Engineering Microscale Interfaces Using Convection and Curvature

T.M. Moyle, L.M. Walker, S.L. Anna
Carnegie Mellon University, US

Keywords: microfluidics, emulsions, surfactant, adsorption, tipstreaming

Summary:

Microfluidic technologies enable production of highly uniform droplets tens of micrometers in size. The ability to control the volume, motion, coalescence, and mixing of droplets has led to unique concepts in the fabrication of multiphase materials and “digital” lab-on-a-chip devices. However, synthesis of submicron droplets is still a challenge since the smallest geometric feature controls droplet breakup. To overcome this limitation, we use a fluid dynamical phenomenon called ‘tipstreaming’, in which submicron-scale droplets stream from the tip of a much larger parent droplet. Controlling this process is essential in its successful application; however, models that predict operating conditions for tipstreaming do not currently exist. We have developed a simplified semi-analytical model that captures the essential physics of the mechanism and provides design criteria for generating submicron droplets via tipstreaming. The model balances the mass of surfactant adsorbed to the interface during tipstreaming with the specific amount needed to maintain the fixed interfacial shape characteristic of the process. Model predictions are found to agree well with experimental operating conditions. Although this work focuses on a specific process for generating submicron droplets, the results rely on generic interfacial processes that can be exploited in the successful design of novel multiphase materials.