(163h) Confinement Induced Alteration of Morphologies of Oil-Water Emulsion

Many scientific and engineering applications of emulsions demand alteration in morphology of the dispersed droplets, e.g. from core-shell to Janus and vice-versa. Such alterations require small yet definite changes in the interfacial energy between the dispersed and the continuous phases which are conventionally achieved by using a surface-active agent. Sometimes, two different surfactant molecules, that lead to two different emulsion morphologies, are used at a desired relative quantity, to reversibly alter the interfacial energies and consequent switch between the morphologies. Yet in others, an external trigger like temperature, pH or UV/blue light irradiation is employed which causes such transitions. It has been shown also that an anionic liquid motif can alter the interfacial energies to change the drop morphology. While all these methods involve actual alteration of the interface between liquids, we have shown that the morphology of dispersed droplets can be altered also by sufficiently confining the emulsion, without really chemically altering the interface.

We have demonstrated this effect by using three immiscible liquids: silicone oil, paraffin oil and aqueous solution of surface-active agents like Agarose, SDS, AOT, CTAB to form oil in water emulsions. The interfacial energy between these liquids are such that at the unconfined state they form core-shell drops with paraffin oil at the core and silicone oil at the shell. The core-shell morphology remains stable over long time and even after strong agitation. However, when a drop of the emulsion is confined between two parallel plates, although initially the dispersed droplets of oil continue to remain core-shell, with decrease in gap between the plates, the droplets turn Janus. Interestingly, even drops of diameter much smaller than this spacing, alter their morphology. We have shown that the continuous aqueous phase forms a liquid disk with a concave meniscus between the two plates. The excess surface energy of this meniscus alters the energetics of the system so that the core-shell, which remains energetically favored at the unconfined state, alters to Janus, the more favored one when confined. Examination under high frame rate camera show that this transition happens via a sequence of steps: the core liquid moves to one side of the shell, the shell liquid drains out from the thinnest region leading
to its further thinning and then it finally ruptures; following rupture, the shell liquid dewets on the core eventually assuming the Janus morphology. The rupture of the thin film and the subsequent flow of the shell liquid involves viscous energy dissipation which acts as an energy barrier, for transition in both forward and the reverse direction.

It is worth noting that while the morphology of dispersed droplets in an emulsion is routinely characterized by confining an emulsion droplet between two parallel plates and then examining it under optical microscope, the effect of confinement is largely neglected. Our results show that the excess surface energy of the meniscus formed at the periphery of such confined drop of the emulsion is an important parameter.