according to Marshall-Palmer's law (1948), is quantitatively understood from the fragmentation products of non-interacting, isolated drops. Both the shape of the drops' size distribution, and its parameters are related from first principles to the dynamics of a single drop deforming as it falls in air, ultimately breaking into a dispersion of smaller fragments containing the whole spectrum of sizes observed in rain. The topological change from a big drop into smaller stable fragments-the raindrops-is accomplished within a timescale much shorter than the typical collision time between the drops

International audienceWe address the sequence of events accompanying the transition from an initially compact volume of liquid a drop into dispersed fragments when it impacts a solid surface. We describe the change of topology of the drop to a radially expanding sheet and discuss the reasons of its rim destabilization, responsible for the emergence of radial ligaments which ultimately fragment into smaller drops. The dynamics ruling the radius of the sheet, its stability and the resulting fragment drop size distribution are documented experimentally. The radius dynamics results from a simple balance between inertia of the initial drop and capillary restoring forces at the rim, with damping due to the continuous transfer of momentum from the sheet to the rim. The ligaments expelled from the rim originate from a Rayleigh Taylor mechanism localized at the rim. The final drop size distribution in the spray is shown to be a linear superposition of gamma distributions characteristic of ligament breakup, leading generically to Bessel functions

International audienceThe present study aims at documenting, making use of an original set-up allowing to acquire well-converged data, the coarsening of foams in two dimensions. Experiments show that a foam behaves quite differently depending on the way it has been prepared. We distinguish between an initially quasi-monodisperse foam and a polydisperse foam. The coarsening laws are initially different, although both foams reach a common, time-dependent asymptotic regime. The ageing process relies on exchanges between adjacent foam cells (von Neumann's law), and on topological rearrangement (T(1) and T(2) processes) whose rates are measured in all regimes. We attempt to make their contribution to the evolution of the area S and facet number n distribution of probability P(S, n, t) quantitative. The corresponding mean field theory predictions represent well the phenomenon qualitatively, and are sometimes in quantitative agreement with the measurements. A simplified version of this theory, taking the form of a Langevin model, explains in a straightforward manner the different scaling laws in the different regimes, for the different foams