Nematic director reorientation at solid and liquid interfaces under flow: SAXS studies in a microfluidic device

Area of Science:

• Soft Matter Physics
• Fluid Dynamics
• Materials Science

Background:

• Understanding liquid crystal behavior under flow and confinement is crucial for advanced material applications.
• The interplay between hydrodynamic forces and surface interactions dictates the orientation of liquid crystal molecules.

Purpose of the Study:

• To investigate how flow rate and boundary conditions influence the orientation field of thermotropic nematic liquid crystals in microfluidic channels.
• To explore the director orientation under hydrodynamic focusing in confined geometries.

Main Methods:

• Utilized synchrotron small-angle X-ray-scattering (SAXS) for high-resolution structural analysis.
• Conducted experiments in a microfluidic device with a square-channel cross-section and employed hydrodynamic focusing.
• Measured nematic director orientation projected onto the velocity/velocity gradient plane using a 2D detector.

Main Results:

• At moderate-to-high flow rates, nematic director aligns with flow, exhibiting a small tilt in the velocity gradient direction, consistent with Ericksen-Leslie-Parodi (ELP) theory.
• At low flow rates, boundary conditions become dominant, leading to an escaped radial director configuration.
• Hydrodynamic focusing shifts director orientation towards boundaries, with ambiguity regarding tilt angle variation within the confined nematic sheet.

Conclusions:

• Demonstrated the significant influence of both flow and boundary conditions on nematic liquid crystal orientation in microfluidic systems.
• The presented SAXS technique offers a powerful tool for high-throughput assessment of surfactants and understanding defect structures in liquid crystals.
• Findings have implications for boundary lubrication and the development of liquid crystal displays (LCDs).