March 28, 2003 Colloquium

Drying is familiar, ubiquitous, and deceptively simple. Classical thermodynamics and the kinetic theory of gases accurately describe the energetics and dynamics of evaporating simple liquids. However, even slight departures from ideality can lead to rich phenomenology that extends well beyond evaporation. For example, drying can drive sol-gel transitions and pattern-forming hydrodynamic instabilities. The theoretical descriptions of such complex phenomena may require a clear understanding of molecular-level transport and interfacial forces. We describe the drying of aqueous suspensions of monodisperse silica nanoparticles. As the films dry, they transform from a sol to a gel and crack. Cracks are evenly spaced and invade from the drying surface with stick/slip dynamics. We observe drying and cracking with Coherent Anti-Stokes Raman Scattering (CARS) microscopy, bright-field microscopy and the naked eye. Our findings highlight the role of interfacial forces and the film's nanometer-scale structure, and call into question existing models of drying and cracking in these systems.