On the structure of turbulent liquid—liquid dispersed flows in an agitated vessel

Abstract

The structure and stability of dispersed liquid—liquid flow have been studied in a stirred vessel under semi-batch (wash-out) and batch conditions. Four aqueous-organic systems were used with the density and viscosity of the organic phase close to and straddling the values for water. A recently developed video technique was employed, capable of giving sharp images of the dispersed phase droplets down to 25 μm. In the wash-out system, the dispersed phase fraction was increased from zero continuously until phase inversion occurred; and in the batch case, concentration from 0.05 in steps of 0.1 volume phase fraction up to phase inversion were used. In all cases, for water-in-oil dispersions above ∼ 0.25 volume fraction up to inversion, droplets of oil were found in the drops of water, and within a few seconds of phase inversion, such a structure disappeared. Conversely, for all fractions of oil-in-water, droplets-in-drops were not observed; but, in less than a second, following inversion, droplets of oil in drops of water formed. These findings are used to explain qualitatively: (i) the larger drop sizes found in water-in-oil dispersions at the same concentrations compared to those in oil-in-water ones under equivalent hydrodynamic conditions; (ii) the lower concentration in water-in-oil systems at which phase inversion occurs compared to oil-in-water systems; (iii) the presence of a delay time with water-in-oil inverting to oil-in-water compared to the absence of a delay time in the reverse case. It would appear that the general fluid dynamic treatment of such systems which only utilises the two-phase densities and viscosities and the symmetrical property, interfacial tension, is unable to account for these observations.