The transport equation (Equation 20–6) governing the bubble radius R involves a source term, which has been pre-programmed as a PMAT function. The general form of this function is given by

(20–7)

where and respectively stand for the bubble radius and the pressure in the fluid. A comparison between equations Equation 20–6 and Equation 20–7 easily allows you to identify the meaning and value of the parameters a to e. More precisely, you will find a = 1, b is a typical viscosity of the mixture at the end of the process, c is the initial gas pressure, d is the initial gas bubble radius and e is a surface tension coefficient.