Effect of surfactants on drop formation in microfluidic channels*

Emulsions are widely used in applications including inkjet printing, healthcare, pharmaceuticals [1,2]. Surfactants are commonly added during drop formation to modify the interfacial properties and improve the stability of emulsions. The presence of surfactants affects the drop formation process and the final size.  In recent studies [3,4,5] it was found that high dispersed phase flowrates enhance convection and surfactant mass transfer to the interface, which justifies the reduced interfacial tension values at short formation times.

 

In order to investigate velocity fields and possible circulation patterns during the fast drop formation process, high-speed μPIV was used. Drops were generated in a flow-focusing glass microchannel. Previously it was found that absorption and mass transfer rates compete with drop formation times [3], thus it is important to study mixing inside the drops. Results revealed that convection is more prominent in the surfactant-free cases and a decrease in circulation patterns is seen with increasing surfactant concentration, which is in agreement with previous works [6]. Numerical simulations were also performed of the drop formation in the microfluidic channel and excellent agreement was found for drop size, drop formation time and circulation patterns.