(236g) 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.