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

Abstract

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.

Dynamic Flow Control over Optical Properties of Liquid Crystal–Quantum Dot Hybrids in Microfluidic Devices

“…This work continues our research in nematic LC systems [47] and LC-QD composites [43,48] at the microscale and macroscale. In our previous works, we focused on analyzing the impact of flow conditions in microchannels on the orientation behavior of liquid crystals in single-phase and multiphase microflows and characterized the sensitivity of such systems to variable flows in polarized light.…”

Emulating paper: a review of reflective display technologies

“…The functional mechanism intricately controls the orientation of nematic LC molecules using a precisely patterned grating surface. 56 This grating, characterized by its distinct grooves, shapes the energy landscape, leading to the elastic deformation of the LCs and the formation of disclinations, or irregular alignment points, known as defects. 57 These disclinations stabilize the LC molecules in two distinct states: the defect (D) state, in which molecules are aligned at a low tilt, and the continuous (C) state, characterized by near-vertical alignment.…”

“…57 These disclinations stabilize the LC molecules in two distinct states: the defect (D) state, in which molecules are aligned at a low tilt, and the continuous (C) state, characterized by near-vertical alignment. 56 Central to the switching mechanism between these bistable states is the flexoelectric effect, a result of the significant elastic deformation around the grating grooves. 58,59 This effect generates polarization that responds to external electrical pulses, facilitating the controlled reorientation of the LC molecules between the D and C states.…”

“…The functional mechanism intricately controls the orientation of nematic LC molecules using a precisely patterned grating surface 56 . This grating, characterized by its distinct grooves, shapes the energy landscape, leading to the elastic deformation of the LCs and the formation of disclinations, or irregular alignment points, known as defects 57 .…”

“…This grating, characterized by its distinct grooves, shapes the energy landscape, leading to the elastic deformation of the LCs and the formation of disclinations, or irregular alignment points, known as defects 57 . These disclinations stabilize the LC molecules in two distinct states: the defect (D) state, in which molecules are aligned at a low tilt, and the continuous (C) state, characterized by near-vertical alignment 56 . Central to the switching mechanism between these bistable states is the flexoelectric effect, a result of the significant elastic deformation around the grating grooves 58 , 59 .…”