The velocity of bubbles rising in pure liquids is affected by the boundary conditions at the liquid–air interface, with bubbles rising the fastest when the bubble surface is fully mobile. The presence of even very small amounts of surface-active molecules causes tangential immobility at the liquid–air interface and subsequently results in slower bubble rise velocity. The existing literature on the rise velocities of air bubbles in high-purity water does not provide a conclusive picture on whether or not the water–air interface is immobile, with the most discrepancies reported for very small bubbles. This paper presents the first systematic study of bubble rise velocities in high-purity water for a sufficiently wide range of bubble sizes (bubble diameter between 48 μm and 1.5 mm) and for the same experimental conditions, which will allow firm conclusions to be drawn on this issue. For bubbles of diameter 800 μm or larger, the measured rise velocity is in a good agreement with the theoretical predictions for a completely mobile water–air interface, but the velocity starts to deviate from that of mobile bubbles as the size becomes smaller. The smaller the bubble, the closer the rise velocity corresponds to an immobile water–air interface, and for Re < 1, the bubble rise velocity agrees with the Stokes law. We use the rear stagnant cap model to explain why smaller bubbles are significantly more sensitive to any surface-active impurities than larger ones.

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The influence of two biopolymers (xanthan gum and locust bean gum) on the interaction between bubbles and graphite has been elucidated using a combination of direct measurement techniques. Bubble–surface collisions (monitored using high speed video capture) reveal that when graphite is exposed to low concentration solutions of the two polymers, the time scale of bubble attachment is prolonged by 1–2 orders of magnitude, and the final receding water contact angle achieved on such surfaces is reduced by approximately 30 deg. Single bubble flotation studies confirm the significant effect of such aspects of bubble–particle collisions on the collection efficiency of graphite particles, with marked reduction in flotation recovery across the particle size range, with greatest effect on the coarser particle sizes. The differences in performance of the two polymers in reducing bubble–particle attachment is seen to be partly due to variation in adsorbed layer coverage of the two polymers on the graphite surface, as revealed by atomic force microscopy imaging. Both polymers can be expected to perform well in the prevention of flotation of graphitic/carbonaceous minerals.

Currently there are no available methods for in-line measurement of gas-liquid interfacial tension during the flotation process. Microfluidic devices have the potential to be deployed in such settings to allow for a rapid in-line determination of the interfacial tension, and hence provide information on frother concentration. This paper presents the development of a simple method for interfacial tension determination based on a microfluidic device with a flow-focusing geometry. The bubble generation frequency in such a microfluidic device is correlated with the concentration of two flotation frothers (characterized by very different adsorption kinetic behavior). The results are compared with the equilibrium interfacial tension values determined using classical profile analysis tensiometry.