The mechanism of deinking by flotation has been studied in a bench top deinking loop, in which a small quantity of dilute pulp is circulated downwardly through a small transparent observation tube. Bubbles are introduced by a syringe needle, into the observation tube, to be observed through high speed micro video recordings. It can be seen how ink particles themselves to bubbles, are in free Different bubble sizes can be observed, depending on the fluid velocity chosen. The above work is related to earlier work by Professor Ed Thompson at the University of Maine. On a hydrophobic surface, a water drop forms a bead, the surface tries to minimize its contact with the water phase. Or, alternatively air squeezes itself between the solid and the water to maximize the air contact with the solid. Conventionally, the same situation should apply to air bubbles and hydrophobic particles in water. The particle would be happy to find a bubble so that it can "dry" or "unwet" some of its surface, which hates the predominantly flat toner particle, which has a hydrophobic surface, attach itself to the bubble? In the of the real observations, have created mental models, Figure 1, and I would have sworn that the bubble must actually try to swallow the particle, in order to push the water phase away and to surround the particle with air. Of course, the situation could also be discussed in terms of contact angles, but for our purposes, the concept wetting or unwetting can be sufficient. As illustrated in Figure 1, only the model whereby the air bubble surrounds the particle really fulfills the concept of unwetting and the common sense requirement of having a basically overall round and convex bubble. Unfortunately, my belief was shattered a couple of years ago, when I saw micro video recording of a stationary air bubble which had caught a toner particle (Reference 1) Attachment had occurred in none of the three ways of Figure 1, but, alas, in a way which actually minimized the contact with the air, to a single point. This observation completely contradicted the conventional wisdom which would require a maximum contact with the air. Immediately, I tried to explain the phenomenon as a systematic experimental error. Perhaps, the unexpected mode of attachment was due to the fact that the bubble was fixed on a syringe. Was some electrostatic potential at play which caused the aberration? The observation irritated me so much that we set out to find a way to observe air bubbles which were free floating in a stream of water or pulp, as discussed in this paper.
