Title: The spontaneous motion of a slug of miscible liquids in a capillary tube

Authors: Mathieu Sellier; Claude Verdier; Volker Nock

Addresses: Department of Mechanical Engineering, University of Canterbury, Private Bag, 4800, Christchurch, 8140, New Zealand ' Laboratoire Interdisciplinaire de Physique (LIPhy), CNRS and Université Grenoble Alpes (UMR 5588), F-38000, France ' Department of Electrical and Computer Engineering, University of Canterbury, Private Bag, 4800, Christchurch, 8140, New Zealand

Abstract: This contribution explores a droplet actuation mechanism which involves mixing slugs of two different liquids in a glass capillary. The resulting contrast in surface tension which arises provides the necessary propulsive power for the droplet. The conceptual idea is demonstrated for an ethanol-water system. The droplet is observed to rapidly reach a peak velocity which then gradually decreases with time as the two miscible liquids mix. A model is proposed based on Newton's second law which is able to capture the main observed flow phenomena and explain the driving and dissipative mechanisms simultaneously at play in the droplet. This passive actuation mechanism could prove an attractive alternative in digital microfluidics systems for which bulky pumping systems are often required.

Keywords: droplets; droplet actuation; self-propulsion; digital microfluidics; multiphase flow; miscible liquids; contact line; wetting; spontaneous motion; capillary tubes; surface tension; ethanol-water systems.

DOI: 10.1504/IJNT.2017.082475

International Journal of Nanotechnology, 2017 Vol.14 No.1/2/3/4/5/6, pp.530 - 539

Published online: 21 Feb 2017 *

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