Laurent Courbin

Daughter bubble cascades produced by folding of ruptured thin films

Thin liquid films, such as soap bubbles, have been studied extensively for over a century because they are easily formed and mediate a wide range of transport processes in physics, chemistry and engineering. When a bubble on a liquid–gas or solid–gas interface (referred to herein as an interfacial bubble) ruptures, the general expectation is that the bubble vanishes. More precisely, the ruptured thin film is expected to retract rapidly until it becomes part of the interface, an event that typically occurs within milliseconds. The assumption that ruptured bubbles vanish is central to theories on foam evolution and relevant to health and climate because bubble rupture is a source for aerosol droplets. Here we show that for a large range of fluid parameters, interfacial bubbles can create numerous small bubbles when they rupture, rather than vanishing. We demonstrate, both experimentally and numerically, that the curved film of the ruptured bubble can fold and entrap air as it retracts. The resulting toroidal geometry of the trapped air is unstable, leading to the creation of a ring of smaller bubbles. The higher pressure associated with the higher curvature of the smaller bubbles increases the absorption of gas into the liquid, and increases the efficiency of rupture-induced aerosol dispersal.

Imbibition in geometries with axial variations

When surface wetting drives liquids to invade porous media or microstructured materials with uniform channels, the penetration distance is known to increase as the square root of time. We demonstrate, experimentally and theoretically, that shape variations of the channel, in the flow direction, modify this ‘diffusive’ response. At short times, the shape variations are not significant and the imbibition is still diffusive. However, at long times, different power-law responses occur, and their exponents are uniquely connected to the details of the geometry. Experiments performed with conical tubes clearly show the two theoretical limits. Several extensions of these ideas are described.

Three-Dimensional Self-Assembling of Gold Nanorods with Controlled Macroscopic Shape and Local Smectic B Order

We describe a method of controlled evaporation on a textured substrate for self-assembling and shaping gold-nanorod-based materials. Tridimensional wall features are formed over areas as large as several square millimeters. Furthermore, analyses by small-angle X-ray scattering and scanning electron microscopy techniques demonstrate that colloids are locally ordered as a smectic B phase. Such crystallization is in fact possible because we could finely adjust the nanoparticle charge, knowledge that additionally enables tuning the lattice parameters. In the future, the type of ordered self-assemblies of gold nanorods we have prepared could be used for amplifying optical signals.

Splashing on elastic membranes: The importance of early-time dynamics

We study systematically the effect of substrate compliance on the threshold for splashing of a liquid drop using an elastic membrane under variable tension. We find that the splashing behavior is strongly affected by the tension in the membrane and splashing can be suppressed by reducing this tension. The deflection of the membrane upon droplet impact is measured using a laser sheet, and the results allow us to estimate the energy absorbed by the film upon drop impact. Measurements of the velocity and acceleration of the spreading drop after impact indicate that the splashing behavior is set at very early times after, or possibly just before, impact, far before the actual splash occurs. We also provide a model for the tension dependence of the splashing threshold based on the pressure in the drop upon impact that takes into account the interplay between membrane tension and drop parameters.

Observation of Droplet Size Oscillations in a Two-Phase Fluid under Shear Flow

Experimental observations of droplet size sustained oscillations are reported in a two-phase flow between a lamellar and a sponge phase. Under shear flow, this system presents two different steady states made of monodisperse multilamellar droplets, separated by a shear-thinning transition. At low and high shear rates, the droplet size results from a balance between surface tension and viscous stress, whereas for intermediate shear rates it becomes a periodic function of time. A possible mechanism for such kinds of oscillations is discussed.

Passive breakups of isolated drops and one-dimensional assemblies of drops in microfluidic geometries: experiments and models

Using two different geometries, rectangular obstacles and asymmetric loops, we investigate the breakup dynamics of deformable objects, such as drops and bubbles, confined in microfluidic devices. We thoroughly study two distinct flow configurations that depend on whether object-to-object hydrodynamic interactions are allowed. When such interactions are introduced, we find that the volumes of the daughter objects created after breakup solely depend on the geometrical features of the devices and are not affected by the hydrodynamic and physicochemical variables; these results are in sharp contrast with those obtained for non-interacting objects. For both configurations, we provide simple phenomenological models that capture well the experimental findings and predict the evolution of the volumes of the daughter objects with the controlling dimensionless quantities that are identified. We introduce a mean-field approximation, which permits accounting for the interactions between objects during breakup and we discuss its conditions of validity.

Energy absorption in a bamboo foam

We discuss the ability of soap films to absorb the energy of an impacting projectile. If the impact velocity is large enough, solid objects or water drops pass through the films without breaking them. However, during each impact a small part of the original kinetic energy is extracted, which is revealed by considering collections of parallel films (so-called bamboo foams). Then, the impacting object is observed to stop after crossing numerous films. The total energy absorbed by the foam is found to be the total energy of distortion of the films, integrated over the multiple crossings. The case of inclined films is also considered, and found to affect the trajectory of the projectile.

Splash and anti-splash: Observation and design

We experiment with releasing ethanol droplets, from a specific height, onto both flexible and rigid surfaces. The study of the resulting impact and splashing provides new insights into the mechanisms of the short time-scale dynamics. Tuning and even suppressing a splash can be achieved by the use of elastic membranes with controlled tension. Under certain experimental conditions, i.e., droplet size and velocity of impact, for which a splash occurs after the impact of a droplet on a Petri dish Fig. 1 a , striking changes in the response occur when elastic membranes are used: as the film tension is decreased, the ability of the membrane to deflect during the impact increases until the complete suppression of the splash Fig. 1 b . We also use soft-lithography techniques to manufacture topographically microdecorated surfaces which consist of micropillars arranged on a square lattice with various lattice parameters. The use of these substrates allows us to alter the impact dynamics and, in particular, to achieve the tuning of a splash until the complete inhibition of the ejection of matter during the impact Fig. 2 . Rapidly moving the target surface is another way to control a splash. When the drop impacts a stationary surface, it splashes uniformly Fig. 3 a . However, when the target surface moves here a rotating disk is used , the splash is enhanced in the direction opposed to the motion and attenuated in the direction of motion Fig. 3 b . When a droplet impacts an inclined surface, asymmetric splashing is also observed Fig. 4 . In this example, a drop impacts a solid, smooth, and dry surface slightly above the splashing threshold. Above a critical inclination, splashing becomes suppressed only on the upward side Fig. 4 b . From the experiments presented above, we expect to better understand the fundamental physics of splashing. FIG. 1.

Inertia dominated thin-film flows over microdecorated surfaces

We analyze the inertia dominated flow of thin liquid films on microtextured substrates, which here are assemblies of micron-size posts arranged on regular lattices. We focus on situations for which the thin-film thickness and the roughness characteristic length scale are of the same order of magnitude, i.e., a few hundred microns. We assume that the liquid flows isotropically through the roughness at a flow rate that depends on the geometrical features of the porous layer; above the texture, the flow is characterized by a larger Reynolds number and modeled using a boundary layer approach. The influence of the microtexture on the thin-film flow above the microposts is captured by a reduction of the flow rate due to the leakage flow through the texture and a slip boundary condition, which depends on the flow direction as well as on the lattice properties. In this way, the velocity field in the free surface flow adopts the symmetry of the microtexture underneath. The results of this model are in good agreeme...

Droplet traffic regulated by collisions in microfluidic networks

We study droplet traffic at the inlet node of an asymmetric microfluidic loop. We observe that collisions between successive droplets may occur at the junction. We show that this phenomenon has an impact on the repartition of the droplets in the arms of the loop since it modifies the nature of the collective hydrodynamic feedback mechanism that usually regulates traffic. We present a robust indirect method to measure the excess hydrodynamic resistance added by each droplet to a microfluidic channel, and we rationalize our experimental finding using simple physical arguments.