Consider a cloud of droplets and assume that they all have the same diameter r. According to the Laplace formula (exercise 4.8), the pressure p (r) inside the droplets of radius r is related to the vapour pressure p0 (r) by,

Principle of a heat pipe: At the hot side, the liquid passes through a wick and vaporises at pressure p+Δp. At the cold side, the vapour condenses at pressure p. where γ is the surface tension.We note p∞ the vapour pressure for an infinite radius. At temperature T, show that,

where v is the molar volume of liquid in the limit where p∞ v RT since the molar volume of liquid is much smaller than the molar volume of gas.

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p(r) = po (r) + 27 Y