A deformable bubble passing through an interface between two miscible liquids*

We conducted experiments to understand interfacial phenomena when a deformable bubble passes through an interface between two miscible liquids. We used tap water and water-glycerine mixtures at the upper (liquid 1; μ₁ = 1 mPa·s, ρ₁ = 998 kg/m³) and lower (liquid 2; μ₂ = 1.3 - 900 mPa·s, ρ₂ = 1021 - 1257 kg/m³) liquids, respectively, and the cap bubble (R_cap = 12 - 35 mm) was considered. Varying the viscosity ratio (μ₂/μ₁) and R_cap, we measure how the interface deforms. When μ₂/μ₁ = 1.3 - 5, wake fragmentation and path instability happened so fast that the interface of the miscible liquids generated by the bubble collapsed instantly. When R_cap ≤ 18 mm, the interface collapsed due to bubble path instability at μ₂/μ₁ = 10 - 40 and small interfacial disturbance occurred at μ₂/μ₁ > 40. When R_cap ≥ 23 mm, the Kelvin-Helmholtz (KH) and azimuthal instability were observed simultaneously at μ₂/μ₁ = 10 - 210, and the KH instability was not clearly observed at μ₂/μ₁ > 210. Based on these, we suggested an instability regime according to μ₂/μ₁ and interfacial Eötvös number. We also found that these interfacial phenomena were related to bubble deformation characterized by the large-amplitude waves.