Devendra S. Parmar

A shear sensitive monomer‐polymer liquid crystal system for wind tunnel applications

Characteristics of a liquid crystal system, comprised of a shear sensitive cholesteric monomer liquid crystal thin‐film coated on a liquid crystal polymer substrate, are described. The system provides stable Grandjean texture, a desirable feature for shear stress measurements using selective reflection from the monomer liquid crystal helix structure. Impingement of gas or air flow on the monomer liquid crystal free surface changes the wavelength of the selective reflection for an incident white light from red towards blue with increase in the rate of gas flow. The contrast of the selectively reflected light improves considerably by providing a thin (∼5 μm) black coating at the monomer‐polymer interface. The coating thickness is such that the steric interactions are still sufficiently strong to maintain Grandjean texture. For a small angle of incidence (∼15°) of a monochromatic light, the measurement of the reflected light intensity normal to the monomer‐polymer liquid crystal interface enables us to deter...

Partially exposed polymer dispersed liquid crystals for boundary layer investigations

A new configuration termed partially exposed polymer dispersed liquid crystal in which the liquid crystal microdroplets dispersed in a rigid polymer matrix are partially entrapped on the free surface of the thin film deposited on a glass substrate is reported. Optical transmission characteristics of the partially exposed polymer dispersed liquid crystal thin film in response to an air flow induced shear stress field reveal its potential as a sensor for gas flow and boundary layer investigations.

Pressure dependence of the electro‐optic response function in partially exposed polymer dispersed ferroelectric liquid crystals

Ferroelectric liquid crystals in a new configuration, termed partially exposed polymer dispersed ferroelectric liquid crystal (PEPDFLC), respond to external pressures and demonstrate pressure‐induced electro‐optic switching response. When the PEPDFLC thin film is sandwiched between two transparent conducting electrodes, one a glass plate and the other a flexible sheet such as polyvenylidene fluoride, the switching characteristics of the thin film are a function of the pressure applied to the flexible transparent electrode and the bias voltage across the electrodes. Response time measurements reveal a linear dependence of the change in electric field with external pressure.

Skin friction measurement with partially exposed polymer dispersed liquid crystals

Partially exposed polymer dispersed liquid crystal thin film (10–25 μm) deposited on a flat glass substrate has been used for the first time to measure skin friction. Utilizing the shear stress induced director reorientation in the partially exposed liquid crystal droplets, optical transmission under crossed polarization has been measured as a function of the air flow differential pressure. Direct measurement of the skin friction with a skin friction drag balance, under the same aerodynamic conditions, lets us correlate the skin friction with optical transmission. This provides a unique technique for the direct measurement of skin friction from the transmitted light intensity. The results are in excellent agreement with the model suggested in this paper.

Boundary layer elasto-optic switching in ferroelectric liquid crystals

Utilizing a simple new configuration, one rigid surface and the other exposed directly to the gas flow, elasto‐optic switching in thin (∼2 μm) ferroelectric liquid crystal films is observed when a gas flow is impingement in the plane of smectic layers. The elasto‐optic response caused by the shear stress is large, fast, and reversible.

Ferroelectric liquid crystals in aerodynamic testing

The process of simultaneous optical visualization and quantitative measurement of aerodynamic boundary layer parameters requires new concepts, materials and utilization methods. Measurement of shear stress in terms of the transmitted or the reflected light intensity from an aligned ferroelectric liquid crystal (FLC) thin (∼ 1μm) film deposited on a glass substrate has been the first step in this direction. In this paper, recent progress in utilization of FLC thin films for skin friction measurement and for studying the state of the boundary layer in a wind tunnel environment is reviewed. The switching characteristics of FLCs have been used to measure pressure from the newly devised system of partially exposed polymer dispersed ferroelectric liquid crystals (PEPDFLCs). In this configuration, a PEPDFLC thin film (∼10-25 μm) is sandwiched between two transparent conducting electrodes, one a rigid surface and the other a flexible sheet such as polyvinylidene fluoride or mylar. The switching characteristics of...

Ferroelectric liquid crystals as flow-field sensors in boundary layer investigation

Abstract Utilization of Ferroelectric Liquid Crystals in a simple new configuration, one rigid surface and the other exposed directly to the flow field, for non-intrusive global as well as local 2D surface stress field determination is reported. Shear flow-field induced optical switching on the boundary layer has been used both in the transmission as well as in the reflection mode operation of the sensor. Characteristics of domain formation in an A - C* and a N* - C* material and the optical response of domains to an applied shear stress in the plane of the C* layers are described. Optical polarization microscopy has been used to measure the transmitted and the reflected light intensities as function of flow parameters. Using micro wind tunnel in conjunction with polarizing microscopy, change in director orientation as a function of the shear stress field has been determined. Director dynamics has been discussed from free energy considerations in the present context.