Dynamic Wetting and Adhesion at Structured Surfaces

Laureate Professor John Ralston AO FAA FTSEM
Ian Wark Research Institute
University of South Australia
Mawson Lakes Campus

ICI Distinguished Lecture - Part 2

3:30pm - September 29, 2011
ETLC 1-001

Abstract:

A conducting droplet of an aqueous salt solution or an ionic liquid, in contact with air or immersed in an immiscible liquid, will electrowet a smooth, hydrophobic , insulator-coated electrode when a potential is applied. The advancing liquid contact angle decreases below the value at zero voltage as the potential is either increased or decreased. The electrowetting curves (contact angle versus voltage) are reversible and symmetric with respect to zero voltage, displaying minimal hysteresis and follow the Young-Lippmann equation below saturation. For a partially hydrophobized metal oxide surface such as titania, the advancing water contact angle is a maximum at the point of zero charge, decreasing symmetrically on either side in a Lippman-like manner. When a surface is coated with a thin organic layer capable of being stimulated by light, or when certain metal oxide surfaces are heat treated, comparable changes changes in wettability may be obtained. In all of these examples of "stimulated wetting" the increase in wettability of the solid surface arises from a reduction in the interfacial tension at the solid-liquid interface.

During dynamic wetting or dewetting of a partially wetting surface, the dependence of dynamic contact angle on wetting velocity can be predicted by the molecular kinetic theory (MKT) and /or by a hydrodynamic approach, depending upon the relevant velocities and contact angles, as well as liquid and interface type. There is a strong connection between liquid structure, work of adhesion, surface friction and molecular size. Surface heterogeneity, has a strong influence on dynamic wetting behaviour- pinning of the three phase contact line can play an important role.

This dynamic wetting behaviour, irrespective of whether it is spontaneous [e.g. a droplet spreading during partial wetting] or forced [e.g. during electrowetting, or when a plate is immersed in a liquid] has significant ramifications for control of micro and nanofluidic flow in open channels as well as closed capillaries. Surface friction and slip length are closely connected. The science underlying these various phenomena is illustrated by practical examples ranging from electro-optical imaging to advanced surface coatings processes and biomedical detection devices.