laboratoire de physique statistique
laboratoire de physique statistique


Inverse Leidenfrost Effect: Levitating Drops on Liquid Nitrogen - Adda-Bedia, M. and Kumar, S. and Lechenault, F. and Moulinet, S. and Schillaci, M. and Vella, D.
LANGMUIR 324179-4188 (2016) 

Abstract : We explore the interaction between a liquid drop (initially at room temperature) and a bath of liquid nitrogen. In this scenario, heat transfer occurs through film-boiling: a nitrogen vapor layer develops that may cause the drop to levitate at the bath surface. We report the phenomenology of this inverse Leidenfrost effect, investigating the effect of the drop size and density by using an aqueous solution of a tungsten salt to vary the drop density. We find that (depending on its size and density) a drop either levitates or instantaneously sinks into the bulk nitrogen. We begin by measuring the duration of the levitation as a function of the radius R and density rho(d) of the liquid drop. We find that the levitation time increases roughly linearly with drop radius but depends weakly on the drop density. However, for sufficiently large drops, R >= R-c(rho(d)), the drop sinks instantaneously; levitation does not occur. This sinking of a (relatively) hot droplet induces film-boiling, releasing a stream of vapor bubbles for a well-defined length of time. We study the duration of this immersed-drop bubbling finding similar scalings (but with different prefactors) to the levitating drop case. With these observations, we study the physical factors limiting the levitation and immersed-film-boiling times, proposing a simple model that explains the scalings observed for the duration of these phenomena, as well as the boundary of (R,rho(d)) parameter space that separates them.
Popping Balloons: A Case Study of Dynamical Fragmentation - Moulinet, Sebastien and Adda-Bedia, Mokhtar

Abstract : Understanding the physics of fragmentation is important in a wide range of industrial and geophysical applications. Fragmentation processes involve large strain rates and short time scales that take place during crack nucleation and propagation. Using rubber membranes, we develop an experimental analysis that enables us to track the fragmentation process in situ in both time and space. We find that bursting a highly stretched membrane yields a treelike fragmentation network that originates at a single seed crack, followed by successive crack tip-splitting events. We show that a dynamic instability drives this branching mechanism. Fragmentation occurs when the crack tip speed attains a critical velocity for which tip splitting becomes the sole available mechanism of releasing the stored elastic energy. Given the general character of the fragmentation processes, this framework should be applicable to other crack networks in brittle materials.
Viscous fingering at ultralow interfacial tension - Setu, Siti Aminah and Zacharoudiou, Ioannis and Davies, Gareth J. and Bartolo, Denis and Moulinet, Sebastien and Louis, Ard A. and Yeomans, Julia M. and Aarts, Dirk G. A. L.
SOFT MATTER 910599-10605 (2013) 

Abstract : We experimentally study the viscous fingering instability in a fluid-fluid phase separated colloid-polymer mixture by means of laser scanning confocal microscopy and microfluidics. We focus on three aspects of the instability. (i) The interface between the two demixed phases has an ultralow surface tension, such that we can address the role of thermal interface fluctuations. (ii) We image the interface in three dimensions allowing us to study the interplay between interface curvature and flow. (iii) The displacing fluid wets all walls completely, in contrast to traditional viscous fingering experiments, in which the displaced fluid wets the walls. We also perform lattice Boltzmann simulations, which help to interpret the experimental observations.
Drop Formation in Non-Newtonian Fluids - Aytouna, Mounir and Paredes, Jose and Shahidzadeh-Bonn, Noushine and Moulinet, Sebastien and Wagner, Christian and Amarouchene, Yacine and Eggers, Jens and Bonn, Daniel

Abstract : 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.
Effect of wetting on capillary pumping in microchannels - Javadi, Arman and Habibi, Mehdi and Taheri, Fereshte Samadi and Moulinet, Sebastien and Bonn, Daniel

Abstract : We investigate capillary pumping in microchannels both experimentally and numerically. Putting two droplets of different sizes at the in/outlet of a microchannel, there will in general be a flow from the smaller droplet to the larger one due to the Laplace pressure difference. We show that an unusual flow from a larger droplet into a smaller one is possible by manipulating the wetting properties, notably the contact line pinning. In addition, we propose a way to actively control the flow by electrowetting.
Spooling and disordered packing of elastic rods in cylindrical cavities - Pineirua, M. and Adda-Bedia, M. and Moulinet, S.
EPL 104 (2013) 

Abstract : The compaction of elastic rods in rigid cylindrical cavities is experimentally performed. The results show two main packing behaviours: an ordered regime in which the rod spools on the internal surface of the cavity and a disordered phase where the orientation of the coils is randomly distributed. The phase diagram separating these two packing configurations is determined as a function of the aspect ratio of the container and of the intrinsic curvature of the rod. A theoretical stability analysis and an experimental study of the dynamics of the rod at the injection point allow to describe different instability mechanisms that drive the transitions from ordered to disordered packing, leading to the identification of the different disordering scenarios. Copyright (C) EPLA, 2013
Height fluctuations of a contact line: A direct measurement of the renormalized disorder correlator - Le Doussal, P. and Wiese, K. J. and Moulinet, S. and Rolley, E.
EPL 87 (2009) 

Abstract : We measure the center-of-mass fluctuations of the height of a contact line at depinning for two different systems: liquid hydrogen on a rough cesium substrate and isopropanol on a silicon wafer grafted with silanized patches. The contact line is subject to a confining quadratic well, provided by gravity. From the second cumulant of the height fluctuations, we measure the renormalized disorder correlator Delta(u), predicted by Functional RG to attain a fixed point, as soon as the capillary length is large compared to the Larkin length set by the microscopic disorder. The experiments are consistent with the asymptotic form for Delta(u) predicted by Functional RG, including a linear cusp at u = 0. The observed small deviations could be used as a probe of the underlying physical processes. The third moment, as well as avalanche-size distributions are measured and compared to predictions from Functional RG. Copyright (C) EPLA, 2009
Life and death of a fakir droplet: Impalement transitions on superhydrophobic surfaces - Moulinet, S. and Bartolo, D.

Abstract : We show that the equilibrium state of a water drop deposited on a superhydrophobic surface cannot be solely determined by its macroscopic contact angle but also depends on the drop size. Following the evolution of the interface of evaporating droplets, we demonstrate that the liquid can explore a succession of equilibrium conformations which are neither of the usual fakir nor Wenzel types. A comprehensive description of the transition between these wetting states is provided. To do so, we have taken advantage of microfabrication techniques and interference microscopy which allows for the ``3D'' imaging of the liquid interface. In addition, we propose a simple theoretical description of the interface geometry which goes beyond the standard two-state picture for superhydrophobicity. This model accounts correctly for all our experimental observations. Finally, guided by potential microfluidic applications we propose an efficient design strategy to build robust liquid repellant surfaces.