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Dr. Jean-Christophe Loudet: Mixing ellipsoids with liquid crystals and ellipsoidal dance under laser illumination

Date: 5. 1. 2012
Source: Monday physics colloquium
Ponedeljek, 9. januarja 2012, ob 16:15 v predavalnici F1, FMF UL, Jadranska 19, Ljubljana

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Povzetek:

    Mixing ellipsoids with liquid crystals and ellipsoidal dance under laser illumination

    Dr. Jean-Christophe Loudet, Université de Bordeaux, Centre de Recherche Paul Pascal - CNRS, Pessac, France

In this talk, I will present two recent experimental works dealing with soft matter colloidal systems. The first study reports on the behaviour of non-spherical solid bodies (a few microns in size) embedded in a liquid crystal matrix while the second one, more exploratory, focuses on the optical levitation of the very same particles. Although not tightly correlated, both examples highlight the importance of particle geometry in colloidal systems and this is what I will try to emphasize in this lecture.

            1/ In the first part, as aforesaid, we study the behavior of micrometer-sized prolate ellipsoidal particles dispersed in a nematic liquid crystal. Whereas small aspect ratio ellipsoids aggregate and form anisotropic structures (as observed for spheres), this is no longer the case when the aspect ratio reaches a well-defined value: anisotropic objects can remain homogeneously dispersed in an anisotropic solvent provided they are elongated enough. We put forward simple arguments to rationalize our observations.

            2/ In a second part, we briefly report on the mechanical effects of lights on elongated dielectric particles (the same as those in part 1/). Those particles are not chiral, absorbing or birefringent (or negligibly so), and yet display a variety of dynamical states under laser illumination. Depending on laser beam diameter and ellipsoid aspect ratios, simple static equilibrium, periodic oscillations or irregular (chaotic) motions are observed. Being a preliminary work, we do not have yet a deep understanding of those phenomena and will suggest possible routes.