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.

It has been conjectured that roughness plays a role in surface nucleation, the tendency for freezing to begin preferentially at the liquid-gas interface. Using high speed imaging, we sought evidence for freezing at the contact line on catalyst substrates with imposed characteristic length scales (texture). Length scales consistent with the critical nucleus size and with δ∼τ/σ, where τ is a relevant line tension and σ is the surface tension, range from nanometers to micrometers. It is found that nanoscale texture causes a shift in the nucleation of ice in supercooled water to the three-phase contact line, while microscale texture does not.

on liquid−solid nucleation arrived at conflicting results regarding a preferred status of the triple line between water, air, and an ice-catalyzing substrate. Temperature nonuniformity within drops and substratedependent contact angles have been suggested as culprits in a recent review by Sear (Int. Mater. Rev. 2012, 57, 328−356).To that end, we redesigned our earlier experiment to allow substrate-induced cooling and a side view with a second high-speed camera. The two camera views pinpoint the spatial location of nucleation sites in both the vertical and horizontal directions. Here we report such nucleation positioning results measured within drops freezing on a substrate. The role of thermal gradients was explored in three ways: (i) implementing direct cooling of the substrate; (ii) mimicking (higher) cooling rates used by Suzuki et al.; and (iii) varying the drop−substrate contact angle. No influence of thermal gradients on the preference for freezing at the triple line has been observed. Thermal simulations of the drop−substrate system confirm that horizontal temperature gradients are extremely small. Furthermore, treatment of the substrate to obtain a range of contact angles also yielded no preference for freezing at the triple line. The combined top and side views of the freezing drops suggest that apparent triple-line nucleation can be a spurious result of the viewing geometry.

Supercooled water is found to have a significantly enhanced freezing temperature during transient electrowetting with electric fields of order 1 V/lm. High speed imaging reveals that the nucleation occurs randomly at the three-phase contact line (droplet perimeter) and can occur at multiple points during one freezing event. Possible nucleation mechanisms are explored by testing various substrate geometries and materials. Results demonstrate that electric field alone has no detectable effect on ice nucleation, but the moving boundary of the droplet on the substrate due to electrowetting is associated with the triggering of nucleation at a much higher temperature. V

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