  For well-stirred multiphase fluid systems the mean interface area per unit
volume, or "specific interface area" $S_V$, is a significant characteristic of
the system state. In particular, it is important for the dynamics of systems of
immiscible liquids experiencing interfacial boiling. We estimate the value of
parameter $S_V$ as a function of the heat influx $\dot{Q}_V$ to the system or
the average system overheat $\langle\Theta\rangle$ above the interfacial
boiling point. The derived results can be reformulated for the case of an
endothermic chemical reaction between two liquid reagents with the gaseous form
of one of the reaction products. The final results are restricted to the case
of thin layers, where the potential gravitational energy of bubbles leaving the
contact interface is small compared to their surface tension energy.
