Pressure-driven microfluidic flow-focusing of air through a surfactant-doped dilute polymer liquid

In this paper, we present experimental results on the formation of air bubbles in a non-Newtonian liquid. Flow-focusing technology with pressure-driven fluid pumping is used to demonstrate and study the bubble production process. The liquid phase consists of aqueous polyacrylamide at concentrations ranging from 0.01 to 0.10 wt%. Additionally, the solution contains sodium lauryl sulfate, an anionic surfactant, at concentrations ranging from 0.01 to 0.10 wt%. We report on bubble production with pressure ratios (dispersed/continuous phases) ranging from 0.6 to 0.9; the range spans from the minimum ratio required to produce bubbles to the pressure ratio required to produce an air jet. Within this range of pressure ratios, the bubbles lengths are polydisperse, a clear deviation from previous studies involving Newtonian continuous phase fluids. Although the continuous phase contains two additives, we deduce that the polymer is mainly responsible for this change in behavior from Newtonian fluids. The addition of surfactant though provides a means of manipulating this behavior for a certain range of polymer concentrations. We characterize the efficiency of the surfactant by performing a mass transport analysis using Schmidt (Sc), Sherwood (Sh), and Reynolds (Re) numbers. Despite this observation, the average bubble lengths follow trends established for Newtonian fluids.