No abstract

A high surface area-to-volume ratio in microchannels increases the importance of surface interactions within them. In layered liquids, such as smectic liquid crystals, surface interactions play an important role in the formation of defect textures. We use 8CB liquid crystal, which is in the smectic-A phase at room temperature, as a model layered liquid. PDMS surfaces can be tuned to be hydrophilic or hydrophobic, and due to the nature of liquid crystalline molecules, we show that this results in planar or homeotropic anchoring conditions, respectively. In a confined system, contrary to the bulk, generated defects cannot grow freely. In the present work, we show that the confinement offered by PDMS microchannels along with the capability of creating mixed anchoring conditions within them results in the formation of particular ordered defect textures through increased surface interactions in smectic-A liquid crystals. Our observations imply that microscale confinement is useful for controlling the size, size distribution, and packing structure of microscale defect structures within these materials. In addition, we show that by placing a droplet of smectic-A liquid crystal on a PDMS surface containing microscale parallel cracks, ordered focal conic defects form between two adjacent cracks. The distance between two adjacent cracks dictates the size of the defects. These observations could lead to useful ideas for exploring new technologies for flexible optical devices or displays that utilize smectic-A liquid crystals.

We study the capillary interactions between ellipsoidal Janus particles adsorbed at flat liquid-fluid interfaces. In contrast to spherical particles, Janus ellipsoids with a large aspect ratio or a small difference in the wettability of the two regions tend to tilt at equilibrium. The interface deforms around ellipsoids with tilted orientations and thus results in energetic interactions between neighboring particles. We quantify these interactions through evaluation of capillary energy variation as a function of the spacing and angle between the particles. The complex meniscus shape results in a pair interaction potential which cannot be expressed in terms of capillary quadrupoles as in homogeneous ellipsoids. Moreover, Janus ellipsoids in contact exhibit a larger capillary force at side-by-side alignment compared to the tip-to-tip configuration, while these two are of comparable magnitude for their homogeneous counterparts. We evaluate the role of particles aspect ratio and the degree of amphiphilicity on the interparticle force and the capillary torque. The energy landscapes enable prediction of micromechanics of particle chains, which has implications in predicting the interfacial rheology of such particles at fluid interfaces.