Dr Peter Ireland

We report the manufacture of complex structures of silica, coal or sphalerite particles around a water droplet, driven by an electrostatic field. A particle bed was deposited on an electrically biased substrate and an earthed water drop brought close, such that the particles jumped to the drop. These structures' shape and internal composition were determined by a combination of the particles' wettability and electrical properties, and other attributes such as shape, size and density were also thought to play a role. Hydrophilic particles tend to be internalised by the drop, while hydrophobic ones tend to form a layer or shell on the surface. Thus, one example of these structures was a 'complex liquid marble', with a hydrophilic particle suspension core and a stabilising shell of hydrophobic particles.

We report the first study of the influence of drop and particle size on the electrostatic manufacture and subsequent stability of liquid marbles. It is clear from this study that the ¿rules¿ for electrostatic formation of liquid marbles are quite different for those for conventional direct-contact manufacture. Formation of liquid marbles was observed when an earthed water drop of volume 3¿7¿µL was brought into proximity with a bed of highly-charged polystyrene particles of diameter 22¿153¿µm. Under appropriate conditions the particles jumped to and coated the drop, producing a particle-liquid aggregate that dropped to the bed surface in the form of either a stable liquid marble or a particle-stabilised sessile drop. The subsequent evolution of the physical dimensions of the metastable aggregate was measured as the liquid drained into the bed, and its stability assessed. Formation of stable liquid marbles appeared to occur more easily for smaller drops and larger particles, and some of these considerably exceeded the conventionally-understood limit for the ratio of particle to drop size of stable liquid marbles.

In this paper we investigate the hypothesis that when bubbles carrying attached hydrophobic particles arrive at an air-liquid interface, the abrupt change in velocity is sufficient to dislodge attached particles, which fall back into the liquid. For the first time, experiments have demonstrated a case in which the particles do not detach, but move smoothly over the surface of the bubbles. The kinetic energy of arrival is dissipated by the motion of the particles through the liquid, as they move over the surface of the bubble while remaining attached. Some energy is also dissipated by the pulsations of the bubbles. The pulsations themselves do not lead to detachment of particles. A theory has been developed to explain the observed phenomena. © 2014 Published by Elsevier B.V.

Buoyancy-driven convective flows have a substantial effect on the performance of the froth layer in flotation cells, particularly when wash water is applied, but are relatively poorly understood. This study presents some experiments on convective flows in a foam undergoing forced drainage. A flat cell was used to create a planar foam, and a dye tracer was used to reveal the flow patterns, which were digitally imaged. The eddy scales and mixing behaviour of the flows are assessed using several different metrics, and their dependence on liquid and gas flow rates in the foam is assessed and compared. Finally, the implications of these findings for the effectiveness of wash water in flotation froths are discussed. © 2013.

Transport of dry solid particles to a liquid is relevant to a number of emerging applications, including 'liquid marbles'. We report experiments where the transport of dry particles to a pendent water droplet is driven by an external electric field. Both hydrophilic and hydrophobic materials (silica, PMMA) were studied. For silica particles (hydrophilic, poorly conductive), a critical applied voltage initiated transfer, in the form of a rapid 'avalanche' of a large number of particles. The particle-loaded drop then detached, producing a metastable spherical agglomerate. Pure PMMA particles did not display this 'avalanche' behaviour, and when added to silica particles, appeared to cause aggregation and change the nature of the transfer mechanism. This paper is largely devoted to the avalanche process, in which deformation of the drop and radial compaction of the particle bed due to the electric field are thought to have played a central role. Since no direct contact is required between the bed and the drop, we hope to produce liquid marble-type aggregates with layered structures incorporating hydrophilic particles, which has not previously been possible.