Manipulation of aqueous droplets in microchannels has great significance in various emerging applications such as biological and chemical assays. Magnetic-field based droplet manipulation that offers unique advantages is consequently gaining attention. However, the physics of magnetic field-driven cross-stream migration and the coalescence of aqueous droplets with an aqueous stream are not well understood. Here, we unravel the mechanism of cross-stream migration and the coalescence of aqueous droplets flowing in an oil based ferrofluid with a coflowing aqueous stream in the presence of a magnetic field. Our study reveals that the migration phenomenon is governed by the advection (τa) and magnetophoretic (τm) time scales. Experimental data show that the dimensionless equilibrium cross-stream migration distance δ* and the length required to attain equilibrium cross-stream migration depend on the Strouhal number, St = (τa/τm), as δ* = 1.1 St0.33 and , respectively. We find that the droplet-stream coalescence phenomenon is underpinned by the ratio of the sum of magnetophoretic (τm) and film-drainage time scales (τfd) and the advection time scale (τa), expressed in terms of the Strouhal number (St) and the film-drainage Reynolds number (Refd) as ξ = (τm + τfd)/τa = (St−1 + Refd). Irrespective of the flow rates of the coflowing streams, droplet size, and magnetic field, our study shows that droplet-stream coalescence is achieved for ξ ≤ 50 and ferrofluid stream width ratio w* < 0.7. We utilize the phenomenon and demonstrated the extraction of microparticles and HeLa cells from aqueous droplets to an aqueous stream.
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November 2019
Research Article|
November 13 2019
Cross-stream migration and coalescence of droplets in a microchannel co-flow using magnetophoresis
U. Banerjee;
U. Banerjee
Department of Mechanical Engineering, Indian Institute of Technology Madras
, Chennai 600036, India
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C. Mandal;
C. Mandal
Department of Mechanical Engineering, Indian Institute of Technology Madras
, Chennai 600036, India
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S. K. Jain;
S. K. Jain
Department of Mechanical Engineering, Indian Institute of Technology Madras
, Chennai 600036, India
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A. K. Sen
A. K. Sen
a)
Department of Mechanical Engineering, Indian Institute of Technology Madras
, Chennai 600036, India
a)Author to whom correspondence should be addressed: ashis@iitm.ac.in
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a)Author to whom correspondence should be addressed: ashis@iitm.ac.in
Physics of Fluids 31, 112003 (2019)
Article history
Received:
August 07 2019
Accepted:
October 28 2019
Citation
U. Banerjee, C. Mandal, S. K. Jain, A. K. Sen; Cross-stream migration and coalescence of droplets in a microchannel co-flow using magnetophoresis. Physics of Fluids 1 November 2019; 31 (11): 112003. https://doi.org/10.1063/1.5123533
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