Dissolution behaviour of a binary mixture in a capillary tube

Abstract

We develop a pore-level physical model for the process of miscible displacement through porous media. Using the network model, the current task is reduced to the study of the dissolution dynamics of a binary mixture within a single capillary tube. Tubes of rather small diameters are considered when the typical diffusion and convective time scales are comparable. The test tube filled with the solute is immersed into the solvent-filled thermostatic bath; no pressure difference between the ends of the tubes is applied. Using a high-resolution video-camera, we study the solvent penetration into the test tube. We examine the evolution of the isobutyric acid/water mixture far from and close to the critical (consolute) point (which is 26C for this mixture). The mixture fills the circular glass tubes of diameters 0.4mm-0.8mm and of various lengths. The shape of the interface and its position are tracked and analysed. Based on our observations the following conclusions can be drawn. In all experiments, we observe a front-type propagation of the solvent phase into the tube with a clearly visible interface. The gravity force significantly affects the shape of the interface and the dissolution dynamics in all undertaken experiments. If the mixture temperature is below the critical point, then the uneven one-sided penetration of the solvent into the tube was consistently observed. The solute/solvent interface experiences oscillations of its shape (being either concave or convex at different time moments). If the mixture temperature is above the critical point, then the solvent penetrates evenly from both ends. In both under- and supercritical conditions, the contact line moves with the same speed as the interface, but the apparent contact angle is time- and coordinate-dependent. The rate of the interface propagation varies at different stages of the dissolution process and does not follow the predictions of the diffusion theory