From flow birefringence in the isotropic phase of liquid crystals to the identification of shear elasticity in liquids

ABSTRACT Recent experimental developments show that without reticulation links or entanglements, the isotropic phase of liquid crystals can work mechanically as an elastomer and optically as a harmonic oscillator at the sub-millimetre scale. The strength of the elastic response depends on the molecular architecture and is enhanced when the liquid-crystal molecules are attached as side-chain moieties to a chain backbone. This elastomeric behaviour of the isotropic phase is evidenced via stress and optical responses to a low-frequency mechanical excitation. Visible far away from any phase transition, this spectacular effect contains important information: the liquid state is a low threshold elastic medium; molecules are dynamically correlated and this elastic correlation is observed up to the sub-millimetre scale. This discovery sheds a new light on the origin of well-known effects as flow birefringence, shear banding in complex fluids or flow instabilities in simple liquids. The present paper emphasizes the key role of long range elastic interactions for a comprehensive approach to the flow birefringence, from its origins and to new possible applications.