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Issue 37, 2018
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Two-component marangoni-contracted droplets: friction and shape

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When a mixture of propylene glycol and water is deposited on a clean glass slide, it forms a droplet of a given apparent contact angle rather than spreading as one would expect on such a high-energy surface. The droplet is stabilized by a Marangoni flow due to the non-uniformity of the components' concentrations between the border and the apex of the droplet, itself a result of evaporation. These self-contracting droplets have unusual properties such as absence of pinning and the ability to move under an external humidity gradient. The droplets' apparent contact angles are a function of their concentration and the external humidity. Here we study the motion of such droplets sliding down slopes and compare the results to normal non-volatile droplets. We precisely control the external humidity and explore the influence of the volume, viscosity, surface tension, and contact angle. We find that the droplets suffer a negligible pinning force so that for small velocities the capillary number (Ca) is directly proportional to the Bond number (Bo): Ca = Bo sin α with α the angle of the slope. Lastly we study the successive shapes the droplets take when sliding at larger and larger velocities.

Graphical abstract: Two-component marangoni-contracted droplets: friction and shape

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Article information

01 Dec 2017
23 Aug 2018
First published
24 Aug 2018

Soft Matter, 2018,14, 7724-7730
Article type
Author version available

Two-component marangoni-contracted droplets: friction and shape

A. Benusiglio, N. J. Cira and M. Prakash, Soft Matter, 2018, 14, 7724
DOI: 10.1039/C7SM02361H

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