Rational Design of Foamed Materials : Rheological and Interfacial Aspects (Rationeel ontwerp van geschuimde materialen: reologische en interfase effecten)

Abstract:This doctoral dissertation describes the role of interfacially active solid particles in the stabilization of multiphase materials as foams and emulsions. As the factors controlling their stability are, at least qualitatively, known, this thesis addresses three specific key challenges. The first key challenge is the improvement of the understanding of the relation between interfacial thermodynamic and rheological properties on the one hand and foam and emulsion stability on the other hand. A home-made thin film balance has been developed which enables the study of the properties of thin liquid films. It has been validated by benchmarking the set-up with available literature data on surfactant stabilized foam films. The large range of potential applications of the thin film balance has been demonstrated by the in vitro formation of large scale, planar lipid bilayers. The melting point of the lipids appeared to be a determining system parameter. Concerning the design, the careful control of the applied capillary pressure in the low range is important. Using model systems with known interfacial characteristics, this device, operated in the dynamic mode, can establish a link between interfacial rheology and film thinning. Furthermore it can be used as a thin film rheometer, provided that suitable constitutive models for the interface are available. The stability of aqueous foams and emulsions can be improved by means of strong, elastic layers covering the individual bubbles and droplets. For particle layers, the intrinsic lateral interfacial particle interactions are at the origin of their strength and elasticity. Shear interfacial rheological measurements of monolayers of rough carbon black particles with attractive capillary interactions at the n-octane-water interface have confirmed theoretical predictions of elastic monolayers formed by particles with undulated contact lines. The attractiveness between the particles is set by the roughness scale of the particles. The effects of both particle concentration and interaction strength on the rheological properties can be scaled onto a single master curve. The carbon black monolayers behave as elastic soft glasses, including the process of ageing with time, i.e. thelayers become more solid like with time. These particles can successfully be used as an emulsion stabilizer. These emulsions show nonspherical droplets that do not relax with time. Finally, insights from the low viscous systems, namely the importance of the wetting properties of the solid particles, have been applied to an industrial relevant polyurethane foam system. It is shown that the improvement of the mechanical properties of rigid polyurethane foams by the addition of clay particles is not trivial. The expected beneficial effect of the clay particles can be balanced or be outweighed by a reduction in the strength of the matrix. On the contrary, clay particles, which are likely to get trapped at the cell wall, are indeed able to improve the mechanicalproperties, even though they are more difficult to disperse initially, before foaming.

Publication year:2014

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