Microemulsion droplets in optical traps

Alex L. Hargreaves, Florence Gregson, Andrew K. Kirby, Sandra Engelskirchen, Colin D. Bain

While the optical manipulation of solid particles is increasingly familiar, complex liquids represent a relatively new area of investigation. In this study, microscopic liquid–liquid interfaces are deformed at ultralow interfacial tension using near-infrared optical tweezers. The chosen emulsions, of hydrocarbon oil (heptane and decane) in aqueous solutions of surfactant (AOT, C12E4, C12E5, Brij-L4) and salt, are capable of forming Winsor microemulsions, although the macroscopic phase volumes are unaffected by changes in ambient temperature. The amphiphilic ratio of nonionic (C12E4,5) and anionic (AOT) surfactants is chosen to minimise the temperature dependence of the monolayer curvature. These phases and their dispersions are examined, both with and without equilibration of the compositions. Under laser action at powers > 0.1 W, a plethora of metastable phases, multiple emulsions and vesicles are produced.
The rates of phase separation of microemulsion increase near the coverglass, which absorbs the laser more strongly. Conversely, substitution with heavy water reduces the absorptive heating, whilst maintaining a similar refractive index contrast. Localised laser heating is found to be the major driving force behind phase separation. The observed changes are rationalised in terms of the phase diagram of the microemulsion and the value of the amphiphilic ratio. The relative importance and origin of the thermal and optical effects on phase separation are discussed.

DOI