The role of particles in stabilising foams and emulsions

Porous metals and metallic foams are presently the focus of very active research and development activities. There are currently around 150 institutions working on metallic foams worldwide, most of them focussing on their manufacture and characterisation. Various companies are developing and producing these materials which are now being used in numerous industrial applications such as lightweight structures, biomedical implants, filters, electrodes, catalysts, and heat exchangers. This review summarizes recent developments on these materials, with particular emphasis on research presented at the latest International Conference on Porous Metals and Metallic Foams (MetFoam 2007).

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Porous Metals and Metallic Foams: Current Status and Recent Developments†

1. Particles at liquid interfaces - an introduction B. P. Binks and T. S. Horozov 2. Structure and formation of particle monolayers at liquid interfaces L. Bergstrom 3. Theory for interactions between particles in monolayers J. C. Fernandez-Toledano, A. Moncho-Jorda, F. Martinez-Lopez and R. Hidalgo-Alvarez 4. Particle-assisted wetting W. A. Goedel 5. Particle-laden interfaces: rheology, coalescence, adhesion and buckling G. G. Fuller, E. J. Stancik and S. Melle 6. Solids-stabilized emulsions: a review R. J. G. Lopetinksy, J. H. Masliyah and Z. Xu 7. Novel materials derived from particles assembled on liquid surfaces K. P. Velikov and O. D. Velev 8. Interfacial particles in food emulsions and foams E. Dickinson 9. Collection and attachment of particles by air bubbles in froth flotation A. V. Nguyen, R. J. Pugh and G. J. Jameson 10. Antifoam effects of solid particles, oil drops and oil-solid compounds in aqueous foams N. D. Denkov and K. G. Marinova 11. Metal foams: towards high temperature colloid chemistry N. Babcsan and J. Banhart.

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Colloidal Particles at Liquid Interfaces

The history of metallic foams and the key innovations that have led to the variety of processing methods known today are reviewed. It is evident that the idea of foaming metals is very old and that most of the techniques used today have been proposed already in the 1950s. The most important milestones in the development of foaming technologies and the some of the attempts to commercialise metal foams are reviewed.

Light‐Metal Foams—History of Innovation and Technological Challenges

Liquid metal foam can be generated by creating gas inside a liquid or semi-liquid metallic melt which has been pre-treated in a suitable way. Such foams are self-supporting disordered structures in complete analogy to aqueous foams. The stabilisation mechanism of metallic foams is still not fully understood. In order to facilitate discussion of foam stabilisation the various methods for making such foams are classified with respect to the mode of gas generation and the type of melt used for foaming. Metal foams can be produced by gas injection from an external source or by in-situ nucleation of gas bubbles in the melt for which various possibilities are known. Melts amenable to foaming range from almost pure molten alloys to melts to which particles have been added, formed by in-situ reactions or which have been already present in the solid precursor prior to melting. Some of the available experimental evidence for the action of stabilising particles in metallic foams is presented. It is found that although stabilisation seems to be based on the presence of a solid constituent in all the foaming processes, the mechanisms might vary. At present the nature of stabilisation is not yet fully understood and many questions remain

Metal Foams: Production and Stability†

Metal foams—high temperature colloids: Part I. Ex situ analysis of metal foams

Metal foams were produced by blowing gas into aluminium alloy melts. The effect of oxygen content of the blowing gas on composition and structure of the inner surface of the foam cells is studied by varying gas composition from argon, nitrogen and air to pure oxygen. Scanning Electron Microscopy, Auger Electron Spectroscopy and Transmission Electron Microscopy are used to analyse the surfaces. Initially particle-free melts are pre-treated by bubbling air through them after which a certain degree of foam stability is achieved. The oxidation products are characterised by microscopy on such foams.

The Role of Oxidation in Blowing Particle‐Stabilised Aluminium Foams

Rupture of an individual film in an evolving liquid metal foam is investigated by means of high-speed x-ray radioscopy using white synchrotron radiation. At a frame rate of 5000frames∕s, the rupture event is spread over three to four images. The images show that the remnants of the rupturing film are pulled into the surrounding plateau borders in 600±100μs which conforms well with a liquid movement governed by inertia and not by viscosity. Within one order of magnitude, the viscosity of the liquid involved must be similar to the viscosity of pure liquid aluminium.

Fast processes in liquid metal foams investigated by high-speed synchrotron x-ray microradioscopy

X-ray radioscopy of liquid metalfoams: influence of heating profile, atmosphere and pressure

In the present paper the authors studied isolated metallic films made from the same material used for making metallic foams, and then characterised their properties. Metal films were made from a liquid aluminium alloy reinforced with ceramic particles of known concentration. Melts without such particles were also investigated. It is shown that stable films could not be made from Al–Si alloy having no particles, and just extremely thin and fragile films could be made from commercially-pure Al. In contrast, aluminium alloys containing particles such as SiC and TiB2 allowed pulling thin, stable films, which did not rupture. Significant thinning of films was observed when the particle concentration in the melt decreased. By in situ X-ray monitoring of liquid films during pulling, film thickness and drainage effects within the liquid film could be studied. The morphology and microstructure of films was characterised after solidification. Our work shows that the question of how foams are stabilised can be studied using a simplified system such as a film, instead of having to deal with the multitude of different structural elements present in a foam.

Norbert Babcsan

Papers

Metal foams—high temperature colloids: Part I. Ex situ analysis of metal foams

The Role of Oxidation in Blowing Particle‐Stabilised Aluminium Foams

Fast processes in liquid metal foams investigated by high-speed synchrotron x-ray microradioscopy

X-ray radioscopy of liquid metalfoams: influence of heating profile, atmosphere and pressure

Study on aluminium-based single films