Role of bubble deformation in interfacial instability across a two-layer miscible liquid

We conducted experiments to investigate interfacial phenomena when soft/rigid bodies cross a two-layer miscible stratified liquid, focusing on the role of bubble deformation. We used water and water-glycerol mixtures at the upper (μ_u = 1 mPa·s, ρ_u = 998 kg/m³) and lower (μ_l = 91 - 872 mPa·s, ρ_l = 1021 - 1257 kg/m³) liquids, respectively, in which the cap bubble (R_cap = 30 - 35 mm) and a rigid hollow sphere rise due to buoyancy at the speed of 0.3 - 0.4 and 0.2 - 0.9 m/s, respectively. According to the viscosity ratio (μ_l/μ_u = 97.3 - 933), we analyzed how the interfacial phenomena on the surface of soft and rigid bodies grow. When μ_l/μ_u = 97.3, bulges were generated on the bubble surface and the Kelvin-Helmholtz (KH) instability was triggered due to bubble deformation. At a higher viscosity ratio (436 and 933), however, the bulges could not grow into the roll-up waves. Without the surface deformation (rigid body), no bulges were generated in all viscosity ratios. Instead, the roll-up waves directly occurred at the surface of the rigid sphere when the rising velocity is as fast as 0.7 m/s, of which the wavelength becomes longer with increasing the viscosity ratio. We will explain more detail on this difference.