Liquid crystal droplet production in a microfluidic device

BD Hamlington, B Steinhaus, JJ Feng, D Link, MJ Shelley & AQ Shen

Liq. Cryst. 34, 861-870 (2007).

Abstract - Liquid crystal drops dispersed in a continuous phase of silicone oil are generated with a narrow distribution in droplet size in microfluidic devices both above and below the nematic to isotropic transition temperature. Our experiments show that the surface properties of the channels can be critical for forming droplets. We observe different dynamics in liquid crystal droplet generation, coalescence, and distinct droplet morphology by altering the microchannel surface energy. This is explained by the thermodynamic description of the wetting dynamics of the system. The effect of the nematic-to-isotropic transition on the formation of liquid crystal droplets is also observed and related to the capillary number. We also investigate how the nematic droplet size varies with the flow rate ratio and compare this behavior with a Newtonian reference system. The effect of the defect structures of the nematic liquid crystal can lead to distinctly different scaling of droplet size in comparison to the Newtonian system. When the nematic liquid crystal phase is stretched into a thin filament before entering the orifice, different defect structures and number of defect lines can introduce scatter in the drop size. Capillary instabilities in thin nematic liquid crystal filament has additional contribution from anisotropic effects such as surface gradients of bending stress which can provide extra instability modes compared to that of isotropic fluids.