Consider a large amount of soap foam (or any other substance producing foam), of mass density $\rho$ in a gravityless environment. What is the internal pressure $p$ of that foam, viewed as a fluid on a large scale, in relation with its density? \begin{equation}\tag{1} p(\rho) = \: ? \end{equation} I'm not interested in the air or liquid pressure inside the small scale bubbles. I'm only interested in the large scale (or "macroscopic") properties of the foam considered as a smooth entity, without knowing the "microscopic" details (i.e bubbles size and their internal pressure, etc). What should be the general expression for (1), maybe with "thermodynamic-like" parameters depending on the microscopic details?


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