(98ap) Experiments and Simulations of Droplet Flow Through Sharp Contractions

The stability of droplets flowing in channels is an important issue in the design of microfluidic devices. A sudden 90 degree contraction in a cylindrical channel geometry can create large extensional strains leading to droplet breakup. The critical capillary number for breakup depends strongly on the viscosity ratio as well as the geometry of the contraction. In this study, the dynamics of a droplet as it flows through a sudden contraction is studied both experimentally and numerically. Experimentally, we fabricate cylindrical PDMS channels with sharp contractions and varying contraction ratios. The deformation and break up of glycerol-water drops in silicon oil of varying viscosities is recorded at varying flow rates. Critical capillary numbers at which drop break up occurs is identified. The experimental results are validated by comparing them with numerical simulations using a front tracking volume of fluid method. In addition, the effect of surfactants on droplet breakup is also studied. Drop breakup pattern similar to tip streaming is observed in the presence of surfactants.