Abstract
There is little literature on the flow properties of the isotropic phase of liquid crystalline fluids. However, this phase is an ideal tool to bridge the physics of liquid crystals with those of (ordinary) fluids. Optical and mechanical studies are presented, demonstrating that away from any phase transition, the isotropic phase of liquid crystalline molecules (LCs) and liquid crystalline polymers (LCPs) can work as an optical oscillator in response to low-frequency mechanical excitation, establishing the elastic origin of the flow birefringence and “visualizing” the very existence of the elastic nature of the liquid state. Additionally, mimicking the excellent anchoring ability of liquid crystals, an alternative rheological protocol optimizing the fluid/substrate interfaces is presented to access the low-frequency shear elasticity in various one-component liquids and salt-free aqueous solutions.
Highlights
Liquid crystalline molecules (LCs) are typically made of one or several flexible alkyl chains linked to rigid and polar cores (~5.0 D)
We describe recent developments indicating that it is possible to produce a strong strain-induced optical signal of the isotropic phase of low molecular weight liquid crystalline polymers (LCPs) upon applying low-frequency (Hertz-range) mechanical excitation
We show here that low-frequency shear elasticity has been identified in worm-like micellar solutions (Figure 12) bringing possible new input in the long debate on the origin of shear-induced phase transitions in lyotropic solutions
Summary
Liquid crystalline molecules (LCs) are typically made of one or several flexible alkyl chains linked to rigid and polar cores (~5.0 D). The first part of this contribution is devoted to the new identified strain optical properties induced at low frequency in the isotropic phase of LCPs and LCs, while the second part describes the origin of the low-frequency birefringence on the basis of the identification of shear elasticity in LCPs and LCs and in various fluids away from any phase transition. Both optical and stress behaviors challenge the Maxwell (molecular) definition of liquids (that are supposed to exhibit shear elasticity at high frequency only (typically at MHz or GHz)) and show that low-frequency (around Hz) shear elasticity exists. This long wavelength excitation reveals collective effects largely ignored in fluidics and points out a new no man’s land to explore
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