Monodisperse Droplet Generation for Microscale Mass Transfer Studies

Monodisperse droplet generation for microscale
mass transfer studies

Christine C. Roberts¹, Rekha R. Rao², Anne M. Grillet¹, Carlos F Jove-Colon³,
Carlton F. Brooks¹, Martin B. Nemer¹

¹Thermal and Fluid Experimental Sciences, MS 0345
²Thermal and Fluid Processes, MS 0836
³Radiological Consequence Management and Response Technologies
Sandia National Laboratories
P.O. Box 5800
Albuquerque, New Mexico 87185

            Understanding interfacial mass transport on a droplet scale is essential for modeling liquid-liquid extraction processes, such as those used in nuclear waste reprocessing.  A thin flow-focusing microfluidic channel is evaluated for generating monodisperse liquid droplets for microscale mass transport studies.  Surface treatment of the microfluidic device allows creation of both oil in water and water in oil emulsions, facilitating a large parameter study of viscosity ratios ranging from 0.05 < λ < 96 and flow rate ratios ranging from 0.01 < Q_i/Q_o < 2 in one chip.    The unusually thin channel height promotes a flow regime where the inner fluid wets the top and bottom of the channel in the orifice and no droplets form.  Through confocal microscopy, this fluid stabilization is shown to be highly influenced by the contact angle of the liquids in the channel.  Drop sizes of oil in water emulsions follow previously proposed scaling behavior (Lee W, Walker LM, Anna SL, Phys Fluids. 2009 21: 032103-1-14), but oil in water droplet sizes are found to scale with a modified capillary number.  Liquid streamlines within the droplets are inferred by high speed imagery of microparticles dispersed in the droplet phase.   Finally, species mass transfer to the droplet fluid is quantitatively measured using a spectrophotometer.

Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.  This research is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories.