  This letter presents a scaling theory of the coalescence of two viscous
spherical droplets. An initial value problem was formulated and analytically
solved for the evolution of the radius of a liquid neck formed upon droplet
coalescence. Two asymptotic solutions of the initial value problem reproduce
the well-known scaling relations in the viscous and inertial regimes. The
viscous-to-inertial crossover experimentally observed by Paulsen et al. [Phys.
Rev. Lett. 106, 114501 (2011)] manifests in the theory, and their fitting
relation, which shows collapse of data of different viscosities onto a single
curve, is an approximation to the general solution of the initial value
problem.
