Interfacial instabilities due to immiscible fluid displacement in circular and non-circular microchannels

Interfacial instabilities caused by the displacement of one fluid by another were studied experimentally in three horizontal channels of different shape of cross section with hydraulic diameters ranging from 100 to 200 µm. Flow instabilities were induced by the displacement of a more viscous fluid (silicone oil) by an immiscible, less viscous fluid (aqueous solutions of glycerol) with viscosity ratios between the two fluids ranging from 20 to 100. In addition, the effect of surfactant was studied by the addition of Sodium Dodecyl Sulfate to the displacing fluid. Flow regime maps were developed for the different types of instability observed, with more complex 3-D instabilities shown to occur as the capillary number increases. Whilst fluid viscosities, channel shape and wall wettability were shown to affect the threshold capillary numbers for instabilities, the addition of SDS did not have a significant impact, which is believed to be a consequence of the long contact time between the two fluids during the whole displacement process. It was found that higher flow rates of the displacing fluid (resulting in more complex interfacial instabilities) did not cause a proportionally faster removal of the displaced fluid, which is an important finding for practitioners.