Soft Condensation

When moist air meets a cold surface, it creates a breath figure characterized by numerous small droplets. The central question is how the vapor flux is distributed between the growth of condensed drops and the nucleation of new ones. Here, we investigate the nucleation, growth, and coalescence of droplets on soft substrates consisting of cross-linked polymer networks. The number of droplets initially remains constant, until drops start to coarsen according to a universal law; both phenomena are explained via the formation of a saturated boundary layer. We quantitatively unveil an algebraic sensitivity of the number of droplets on substrate elasticity, which is unexpected since nucleation occurs at a scale where the polymer network resembles a melt. Breath figures thus offer a macroscopic probe of the molecular characteristics of the polymer surface, and suggest a surprisingly low-energy pathway for nucleation.