Micromechanics of particle-coated bubbles: deformation from quasistatic to millisecond timescales

Abstract

Particles adsorbed at fluid-fluid interfaces confer stability to dispersed systems such as foams and emulsions. The emergent properties associated with the interfacial microstructure underpins the creation of functional materials. In the design, synthesis and application of such materials, it is essential to understand the dynamic behaviour of structured interfaces at deformation timescales that are relevant in practical scenarios. In this experiment-driven study, a bubble is used as a probe to understand the stability mechanisms and dynamics of fluid-fluid interfaces coated with particles. First, in a model wax-based oil foam, or oleofoam, bubble dissolution time, under controlled conditions, is used as a parameter to assess the bulk and interfacial rheological contributions responsible for the remarkable stability observed. Focus is then drawn to interfacial phenomena, by removing bulk effects, through microscopic observations of crystal-coated bubbles undergoing deformation due to either bubble dissolution or ultrasound-driven volumetric oscillations. In this way phenomena at two extreme timescales, 10,000 s and 0.0001 s, are observed and interpreted. Finally, the effect of unsteady, fast deformation on a complex interface is systematically studied using a well characterised model interface, comprising of bubbles coated with optically resolvable, monodisperse latex microspheres. The bubbles are subjected to acoustic forcing, leading to the rapid cyclic compression and expansion of the colloidal monolayer. Effects of pressure amplitude, particle size and surface coverage on bubble excursions are studied. The results signify the importance of local mesoscopic phenomena in explaining the stability of oleofoams, where invoking macroscopic rheological reasoning alone is somewhat inadequate. Experimental timescales strongly influence the nature, integrity and response of complex interfaces to imposed stresses. Further, a bubble driven by ultrasound has potential in studying time-dependent interfacial mechanics.