In this expression, Δ Δ p = p l − − p v is the pressure difference between the (bulk) pressure of the liquid inside and the pressure of the vapour outside, and σ σ is the surface tension of the planar interface, i.e. the interface with zero curvature R = ∞ ∞ . The Tolman length δ δ is thus defined as the leading order correction in an expansion in 1 / R .