Evaporation of water:Evaporation rate and collective effects

We study the evaporation rate from single drops as well as collections of drops on a solid substrate, both experimentally and theoretically. For a single isolated drop of water, in general the evaporative flux is limited by diffusion of water through the air, leading to an evaporation rate that is proportional to the linear dimension of the drop. Here, we test the limitations of this scaling law for several small drops and for very large drops. We find that both for simple arrangements of drops, as well as for complex drop size distributions found in sprays, cooperative effects between drops are significant. For large drops, we find that the onset of convection introduces a length scale of approximately 20 mm in radius, below which linear scaling is found. Above this length scale, the evaporation rate is proportional to the surface area

Drop Formation in Non-Newtonian Fluids

International audienceWe study the pinch-off dynamics of droplets of yield stress and shear thinning fluids. To separate the two non-Newtonian effects, we use a yield stress material for which the yield stress can be tuned without changing the shear thinning behavior, and a shear thinning system (without a yield stress) for which the shear thinning can be controlled over a large range, without introducing too much elasticity into the system. We find that the pinch-off remains very similar to that of constant viscosity Newtonian liquids, and consequently thinning in shear flow does not imply a thinning in elongational flow

Effect of Surface Tension Variations on the Pinch-Off Behavior of Small Fluid Drops in the Presence of Surfactants

It is shown experimentally that surfactants can change the thinning rate of fluid necks undergoing rupture. In the case of two-fluid pinch-off, two or three linear regimes are observed for the variation of the neck radius versus time. The surface tension in the neck region changes with time, as a result of surfactant depletion. Similar results are obtained for the case of a single fluid pinching in air. The depletion of surfactant can be either partial or complete depending on the rate of transport of the surfactant from the bulk to the surface

Impact dynamics of surfactant laden drops : dynamic surface tension effects

International audienceWe study the impact and subsequent retraction of aqueous surfactant-laden drops upon high-speed impact on hydrophobic surfaces. Without surfactants, a rapid expansion of the drop due to the fluid inertia is followed by a rapid retraction, due to the wetting incompatibility. With surfactants, the retraction can be partly or completely inhibited. We provide quantitative measurements showing that both the expansion and the retraction dynamics depend not only on the equilibrium surface tension (ST) but also on the dynamic tension of the surfactant solutions; the latter varies significantly between different surfactants

Dynamic Interfacial Tension Effects in the Rupture of Liquid Necks

By examining the rupture of fluid necks during droplet formation of surfactant-laden liquids, we observe deviations from expected behaviour for the pinch-off of such necks. We suggest that these deviations are due to the presence of a dynamic (time-varying) interfacial tension at the minimum neck location and extract this quantity from our measurements on a variety of systems. The presence of such dynamic interfacial tension effects should change the rupture process drastically. However, our measurements show that a simple ansatz, which incorporates the temporal change of the interfacial tension, allows us to understand the dynamics of thinning. This shows that this dynamics is largely independent of the exact details of what happens far from the breakup location, pointing to the local nature of the thinning dynamics

Drop Formation in Non-Newtonian Fluids

We study the pinch-off dynamics of droplets of yield stress and shear thinning fluids. To separate the two non-Newtonian effects, we use a yield stress material for which the yield stress can be tuned without changing the shear thinning behavior, and a shear thinning system (without a yield stress) for which the shear thinning can be controlled over a large range, without introducing too much elasticity into the system. We find that the pinch-off remains very similar to that of constant viscosity Newtonian liquids, and consequently thinning in shear flow does not imply a thinning in elongational flow