Instability of miscible interfaces in a cylindrical tube

We report experimental results on the displacement of a less viscous liquid by a more viscous, miscible liquid in a vertical tube of small diameter. The liquids used are silicone oils of various viscosities. The more viscous fluid has a higher density than the less viscous fluid. Both upward displacement and downward displacement have been studied. Our observations show that for downward displacement, the “interface” between the liquids can be unstable and exhibits an asymmetric, sinuous shape. During upward displacement, the interface forms an axisymmetric finger of the intruding liquid. At the tip of the main finger, a spike of the more viscous liquid is observed under some conditions. The Reynolds number is negligibly small in these experiments and the Peclet number for mass transfer is O(50)–O(105). The influence of gravity is characterized by the dimensionless parameter F=gΔρd2∕μU where g is the acceleration due to gravity, Δρ is the density difference between the fluids, d is the tube diameter, μ is...