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Droplets coalescence and mixing with identical and distinct surface tension on a wettability gradient surface

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Abstract

The influence of identical and distinct surface tensions on the coalescence and mixing of droplets after a direct collision on a wettability gradient surface (made from a self-assembled monolayer, SAM technique) was investigated. The results indicate that their mixing is driven sequentially by interior convection and diffusion; the convection endures less than 100 ms but dominates more than 60 % of the mixing. If the stationary droplet has a large surface tension (73.28 mN × m−1), whether the moving droplet has a large surface tension (73.28 mN × m−1) or a small surface tension (38.63 mN × m−1), the mushroom-shaped mixing pattern is generated within the coalesced droplet that enhances the convective mixing and also significantly enlarges the interface for mass diffusion. The mixing index of these two cases was greater than 0.8 at 120 s after the collision. For the cases in which the stationary droplet with a small surface tension collided by the moving droplet with a large surface tension, a mixing pattern with a round-head shape developed, which was insufficient to benefit the mixing. When the stationary and moving droplets both had small surface tension, the moving droplet was unable to merge with stationary droplet and had poor mixing quality due to the small surface Gibbs energy of both stationary and moving droplets. For the collision of droplets of identical surface tension, the surface tension affects the coalescence behavior; for the collision of droplets with distinct surface tension, the coalescence behavior and mixing quality depend on the colliding arrangement of stationary and moving droplets.

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Acknowledgments

National Science Council of Taiwan, ROC supported this work under grant number NSC-100-2120-M-002-013.

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Authors and Affiliations

  1. Institute of Applied Mechanics, National Taiwan University, 1 Sec. 4 Roosevelt Rd., Taipei, 106, Taiwan

    Szu-I Yeh & Horn-Jiunn Sheen

  2. Department of Mechanical Engineering, National Taiwan University, 1 Sec. 4 Roosevelt Rd., Taipei, 106, Taiwan

    Wei-Feng Fang & Jing-Tang Yang

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  1. Szu-I Yeh
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  2. Wei-Feng Fang
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  4. Jing-Tang Yang
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Correspondence to Jing-Tang Yang.

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Yeh, SI., Fang, WF., Sheen, HJ. et al. Droplets coalescence and mixing with identical and distinct surface tension on a wettability gradient surface. Microfluid Nanofluid 14, 785–795 (2013). https://doi.org/10.1007/s10404-012-1096-2

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  • DOI: https://doi.org/10.1007/s10404-012-1096-2

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