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by solid substrates, the particle creates a hyperbolic hedgehogdefect, usually located near the bead (at a distance of order of its size) and the pair particle-defectforms a neutral unit, stable in time. Here we show theoretically and experimentally how capillaryeffects strongly modify the behaviour of particles trapped in a thin nematic film with hybrid anchoringconditions at free surfaces. For a certain range of thickness values (films thinner than the particles’size) two new interesting patterns are formed by isolated particles: the giant dipole [3] and the“butterfly” texture (see Fig.1). In the giant dipole, a micron-sized sphere is accompanied by a pointdefect which is located at a distance up to several hundreds microns. The situation is quite different inthe “butterfly” texture: the particle still produces an accompanying defect in its close neighbourhood,but a p-wall is formed on the opposite side. Using spatially resolved retardation and easy-axis maps,we analysed quantitatively and separately the 2D interfaces deformation and the nematic textures.Both behaviours are due to the same axisymmetric capillary deformation of the thin film around thebeads but with different boundary conditions for the polar 2D c-director. Using a simple 2D Ansatz,we were able to reproduce the c-director patterns found in the films. The local 3D textures have beenalso investigated in the framework of the Landau-de Gennes theory. Although capillary interactionsbetween inclusions in a thin film are always attractive [4], these new spontaneous organizations in thinnematic films offer new ways to self-assemble complex colloidal systems in 2D.Figure1: The two different birefringence patterns formed by microparticles trapped in a thin nematicfilm, observed a) between crossed polarizers and b) with an Abrio birefringence measurement system.[1] I. Musevic, M.Skarabot, U.Tkaler, M.Ravnik and S.Zumer, Science, 313,954 (2006).[2] P.Poulin, H.Stark, T.Lubensky and D.Weitz, Science, 275,1770 (1997).[3] H.Jeridi, M.A.Gharbi, T.Othman, C.Blanc, Proc Natl Acad Sci USA, 112,14771 (2015).[4] P.A. Kralchevsky, K. Nagayama K, Adv Colloid Interface Sci , 85, 145–192 (2000).

Ref HAL: hal-01371450_v1
Ref Arxiv: 1511.03573
DOI: 10.1103/PhysRevE.94.032605
WoS: 000383878500004
Ref. & Cit.: NASA ADS
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16 Citations
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The growing sluggishness of glass-forming liquids is thought to be accompanied by growing structural order. The nature of such order, however, remains hotly debated. A decade ago, point-to-set (PTS) correlation lengths were proposed as measures of amorphous order in glass formers, but recent results raise doubts as to their generality. Here, we extend the definition of PTS correlations to agnostically capture any type of growing order in liquids, be it local or amorphous. This advance enables the formulation of a clear distinction between slowing down due to conventional critical ordering and that due to glassiness, and provides a unified framework to assess the relative importance of specific local order and generic amorphous order in glass formation.

Commentaires: 12 pages, 8 figures; v2: Fig.4 added, presentation improved. Data relevant to this work can be accessed at http://doi.org/10.7924/G8BG2KWP. Réf Journal: Phys. Rev. E 94, 032605 (2016)

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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.

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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-01368422_v1
DOI: 10.1002/chem.201602746
WoS: 000382925000014
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The highly constrained b-amino acid ABOC induces different types of helices in b urea and 1:1 a/b amide oligomers. The latter can adopt 11/9- and 18/16-helical folds depending on the chain length in solution. Short peptides alternating proteinogenic a-amino acids and ABOC in a 2:1 a/b repeat pattern adopted an unprecedented and stable 12/14/14-helix. The structure was established through extensive NMR, molecular dynamics, and IR studies. While the 1:1 a-AA/ABOC helices diverged from the canonical a-helix, the helix formed by the 9-mer 2:1 a/b-peptide allowed the projection of the a-aminoacid side chains in a spatial arrangement according to the a-helix. Such a finding constitutes an important step toward the conception of functional tools that use the ABOC residue as a potent helix inducer for biological applications.