Of Bubbles, Particles, Interfacial Fluid Flow and Why Things Get Wet

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

ICI Distinguished Lecture - Part 1

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

Abstract:

Wetting and interface science largely influence the success (or failure) of many natural and industrial processes that rely on the wetting or non-wetting of surfaces. For example nature has designed the lotus leaf to give a strongly non-wetting exterior, so that a water droplet will sit proud of the surface. Yet for many solids a strongly wetting surface is desirable, as in coating processes, the formation of particle dispersions and many solid-liquid separations.
Wetting and fluid flow are also central to understanding and controlling the behaviour of small quantities of liquids on solid surfaces and through narrow channels, the science of microfluidics. The structure, both chemical and topographical, of the interfaces involved and the manner in which a solid surface interacts with a liquid are crucial to manipulating the processes involved.

It is essential to identify and measure the forces involved when particles, bubbles or droplets interact, how energy is dissipated when a liquid moves over a solid surface and what sort of interfacial liquid structure may exist, for the latter impinges upon slip length.

Delicate and precise measurements of forces, coupled to the direct interferometric measurement of distance, and supported by stability investigations of particle dispersions, permit static and dynamic interaction behaviour to be quantified, when solid particles interact with each other or with soft matter. X-ray reflectivity studies of solid liquid interfaces reveal that liquids are structured in a different manner from the bulk liquid, over just a few molecular layers, yet this persistent structure can have a profound influence upon interfacial properties. When a liquid flows over a surface or through a narrow channel, energy may be dissipated at the three phase contact line, by a thermally activated, interfacial rate process or in the bulk liquid. Which mode dominates depends on the fluid flow rates, the strength of the solid-liquid interactions, along with the structure and topography of the surface.

The exciting science underlying these different phenomena is discussed, accompanied by diverse applications that include flotation, solvent extraction using microfluidics and the rheology of dense particle dispersions.