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Guiding of emulsion droplets in microfluidic chips along shallow tracks defined by laser ablation

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Abstract

We demonstrate controlled guiding of nanoliter emulsion droplets of polar liquids suspended in oil along shallow hydrophilic tracks fabricated at the base of microchannels located within microfluidic chips. The tracks for droplet guiding are generated by exposing the glass surface of polydimethylsiloxane (PDMS)-coated microscope slides via femtosecond laser ablation. The difference in wettability of glass and PDMS surfaces together with the shallow step-like transverse topographical profile of the ablated tracks allows polar droplets wetting preferentially the glass surface to follow the track. In this study, we investigate guiding of droplets of two different polar liquids (water/ethylene glycol) with and without surfactant suspended in an oil medium along surface tracks of different depths of 1, 1.5, and 2 \(\upmu\)m. The results of experiments are also verified with computational fluid dynamics simulations. Guiding of droplets along the tracks as a function of the droplet composition and size and the surface profile depth is evaluated by analyzing the trajectories of moving droplets with respect to the track central axis, and conditions for stable guiding are identified. The experiments and numerical simulations indicate that while the track topography plays a role in droplet guiding using 1.5- and 2-\(\upmu\)m deep tracks, for the case of the smallest track depth of 1 \(\upmu\)m, droplet guiding is mainly caused by surface energy modification along the track rather than the presence of a topographical step on the surface. Our results can be exploited to sort passively different microdroplets mixed in the same microfluidic chip, based on their inherent wetting properties, and they can also pave the way for guiding of droplets along reconfigurable tracks defined by surface energy modifications obtained using other external control mechanisms such as electric field or light.

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Acknowledgements

This work was supported by TÜBİTAK (Grant No. 112T972). Z. Rashid and B. Morova thank HEC Pakistan and ASELSAN A.S., respectively, for Ph.D. scholarships. The authors thank M. Waqas Nawaz and Z. Emami for help with initial experiments, A. Ijaz for help with interfacial tension measurements, and M. B. Yağcı for help with SEM and Raman measurements in Koç University Surface Science and Technology Center (KUYTAM).

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Authors and Affiliations

  1. Department of Electrical and Electronics Engineering, Koç University, 34450, Sariyer, Istanbul, Turkey

    Zeeshan Rashid & Alper Kiraz

  2. Department of Mechanical Engineering, Istanbul Technical University, 34437, Gümüşsuyu, Istanbul, Turkey

    Umut Can Coşkun

  3. Department of Physics, Istanbul Technical University, 34469, Maslak, Istanbul, Turkey

    Yağız Morova, Berna Morova & Alexandr Jonáš

  4. Department of Physics, Yıldız Technical University, 34220, Esenler, Istanbul, Turkey

    Asuman Aşıkoğlu Bozkurt

  5. Department of Physics, Koç University, 34450, Sariyer, Istanbul, Turkey

    Ahmet Erten & Alper Kiraz

  6. Egercili Mahallesi, Gokler Sokak No: 9/2, 55500, Carsamba, Samsun, Turkey

    Selçuk Aktürk

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  1. Zeeshan Rashid
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  2. Umut Can Coşkun
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  8. Selçuk Aktürk
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Correspondence to Alper Kiraz.

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Rashid, Z., Coşkun, U.C., Morova, Y. et al. Guiding of emulsion droplets in microfluidic chips along shallow tracks defined by laser ablation. Microfluid Nanofluid 21, 160 (2017). https://doi.org/10.1007/s10404-017-1997-1

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  • DOI: https://doi.org/10.1007/s10404-017-1997-1

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