A \b[droplet] is a bounded amount of <liquid> surrounded by another phase, such as a <gas> or an immiscible <liquid>. Its interface carries <surface energy>. For a free droplet at fixed <volume> with uniform positive <surface tension>, reducing <surface area> lowers this contribution to the <free energy>; when other effects are negligible, the equilibrium shape is a <sphere>. This is the physical setting behind the <isoperimetric inequality>.

For a spherical <droplet> of radius $R$ and constant <surface tension> $\gamma$, write $F_s=4\pi\gamma R^2$ and $V=4\pi R^3/3$. Quasistatic expansion against the pressure difference gives
$$
\Delta p=\frac{dF_s/dR}{dV/dR}=\frac{8\pi\gamma R}{4\pi R^2}=\frac{2\gamma}{R}.
$$
This is the spherical <Young–Laplace equation>: the <pressure> inside exceeds the outside pressure. An illustrative $\gamma=0.072\,\mathrm{N\,m^{-1}}$ gives $\Delta p=144\,\mathrm{Pa}$ at $R=1\,\mathrm{mm}$, and twice that pressure difference at half the radius.

Real <surface tension> depends on material and temperature; the IAPWS release, for example, tabulates measured <water> values and supplies an interpolation formula.<Revised Release on Surface Tension of Ordinary Water Substance by International Association for the Properties of Water and Steam>[Printed p. 3, correlation and validity range; Table 1, measured values.]{ref} <Gravity>, contact with a solid and motion can deform a <droplet>, so the spherical calculation states a regime rather than defining every possible droplet shape.


 Droplet (source code)