Bubbles in water are studied intensively. For modeling and measurement purposes it is sometimes assumed that bubbles are spherical, but this is only true up to a certain size: When the hydrostatic pressure difference from top to bottom of the bubble is larger than the internal pressure inside the bubble it flattens. Think of a round balloon: The rubber (surface tension) produces a pressure difference between the inside and outside of the balloon (air bubble) that makes it round. If you squeeze the balloon between your hands it flattens. Same for an air bubble: If the hydrostatic pressure difference from bottom to top is larger than the pressure difference induced by the surface tension, the bubble flattens. The table below shows that for air bubbles in water, this happens at a bubble size of around 5.4 mm.
| Bubble diameter | ||||||
|---|---|---|---|---|---|---|
| Micrometers | 10 | 100 | 1000 | 5442 | 5443 | 10000 |
| Centimeters | 0.001 | 0.01 | 0.1 | 0.5442 | 0.5443 | 1 |
| Meters | 1.0E-5 | 0.0001 | 0.001 | 0.005442 | 0.005443 | 0.01 |
| Bubble Radius (m) | 5.0E-6 | 5.0E-5 | 0.0005 | 0.002721 | 0.0027215 | 0.005 |
| Water Surface Tension, σ (J/m2) | 0.07272 | 0.07272 | 0.07272 | 0.07272 | 0.07272 | 0.07272 |
| ΔPbubble = 2*σ/r (N/m2) | 29088 | 2908.8 | 290.88 | 53.45 | 53.44 | 29.088 |
| Hydrostatic Pressure Difference, ΔP (N/m2) | 0.0982 | 0.982 | 9.82 | 53.44 | 53.45 | 98.2 |
| Spherical bubble? | Yes | Yes | Yes | Yes | No | No |
The hydrostatic pressure difference ΔP is computed as ρ*g*h, where ρ is assumed at 1,000 kg/m3 and g is 9.82 m/s2 and h is the bubble diameter.
If ΔPbubble > ΔP, the bubble is spherical.
When ΔP > ΔPbubble the bubble gets squeezed, like a balloon, and is no longer spherical.