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Résumé:

Colloidal particles trapped at the interface between two immiscible fluids usually display various bidimensional phases depending on the competition between direct and capillary interactions [1]. On the other hand, the physics of colloids confined in a nematic cell has been thoroughly investigated in the recent years. The structures formed by embedded solid spheres strongly depend on the nematic elasticity, the nematic topological defects and the anchoring at interfaces. Only few studies are devoted to colloids at nematic interfaces [2-5] where these interactions combine. Here, we examine the behavior of spherical solid particles at nematic liquid crystal (NLC) interfaces with different geometries. First, we will detail the phase behavior of beads with a strong homeotropic anchoring at an air-NLC interface [6]. Depending on their area density, the nematic thickness and the anchoring, colloids spontanously form various structures (Fig.1). Using optical tweezers, we determine the pair potential and we probe the different roles of capillary and elastic forces. Subsequently, we report the behavior of silica beads at more complex NLC interfaces. By controlling the geometry of the interfaces and the colloids density with microfluidic techniques, several structures have been observed. The competition between anchoring conditions and liquid crystal elasticity, but also the topology of the curved surface is responsible of long-range interactions and the formation of new ordered structures. In particular we examine how the behavior of colloids trapped at the interface of a curved nematic interface modifies the overall director field (Fig.2).