Damping-Driven Time Reversal for Waves.

Damping is usually associated with irreversibility. Here, we present a counterintuitive concept to achieve time reversal of waves propagating in a lossless medium using a transitory dissipation pulse. Applying a sudden and strong damping in a limited time generates a time-reversed wave. In the limit of a high damping shock, this amounts to "freezing" the initial wave by maintaining the wave amplitude while canceling its time derivative. The initial wave then splits in two counterpropagating waves with half of its amplitude and time evolutions in opposite directions. We implement this damping-based time reversal using phonon waves propagating in a lattice of interacting magnets placed on an air cushion. We show with computer simulations that this concept also applies to broadband time reversal in complex disordered systems.

Experimental Implementation of Wave Propagation in Disordered Time-Varying Media.

Here, we study and implement the temporal analog in time disordered sytems. A spatially homogeneous medium is endowed with a time structure composed of randomly distributed temporal interfaces. This is achieved through electrostriction between water surface and an electrode. The wave field observed is the result of the interferences between reflected and refracted waves on the interfaces. Although no eigenmode can be associated with the wave field, several common features between space and time emerge. The waves grow exponentially depending on the disorder level in agreement with a 2D matrix evolution model such as in the spatial case. The relative position of the momentum gap appearing in the time modulated systems plays a central role in the wave field evolution. When tuning the excitation to compensate for the damping, transient waves, localized in time, appear on the liquid surface. They result from a particular history of the multiple interferences produced by a specific sequence of time boundaries.

Space-Time Folding of the Wake Produced by a Supervelocity Rotating Point Source.

Wave sources moving faster than the waves they emit create a wake whose topological features are directly related to the geometry of the source trajectory. These features can be understood by considering space-time surfaces representing past emitted wave fronts. Specifically, for a supervelocity source moving along a circular path the space-time envelope folds and a cusp appears on the inner part of the wake. As a result, the wake is ultimately contained within two parallel corotating spiraling branches. In this Letter we take advantage of the low phase speed of water waves to study experimentally supervelocity sources moving at velocities up to several time the wave speed. We image in real time their emission patterns and characterize the topological features of their wakes.

Fast Dynamics of Water Droplets Freezing from the Outside In.

A drop of water that freezes from the outside in presents an intriguing problem: the expansion of water upon freezing is incompatible with the self-confinement by a rigid ice shell. Using high-speed imaging we show that this conundrum is resolved through an intermittent fracturing of the brittle ice shell and cavitation in the enclosed liquid, culminating in an explosion of the partially frozen droplet. We propose a basic model to elucidate the interplay between a steady buildup of stresses and their fast release. The model reveals that for millimetric droplets the fragment velocities upon explosion are independent of the droplet size and only depend on material properties (such as the tensile stress of the ice and the bulk modulus of water). For small (submillimetric) droplets, on the other hand, surface tension starts to play a role. In this regime we predict that water droplets with radii below 50 µm are unlikely to explode at all. We expect our findings to be relevant in the modeling of freezing cloud and rain droplets.

Electric field makes Leidenfrost droplets take a leap.

Leidenfrost droplets, i.e. droplets whose mobility is ensured by a thin vapor film between the droplet and a hot plate, are exposed to an external electric field. We find that in a strong vertical electric field the droplet can start to bounce progressively higher, defying gravitational attraction. From the droplet's trajectory we infer the temporal evolution of the amount of charge on the droplet. This reveals that the charge starts high and then decreases in steps as the droplet slowly evaporates. After each discharge event the charge is in a fixed proportion to the droplet's surface area. We show that this behavior can be accurately modeled by treating the droplet as a conducting sphere that occasionally makes electrical contact with the hot plate, at intervals dictated by an electro-capillary instability in the vapor film. An analysis of the kinetic and potential energies of the bouncing droplet reveals that, while the overall motion is damped, the droplet occasionally experiences a sudden boost, keeping its energy close to the value for which the free fall trajectory and droplet oscillation are in sync. This helps the droplet to escape from the hot surface when finally the electrical surface forces overtake gravity.

Dynamics of high-speed micro-drop impact: numerical simulations and experiments at frame-to-frame times below 100 ns.

Technologies including (3D-) (bio-)printing, diesel engines, laser-induced forward transfer, and spray cleaning require optimization and therefore understanding of micrometer-sized droplets impacting at velocities beyond 10 m s(-1). However, as yet, this regime has hardly been addressed. Here we present the first time-resolved experimental investigation of microdroplet impact at velocities up to V0 = 50 m s(-1), on hydrophilic and -phobic surfaces at frame rates exceeding 10(7) frames per second. A novel method to determine the 3D-droplet profile at sub-micron resolution at the same frame rates is presented, using the fringe pattern observed from a bottom view. A numerical model, which is validated by the side- and bottom-view measurements, is employed to study the viscous boundary layer inside the droplet and the development of the rim. The spreading dynamics, the maximal spreading diameter, the boundary layer thickness, the rim formation, and the air bubble entrainment are compared to theory and previous experiments. In general, the impact dynamics are equal to millimeter-sized droplet impact for equal Reynolds-, Weber- and Stokes numbers (Re, We, and St, respectively). Using our numerical model, effective scaling laws for the progression of the boundary layer thickness and the rim diameter are provided. The dimensionless boundary layer thickness develops in time (t) according to δBL ~ D0/√Re(t/τ)0.45, and the diameter of the rim develops as DRim ~ D0/√We(t/τ)0.68, with drop diameter D0 and inertial time scale τ = D0/V0. These scalings differ from previously assumed, but never validated, values. Finally, no splash is observed, at variance with many predictions but in agreement with models including the influence of the surrounding gas. This confirms that the ambient gas properties are key ingredients for splash threshold predictions.

Tribonucleation of bubbles.

We report on the nucleation of bubbles on solids that are gently rubbed against each other in a liquid. The phenomenon is found to depend strongly on the material and roughness of the solid surfaces. For a given surface, temperature, and gas content, a trail of growing bubbles is observed if the rubbing force and velocity exceed a certain threshold. Direct observation through a transparent solid shows that each bubble in the trail results from the early coalescence of several microscopic bubbles, themselves detaching from microscopic gas pockets forming between the solids. From a detailed study of the wear tracks, with atomic force and scanning electron microscopy imaging, we conclude that these microscopic gas pockets originate from a local fracturing of the surface asperities, possibly enhanced by chemical reactions at the freshly created surfaces. Our findings will be useful either for preventing undesired bubble formation or, on the contrary, for "writing with bubbles," i.e., creating controlled patterns of microscopic bubbles.