Tolman length
The Tolman length δ {\displaystyle \delta } (also known as Tolman's delta) measures the extent by which the surface tension of a small liquid drop deviates from its planar value. It is conveniently defined in terms of an expansion in 1 / R {\displaystyle 1/R} , with R = R e {\displaystyle R=R_{e}} the equimolar radius (defined below) of the liquid drop, of the pressure difference across the droplet's surface: Δ p = 2 σ R ( 1 − δ R + … ) {\displaystyle \Delta p={\frac {2\sigma }{R}}\left(1-{\frac {\delta }{R}}+\ldots \right)} (1) In this expression, Δ p = p l − p v {\displaystyle \Delta 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 {\displaystyle \sigma } is the surface tension of the planar interface, i.e.