Fast and viscous!

The dynamics of droplets on repellent materials is quick, owing to the conjunction of a low lateral adhesion with a small dynamical friction. This mobility is especially spectacular for viscous liquids, such as glycerol, where liquid pearls can commonly be 100 times faster than on a wetting solid at a given driving force – gravity for instance. Here we report that viscous droplets on repellent inclines can have two modes of descent, depending on the way they are deposited: either they run at the high speed known for pearls, or they are 30 to 60 faster, which defines a super-quick mode of motion. We show that this effect relies on the fragility of the solid/liquid contact that can easily be "erased" by the insertion of a dynamic air cushion, so as to make droplets glide at a velocity independent of their viscosity. We characterize these lubricating films (thickness and onset of appearance), and show that a super-quick pearl can maintain its velocity and shape on a hydrophilic plate, once it is in this state.