Applied Mechanics Colloquia Applied Physics Colloquia

Anomalously Stable Mixtures of Water and Oil

Jérôme Bibette (Pierre-Gilles de Gennes Institute)

Mar 17, 2023
12:00 pm to 1:00 pm | Pierce Hall, 209 | Remote option

Oil and water do not mix. Upon agitation they do mix, with one phase being dispersed into the other as droplets, forming an emulsion. However, as soon as the agitation is removed coalescence rapidly recombines the dispersed phase and reestablishes the original phase-separated coexistence. Addition of surfactant can delay this recombination and can produce a metastable emulsion. However, in the absence of some type of surfactant an emulsion can not exist. Here we report the surprising observation that metastable water-in-oil emulsions can exist, even at surprisingly large volume fractions of the dispersed phase. We show that this behavior is the result of an unexpected weak adhesion between droplets due to adsorption of oil molecules at the interface.  This results in a critical value of the internal pressure that droplets can sustain before coalescing, manifested by a maximum volume fraction at which a stable emulsion can be produced. Above, the emulsion breaks under shear forming double emulsion droplets composed of the primary water-in-oil dispersion with a characteristic internal volume fraction. These results provide a new understanding of the interactions of oil molecules at the water interface which will allow surfactant-free stable emulsions to be produced for a broad range of fluid mixtures; moreover, this will lead to a new method for formulating materials through double emulsions made with a single step. 


Douglas Woodhouse