Droplets in Microchannels: Dynamical Properties of the Lubrication Film

International audienceWe 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. Our experiments yield highly resolved topographies of the shape of the interface and allow us to bring new insights into droplet dynamics in microfluidics. We report the novel 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 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 that yield direct confirmation of boundary condition in viscous regime. The dynamics of a droplet confined between solid walls and pushed by a surrounding liquid is an old problem, however recent theories are still being developed to describe unexplored regimes and experimental characterizations are still lacking to shed light on these novel developments. A complete understanding of the droplet velocity calls for accurate knowledge of the dissipation mechanisms involved, particularly in the lubrication film. Our understanding of the lubrication properties of menisci travelling in confined geometries has been steadily refined since the pioneering work of Taylor & Saffman [1]. Notably, the influence of the lubrication film left along the wall by the moving meniscus was first taken into account by Bretherton, who investigated the motion of an inviscid bubble in a cylindrical tube [2]. Far from the meniscus, this dynamical film reaches a uniform thickness h ∞ , related to the bubble velocity through the capillary number Ca = µ f U d /γ, where U d is the bubble velocity , µ f the viscosity of the continuous phase, and γ the surface tension. When the capillary pressure dominates over the viscous stress, i.e. in the regime where Ca 1, the thickness of the film follows h Breth = 1.34 r Ca 2/3 , where r is the radius of the capillary tube. Besides bubbles , the case of viscous droplets remains however largely unexplored. A recent theoretical advance in the field by Hodges et al. [3] shows by numerical calculations of the whole flow pattern that significant corrections in the thickness of lubrication films can arise at very low Ca. Furthermore, the regime of the Bretherton theory is only valid for lubrication films thicker than the molecular sizes or than the range of interfacial interactions. The typical velocities and lengthscales involved in droplet-based microfluidics would lead to lubrication films h ∞ ∼

New modeling of reflection interference contrast microscopy including polarization and numerical aperture effects: application to nanometric distance measurements and object profile reconstruction.

International audienceWe have developed a new and improved optical model of reflection interference contrast microscopy (RICM) to determine with a precision of a few nanometers the absolute thickness h of thin films on a flat surface in immersed conditions. The model takes into account multiple reflections between a planar surface and a multistratified object, finite aperture illumination (INA), and, for the first time, the polarization of light. RICM intensity I is typically oscillating with h. We introduce a new normalization procedure that uses the intensity extrema of the same oscillation order for both experimental and theoretical intensity values and permits us to avoid significant error in the absolute height determination, especially at high INA. We also show how the problem of solution degeneracy can be solved by taking pictures at two different INA values. The model is applied to filled polystyrene beads and giant unilamellar vesicles of radius 10-40 microm sitting on a glass substrate. The RICM profiles I(h) can be fitted for up to two to three oscillation orders, and extrema positions are correct for up to five to seven oscillation orders. The precision of the absolute distance and of the shape of objects near a substrate is about 5 nm in a range from 0 to 500 nm, even under large numerical aperture conditions. The method is especially valuable for dynamic RICM experiments and with living cells where large illumination apertures are required

Laplace pressure based disjoining pressure isotherm in non symmetric conditions

International audienceUnderstanding 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 Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory by building disjoining pressure isotherms. This paper reports on a technique allowing the measurement of disjoining pressure isotherms in a thin liquid film squeezed by either a gas or a liquid phase on a solid substrate. We couple a Reflection Interference Contrast Microscopy 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 sodium dodecyl sulfate (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 for investigating other working fluids and intermolecular interactions at smaller scales

Topography of the lubrication film under a pancake droplet travelling in a Hele-Shaw cell

International audienceUnderstanding the dynamics of a droplet pushed by an external fluid in a confined geometry calls for the identification of all the dissipation mechanisms at play in the lubrication film between droplet and cell wall. Experimentally, reflection interference contrast microscopy has proven an efficient tool to measure the thickness of such lubrication films for microfluidic droplets, with a precision of a few nanometres (Huerre et al., Lab on a Chip, vol. 16 (5), 2016, pp. 911-916). The present work takes advantage of the high accuracy of this technique to chart quantitatively the lubrication film between oil droplets and the glass wall of a microfluidic chamber. We find that the lubrication films exhibit a complex three-dimensional shape, which we are able to rationalize using a hydrodynamical model in the lubrication approximation. We show that the complete topography cannot be recovered using a single model boundary condition along the whole interface. Rather, surface tension gradients are negligible at the front of the droplet, whereas they significantly modify the film profile at the rear, where surfactant accumulation induces local thickening of the lubrication film. The presence of ravines on the sides of the droplet is due to three-dimensional effects which can be qualitatively reproduced numerically. To our knowledge, this is the first experimental investigation of such local effects on travelling droplets

Like a rolling droplet : dynamical properties of the lubrication film in confined microchannels

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Sensitive detection of ultra-weak adhesion states of vesicles by interferometric microscopy

International audienceWe have used an original analysis of reflection interference contrast microscopy (RICM) to detect an ultra-weak specific interaction between a glycolipid vesicle and a lectin-coated substrate. The membrane height fluctuations in the contact zone are observed with a high illumination aperture; the membrane profile and the membrane-substrate distance are quantitatively determined using the new analysis, which accounts for multiple interfaces and multiple incidence rays. We showed that this refined version of RICM theory is necessary, specifically in the case of intermediate membranesubstrate distance (30 nm) and helped to discriminate between the ultra-weak interaction and pure gravitational sedimentatio

Lymphocytes can self-steer passively with wind vane uropods

International audienceA wide variety of cells migrate directionally in response to chemical or mechanical cues, however the mechanisms involved in cue detection and translation into directed movement are debatable. Here, we investigate a model of lymphocyte migration on the inner surface of blood vessels. Cells orient their migration against fluid flow, suggesting the existence of an adaptive mechano-tranduction mechanism. We find that flow detection may not require molecular mechano-sensors of shear stress and detection of flow direction can be achieved by the orientation in the flow of the non-adherent cell rear, the uropod. Uropods act as microscopic wind vanes that can transmit detection of flow direction into cell steering via the on-going machinery of polarity maintenance, without need for novel internal guidance signalling triggered by flow. Contrary to chemotaxis, which implies active regulation of cue-dependant signalling, upstream flow mechanotaxis of lymphocytes may only rely on a passive self-steering mechanism

Microfluidic tools to investigate pathologies in the blood microcirculation

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