Abstract
We demonstrate passive sorting of emulsion microdroplets based on differences in their interfacial tension and contact angle. The sorted droplets are flowing inside a microfluidic channel featuring a shallow guiding track (depth \(\sim 0.6\,\upmu {\text {m}}\)) defined by femtosecond laser micromachining in polydimethylsiloxane coating deposited on glass. Under these flow conditions, the droplets experience a confinement force that pulls them into the track; this force depends on the interfacial tension and the difference between the contact angles inside and outside the ablated track. The interplay between the confinement force, fluid drag, and wall friction then determines the trajectory of the droplet along the guiding track. We investigate experimentally the droplet trajectory as a function of droplet velocity and angle between the track and the channel axis and demonstrate precise control of droplet direction by adjusting the track angle. Moreover, we show that droplets of liquids with different interfacial tensions and contact angles travel different distances along the guiding track at a constant flow rate, which can be used for droplet sorting. We develop a theoretical model that incorporates the droplet position with respect to the ablated track, interfacial tension, and contact angles to predict the droplet trajectory under given experimental conditions. Thus, the dynamic behavior of the droplets leading to different guiding scenarios can be studied without the need of computationally expensive fluid dynamics simulations. The presented study paves the way for designing and optimizing new systems for advanced manipulation of droplets of different content using potentially reconfigurable guiding tracks.
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Acknowledgements
This work was supported by TÜBİTAK (Grants nos. 112T972 and 117F348) and HEC Pakistan (Ph.D. scholarship of Z.R.). We also thank KUYTAM (Koç University Surface Science and Technology Center) for laser ablation experiments and characterization measurements.
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Experimental details, CA measurements, surface topography measurements, further details on droplet geometrical parameters and on modeling of confinement force, together with experimental images and movies. (MP4 17533KB)
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Rashid, Z., Erten, A., Morova, B. et al. Passive sorting of emulsion droplets with different interfacial properties using laser-patterned surfaces. Microfluid Nanofluid 23, 65 (2019). https://doi.org/10.1007/s10404-019-2236-8
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DOI: https://doi.org/10.1007/s10404-019-2236-8