Why bubbles start at particular spots
Bubbles need a starting point. On a smooth heating surface, water can't easily form a vapor bubble from nothing because it would have to create a whole new surface between liquid and vapor. Instead, tiny pits, scratches, or trapped gas pockets on the surface act as ready-made starting points. These are called nucleation sites. When the surface gets hot enough, vapor begins to form at these sites and grows into a bubble [4][5].
The bubble doesn't just sit there. As it grows, a thin film of liquid gets trapped between the bubble and the hot surface. This film is called the microlayer, and it evaporates quickly, feeding the bubble's growth [4]. So the bubble is not just a passive pocket of vapor; it actively draws energy from the hot surface through this thin liquid layer.
How bubbles grow and let go
As the bubble grows, it eventually reaches a size where it detaches from the surface and rises. The timing of this departure depends on pressure and flow conditions. In high-pressure flow boiling, for example, bubbles stay spherical as they grow and slide along the heated surface, and their departure size and growth time can be predicted from the water's physical properties and how fast the bubble is growing [5].
Bubbles don't always act alone. When multiple bubbles form close together, they interact. A second bubble forming nearby can change how the first bubble's microlayer depletes, sometimes speeding up its lift-off or affecting its movement. This means that in real boiling, bubbles influence each other's behavior, and models that ignore these interactions may overestimate how much the microlayer contributes to bubble growth [4].
What bubble patterns reveal about boiling
The way bubbles appear and disappear over time is not completely random. A laser scattering experiment that passed light through boiling water found that the time intervals between bubble events did not follow a simple exponential distribution—the pattern expected if bubbles formed independently at a constant rate. Instead, the intervals had a heavy-tailed distribution, meaning long gaps between events were more common than a simple random process would predict [1]. This suggests that bubbles interact and influence each other's timing, consistent with the idea that boiling is a collective process rather than a series of isolated events.
The details of bubble formation also depend on the liquid and the conditions. Studies of pool boiling with ethanol and water have examined heat transfer near the contact line where the bubble meets the surface, showing that local heat transfer is concentrated in that tiny region [2]. And in water boiling at subatmospheric pressures, bubble growth rates have been measured, with the waiting time between bubbles varying substantially while other aspects remained more constant [3]. Together, these findings show that while the basic picture—vapor forming at nucleation sites and growing into bubbles—holds, the exact timing and size of bubbles depend on pressure, liquid, and surface conditions.
Sources used in this answer
On the dynamics of bubbles in boiling water
Laser scattering experiments on boiling water show that the time intervals between bubble events follow a heavy-tailed distribution rather than a simple exponential one, suggesting interacting bubble dynamics.
Study on local heat transfer in the vicinity of the contact line under vapor bubbles at pool boiling
Local heat transfer near the contact line of vapor bubbles during pool boiling of ethanol and water was studied with high spatial resolution, highlighting the importance of this region for bubble formation.
Bubble growth rates in nucleate boiling of water at subatmospheric pressures
Bubble growth rates in nucleate boiling of water at subatmospheric pressures were measured, with the waiting time between bubbles varying substantially.
Microlayer dynamics of hydrodynamically interacting vapour bubbles in flow boiling
In flow boiling of water, a thin liquid microlayer trapped under growing bubbles contributes to bubble growth through evaporation, and interactions between multiple bubbles can alter microlayer depletion and bubble lift-off.
Bubble departure and sliding in high-pressure flow boiling of water
In high-pressure flow boiling of water above 1 MPa, bubbles remain spherical during growth and sliding, and their departure diameter and growth time can be predicted from physical properties and bubble growth rate.