# What is the equation of state of foam, on a macroscopic scale?

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? $$$$\tag{1} p(\rho) = \: ?$$$$ 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?