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Surfing of drops on moving liquid–liquid interfaces

Published online by Cambridge University Press:  08 April 2020

Teng Dong Open the ORCID record for Teng Dong [Opens in a new window] ,
Fei Wang ,
Weheliye H. Weheliye  and
Panagiota Angeli Open the ORCID record for Panagiota Angeli [Opens in a new window]
Teng Dong
Affiliation:
ThAMeS Multiphase, Department of Chemical Engineering, University College London, LondonWC1E 7JE, UK
Fei Wang
Affiliation:
ThAMeS Multiphase, Department of Chemical Engineering, University College London, LondonWC1E 7JE, UK Beijing Institute of Space Long March Vehicle, No. 1 Nandahongmen Road, Fengtai District,Beijing100076, PR China
Weheliye H. Weheliye
Affiliation:
ThAMeS Multiphase, Department of Chemical Engineering, University College London, LondonWC1E 7JE, UK
Panagiota Angeli*
Affiliation:
ThAMeS Multiphase, Department of Chemical Engineering, University College London, LondonWC1E 7JE, UK
*
†Email address for correspondence: p.angeli@ucl.ac.uk
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Abstract

The delayed coalescence of drops with the interface between a moving aqueous layer and an oil phase is investigated in a novel flow channel. Drops are released onto oil–aqueous interfaces moving at velocities from $0~\text{cm}~\text{s}^{-1}$ up to $3.4~\text{cm}~\text{s}^{-1}$. The evolution of the drop shape, the film thickness between the drop and the bulk liquid, and the velocities of the drop surface and the bulk interface were measured with planar laser-induced fluorescence. As the interface speed increases, the drop coalescence is delayed. This is attributed to the lubrication pressure that develops in the draining film. This pressure was calculated by using the drop shape and the tangential velocities of the drop surface and the bulk interface, and was shown to increase with the interface velocity. The film forming between the drop and the bulk liquid has a dimple shape, symmetric about the centreline. With increasing interface velocity, the dimple shifts to the front part of the drop, resulting locally in a low pressure, which leads to film rupture. As the film breaks, ‘oil drops on a string’ formations are entrained into the water phase, which is rarely seen when a drop coalesces with a stationary liquid–liquid interface. The velocity fields in the drop were investigated with particle image velocimetry. It is found immediately after reaching the interface that the drops accelerate to reach the interface speed. Initially there is a strong internal circulation in the drops, which decays quickly as the drops approach the speed of the interface.

JFM classification

Drops and Bubbles: Drops Drops and Bubbles: Breakup/coalescence Multiphase and Particle-laden Flows: Multiphase flow
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 892 , 10 June 2020 , A36
Copyright
© The Author(s), 2020. Published by Cambridge University Press

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Dong et al. supplementary movie 1

The traveling drop along with the interface at v =3.4 cm/s from the impact position

Download Dong et al. supplementary movie 1(Video)
Video 15.6 MB

Dong et al. supplementary movie 2

Non-coalescence of pendent drop at the moving interface of v = 2.1 cm/s.

Download Dong et al. supplementary movie 2(Video)
Video 5.9 MB

Dong et al. supplementary movie 3

The coalescence where the rupture point locates in the front bottom of the drop.

Download Dong et al. supplementary movie 3(Video)
Video 18.1 MB

Dong et al. supplementary movie 4

Formation of the drops-on-string during the film breakage at the interface of v = 3.4 cm/s.

Download Dong et al. supplementary movie 4(Video)
Video 6.9 MB