High-Speed Imaging of Freezing Drops: Still No Preference for the Contact Line

Recent experiments by Suzuki et al. (Chem. Phys. Lett. 2007, 445, 37−41) and Gurganus et al. (J. Phys. Chem. Lett. 2011, 2, 1449−1454) 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.