International audienceWe studied the dynamics of water sessile droplets advancing on hydrophobic and visco-elasticpoly(styrene-butadiene-styrene)(SBS)–paraffin gel substrates at various inflation rates. During theadvancing process, the droplet contact line exhibits three different regimes of motions. When thecontact line advances at a high velocity, it moves continuously with a constant contact angle. As thecontact line slows down, it starts a stick-slip motion: the contact line is pinned at a certain position andthen suddenly slips forward. With further decrease of the velocity, the contact line stops the stick-slipmotion and continuously advances again. The observed threshold values for the transitions of thecontact line motions (continuous–stick-slip–continuous) indicate that the rheology of the gel drasticallyaffects the dynamics of liquid on its surface. We suggest that on visco-elastic gels, the moving contactline exhibits both aspects of wetting on elastic solids and wetting on viscous liquids depending on thecharacteristic frequency of the gel surface deformation. At an intermediate regime, the stick-slip motionof the contact line appears. We also propose a simple geometrical model in the stick-slip regime whichallows us to relate the jumps of the droplet radius to the jumps of the apparent contact angl

In this study we investigate the effect of geometrical or thermal discontinuities on the growth of water droplets condensing on a cooled substrate. Edges, corners, and cooled and noncooled boundaries can have a strong effect on the vapor concentration profile and mass diffusion around the drops. In comparison to growth in a pattern where droplets have to compete to catch vapor, which results in a linear water concentration profile directed perpendicularly to the substrate, droplets near discontinuities can get more vapor (outer edges, corners), resulting in faster growth or less vapor (inner edges), giving lower growth. When the cooling heat flux limits growth instead of mass diffusion (substrate with low thermal conductivity, strong heat exchange with air), edge effects can be canceled. In certain cases, growth enhancement can reach nearly 500% on edges or corners.

Acoustic streaming near a sharp structure and its mixing performance characterization, Microfluidics and Nanofluidics

Gravity-driven drainage of small volumes of condensates, such as natural dew, is a challenge because small drops usually remain pinned to inclined surfaces. We report that submillimetric grooves substantially reduce dew retention by modifying the repartition of liquid: Because of a long-range coalescence mechanism mediated by grooves imbibition, the growth and shedding of large drops are accelerated. Such findings can be applied to increase the passive harvesting of dew as well as to accelerate the drainage of other condensates.

Preprint published in Applied Thermal EngineeringPavement-watering as a technique of cooling dense urban areas and reducing the urban heat island effect has been studied since the 1990's. The method is currently considered as a potential tool for and climate change adaptation against increasing heat wave intensity and frequency. However, although water consumption necessary to implement this technique is an important aspect for decision makers, optimization of possible watering methods has only rarely been conducted. We propose an analysis of pavement heat flux at a depth of 5 cm and solar irradiance measurements to attempt to optimize the watering period, cycle frequency and water consumption rate of a pavement-watering method applied in Paris over the summer of 2013. While fine-tuning of the frequency can be conducted on the basis of pavement heat flux observations, the watering rate requires a heat transfer analysis based on a relation established between pavement heat flux and solar irradiance during pavement insolation. From this, it was found that watering conducted during pavement insolation could be optimized to a frequency of every 30 minutes and water consumption could be reduced by more than 76% while reducing the cooling effect by less than 10%

We studied the dynamics of a liquid contact line receding on a hydrophobic soft gel (SBS-paraffin). In order to realize a well-defined geometry with an accurate control of velocity, a dip-coating setup was implemented. Provided that the elastic modulus is small enough, a significant deformation takes place near the contact line, which in turn drastically influences the wetting behaviour. Depending on the translation velocity of the substrate, the contact line exhibits different regimes of motions. Continuous motions are observed at high and low velocities, meanwhile two types of stick-slip motion - periodic and erratic - appear at intermediate velocities. We suggest that the observed transitions could be explained in terms of the competition between different frequencies, i.e., the frequency of the strain field variation induced by the contact line motion and the crossover frequency of the gel related to the material relaxation. Our results provide systematic views on how the wetting of liquid is modified by the rheological properties of a complex soft substrate.

Acoustic waves can generate steady streaming within a fluid owing to the generation of viscous boundary layers near walls of typical thickness δ. In microchannels, the acoustic wavelength λ is adjusted to twice the channel width w to ensure a resonance condition, which implies the use of MHz transducers. Recently, though, intense acoustic streaming was generated by acoustic waves of a few kHz (hence with λ w), owing to the presence of sharp-tipped structures of curvature radius at the tip r c smaller than δ. The present study quantitatively investigates this sharp-edge acoustic streaming via the direct resolution of the full Navier-Stokes equation using the finite element method. The influence of δ, r c , and viscosity ν on the acoustic streaming performance is quantified. Our results suggest choices of operating conditions and geometrical parameters, in particular the dimensionless tip radius of curvature r c /δ and the liquid viscosity.

We studied the dynamics of the wetting and diffusing processes of water droplets on hydrogel (Poly (2-acrylamido-2-methyl-propane-sulfonic acid-co-acrylamide)(PAMPS-PAAM)) substrates. The profiles of the droplet and substrate were measured simultaneously using a grid projection method. We observed that as the water droplet diffuses into the gel, the contact line of the droplet exhibits successively two different behaviors: pinned and receding, and the transition between these two behaviors is closely related to the local deformation of the gel substrate. The contact line is pinned at an early stage. As the water diffusion proceeds, the contact angle of the droplet decreases while the angle of the local slope of the gel surface near the contact line increases. At the moment where these two angles almost correspond to each other, the contact line starts to recede. Our results indicate that due to the water diffusion, a locally swollen region is formed in the vicinity of the droplet-gel interface, and whether the contact line is pinned or recedes is determined by the surface property of this swollen region.