Investigating the role of the contact line in heterogeneous nucleation with high speed imaging

While nucleation of solids in supercooled liquids is ubiquitous, surface crystallization, the tendency for freezing to begin preferentially at the liquid-gas interface, has remained puzzling. Here we employ high-speed imaging of supercooled water drops to study the phenomenon of heterogeneous surface crystallization. Our geometry avoids the "point-like contact" of prior experiments by providing a simple, symmetric contact line (triple line defined by the substrate-liquid-air interface) for a drop resting on a homogeneous silicon substrate. We examine three possible mechanisms that might explain these laboratory observations: (i) Line Tension at the triple line, (ii) Thermal Gradients within the droplets and (iii) Surface Texture. In our first study we record nearly perfect spatial uniformity in the immersed (liquid-substrate) region and, thereby, no preference for nucleation at the triple line. In our second study, no influence of thermal gradients on the preference for freezing at the triple line was observed. Motivated by the conjectured importance of line tension (tau) for heterogeneous nucleation, we also searched for evidence of a transition to surface crystallization at length scales on the order of delta ~ tau /sigma, where sigma is the surface tension; poorly constrained tau leads to delta ranging from microns to nanometers.