Innovations for the full measurement of the viscoelastic properties of fluids

Due to combined advances in terms of surface studies, tribology and wettability for rheology protocols, a new method of study of fluids has been developed and is presented in this article. The dynamic relaxation probes the dynamics at the macroscopic scale, by imposing through contact with the substrate a shear stress of variable frequency. This optimization of contact demonstrates the existence of a macroscopic elastic (solid) component, far from any transition. This macroscopic elasticity is induced by intermolecular interactions which are generally not taken into account in traditional approaches. This new measurement protocol proves that, by optimizing the interaction between the fluid and the surface, the dynamic response of the fluid may be higher of several tens and that the shear modulus still occurs at low frequencies. This observation illustrates the high degree of interconnectivity between molecules. The existence of this long-range elastic cohesion explains the appearance of flow instabilities, slow relaxations and flow-induced transitions.