(435f) Modeling, Predicting, and Controlling Microscale Tipstreaming

Microscale tipstreaming is one 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 tipstreaming in microfluidics do not currently exist and the periodic nature of thread formation has hindered its use in applications.  In this work, we develop an analytical model aimed at capturing the essential physics of the tipstreaming mechanism.  The model relies on observations of a fixed interfacial shape indicative of thread formation 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.  Good agreement is found between model predictions and experiments.  Lastly, modeling the interfacial shape in this manner evokes the idea of fixing the interfacial shape experimentally.  We implement an active feedback control algorithm using a custom LabVIEW code for image analysis to hold the interfacial shape fixed during tipstreaming.  Controlling the interfacial shape experimentally increases thread production, potentially expanding the number of applications where tipstreaming can be used.