At all points along the length of the heat pipe, at the interface between the vapor and the liquid, the difference in the static pressures of the phases is balanced by the local capillary pressure difference. The equilibrium condition is expressed as

\[\label{eq1}p_v-p_l=\frac{2\sigma\cos\alpha}{r}\tag{1}\]

The interface adjusts its location within the wick pore structure such that there is an equilibrium condition: the interface finds the pore radius r, which satisfies Equation 1.

Figure 1 illustrates the variation of pressure and mass flow rate along the heat pipe and also the associated interfaced distribution. Maximum circulation will occur when there is no pressure difference between the phase at one point (in the case illustrated at the end of the condenser), whereas at the end of the evaporator the liquid interface is at the plane where the pore radius is smallest. The capillary pressure difference over the heat pipe length is then