At the interface, as in the bulk, the Brownian motion rules the diffusion of colloids. The motor of the motion is the thermal energy kT; and the friction coefficient accounts for viscous dissipations at the interface. These physical conditions constrain the colloid to move following random walk trajectories. To observe directional trajectories, overcoming the limitation of Brownian motion, we have investigated Janus Silica-Platinum active colloids. The catalytic motor of the motion in our case is provided by the Platinum face, on which H2O2 (the fuel) decomposes into oxygen gas and water. To study the motion at the interface we have used a particle-tracking video-microscopy method. We start presenting the results on the motion of Silica colloids (Radius = 1 µm) at air-water interface. Then we describe the fabrication of Janus Silica-Platinum colloids. Presenting the results on the active motion, we discuss the effect of H2O2 fuel concentration and the competition between directional and Brownian motion.

In this work, we experimentally investigate the ability of topological defects to guide interfacial assembly of spherical particles with homeotropic anchoring confined to nematic interfaces. We propose two different systems: In the first one, particles are trapped at an air/nematic interface where they spontaneously form various 2D patterns. We demonstrate that the phase transition between these patterns can be controlled by defects formed in the nematic bulk. In the second system, we explore the behavior of particles at the surface of bipolar nematic drops. We found that particles assemble into linear chains and interact with surface defects at the North and South poles of the drop, giving rise to the formation of star structures in a self-assembly process. We detail the mechanism that guides the behavior of particles and discuss the role of defects in the formation of the observed patterns.

In a seminal work, [1] D. R. Nelson proposed that nematic coatings could be used to promote localized functionalization on colloidal particles. Topological defects are indeed necessarily present on genus-0 surfaces and it could be possible to attach ligands to them. Such a realization, however, still remains an experimental challenge. As a first step, the fabrication of thin nematic shells from double emulsions drops was reported a few years ago in Ref. [2]. Since then, the equilibrium structure of the topological defects on a sphere has been studied in details both theoretically and experimentally. It not only depends strongly on the homogeneity of the shell thickness [3,4] but also on the nematic elastic constants [4,5]. The defects structure on a nematic shell can therefore be finely controlled. As a next step, we have recently examined, both experimentally and theoretically, how a small number of solid particles can be trapped on nematic shells[6]. We have used water/5CB/water double emulsions produced by microfluidics techniques. The anchoring of the nematic at the water-5CB interface is strongly planar due to the presence of polyvinyl alcohol in aqueous solutions. The colloids are silica beads treated with silane to provide a homeotropic alignment. We especially have examined how the particles interact with the topological defects, where they get trapped, and how the equilibrium textures depend on their size. A great diversity of defect structures and patterns, different from the ones known in simple nematic shells, can be observed. I will describe their main features and explain how they result from the competition between elasticity and capillarity effects, during the formation of the shells. I will show how this last mechanism can be used to control the position of ligands on a spherical shell.

The fabrication of novel materials with revolutionary technological applications, such as photonic crystals with large band gaps or metamaterials, could perhaps be based on colloidal particles. However, common particles lack the ability to interact along specic directions, which is needed to build colloidal crystals with desired structures. A possibility is to use either anisotropic building blocks, or to induce tunable directionality and controllable valence of building blocks [1]. Liquid crystal shells oer a way to induce controllable valence by using double emulsions of nematic liq- uid crystal in a host liquid [2,3]. The nematic liquid crystal is an anisotropic medium with local orientational order that exhibits topological singularities (defects). And it is by functionalizing these singularities with linkers, that one could induce directional interactions and bonds between the colloidal particles. Here, we study theoretically and experimentally, the behaviour of microspheres trapped to a spher- ical nematic shell [4]. We demonstrate dierent { director and defect { organizations around a sin- gle particle, such as congurations where the colloidal particle is accompanied by an escaped

In many aspects, colloids behave as big atoms [1]; however, the usual absence of directionality in the interaction between colloids has limited the complexity of the structures that they can spontaneously form [2]. As a result, low-coordination structures, common in atomic and molecular systems, are rare in the colloidal domain. One way to address this is to exploit the anisotropy that spontaneously arises when a colloidal particle is coated by a nematic liquid crystal. In this spherical geometry, the orientational molecular order of the nematic liquid crystal is disrupted by the presence of singularities or topological defects, which appear symmetrically organized on the particle surface. These topological defects are not only mathematic concepts, but also high energy spots suitable for chemical attack that could be functionalized with ligands and act as attractive patches between particles [3,4]. The number and arrangement of these defects can vary, providing flexibility for tuning directional interactions. We have recently shown that these defects can be engineered to emulate the linear, trigonal and tetrahedral geometries of sp, sp2, and sp3carbon bonds [5]. These symmetries represent just a small sample of the variety of configurations that we can generate by tuning parameters such as shell geometry, temperature, or symmetry of the liquid crystal phase, which has revealed a vast playground for the formation of complex colloidal superlattices [5,6].

We report theoretical predictions and measurements of the capillary force acting on a spherical colloid smaller than the capillary length that is placed on a curved uid interface of arbitrary shape. By coupling direct imaging and interferometry, we are able to measure the in situ colloid contact angle and to correlate its position with respect to the interface curvature. Extremely tiny capillary forces down to femto-Newton can be measured with this method. Measurements agree well with a theory relating the capillary force to the gradient of Gaussian curvature and to the mean curvature of the interface prior to colloidal deposition. Numerical calculations corroborate these results.

A seminal paper [D. R. Nelson, Nano Lett., 2002, 2, 1125.] has proposed that a nematic coating could be used to create a valency for spherical colloidal particles through the functionalization of nematic topological defects. Experimental realizations however question the complex behaviour of solid particles and defects embedded in such a nematic spherical shell. In order to address the related topological and geometrical issues, we have studied micrometer-sized silica beads trapped in nematic shells. We underline the mechanisms that strongly modify the texture of the simple (particle-free) shells when colloidal particles are embedded. Finally, we show how the coupling between capillarity and nematic elasticity offers new ways to control the valence and directionality of shells.