2007-12-05
10:15 at HCI J 574

Flow Visualization of Polymer/Water/Surfactant Droplets in Oil using Coflow and Flow-Focusing Channels with microPIV Technique

Manuela R. Duxenneuer

Laboratory of Food Process Engineering, Department of Agricultural and Food Science, ETH Zürich

In this study, we have investigated the drop formation and breakup (dripping mode) in coflow and flow-focusing microchannels, producing pure water, Tween20-surfactant/water (0.1cmc to 100cmc), and surfactant/polymer/water (0.1wt% to 0.2wt% hydroxypropylether guar gum) droplets (Ddrop=0.010 to 0.3mm) in sunflower oil. The flow fields with the surrounding continuous and disperse phase were analysed. Fluorescent particles with diameters ranging from 0.300 to 1mm were seeded into both the oil and water phases allowing the flow inside and outside of different sized droplets during formation and breakup to be visualized and analysed simultaneously. Droplet and continuous phase velocity distributions were quantified during droplet formation, pinch-off, and drop convection along the length of the channel. The velocity profiles show that fluid recirculation occurs inside the drop only when the drop is in contact with the wall and is blocking the continuous phase passing by. The recirculation can be attributed to wall slip and shearing at the leading edge and backside of the droplet. This is consistent with previous two-phase studies utilizing confocal ?PIV or ordinary ?PIV for water/glycerol droplets in silicon oil. However, this phenomenon was not observed when the droplet creation event was very fast, resulting in the droplet size being much smaller than the channel depth (Dcell=0.5mm, Hcell=0.1 to 0.2mm, Lcell=3-4cm). The effect of elasticity on drop formation and break-up has also been explored by using various solutions as the disperse phase (water, surfactant [0.1-100cmc], surfactant/polymer [0.1-0.2wt]). At constant flow rate, it was found that with increasing polymer concentration, the elasticity of the fluid increases and therefore the pinch-off distance of the droplet increases. The elongated neck of the drop also in the production results in small satellite droplets.