Abstract
In this work we tackle the challenging task of investigating the effective interfacial tension (EIT) between molecular miscible fluids. We first investigate the dynamics of spinning drops when they are subject to a sudden rotation speed jump. Immiscible drops show an exponential decay towards their equilibrium shape, with a characteristic time entirely determined by the viscosities of the two fluids, the drop size and the interfacial tension. Contrarily, a completely different elongation dynamics is found for miscible drops with a small difference in concentration with respect to the background fluid: in this case the dynamics are well captured by a power law. Moreover, for sufficiently low interfacial tension drops deform radially developing a “dumbbell" shape, consisting in a thin central body connecting two larger heads. We investigate the origin of such shapes, and demonstrate that they can be used to measure the interfacial tension at the boundary between the drop and the background fluids. By developing a simple model in which we balance the normal stress imposed on the drop surface, the shear stress opposing the deformation and a Laplace-like term containing the surface tension, we exploit the deformation dynamics of miscible drops to measure the EIT between water and glycerol as a model for simple miscible liquids. In particular, we find an EIT of 250±50 nN/m for pure water in contact with pure glycerol, a value orders of magnitude lower than the experimental limits of more conventional tensiometry techniques. In this thesis we also explore for the first time the systematic use of spinning drop tensiometry to measure the elastic modulus and interfacial tension of soft elastic beads, showing that this technique allows for the simultaneous measurement of the two quantities. Finally, we investigate the stability of viscosity-stratified coflows as a second route to study the EIT. Two different regimes are observed, as the instability wavelength varies non monotonically with respect to the flow rates. At small to moderate flow rates, the interplay between the interface and the channel walls leads to a confinement effect preventing the excitation of big waves. At higher values of flow rate, the amplitude of the disturbance reaches a steady value. We show that, to understand the stability of viscosity-stratified flows one needs to take into account both kinetic energy dissipation and production, notably considering the shape of the base flow velocity profile.