# Laplace's Law

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## Laplace’s Law

the dependence of the hydrostatic pressure drop *Δp* at the boundary surface between two phases (liquid-liquid, liquid-gas, or liquid-vapor) on the interphase surface tension σ and the mean curvature ∊ of the surface at a given point: Δ*p* = *P*_{1} — *P*_{2} = *∊σ*, where *p*_{1} is the pressure on the concave side of the surface, *p*_{2} is the pressure on the convex side, and ∊ = 1/*R*_{1} + 1/*R*_{2} where *R*_{1} and *R*_{2} are the radii of curvature in two mutually perpendicular normal sections of the surface at the given point (see Figure 1).

The Laplace law, established in 1806 by P. Laplace, determines the magnitude of the capillary pressure, and thereby makes it possible to write down the conditions of mechanical equilibrium for movable (liquid) interfaces.