Bubbles collision with various interfaces
WHAT HAPPENS WHEN BUBBLE REACHES FREE SURFACE???
Bikerman wrote in his text-book ( Bikerman "Foams", Springer-Verlag Berlin, Heidelberg, New York, 1973, p. 57) that:
"When the bubble reaches the upper surface of the liquid, and the liquid has no foaming tendency, the bubble burst at once; that is the film separating it from the bulk gas phase immediately ruptures. When the liquid contains a foaming agent, the above film has a significant persistence, and the bubble lifts a "dome" ...".
Our recent studies showed that the situation is more complicated and fascinating phenomena occurring in time scale of milliseconds, prior to the bubble rupture in distilled water, are illustrated in Fig.1. When a bubble rising in distilled water "collides" with the air/water interface then bouncing from the interface and rapid pulsations of the bubble shape, with a frequency of more than 1000 Hz, can occur. Presence of surface active substance causes diminishing of amplitude of the bubble's bouncing and shape pulsations, and a prolongation of the bubble lifetime at the free surface, as a result of the increased stability of the thin liquid layer (foam film) separating the bubble from the atmosphere. Occurrence of so rapid bubble's shape pulsation and consequently, its surface area variation is a straightforward indication that on such a rapidly distorted interface the adsorption coverage can be very different from that at equilibrium. This fact should be more appropriately taken into account in discussion of the mechanism of formation and stabilization of various dispersed system.
Fig. 1. Sequence of photos showing shape pulsation and bouncing of the bubble during collision with water surface. Each subsequent frame shows the bubble position after a time interval 0.845 ms.
WHAT HAPPENS WHEN BUBBLE COLLIDES WITH A SOLID SURFACE???
Bubble collision with solids and formation of the three phase contact is a necessary condition for flotation separation. It is rather common understanding that immediate attachment should occur in the case of hydrophobic surface, while there should be no attachment in the case of the hydrophilic ones. Recently, we have found that even in the case of such hydrophobic solid surface as Teflon, the bubble attachment didn't need to occur at first collision and in distilled water the bubble can bounce a few times without attachment (see Fig. 2A). Presence of surface active substance facilitates the bubble attachment to hydrophobic solid surface. Time-scale of the TPC formation is very short, of an order of single ms. It was observed that presence of a micro-bubble at the solid surface facilitated drastically an attachment of the colliding bubble (see magnified pictures - Fig. 2B). Roughness of Teflon surface increases probability of the bubble attachment - most probably - as a result of higher probability of micro- and/or nano-bubbles presence at the solid surface
Fig. 2. Bubble collisions with Teflon plate. A)- sequence of photos (time interval 0.845 ms) showing the bubble bouncing, shape pulsations and attachment to Teflon surface. B) - the magnified 4th, 10th, 11th, 41st and 42nd frames of the sequence (A).