Dissolution dynamics of liquid/liquid binary mixtures in porous media
Dr A. Vorobev
Faculty of Engineering and the Environment
University of Southampton
Results of optical observations are reported aimed at understanding the dissolution
dynamics of binary mixtures within porous media. The porous medium is modelled as a
network of capillaries. Our research has been made in two stages: we first investigated the
dissolution dynamics of binary mixtures in a single capillary and then a 2D micromodel built
for a network of capillaries.
The liquid/liquid diffusion process was primarily aimed to investigate. Capillaries with
diameters as small as 0.2mm were used. No external pressure difference was imposed. We
managed to separate the diffusive mass transport from the hydrodynamic motion while using
glycerol/water and soybean oil/hexane binary mixtures in the experiments with the
capillaries. We observed interfaces for quite prolong periods of time despite the fact, that
these are fully miscible liquids. Two phase boundaries moving from the ends into the middle
section of the tube with the speeds v~D1/3t-2/3d2 (D, t and d are the coefficient of diffusion,
time and tube’s diameter, respectively) were observed. The boundaries were slowly
smeared but their smearing occurred much slower than their motion. The motion of the
phase boundaries cannot be explained by the dependency of the diffusion coefficient on
concentration. However, the observed phenomenon can be explained by the influence of
barodiffusion. It is important that the solute/solvent boundaries were endowed with non-zero
surface tension.
The experiments with the micromodel revealed that the solvent penetration is also diffusiondominated in completely miscible binary mixtures (glycerol/water and soybean oil/hexane).
However, this was also non-Fickian diffusion with the dissolution rate, dV/dt, being
proportional to D1/3t-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).
Despite a seeming simplicity of the experiments there is no theory that could correctly
describe the observed diffusional penetration of solvent into a solute-filled capillary, and
hence into a more complex porous structure.