We report optical observations of the dissolution behaviour of glycerol/water, soybean oil/hexane, and isobutyric acid (IBA)/water binary mixtures within horizontal capillary tubes. Tubes with diameters as small as 0.2mm were initially filled with one component of the binary mixture (solute) and then immersed into a solvent-filled thermostatic bath. Both ends of the tubes were open, and no pressure difference was applied between the ends. In the case of glycerol/water and soybean oil/hexane mixtures, we managed to isolate the dissolution (the interfacial mass transfer) from the hydrodynamic motion. Two phase boundaries moving from the ends into the middle section of the tube with the speeds v∼D(1/3)t(-2/3)d(2) (D,t and d are the coefficient of diffusion, time and the diameter of the tube, respectively) were observed. The boundaries slowly smeared but their smearing occurred considerably slower than their motion. The motion of the phase boundaries cannot be explained by the dependency of the diffusion coefficient on concentration, and should be explained by the effect of barodiffusion. The shapes of the solute/solvent boundaries are defined by the balance between gravity and surface tension effects. The contact line moved together with the bulk interface: no visible solute remained on the walls after the interface passage. Changes in temperature and in the ratio between gravity and capillary forces altered the apparent contact angles. The IBA/water system had different behaviour. Below the critical (consolute) point, no dissolution was observed: IBA and water behaved like two immiscible liquids, with the IBA phase being displaced from the tube by capillary pressure (the spontaneous imbibition process). Above the critical point, two IBA/water interfaces could be identified, however the interfaces did not penetrate much into the tube.

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We report optical observations of the dissolution behaviour of glycerol/water, soybean oil/hexane, and isobutyric acid (IBA)/water binary mixtures within a glass micromodel built as a 2D regular network of capillary tubes with diameter of 0.2 mm. The micromodel is initially filled with solute and then is horizontally immersed into a thermostatic solvent-filled bath. The micromodel is open at its corners for solute dissolution to occur with no pressure gradients being applied. Our study shows that the solvent penetration into the micromodel is diffusion-dominated in completely miscible binary mixtures (glycerol/water and soybean oil/hexane). This is, however, non-Fickian diffusion with the dissolution rate, dV /dt, being proportional to D 1/3 t −0.4 for almost the entire duration of the experiment (V is the volume occupied by the solvent, D is the diffusion coefficient, and t is time). For the partially miscible IBA/water mixture the experiments performed at undercritical temperatures revealed that the diffusive transport was negligible despite the mixture being out of its thermodynamic equilibrium. The water phase penetrated into some of the channels, but IBA was never completely displaced/dissolved from the micromodel and numerous interfaces remained visible after very long-time periods.

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