The effect of dissolved gas on coalescence of oil drops studied with microfluidics

In literature it is stated that the stability of oil-in-water emulsions could be enhanced by decreasing the so-called "hydrophobic interactions" between surfaces through removal of dissolved atmospheric gases. Since the effect of the dissolved gases depends on the hydrophobicity of the oil phase, as well as the system pressure, we vary this effect systematically and monitor droplet coalescence in a tailor-made microfluidic device.

The coalescence of oil drops in standard and degassed conditions was studied by direct observation using a microfluidic setup. Two model oils (heptane and xylene) were used to represent different hydrophobicity of the dispersed phases, together with an oil with dynamic interfacial behaviour (diluted crude oil). In addition, the effect of the volume fraction, droplet size and degassing method was studied.

At ambient pressure, the degassing of the continuous phase reduced the extent of coalescence for the model oils, which is in agreement with other reports. No effect of the dissolved gases was found on the drop formation process. At elevated pressures, the dissolved gases influenced only the most hydrophobic oil (heptane), while causing no effect in the other systems. The coalescence frequencies decreased upon the reduction of the drop sizes, which was justified with the theory for two interacting spheres.