Are there really viscous liquids?

International audienceA solid body deforms under shear while a liquid flows. The absence of mechanical resistance to a shear stressindicates a liquid behavior. However, is this common definition really satisfying?Looking at the mesoscopic scale, we show that liquids and melts are actually endowed with shear elasticity thatit is hidden in standard rheological measurements [1,2]. To prove the elastic nature of “liquids and melts”, a new seriesof experiments highlight that ordinary liquids and polymer melts (glycerol, Polypropylene glycol, liquid water ...) are able to emit a modulated thermal (hot and cold) signal when submitted to a mechanical shear stress (figure 1)[3].Thermal response is a collective effect. It is similar to the thermo-elasticity, coupling the deformation of an elastic(solid) body to its temperature and vice-versa. Such properties require long-range elastic interactions that are absent in current viscoelastic descriptions.Our approach is an invitation to consider liquids as not fundamentally different from solids, highlightingsplendid collective effects [3,4]. More than ever, a phononic interpretation is relevant, opening the way to coupling with surface boundaries, whether solid [5] or biological and defining an extended third zone