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.

最近的实验发展表明，如果没有网状连接或纠缠，液晶的各向同性相可以在亚毫米级别上机械地用作弹性体，并在光学上用作谐波振荡器。弹性响应的强度取决于分子结构，并且当液晶分子作为侧链部分附接到链主链时，弹性响应的强度得到增强。各向同性相的这种弹性行为通过对低频机械激励的应力和光学响应得到证明。可见，远离任何相变，这种壮观的效果都包含重要信息：液态是低阈值弹性介质；分子是动态相关的，并且可以观察到这种弹性相关性，直至达到亚毫米级。这一发现为众所周知的效应的起源提供了新的启示，如流动双折射，复杂流体中的剪切带或简单液体中的流体不稳定性。本文强调了远距离弹性相互作用对于从其起源到新的可能应用的流动双折射的综合方法的关键作用。