Electrowetting of nonwetting liquids and liquid marbles

McHale, Glen, Herbertson, Dale, Elliott, Stephen, Shirtcliffe, Neil and Newton, Michael (2007) Electrowetting of nonwetting liquids and liquid marbles. Langmuir, 23 (2). pp. 918-924. ISSN 0743-7463

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Official URL: http://dx.doi.org/10.1021/la061920j

Abstract

Transport of a water droplet on a solid surface can be achieved by differentially modifying the contact angles at either side of the droplet using capacitive charging of the solid-liquid interface (i.e., electrowetting-on-dielectric) to create a driving force. Improved droplet mobility can be achieved by modifying the surface topography to enhance the effects of a hydrophobic surface chemistry and so achieve an almost complete roll-up into a superhydrophobic droplet where the contact angle is greater than 150 degrees. When electrowetting is attempted on such a surface, an electrocapillary pressure arises which causes water penetration into the surface features and an irreversible conversion to a state in which the droplet loses its mobility. Irreversibility occurs because the surface tension of the liquid does not allow the liquid to retract from these fixed surface features on removal of the actuating voltage. In this work, we show that this irreversibility can be overcome by attaching the solid surface features to the liquid surface to create a liquid marble. The solid topographic surface features then become a conformable "skin" on the water droplet both enabling it to become highly mobile and providing a reversible liquid marble-on-solid system for electrowetting. In our system, hydrophobic silica particles and hydrophobic grains of lycopodium are used as the skin. In the region corresponding to the solid-marble contact area, the liquid marble can be viewed as a liquid droplet resting on the attached solid grains (or particles) in a manner similar to a superhydrophobic droplet resting upon posts fixed on a solid substrate. When a marble is placed on a flat solid surface and electrowetting performed it spreads but with the water remaining effectively suspended on the grains as it would if the system were a droplet of water on a surface consisting of solid posts. When the electrowetting voltage is removed, the surface tension of the water droplet causes it to ball up from the surface but carrying with it the conformable skin. A theoretical basis for this electrowetting of a liquid marble is developed using a surface free energy approach.

Item Type: Article
Uncontrolled Keywords: liquid marbles, wetting, EWOD
Subjects: F100 Chemistry
F200 Materials Science
F300 Physics
H600 Electronic and Electrical Engineering
Department: Faculties > Engineering and Environment > Mathematics, Physics and Electrical Engineering
Depositing User: Glen McHale
Date Deposited: 23 Aug 2012 13:12
Last Modified: 24 Oct 2017 08:54
URI: http://nrl.northumbria.ac.uk/id/eprint/8320

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