Method for evaluating the Ostwald ripening kinetics of air microbubbles

The stability of foams (and of aerated systems in general) is affected by three basic mechanisms: drainage, Ostwald ripening (also called disproportionation), and coalescence. Ostwald ripening, in particular, refers to the changes in bubble size due to gas diffusion driven by local differences in Laplace pressure, or by gas diffusion to a head space. This process can be strongly influenced by the bulk and interfacial rheological properties of the system [1]. It is therefore of interest to be able to modulate Ostwald ripening by adjusting the system properties in order to increase the stability of aerated products. To study the Ostwald ripening process, we have developed an observation cell that allows following the shrinkage of multiple bubbles (50-150 m in diameter) simultaneously while they dissolve under controlled conditions (temperature and bulk gas concentration). The bubble size is controlled using microfluidic methods, and bubbles are generated at low frequency. The gas concentration is measured and equilibrated with the atmosphere before measurements. This work describes the application of this set-up to study and compare the Ostwald ripening of bubbles stabilized in various systems: pure water, 0.1 wt% SDS, 0.1 wt% native lactoglobulin at pH 2, 0.1 wt% -lactoglobulin fibers at pH 2 [2]. Bubble shrinkage curves obtained as a result of the analysis accurately illustrate the differences between the systems investigated. As expected, bubbles in pure water and SDS solutions shrunk at rates directly linked to the surface tension values, whereas those stabilized by proteins shrunk at rates influenced by surface properties of the protein films. A simplified model describing the physical phenomenon [3] was used to fit the experimental values. Required physical data was either measured or taken from literature. A good agreement between experimental curves and theoretical values was obtained.