text: Witold Rybczynski and Jacques Hadamard developed an equation to calculate the velocity of bubbles that rise in foam with the assumption that the bubbles are spherical with a radius .
  with velocity in units of centimeters per second. ρ and ρ is the density for a gas and liquid respectively in units of g/cm and ῃ and ῃ is the  dynamic 
   viscosity of the gas and liquid respectively in units of g/cm·s and g is the  acceleration of gravity in units of cm/s.
  However, since the density and viscosity of a liquid is much greater than the gas, the density and viscosity of the gas can be neglected, which yields the new equation for velocity of bubbles rising as:
  However, through experiments it has been shown that a more accurate model for bubbles rising is:
  Deviations are due to the Marangoni effect and capillary pressure, which affect the assumption that the bubbles are spherical.
  For laplace pressure of a curved gas liquid interface, the two principal radii of curvature at a point are R and R. With a curved interface, the pressure in one phase is greater than the pressure in another phase. The capillary pressure P is given by the equation of:
  where  is the surface tension. The bubble shown below is a gas (phase 1) in a liquid (phase 2) and point A designates the top of the bubble while point B designates the bottom of the bubble. 
  At the top of the bubble at point A, the pressure in the liquid is assumed to be p as well as in the gas. At the bottom of the bubble at point B, the hydrostatic pressure is:
  where ρ and ρ is the density for a gas and liquid respectively. The difference in hydrostatic pressure at the top of the bubble is 0, while the difference in hydrostatic pressure at the bottom of the bubble across the interface is gz(ρ − ρ). Assuming that the radii of curvature at point A are equal and denoted by R and that the radii of curvature at point B are equal and denoted by R, then the difference in capillary pressure between point A and point B is:
  At equilibrium, the difference in capillary pressure must be balanced by the difference in hydrostatic pressure. Hence,
  Since, the density of the gas is less than the density of the liquid the left hand side of the equation is always positive. Therefore, the inverse of R must be larger than the R. Meaning that from the top of the bubble to the bottom of the bubble the radius of curvature increases. Therefore, without neglecting gravity the bubbles cannot be spherical. In addition, as z increases, this causes the difference in R and R too, which means the bubble deviates more from its shape the larger it grows.
  Foam destabilization occurs for several reasons. First, gravitation causes drainage of liquid to the foam base, which Rybczynski and Hadamar include in their theory; however, foam also destabilizes due to osmotic pressure causes drainage from the lamellas to the Plateau borders due to internal concentration differences in the foam, and Laplace pressure causes diffusion of gas from small to large bubbles due to pressure difference. In addition, films can break under disjoining pressure, These effects can lead to rearrangement of the foam structure at scales larger than the bubbles, which may be individual (T1 process) or collective (even of the "avalanche" type).