Anisotropic Surface Order in the Isotropic Phase of Nematic Liquid Crystals

Abstract

A spinning-drop technique was used to study the thermal variation of surface tension of two nematic liquid crystals at the interface with an isotropic fluid. A sharp increase in interfacial tension was observed in the vicinity of the nematic-isotropic transition. The interfacial tension-temperature characteristics, unlike monotonically decreasing dependence found in most isotropic fluids, showed regions of positive slope, both in the isotropic and anisotropic phases. A positive slope in the isotropic region of the liquid crystal is interesting since it implies that excess order has developed at the surface while the buk interior remains isotropic.

Interfacial Tension of a Nematic Liquid Crystal/Water Interface with Homeotropic Surface Alignment

“…Above T NI , γ is postulated to increase with temperature because nematic order persists near the interface, a phenomenon called “nematic wetting” . Because the bulk of the liquid crystal is isotropic, this implies a negative value for the excess surface entropy ( S E ), which is related to the temperature derivative of the tension by the thermodynamic equation: , …”

“…The tension at the E 7 −ethylene glycol interface was reported to have a positive temperature gradient below T NI , and for a few kelvins above T NI . The tension at the E 8 −ethylene glycol interface was reported to exhibit these same anomalous features, plus an additional relative minimum in tension occurring at a temperature well below T NI . More recently, Rai et al used the pendant drop method to study the temperature dependence of the interfacial tension between 5CB and a molten isotropic polymer, poly(dimethylsiloxane) (PDMS) …”

“…First, the interfacial tension value of a conventional interface monotonically decreases with increasing temperature, whereas the tension of a liquid crystal-containing interface often exhibits a relative maximum and/or a relative minimum near the nematic−isotropic transition temperature T NI . Positive values for the temperature gradient of the tension imply negative values for the excess surface entropy and enhanced surface ordering . An interesting consequence of this phenomenon is that a liquid crystal droplet should experience zero Marangoni forces when placed in a thermal gradient near the temperature of the relative minimum or maximum in the tension.…”

“…Far fewer reports have been published on the temperature dependence of the interfacial tension between a liquid crystal and an immiscible isotropic liquid. Harthi and George used the spinning drop method to measure the tension between ethylene glycol and two commercial biphenyl-based nematic liquid crystal mixtures denoted “E 7 ” and “E 8 ” . The tension at the E 7 −ethylene glycol interface was reported to have a positive temperature gradient below T NI , and for a few kelvins above T NI .…”

Electrically tunable spatially variable switching in ferroelectric liquid crystal/water system

“…The colloidal interaction between the interfaces of the constituents of a heterogeneous system plays a characteristic role in determining their unique properties. The interfacial surface tension ͑␥͒ at the interface between LC/air 1,20,21 and positive slope ͑d␥ / dT͒, T is the temperature, indicating the excess order at the interface LC/liquid 15,16,[22][23][24] are well reported.Based on these reports, it would be interesting to study the effect on various electro-optical properties of a smectic LC due to the diffusion of an isotropic liquid such as water, ethanol, etc. in between the smectic layers.…”