Orientation Properties of a Nematic Liquid Crystal in Two-Phase Microfluidic Systems

Abstract

In this work, we study orientation properties of nematic liquid crystal systems confined in microfluidic droplets by polarizing optical microscopy. We analyze microfluidic flow conditions that allow for controlling formation of disperse systems in liquid crystal and water flows. The research revealed a different formation behavior of microdroplets depending on the type of disperse systems. In continuous medium of liquid crystals, its molecules tend to align with respect to microchannel walls in tilted position that is close to perpendicular in the presence of added surfactants. Such anchoring was shown to be typical for slow flow below 0.05 mm/s. At higher flow velocities, shear forces realign liquid crystal molecules with respect to the flow axis in a close-to-planar orientation. In small immobilized liquid crystal droplets with a diameter less than half width of the microchannel, a typical orientation of molecules perpendicular to the interfacial boundary is observed at crossed polarizers. At flow velocities up to 10 mm/s, the alignment texture was shown to deform proportionally to the flowrate. In large and long immobile droplets, orientation of the liquid crystal dispersed phase was shown to be similar to that of a single phase system. At average flow velocities above 0.1 mm/s, convection in droplets initiated a transition to irregular dynamics of liquid crystal domains. The behavior of the liquid crystal phase demonstrated in this work allows to perform tailored control of properties of microfluidic liquid crystal droplets and propose them for further potential applications as flow sensors in “lab-on-chip” instruments.