Wetting and Contact Lines of Micrometer-sized Ellipsoids
published: Dec. 22, 2007, recorded: November 2007, views: 3667
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We experimentally and theoretically investigated the wetting of a fluid interface on solid micrometer-sized ellipsoidal particles. The latter were obtained from uniaxial stretching of monodisperse polystyrene spheres (radius R = 5m) and the aspect ratio, k, was varied from 1 to about 10. We have demonstrated that such ellipsoids at oil-water interfaces manifest long range capillary interactions of considerable energies, up to about 105 times the thermal energy. The contact line exhibits saddle-like deformations and has a quadrupolar symmetry: the interface is pulled down near the tips of the ellipsoid and pulled up near the middle of the particle. Two ellipsoids attract each other tip to tip or side by side but repel one another when in a side-to-tip configuration. These trends are indeed in line with the general rule according to which the interaction between capillary charges of the same sign is attractive while it is repulsive between charges of opposite signs. To interpret our experimental data, we numerically solved the partial wetting problem on a single ellipsoid and found that the contact line is indeed a saddle-shaped curve with a quadrupolar symmetry. Furthermore, comparisons of experimental and simulated data allowed us to determine contact angles and rather unexpectedly, the latter were found to decrease significantly with increasing ellipsoid aspect ratio.
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