Competition Between Viscoelasticity and Surfactant Dynamics in Flow Focusing Microfluidics
The goal of this work is to quantify the impact of viscoelasticity on a tipstreaming phenomenon observed in flow focusing microfluidic devices. Tipstreaming, or thread formation, is a phenomenon caused by surfactant transport to, from, and along, deforming interfaces that leads to long filaments in microfluidic devices. Viscoelasticity also leads to stable filaments, so possible synergies are investigated here. The ability to generate extremely long and thin filaments that will break up via interfacial instabilities will lead to the formation of droplets much smaller than the device lengthscales. Viscoelastic Boger fluids are used as the dispersed phase liquid in a flow focusing device and surfactant is added to the continuous phase liquid. We find a region of operating space in which viscoelasticity appears to couple with surfactant transport to form long threads. The phenomenon is described qualitatively and is related to fluid relaxation times.