Sub-micron emulsion ripening and flow through porous media
Description
Emulsions are encountered in a number of industrial and technological applications. In all these applications, emulsion stability and their interaction with surfaces is of utmost importance. This dissertation addresses some of the issues related to the stability of sub-micron emulsions and the interaction of emulsion droplets with solid surfaces while flowing through a porous network One form of instability to which sub-micron emulsions are most susceptible, namely, Ostwald ripening, has been studied both in the presence and absence of micelles using photon correlation spectroscopy. The rates of Ostwald ripening of decane-in-water emulsions stabilized by the anionic surfactant, sodium dodecyl sulfate (SDS) were determined experimentally and compared with those predicted by the Lifshitz-Slezov-Wagner (LSW) theory. Below the critical micelle concentration of SDS, the experimentally determined rates were found to decrease with an increase in SDS concentration, in agreement with the theory. Above the critical micelle concentration, the ripening rates were found to increase with an increase in micellar concentration which is not predicted by the LSW theory. The increase in rates is attributed to the enhancement of mass transfer between droplets by micelles. Volume fraction of the dispersed phase was found to have no significant effect on ripening rates in dilute emulsions The interaction of emulsion droplets with sand surfaces both in the presence and absence of surfactants was studied from flow experiments through sand packed columns. In the absence of surfactants, the effect of pH and ionic strength on the flow of dilute, sub-micron hexadecane-in-water emulsions in sand beds was studied by measuring the effluent concentration and drop size distribution along with the permeability of the porous medium. It is observed that the permeability of a porous medium can be substantially reduced even when the drop size is much smaller than the pore size. The droplets are captured mainly by interception, a mechanism, primarily dictated by electrostatic interactions. The permeability is affected by deposition and coalescence of the drops in the porous medium, though, the latter has a greater effect. The surface charge of the droplets and the sand grains along with the ionic strength of the aqueous phase control the deposition and further coalescence of the droplets within the porous medium. In the absence of any surfactants, oil droplets are more likely to be deposited at low pH values and high ionic strengths where the electrostatic repulsion between the droplets and the sand grains is small. Further, at these conditions, the deposited droplets are more likely to coalesce, thus drastically altering the permeability. At high pH and low ionic strength, no deposition takes place and the permeability is unaffected The cationic surfactant Cetyl Trimethyl Ammonium Bromide (CTAB) was found to significantly alter the surface characteristics of the emulsion droplets and the sand panicles, thereby affecting the deposition of emulsions in porous media. Deposition was found to be a maximum at a CTAB concentration of $5\times10\sp{-6}$ M. The experimental deposition efficiencies were compared with the energy barriers calculated using the DLVO theory and were found to be in qualitative agreement