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A 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 givesThis is the spherical Young–Laplace equation: the pressure inside exceeds the outside pressure. An illustrative $\gamma =0.072\mathrm{N}{\mathrm{m}}^{-1}$ 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.^[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.

Creating a liquid surface changes the molecular environment and costs interfacial free energy. In the dispersion-only cleavage model, the attractive Van der Waals force per area has magnitude $A/(6\pi h^3)$. Separating two contacting halves from a molecular cutoff $h_0$ to infinity takes work per areaHere $A_H$ denotes the Hamaker constant, to distinguish it from an area. Cleavage creates two free surfaces, giving the Hamaker estimate of liquid surface tensionThe continuum attraction diverges at zero separation and neglects molecular impenetrability and short-range repulsion. The cutoff is therefore an effective contact separation of molecular size, commonly of order $0.1$–$0.3\mathrm{nm}$, not a limit that can be taken to zero.

An experimental limitation is the strong temperature dependence of surface tension. For water in equilibrium with its vapour, the recommended experimental values decrease from $71.98\mathrm{mN}{\mathrm{m}}^{-1}$ at $25{}^{\circ }\mathrm{C}$ to $58.92\mathrm{mN}{\mathrm{m}}^{-1}$ at $100{}^{\circ }\mathrm{C}$, and the fitted tension vanishes at the critical point. A fixed pairwise cleavage energy misses interfacial entropy, changing density and many-body correlations.^[ref] The dispersion-only model is unsuitable when cohesion is dominated by hydrogen bonds, ionic interactions or metallic bonding; liquid metals are a clear example of the latter failure.