Line Tension in a Thick Soap Film.

The thickness of freshly made soap films is usually in the micron range, and interference colors make thickness fluctuations easily visible. Circular patterns of constant thickness are commonly observed, either a thin film disc in a thicker film or the reverse. In this Letter, we evidence the line tension at the origin of these circular patterns. Using a well controlled soap film preparation, we produce a piece of thin film surrounded by a thicker film. The thickness profile, measured with a spectral camera, leads to a line tension of the order of 10^{-10} N which drives the relaxation of the thin film shape, initially very elongated, toward a circular shape. A balance between line tension and air friction leads to a quantitative prediction of the relaxation process. Such a line tension is expected to play a role in the production of marginal regeneration patches, involved in soap film drainage and stability.

Self-propulsion of inverse Leidenfrost drops on a cryogenic bath.

When deposited on a hot bath, volatile drops are observed to stay in levitation: the so-called Leidenfrost effect. Here, we discuss drop dynamics in an inverse Leidenfrost situation where room-temperature drops are deposited on a liquid-nitrogen pool and levitate on a vapor film generated by evaporation of the bath. In the seconds following deposition, we observe that the droplets start to glide on the bath along a straight path, only disrupted by elastic bouncing close to the edges of the container. Initially at rest, these self-propelled drops accelerate within a few seconds and reach velocities on the order of a few centimeters per second before slowing down on a longer time scale. They remain self-propelled as long as they are sitting on the bath, even after freezing and cooling down to liquid-nitrogen temperature. We experimentally investigate the parameters that affect liquid motion and propose a model, based on the experimentally and numerically observed (stable) symmetry breaking within the vapor film that supports the drop. When the film thickness and the cooling dynamics of the drops are also modeled, the variations of the drop velocities can be accurately reproduced.

Impact of the Wetting Length on Flexible Blade Spreading.

We study the spreading of a Newtonian fluid by a deformable blade, a common industrial problem, characteristic of elastohydrodynamic situations. Here, we consider the case of a finite reservoir of liquid, emptying as the liquid is spread. We evidence the role of a central variable: the wetting length l_{w}, which sets a boundary between the wet and dry parts of the blade. We show that the deposited film thickness e depends quadratically with l_{w}. We study this problem experimentally and numerically by integration of the elastohydrodynamic equations, and finally propose a scaling law model to explain how l_{w} influences the spreading dynamics.

Inverse leidenfrost drop manipulation using menisci.

Drops deposited on an evaporating liquid bath can be maintained in an inverse Leidenfrost state by the vapor emanating from the bath, making them levitate and hover without effective friction. These perfectly non-wetting droplets create a depression in the liquid interface that sustains their weight, which generates repellent forces when they approach a meniscus rising against a wall. Here, we study this reflection in detail, and show that frictionless Leidenfrost drops are a simple and efficient tool to probe the shape of an unknown interface. We then use the menisci to control the motion of the otherwise elusive drops. We create waveguides to direct and accelerate them and use parabolic walls to reflect and focus them. This could be particularly beneficial in the scale up of droplet cryopreservation processes: capillary interactions can be used to transport, gather and collect vitrified biological samples in absence of contact and contamination.

Water ring-bouncing on repellent singularities.

Texturing a flat superhydrophobic substrate with point-like superhydrophobic macrotextures of the same repellency makes impacting water droplets take off as rings, which leads to shorter bouncing times than on a flat substrate. We investigate the contact time reduction on such elementary macrotextures through experiment and simulations. We understand the observations by decomposing the impacting drop reshaped by the defect into sub-units (or blobs) whose size is fixed by the liquid ring width. We test the blob picture by looking at the reduction of contact time for off-centered impacts and for impacts in grooves that produce liquid ribbons where the blob size is fixed by the width of the channel.

Finite size effects on textured surfaces: recovering contact angles from vagarious drop edges.

A clue to understand wetting hysteresis on superhydrophobic surfaces is the relation between receding contact angle and surface textures. When the surface textures are large, there is a significant distribution of local contact angles around the drop. As seen from the cross section, the apparent contact angle oscillates as the triple line recedes. Our experiments demonstrate that the origin of these oscillations is a finite size effect. Combining side and bottom views of the drop, we take into account the 3D conformation of the surface near the edge to evaluate an intrinsic contact angle from the oscillations of the apparent contact angle. We find that for drops receding on axisymmetric textures the intrinsic receding contact angle is the minimum value of the oscillation while for a square lattice it is the maximum.

Role of kinks in the dynamics of contact lines receding on superhydrophobic surfaces.

We have investigated the depinning of the contact line on superhydrophobic surfaces with anisotropic periodic textures. By direct observation of the contact line conformation, we show that the mobility is mediated by kink defects. Full 3D simulations of the shape of the liquid surface near the solid confirm that kinks account for the measured wetting properties. This behavior, which is similar to the Peierls-Nabarro mechanism for dislocations, may open perspectives for the optimization of wetting hysteresis by design.

Shear thickening in presence of adhesive contact forces: The singularity of cornstarch.

HYPOTHESIS: A number of dense particle suspensions experience a dramatic increase in viscosity with the shear stress, up to a solid-like response. This shear-thickening process is understood as a transition under flow of the nature of the contacts - from lubricated to frictional - between initially repellent particles. Most systems are now assumed to fit in with this scenario, which is questionable. EXPERIMENT: Using an in-house pressure sensor array, we provide a spatio-temporal map of the normal stresses in the flows of two shear-thickening fluids: a stabilized calcium carbonate suspension, known to fit in with the standard scenario, and a cornstarch suspension, which spectacular thickening behavior remains poorly understood. FINDINGS: We evidence in cornstarch a unique, stable heterogeneous structure, which moves in the velocity direction and does not appear in calcium carbonate. Its nature changes from a stress wave to a rolling solid jammed aggregate at high solid fraction and small gap width. The modeling of these heterogenities points to an adhesive force between cornstarch particles at high stress, also evidenced in microscopic measurements. Cornstarch being also attractive at low stress, it stands out of the classical shear-thickening frame, and might be part of a larger family of adhesive and attractive shear-thickening fluids.

Capillary orbits.

Millimeter-sized objects trapped at a liquid surface distort the interface by their weight, which in turn attracts them towards each other. This ubiquitous phenomenon, colloquially called the "Cheerios effect" is seen in the clumping of cereals in a breakfast bowl, and turns out to be a highly promising route towards controlled self-assembly of colloidal particles at the water surface. Here, we study capillary attraction between levitating droplets, maintained in an inverse Leidenfrost state above liquid nitrogen. We reveal that the drops spontaneously orbit around each other - mirroring a miniature celestial system. In this unique situation of negligible friction, the trajectories are solely shaped by the Cheerios-interaction potential, which we obtain directly from the droplet's dynamics. Our findings offer an original perspective on contactless and contamination-free droplet cryopreservation processing, where the Leidenfrost effect and capillarity would be used in synergy to vitrify and transport biological samples.

Water impacting on superhydrophobic macrotextures.

It has been recently shown that the presence of macrotextures on superhydrophobic materials can markedly modify the dynamics of water impacting them, and in particular significantly reduce the contact time of bouncing drops, compared with what is observed on a flat surface. This finding constitutes a significant step in the maximization of water repellency, since it enables to minimize even further the contact between solid and liquid. It also opens a new axis of research on the design of super-structures to induce specific functions such as anti-freezing, liquid fragmentation and/or recomposition, guiding, trapping and so on. Here we show that the contact time of drops bouncing on a repellent macrotexture takes discrete values when varying the impact speed. This allows us to propose a quantitative analysis of the reduction of contact time and thus to understand how and why macrotextures can control the dynamical properties of bouncing drops.