Fast imaging of freezing drops: No preference for nucleation at the contact line

Abstract

We employ high-speed imaging of supercooled water drops to study the recently reported phenomenon of 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. Furthermore, the imaging configuration localizes nucleation sites in the horizontal plane so that their spatial distribution can be examined directly for possible preference near the contact line. Additionally, by using low cooling rates and avoiding substrate cooling, our design minimizes temperature variation within the water drop. The 189 freezing events display nearly perfect spatial uniformity in the immersed (liquid-substrate) region and, thereby, no preference for nucleation at the triple line. This is in contrast to prior experiments where a strong preference for surface freezing (in the contact mode) was observed. © 2011 American Chemical Society.