A bubble is defined as a globule of gas suspended within a liquid or solid. These pockets of gas can be composed of air, water vapor, or any other gas. Understanding bubble formation requires looking at different areas of science, including thermal physics, solution chemistry, and mechanical fluid dynamics. The process is governed by specific physical thresholds and chemical principles that determine when and how a gas phase separates from the liquid phase.
Nucleation and Phase Change
One fundamental way bubbles form is through a phase transition, where the liquid turns into its gaseous state, known as vapor. This process, commonly observed when boiling water, is driven by the addition of heat energy. As the temperature of a liquid rises, the kinetic energy of its molecules increases, which elevates the vapor pressure.
When the liquid’s vapor pressure equals the pressure exerted by the surrounding atmosphere, the bulk phase change begins at the boiling point. For a vapor bubble to form and grow, it must first overcome the liquid’s surface tension. This initial hurdle is overcome at a starting point called a nucleation site.
Nucleation sites are typically microscopic imperfections, such as tiny scratches on the container wall or trapped air pockets within suspended impurities. These sites provide a pre-existing gas-liquid interface where the liquid can more easily transition to vapor, lowering the energy barrier for bubble creation. Once a tiny vapor nucleus forms, it collects surrounding vapor molecules and expands until buoyancy forces it to detach and rise.
Exsolution of Dissolved Gases
A different process involves gas already dissolved in the liquid suddenly coming out of solution, a phenomenon called exsolution. This principle explains the fizz in carbonated beverages and has biological applications. The amount of gas a liquid can hold is directly proportional to the partial pressure of that gas above the liquid, a relationship described by Henry’s Law.
When a bottle of soda is sealed, the liquid is held under high pressure, forcing carbon dioxide gas to dissolve. Upon opening the cap, the pressure above the liquid instantly drops to atmospheric pressure, drastically lowering the gas’s solubility according to Henry’s Law. The liquid becomes supersaturated, and the excess dissolved gas leaves the solution by forming bubbles, causing the drink to fizz.
Temperature also plays a role because the solubility of most gases in water decreases as the temperature rises. This is why bubbles often appear on the bottom of a pan of water before it reaches boiling; the warming liquid can no longer hold the same amount of dissolved air, which then exsolves. Decompression sickness, or “the bends,” affects divers. At depth, higher pressure causes more nitrogen gas to dissolve into the diver’s blood and tissues. If the diver ascends too quickly, the sudden drop in pressure causes the nitrogen to exsolve as bubbles in the bloodstream, potentially blocking circulation and causing severe injury.
Gas Generation via Chemical Processes
Bubbles can also form when a chemical reaction within the liquid generates a new gaseous product. Unlike exsolution, this mechanism involves the creation of new gas molecules that are highly insoluble in the liquid medium. The reaction must produce a gas that is either unstable under the conditions or has an extremely low solubility limit.
A common demonstration is the reaction between an acid and a carbonate-containing substance, such as combining vinegar with baking soda. This reaction yields carbonic acid, which is unstable and immediately decomposes into water and carbon dioxide gas. The carbon dioxide molecules, unable to remain in solution, nucleate to form bubbles and escape.
Biological processes also use this mechanism, such as in the fermentation of bread dough or alcoholic beverages. Yeast consumes sugars and generates ethanol and carbon dioxide gas as byproducts. The carbon dioxide forms small bubbles within the viscous dough or liquid, causing the dough to rise or the beverage to carbonate.
Entrainment and Cavitation
Mechanical forces can introduce or create gas pockets in a liquid through two distinct processes: entrainment and cavitation. Entrainment occurs when external air is trapped and pulled into a liquid through mixing, stirring, or turbulent flow. This happens when air is drawn in from the surface, such as when water free-falls into a tank or when a pump’s suction line is leaky.
The resulting bubbles are composed of the surrounding atmosphere and are simply mixed into the liquid, often leading to a foamy appearance. In contrast, cavitation is the formation of vapor-filled voids caused by a rapid, localized drop in pressure within the liquid. This typically occurs in high-speed fluid systems, such as near the blades of a propeller or a pump impeller.
When the liquid’s pressure falls below its vapor pressure due to the high velocity of the flow, the liquid momentarily boils, creating tiny vapor bubbles. As these bubbles move away from the low-pressure zone into an area of higher pressure, they violently collapse back into a liquid state. This implosion creates intense, localized shockwaves that can cause pitting and damage to machinery components.