8. Microbubbles: Properties, Mechanisms of Their Generation

This chapter discusses microbubbles—small gas bubbles in liquid medium of diameter less than 1 mm. Although they were known to offer a number of advantages, until recently they could be generated only by methods energetically inefficient. New horizons became open by the discovery of generation by aerators provided with an oscillator in their gas supply. Chapter provides in particular an information about no-moving-part fluidic oscillators, recently already almost forgotten but now demonstrated to offer benefits like low manufacturing cost, reliability, long life and absence of maintenance. The empirical fact that small bubbles cannot be obtained simply by making small passages in the aerator is here explained by conjunction of several microbubbles. Because the velocity of bubble motion decreases with decreasing size, small microbubbles tend to dwell near the aerator exits. They then coalesce there into a much larger single bubble (the effect promoted by the latter possessing lower surface energy). The fact that the oscillator prevents this conjunction and thus keeps the microbubbles small has been explained by high-speed camera images which show the effect of oscillatory motions.