Observations of air-bubble mergers in water explain why dissolved salt slows this process and leads to foam.
Air bubbles churned up in pure water can easily merge. But bubbles merge far more slowly in seawater or in other liquids containing dissolved impurities, which is why such liquids often generate enduring foams. Now a team of engineers believes that it has identified the fundamental cause of the difference—subtle forces set up by electrolytes, mobile ions created when substances dissolve in liquids [1]. In a collision between two bubbles, these forces greatly reduce the rate at which the liquid separating the bubbles can flow away. This understanding, the researchers say, explains why foams arise so easily in salty seawater and could be useful in many industrial applications.
Solutions with high electrolyte concentrations often produce persisting foams, so researchers have suspected for decades that dissolved electrolytes somehow slow bubble mergers. The effect has remained mysterious, however, and many theories even suggest that electrolytes should speed up bubble mergers, says mechanical engineer Bo Liu of the University of Alberta in Canada.
Leave a reply