Falling droplets bounce back well from superhydrophobic surfaces. Now it is shown that when a thin air film is made to persist between drop and surface, efficient bouncing is possible for wettable surfaces too, and for drops with low surface tension. The impingement of drops onto solid surfaces1,2 plays a crucial role in a variety of processes, including inkjet printing, fog harvesting, anti-icing, dropwise condensation and spray coating3,4,5,6. Recent efforts in understanding and controlling drop impact behaviour focused on superhydrophobic surfaces with specific surface structures enabling drop bouncing with reduced contact time7,8. Here, we report a different universal bouncing mechanism that occurs on both wetting and non-wetting flat surfaces for both high and low surface tension liquids. Using high-speed multiple-wavelength interferometry9, we show that this bouncing mechanism is based on the continuous presence of an air film for moderate drop impact velocities. This submicrometre ‘air cushion’ slows down the incoming drop and reverses its momentum. Viscous forces in the air film play a key role in this process: they provide transient stability of the air cushion against squeeze-out, mediate momentum transfer, and contribute a substantial part of the energy dissipation during bouncing.

We have developed a scalable, efficient photothermal trap for deicing by localized light absorption and heat spreading at the ice-substrate interface.

Liquid drops hitting solid surfaces deform substantially under the influence of the ambient air that needs to be squeezed out before the liquid actually touches the solid. Nanometer- and microsecond-resolved dual wavelength interferometry reveals a complex evolution of the interface between the drop and the gas layer underneath. For intermediate impact speeds (We$\sim${}1\dots{}10) the layer thickness can develop one or two local minima\char22{}reproduced in numerical calculations\char22{}that eventually lead to the nucleation of solid-liquid contact at a We-dependent radial position, from a film thickness >200\,\,nm. Solid-liquid contact spreads at a speed involving capillarity, liquid viscosity and inertia.

Drops on cylindrical fibers are a familiar sight, for instance in the form of dew drops on spider webs. They can exist in two competing morphologies, a cylindrically symmetric barrel state completely engulfing the fiber and an asymmetric clamshell state, in which the drop touches the fiber only sideways. Despite their omnipresence and their practical relevance, e.g. for the adherence of drops to fibers in separation technology and filter materials, the physical mechanisms governing the stability of the two morphologies remained elusive. Using electrowetting-functionalized fibers we can tune the wettability of the fibers and thereby reversibly switch between the two states. This allows determination of the stability limits of both morphologies as a function of the two relevant control parameters, namely the contact angle and the liquid volume. While clamshells are found to prevail for large contact angles and small volumes, and barrels prevail for small angles and large volumes, there is also a wide range of intermediate parameter values, for which both morphologies are mechanically stable. Mapping out the energy landscape of the system by numerical minimization of the free energy we find that the barrel state is easily deformed by non-axisymmetric perturbations. Such perturbations facilitate the transition to the clamshell state and thereby the removal of drops from the fibers. From a general perspective, the demonstration of electrowetting-based reversible switching of liquid morphologies on fibers opens up opportunities for designing functional textiles and porous materials for various applications in detergency, filtering, and controlled absorption and release of liquids.

A liquid drop approaching a solid surface deforms substantially under the influence of the ambient air which needs to be squeezed out before the liquid can actually touch the solid. We use nanometer- and microsecond-resolved dual wavelength interferometry described in Part I (also published in this issue) to reveal the complex spatial and temporal evolution of the squeezed air layer. In low-velocity droplet impact, i.e., We numbers of order unity, the confined air layer below the droplet develops two local minima in thickness. We quantitatively measure the evolution of the droplet bottom interface and find that surface tension determines the air film thickness below the first kink, after which fluid is diverted outward to form a second even sharper kink. Depending on We, one of the two kinks approaches the surface more closely forming liquid-solid contact. The early time spreading of liquid-solid contact is controlled by the capillary driving force and the inertia of the liquid. The cushioned air film geometry, i.e., a flat micrometer-thin gap, induces an increase of the spreading velocity; the contact area first spreads over the cushioned region, only then followed by radial spreading. This spreading mechanism can lead to the entrapment of one or more air bubbles.