Orientation Behavior of Nematic Liquid Crystals at Flow-Wall Interfaces in Microfluidic Channels

This work characterizes the orientation behavior of nematic liquid crystals in pressure-driven flows of microfluidic channels at interfaces between the flow and microchannel walls. The impact of flow velocity and microchannel geometry on the orientation of liquid crystals in single-phase and two-phase flows is discussed. Polarizing optical microscopy images revealed the homeotropic orientation of liquid crystal molecules at microchannel walls at zero flow velocities, which gradually transitioned into planar alignment along the microchannel axis when the flow velocity increased in the 50 μm/s to 5 mm/s range. Liquid crystal droplets demonstrated homeotropic or planar alignment depending on the sizes of droplets and flow velocities. The polarized light pattern from homeotropically aligned droplets deposited on microchannel walls was found to be logarithmically proportional to the flow velocity in the 2 to 40 mm/s range. The revealed behavior of nematic liquid crystals at microchannel wall surfaces in dynamic flow conditions offers new tools for on-demand control of the optical properties of microfluidic devices and can contribute to the development of analytical lab-on-chip tools with internal continuous or discrete liquid crystal layers for flow characterization in microchannel confinement.