A novel boundary layer sensor utilizing domain switching in ferroelectric liquid crystals

Abstract

A new sensor for optical detection of shear stress field induced by air or gas flow on a rigid surface is reported. The sensor uses the novel effects of shear induced optical switching in ferroelectric liquid crystals. The principle of operation of the sensor is described and a theoretical model for the optical response to shear stress is given. Director dynamics and system-free energy considerations predict a system response time ≊10 ns for an applied shear stress step of ≊700 Torr. A thin (≊1 μm) ferroelectric liquid crystal film coated on a flat glass model surface is exposed to gas flow from a micro wind tunnel attached to the polarizing microscope employed for optical measurements. Schlieren texture of the thin film consists of two stable domains separated by a domain wall. Gas or air flow on the liquid crystal surface induces director reorientation resulting in optical contrast. Transmitted and reflected light intensities from polarization microscopy provide measurement of the flow parameters. System response time τ∼150 μs has been estimated from the material viscosity and the time variation of the applied shear stress in the experiment. Optical response is linear for applied differential pressures up to ≊800 Torr beyond which it tends to saturate. Second-order nonlinear effects are observed for flow rates beyond ≊42 ℓ/min. The configuration used in the present method overcomes many of the limitations of similar measuring techniques including those using cholesteric liquid crystals. The present method offers a preferred alternative for flow visualization and skin friction measurements in wind-tunnel experiments on laminar boundary layer transition investigations.