Substrate Wettability Influences Internal Jet Formation and Mixing during Droplet Coalescence

Sykes, Thomas C. and Harbottle, David and Khatir, Zinedine and Thompson, Harvey M. and Wilson, Mark C. T. (2020) Substrate Wettability Influences Internal Jet Formation and Mixing during Droplet Coalescence. Langmuir, 36 (32). pp. 9596-9607. ISSN 1520-5827

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The internal dynamics during the axisymmetric coalescence of an initially static free droplet and a sessile droplet of the same fluid are studied using both laboratory experiments and numerical simulations. A high-speed camera captured internal flows from the side, visualized by adding a dye to the free droplet. The numerical simulations employ the volume of fluid method, with the Kistler dynamic contact angle model to capture substrate wettability, quantitatively validated against the image-processed experiments. It is shown that an internal jet can be formed when capillary waves reflected from the contact line create a small tip with high curvature on top of the coalesced droplet that propels fluid toward the substrate. Jet formation is found to depend on the substrate wettability, which influences capillary wave reflection; the importance of the advancing contact angle subordinated to that of the receding contact angle. It is systematically shown via regime maps that jet formation is enhanced by increasing the receding contact angle and by decreasing the droplet viscosity. Jets are seen at volume ratios very different from those accepted for free droplets, showing that a substrate with appropriate wettability can improve the efficiency of fluid mixing.

Item Type: Article
Additional Information: ** Article version: VoR ** From Crossref via Jisc Publications Router ** History: epub 06-08-2020; issued 06-08-2020; ppub 18-08-2020. ** Licence for VoR version of this article starting on 07-08-2020:
Identification Number:
20 July 2020Accepted
6 August 2020Published
Uncontrolled Keywords: Spectroscopy, Electrochemistry, General Materials Science, Surfaces and Interfaces, Condensed Matter Physics
Subjects: CAH10 - engineering and technology > CAH10-01 - engineering > CAH10-01-09 - chemical, process and energy engineering
Divisions: Faculty of Computing, Engineering and the Built Environment > School of Engineering and the Built Environment
SWORD Depositor: JISC PubRouter
Depositing User: JISC PubRouter
Date Deposited: 09 Sep 2020 10:38
Last Modified: 12 Jan 2022 13:10

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