Wetting Transition on Hydrophobic Surfaces Covered by Polyelectrolyte Brushes

International audienceWe study the wetting by water of complex “hydrophobic-hydrophilic” surfaces made of a hydrophobic substrate covered by a hydrophilic polymer brush. Polystyrene (PS) substrates covered with polystyrene-block-poly(acrylic acid) PS-b-PAA diblock copolymer layers were fabricated by Langmuir-Schaefer depositions and analyzed by atomic force microscopy (AFM) and ellipsometry. On bare PS substrate, we measured advancing angles θA ) 93 ( 1° and receding angles θR ) 81 ( 1°. On PS covered with poorly anchored PS-b-PAA layers, we observed large contact angle hysteresis, θA ≈ 90° and θR ≈ 0°, that we attributed to nanometric scale dewetting of the PS-b-PAA layers. On well-anchored PS-b-PAA layers that form homogeneous PAA brushes, a wetting transition from partial to total wetting occurs versus the amount deposited: both θA and θR decrease close to zero. A model is proposed, based on the Young-Dupre´ equation, that takes into account the interfacial pressure of the brush Π, which was determined experimentally, and the free energy of hydration of the polyelectrolyte monomers ΔGPAA hyd , which is the only fitting parameter. With ΔGPAA hyd ≈ -1300 J/mol, the model renders the wetting transition for all samples and explains why the wetting transition depends mainly on the average thickness of the brush and weakly on the length of PAA chains

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

Cell motility in confinement: a computational model for the shape of the cell

While cells typically tend to spread their cytoplasm in a flat and thin lamellipodium when moving on a flat substrate, it is widely observed that the cytoplasm has a compact shape in micro-channels, tending to fulfill the cross-section of the microchannel. We propose a minimal mathematical model for a 2D test case which describes the cell lamellipodium deformations when confined in a channel. We then go through a numerical investigation of this mathematical model and show that it allows to recover qualitatively the physiological characteristics of the confined cell

Cell motility in confinement: a computational model for the shape of the cell

International audienceWhile cells typically tend to spread their cytoplasm in a flat and thin lamellipodium when moving on a flat substrate, it is widely observed that the cytoplasm has a compact shape in micro-channels, tending to fulfill the cross-section of the microchannel. We propose a minimal mathematical model for a 2D test case which describes the cell lamellipodium deformations when confined in a channel. We then go through a numerical investigation of this mathematical model and show that it allows to recover qualitatively the physiological characteristics of the confined cell.Alors que les cellules ont généralement tendance à présenter un cytoplasme étendu en un large lamellipode extrêmement fin lors du déplacement sur un substrat plat, il est communément observé que le cytoplasme prend une forme compacte lors du déplacement dans des micro-canaux, remplissant au possible le volume contenu dans le micro-channel. Nous proposons un modèle mathématique minimal pour un cas test en 2D qui décrit les déformations du lamellipode en confinement. Nous proposons une exploration numérique de ce modèle mathématique et nous montrons qu’il permet de retrouver qualitativement les caractéristiques physiologiques de la cellule confinée

Dynamics of a 2D droplet in a Hele-Shaw cell (presenter B. Reichert)

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

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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

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