Advanced #74

Some salts prevent the coalescence of small bubbles

Certain salts prevent small gas bubbles in water from merging into larger ones.

Scientific Explanation

In pure water, small gas bubbles readily merge — they approach, the liquid film between them thins, and they eventually coalesce into a larger bubble. In salt water, however, this coalescence is nearly completely suppressed by certain salts, even at relatively low concentrations.

This is surprising because one would intuitively expect salts that increase surface tension to facilitate bubble merging. Instead, certain salts stabilize the liquid film between two approaching bubbles. As two bubbles draw near, the ions between them become asymmetrically distributed, creating an osmotic pressure that resists the squeezing out of the film water. This effect depends on the specific ion combination — not all salts show it equally.

Interestingly, the effectiveness of salts roughly follows the Hofmeister series: kosmotropic ion combinations are more effective at preventing coalescence than chaotropic ones.

Salt Prevents Bubble Coalescence Two-panel schematic comparing bubble behavior. Left panel shows pure water where two small bubbles merge into one large bubble. Right panel shows salt water where two small bubbles remain separate, prevented from coalescing by salt ions at the interface. Pure Water ↓ Merged Salt Water ↓ Na⁺ Cl⁻ Blocked! Certain salts prevent small bubbles from merging (explains ocean foam persistence)
In pure water, bubbles merge; in salt water, certain salts prevent coalescence.

Everyday Relevance

This phenomenon is most strikingly visible in the ocean: seawater foams much more readily than fresh water because the small bubbles do not merge and instead persist as stable foam. The white foam caps on waves and the long-lasting foam at the shoreline are direct consequences of this effect.

In industry, understanding bubble coalescence is important for froth flotation (where bubble stability is used to separate minerals), wastewater treatment, and the food and beverage industry (for example, in the production of sparkling wines and beer).