Effect of surface coating on droplet generation in flow-focusing microchannels

Different stages of droplet generation are reported in this paper with two immiscible liquids, silicone oil and deionized water, inside a flow-focusing device for hydrophobic and hydrophilic channel walls. Hydrophobic and hydrophilic channels of identical geometry are compared. In this first set of experiments, the efficacy of the hydrophobic channel is compared with a square cross junction for a continuous oil phase with low viscosity. In the hydrophobic case, the flow-focusing design with a diverging outlet delays jetting and allows for the use of higher flow rate ratios in the squeezing regime. For the hydrophilic case, stable and well-structured droplet and slug generation can be achieved using oil and water, resulting in an inverse emulsion. However, the morphology of the fluid interface displays an unusual behavior compared to that of a hydrophobic microchannel. The droplet generation in the hydrophilic channel occurs following the formation of single and double T-junctions, a phenomenon hitherto unreported in the literature. The results demonstrate that the uncoated hydrophobic channels generate monodisperse droplets at a higher capillary number when compared to the hydrophilic channels.