Ref HAL: hal-01370655_v1
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We study the destabilization mechanism of thin liquid sheets expanding in air and show that dilute oilin-water emulsion-based sheets disintegrate through the nucleation and growth of holes that perforatethe sheet [1]. The velocity and thickness fields of the sheet outside the holes are not perturbed by holesand hole opening follows a Taylor-Culick law. We find that a pre-hole, which widens and thins out thesheet with time, systematically precedes the hole nucleation. The growth dynamics of the pre-holefollows the law theoretically predicted for a liquid spreading on another liquid of higher surfacetension due to Marangoni stresses. Classical Marangoni spreading experiments quantitativelycorroborate our findings.

Ref HAL: hal-01369699_v1
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The interface between a liquid and a fluid affects dramatically both the interactions and the motion of colloidal particles. In this talk, the impact of partial wetting dynamics on the motion of passive and active colloids will be presented. First, experimental results on the Brownian dynamics of micrometric spherical silica colloids andpolymeric ellipsoids trapped at a planar air-water interface will be described. Partial wetting defines a contact angle which sets the immersion depth of the colloid. Particle motion is confined in the interfacial plane. For spherical colloids, the contact angle is finely tuned in the range 30°-140° by surface treatments and measured in situ. Translational and rotational diffusion coefficients of colloids trapped at the water interface are obtained by particle tracking video-microscopy. Counter-intuitively, the friction felt by the colloid increases when the contact angle increases; i.e. when particles are less immersed in water and more in air, which has a negligible viscosity. To explain the slowing down of the translational motion for spheres and rotational diffusion for ellipsoids, an extra friction term originating from contact line fluctuations will be introduced.The second part of the talk deals with the motion of isolated active Janus colloids at the surface of water. Spherical catalytic Janus colloids have been prepared coating half surface of silica particles by a thin platinum layer. Immersion depth of the Janus colloids as well as their orientation with respect to the water surface reveal thecomplex wetting properties of Janus particles. The active motion of Janus colloids at the interface in the presence of various concentration of hydrogen peroxide has been studied. The types of trajectories, directional and circular ones observed revealed the effective force and torque induced by the catalytic decomposition of H2O2. At the water surface, active colloids perform more persistent directional motions as compared to the motions performed in the bulk. This has been interpreted as due to the loss of degrees of freedom resulting from the confinement at interfaceand also to the partial wetting conditions that possibly bring new contributions to the rotational friction at interface.

Ref HAL: hal-01369691_v1
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Janus colloidal particles show remarkable properties in terms of surface activity, self-assembly and wetting. Moreover they can perform autonomous motion if they can chemically react with the liquid in which they are immersed. In order to understand the self-propelled motion of catalytic Janus colloids at the air-water interface, wetting and the orientation of the catalytic surface are important properties to be investigated. Wetting plays a central role in active motion since it determines the contact between fuel and catalytic surface as well as the efficiency of transduction of chemical reaction into motion. Active motion is not expected to occur either when the catalytic face is completely out of the aqueous phase or when the Janus boundaries are parallel to the interfacial plane. The design of a Janus colloid possessing two hydrophilic faces is required to allow the catalytic face to react with the fuel (e.g. H2O2 for Platinum) in water and to permit some rotational freedom of the Janus colloid in order to generate propulsion parallel to the interfacial plane.Here, we discuss some theoretical aspects that should be accounted when studying Janus colloids at the surface of water. The free energy of ideal Janus colloidal particles at the interface is modeled as a function of the immersion depth and the particle orientation. Analytical expressions of the energy profiles are established. Energetic aspects are then discussed in relation to the particle ability to rotate at the interface. By introducing contact angle hysteresis we describe how the effects of contact line pinning modifies the scenario described in the ideal case. Experimental observations of the contact angle hysteresis of Janus colloids at the interface reveal the effect of pinning; and orientations of silica particles half covered with a platinum layer at the interface do not comply with the ideal scenarios. Experimental observations suggest that Janus colloids at the fluid interface behave as kinetically driven system, where the contact line motion over defects decorating the Janus faces rules the orientation and rotational diffusion of the particle.

Ref HAL: hal-01366306_v1
Ref Arxiv: 1607.02271
DOI: 10.1021/acsmacrolett.6b00516
WoS: 000385913800002
Ref. & Cit.: NASA ADS
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6 Citations
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We investigate the nucleation and propagation of cracks in self-assembled viscoelastic fluids, which are made of surfactant micellesreversibly linked by telechelic polymers. The morphology of the micelles can be continuously tuned, from spherical to rodlike towormlike, thus producing transient double networks when the micelles are sufficiently long and entangled and transient singlenetworks otherwise. For a single network, we show that cracks nucleate when the sample deformation rate involved is comparable tothe relaxation time scale of the network. For a double network, by contrast, significant rearrangements of the micelles occuras a crack nucleates and propagates. We show that birefringence develops at the crack tip over a finite length, ξ, whichcorresponds to the length scale over which micelle alignment occurs. We find that ξ is larger for slower cracks, suggesting anincrease of ductility.

Ref HAL: hal-01363227_v1
DOI: 10.1364/OE.24.026887
WoS: 000388414600089
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11 Citations
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An original technique that combines digital holography, dual illumination of the sample and cleaning algorithm 3D reconstruction is proposed. It uses a standard transmission microscopy setup coupled with a digital holography detection. The technique is 4D, since it allows to determine, at each time step, the 3D locations (x, y, z) of many moving objects that scatter the dual illumination beam. The technique has been validated by imaging the microcirculation of blood in a fish larvae sample (the moving objects are thus red blood cells RBCs). Videos showing in 4D the moving RBCs superimposed with the perfused blood vessels are obtained.

Ref HAL: hal-01358945_v1
DOI: 10.1126/sciadv.1500465
WoS: WOS:000380072100001
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25 Citations
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Morphological transformations of living cells, such as shape adaptation to external stimuli, blebbing, invagination, or tethering, result from an intricate interplay between the plasma membrane and its underlying cytoskeleton, where molecular motors generate forces. Cellular complexity defies a clear identification of the competing processes that lead to such a rich phenomenology. In a synthetic biology approach, designing a cell-like model assembled from a minimal set of purified building blocks would allow the control of all relevant parameters. We reconstruct actomyosin vesicles in which the coupling of the cytoskeleton to the membrane, the topology of the cytoskeletal network, and the contractile activity can all be precisely controlled and tuned. We demonstrate that tension generation of an encapsulated active actomyosin network suffices for global shape transformation of cell-sized lipid vesicles, which are reminiscent of morphological adaptations in living cells. The observed polymorphism of our cell-like model, such as blebbing, tether extrusion, or faceted shapes, can be qualitatively explained by the protein concentration dependencies and a force balance, taking into account the membrane tension, the density of anchoring points between the membrane and the actin network, and the forces exerted by molecular motors in the actin network. The identification of the physical mechanisms for shape transformations of active cytoskeletal vesicles sets a conceptual and quantitative benchmark for the further exploration of the adaptation mechanisms of cells.