The dynamics of colloidal particles at interfaces between two fluids plays a central role in microrheology, encapsulation, emulsification, biofilm formation, water remediation and the interface-driven assembly of materials. Common intuition corroborated by hydrodynamic theories suggests that such dynamics is governed by a viscous force lower than that observed in the more viscous fluid. Here, we show experimentally that a particle straddling an air/water interface feels a large viscous drag that is unexpectedly larger than that measured in the bulk. We suggest that such a result arises from thermally activated fluctuations of the interface at the solid/air/liquid triple line and their coupling to the particle drag through the fluctuation-dissipation theorem. Our findings should inform approaches for improved control of the kinetically driven assembly of anisotropic particles with a large triple-line-length/particle-size ratio, and help to understand the formation and structure of such arrested materials.

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 fluid 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 femtonewtons 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.

We found the effect of a hidden photoalignment of a dye-doped nematic liquid crystal (LC) on a nonphotosensitive polymer surface after polarized irradiation of the cell in the isotropic phase. We observed that irradiation resulted in a uniform planar orientation of the LC after cooling to the mesophase. The direction of a light-induced easy axis on the polymer can be either parallel or perpendicular to the polarization of the incident light, depending on the light intensity. We attribute this behavior to two mechanisms of photoalignment: light-induced adsorption of dye molecules on the substrate, and anisotropic desorption in a previously adsorbed dye layer. The experimental results on photoalignment of a LC on a thin dye film confirm our model.

We studied the effect of light-induced gliding of the easy axis of dye-doped nematic liquid crystal on an aligning polymer surface. The observed drift of the easy axis is over tens of degrees and is caused by light-induced anisotropic adsorption and/or desorption of dye molecules on or from the aligning layer in the presence of light-induced bulk torque. We present a theoretical model that explains the experimental data in terms of the light-induced changes of the adsorbed dye molecules angular distribution due to their exchange with the dye molecules from the liquid crystal bulk.

A series of experiments was carried out to describe the evolution of light-induced anchoring in dye-doped nematic liquid crystals (LCs) at irradiation with polarized light. The experiments included cells filled with a pure pentyl-cyanobiphenyl (5CB) and containing a layer of azo dye deposited on an aligning film, as well as cells filled with azo dye doped 5CB, which allowed us to distinguish the role of "surface" and "bulk" dye molecules in the evolution of light-induced anchoring. Modifications of the spectra of spontaneously adsorbed dye molecules under illumination enabled us to assert that light-induced desorption is a mechanism responsible for producing an easy orientation axis in a dark-adsorbed layer. We found that the evolution of light-induced anchoring involves a competition between light-induced desorption and adsorption of the dye molecules on the aligning surface, and the final anchoring is determined by the total light irradiation dose. These data allowed introducing a theoretical model of light-induced anchoring of dye-doped nematic LCs that quantitatively described the experimental results and portrayed the whole evolution of the dye-doped LC cell at irradiation.

No abstract

Fast surface reorientation induced by a single 4-ns low-energy laser pulse in dye-doped liquid crystals is reported. The reorientation is due to light-induced modification of the surface anisotropy, which affects the liquid crystal's director through the appearance of a preferred direction on the irradiated surface. The detected signals can be interpreted as being the result of light-induced desorption and adsorption of dye molecules.

We have found that producing a light-induced easy axis in a liquid crystal cell doped with methyl red can result in surface director reorientation both toward and outward the polarization of the incident light. This pointed out two different mechanisms of light-induced anchoring in the system. We proposed that these mechanisms are light-induced anisotropy in the adsorbed MR-layer and light-induced adsorption of MR molecules on the substrate. The study of light-induced anchoring in the isotropic phase showed that the mechanism of light-induced anisotropy in adsorbed MR-layer prevails at small light intensities, whereas the mechanisms of light-induced adsorption dominates at high intensities.