= Interphase mass transfer
{title2=$\partial_t(\alpha_i\rho_i)+\nabla\cdot(\alpha_i\rho_i u_i)=\Gamma_i,\quad\sum_i\Gamma_i=0$}
The source $\Gamma_i$ is the mass gained by phase $i$ per mixture volume and time, through processes such as <evaporation> or condensation. Internal exchange conserves total mass, so its phase sources sum to zero. In an immiscible material-interface model without phase change, every $\Gamma_i$ is zero. The interface may still move: absence of transfer means that mass does not cross it relative to its own motion.
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