Predicting conditions for microscale surfactant mediated tipstreaming

Microscale tipstreaming is a unique method to overcome the limiting length scale in microfluidics allowing for production of submicron-sized droplets. Tipstreaming is the ejection of small drops from a liquid thread formed by interfacial tension gradients and convective transport of surfactant. Controlling and understanding this process is essential for successful application in areas such as synthesis of nano-scale particles, manipulation of biomolecules, enzyme activity studies, and others. However, models that predict operating conditions for microscale tipstreaming do not currently exist. In this work, we develop a semi-analytical model aimed at capturing the essential physics of the tipstreaming mechanism. The model relies on interfacial shape observations indicative of microscale tipstreaming to simplify the fluid flow and surfactant transport equations. The result is an interfacial mass balance of surfactant. Conditions where the mass balance can be satisfied define the operating conditions for microscale tipstreaming. Results from the model are compared with our own experimental results. Good agreement is found between model predictions and experiments. Scaling of each boundary that controls the feasible tipstreaming region is given. Finally, the model is able to guide selection of device geometry and surfactant properties to shift or expand the feasible region where microscale tipstreaming is expected.