Interfacial tension controlled W/O and O/W 2-phase flows in microchannel

In microfluidic systems, interfacial tension plays a predominant role in determining the two-phase flow behavior due to the high surface area to volume ratio. We investigated the influence of both solid-liquid (sigma(SL)) and liquid-liquid (sigma(LL)) interfacial tensions on water-oil two-phase flows in microfluidic devices. Experimental results show that, unlike macroscopic systems, sSL plays a dominant role, determining the emulsion type created in microchannels: oil-in-water (O/W) flow in hydrophilic microchannels and water-in-oil (W/O) flow in the corresponding hydrophobically treated microchannels. Modification of sigma(LL) by surfactants only plays a secondary role. By tuning sigma(LL), oil-water two-phase flow patterns could be changed from droplet-based to stratified flows under constant flow conditions in the same device. In addition, also droplet deformation, coalescence and emulsion inversion could be achieved by tailoring sigma(SL) and sigma(LL) in microfluidic devices. Microfluidic technology therefore provides a valuable additional tool for quantitatively manipulating and evaluating these phenomena which are difficult to realize in the macroscale devices.