Diphasic systems in microfluidics

Introduction under construction

Droplets dynamics in a Hele-Shaw cell: lubrication film

PhD of Axel Huerre, Collaboration with O. Theodoly and M..P. Valignat (LAI), I. Cantat (IPR) and A. Leshansky (Technion Univ.)

We study the motion of droplets in a confined, micrometric geometry, by focusing on the lubrication film between droplet and wall. When capillary forces dominate, the lubrication film thickness evolves non linearly with the capillary number due to viscous dissipation between meniscus and wall. However, this film may become thin enough (tens of nanometers) that intermolecular forces come into play and affect classical scalings. We report the novel experimental characterization of two dynamical regimes as the capillary number increases: (i) at low capillary numbers, the film thickness is constant and set by the disjoinging pressure, while (ii) above a critical capillary number, the interface behavior is well described by a Bretherton-like viscous scenario. At a high surfactant concentration, structural effects lead to the formation of patterns on the interface, which can be used to trace the interface velocity. Our experiments yield highly resolved topographies of the shape of the interface and allow us to bring new insights into droplet dynamics in microfluidics.

Huerre et al., Phys. Rev. Lett., 2015.

Huerre et al., Lab. Chip, 2016.

Understanding the stability and dynamics of two phase systems, such as foams and emulsions, in porous media is still a challenge for physicists and calls for a better understanding of the intermolecular interactions between interfaces. In a classical approach, these interactions are investigated in the framework of DLVO theory by building disjoining pressure isotherms. The paper reports on a technique allowing the measurement of disjoining pressure isotherms in a thin liquid film squeezed either by a gas or a liquid phase on a solid substrate. We couple a Reflection Interference Contrast Microscopy (RICM) set-up to a microfluidic channel that sets the disjoining pressure through the Laplace pressure. This simple technique is found to be both accurate and precise. The Laplace pressure mechanism provides extremely stable conditions and offers opportunity for parallelizing experiments by producing several drops in channels of different heights. We illustrate its potential by comparing experimental isotherms for oil - (water and SDS) - glass systems with different models focusing on the electrostatic contribution of the disjoining pressure. The extracted values of the interface potentials are in agreement with the constant surface potential model and with a full computation. The derived SDS surface concentration agrees with values reported in the literature. We believe that this technique is suitable to investigate other working fluids and intermolecular interactions at smaller scales.

Huerre et al., Applied Physics Letters, 2017.

Droplets dynamics in a Hele-Shaw cell:

predicting droplet velocity

PhD of Benjamin Reichert, Collaboration with O. Theodoly and M..P. Valignat (LAI), I. Cantat (IPR) and A. Leshansky (Technion Univ.)

Under construction

Droplet Breakup at a T-junction

in collaboration with Alexander Leshansky, Shariar Afkhami and Patrick Tabeling

We perform experimental studies of droplet breakup in microfluidic T-junctions in a range of capillary numbers lying between 4.10−4 and 2.10−1 and for two viscosity ratios of the fluids forming the dispersed and continuous phases. The present paper extends the range of capillary numbers explored by previous investigators by two orders of magnitude.We single out two different regimes of breakup. In a first regime, a gap exists between the droplet and the wall before breakup occurs. In this case, the breakup process agrees well with the analytical theory of Leshansky and Pismen [Phys. Fluids 21, 023303 (2009)]. In a second regime, droplets keep obstructing the T-junction before breakup. Using physical arguments, we introduce a critical droplet extension for describing the breakup process in this case. This regime has been modeled by A. Leshansky considering a geometrical construction together with a lubrication approach. The numerical simulations provided by S. Afkhami are in agreement with the model, together with the experimental results.

Jullien et al., Phys of Fluids 2009

Afkhami et al. Phys. Rev. Lett., 2012

Obstructed breakup regime :

2D foam coarsening

PhD thesis of Julien Marchalot, in collaboration with Isabelle Cantat and Jérôme Lambert from IPR

We report an experimental study of 2D microfoam coarsening confined in a micrometer scale geometry, the typical bubbles diameter being of the order of 50–100 μm. These experiments raise both fundamental and applicative issues. For applicative issues: what is the typical time of foam ageing (for a polydisperse foam) in microsystems in scope of gas pocket storage in lab-on-achips? Experimental results show that a typical time of 2–3mn is found, leading to the possibility of short-time storing, depending on the application. For fundamental interests, 2D foam ageing is generally described by von Neumann’s law (von Neumann J., Metal Interfaces (American Society of Metals, Cleveland) 1952, p. 108) which is based on the hypothesis that bubbles are separated by thin films. Does this hypothesis still hold for foams confined in a 40 μm height geometry? This

problematic is analyzed and it is shown that von Neumann’s law still holds but that the diffusion coefficient involved in this law is modified by the confinement which imposes a curvature radius at Plateau borders. More precisely, it is shown that the liquid fraction is high on a film cross-section, in contrast with macrometric experiments where drainage occurs. An analytical description of the diffusion is developped taking into account the fact that soap film height is only a fraction of the cell height. While most of microfoams are flowing, the experimental set-up we describe leads to the achievement of a motionless confined microfoam.

Marchalot et al. EPL, 2008

2D foam acoustics

Post-doc of Lorène Champougny, in collaboration with Juliette Pierre and Valentin Leroy

Under construction