Liquid marbles: topical context within soft matter and recent progress

The study of particle stabilized interfaces has a long history in terms of emulsions, foams and related dry powders. The same underlying interfacial energy principles also allow hydrophobic particles to encapsulate individual droplets into a stable form as individual macroscopic objects, which have recently been called "Liquid Marbles". Here we discuss conceptual similarities to superhydrophobic surfaces, capillary origami, slippery liquids-infused porous surfaces (SLIPS) and Leidenfrost droplets. We provide a review of recent progress on liquid marbles, since our earlier Emerging Area article (Soft Matter, 2011, 7, 5473–5481), and speculate on possible future directions from new liquid-infused liquid marbles to microarray applications. We highlight a range of properties of liquid marbles and describe applications including detecting changes in physical properties (e.g. pH, UV, NIR, temperature), use for gas sensing, synthesis of compounds/composites, blood typing and cell culture

Coupling the Leidenfrost effect and elastic deformations to power sustained bouncing

The Leidenfrost effect occurs when an object near a hot surface vaporizes rapidly enough to lift itself up and hover. Although well-understood for liquids and stiff sublimable solids, nothing is known about the effect with materials whose stiffness lies between these extremes. Here we introduce a new phenomenon that occurs with vaporizable soft solids: the elastic Leidenfrost effect. By dropping hydrogel spheres onto hot surfaces we find that, rather than hovering, they energetically bounce several times their diameter for minutes at a time. With high-speed video during a single impact, we uncover high-frequency microscopic gap dynamics at the sphere-substrate interface. We show how these otherwise-hidden agitations constitute work cycles that harvest mechanical energy from the vapour and sustain the bouncing. Our findings unleash a powerful and widely applicable strategy for injecting mechanical energy into soft materials, with relevance to fields ranging from soft robotics and metamaterials to microfluidics and active matter

Space-Time Resolved Experiments for Water Waves

An overview of recent works on water wave propagation using a full time-space resolved method is given. The experimental method allows us to precisely measure the surface elevation field with spatial and temporal resolutions given by the pixel size and frequency acquisition of a high speed camera. Two typical problems are regarded: (i) the propagation of water waves through surface piercing obstacles with trapped modes or directional emission, a problem of interest notably for its practical applications to the protection of floating structures and to the canalization of the water wave energy, (ii) a study of water wave turbulence is also reported, exhibiting the interest to measure the joint space-time power spectrum to study which hypothesis of weak turbulence theory survives in laboratory experiments

Viscous mechanism for Leidenfrost propulsion on a ratchet

An evaporating drop placed on a ratchet self-propels, as discovered by Linke et al. in 2006. Sublimating platelets do the same, and we discuss here a possible viscous mechanism for these motions. We report that the flow of vapor below the levitating material is rectified by the asymmetric teeth of the ratchet, in the direction of descending slopes along each tooth. As a consequence, the resulting viscous stress can entrain the material in the same direction, and we discuss the resulting self-propelling force

Different Regimes for Water Wave Turbulence

International audienceWe present an experimental study on gravity capillary wave turbulence in water. By using space-time resolved Fourier transform profilometry, the behavior of the wave energy density j k;! j 2 in the 3D ðk; !Þ space is inspected for various forcing frequency bandwidths and forcing amplitudes. Depending on the bandwidth, the gravity spectral slope is found to be either forcing dependent, as classically observed in laboratory experiments, or forcing independent. In the latter case, the wave spectrum is consistent with the Zakharov-Filonenko cascade predicted within wave turbulence theory

Space-Time Resolved Experiments for Water Waves

International audienceAn overview of recent works on water wave propagation using a full time-space resolved method is given. The experimental method allows us to precisely measure the surface elevation field with spatial and temporal resolutions given by the pixel size and frequency acquisition of a high speed camera. Two typical problems are regarded: (i) the propagation of water waves through surface piercing obstacles with trapped modes or directional emission, a problem of interest notably for its practical applications to the protection of floating structures and to the canalization of the water wave energy, (ii) a study of water wave turbulence is also reported, exhibiting the interest to measure the joint space-time power spectrum to study which hypothesis of weak turbulence theory survives in laboratory experiments