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

Abstract

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.

Contact freezing: a review of experimental studies

“…Gurganus et al (2011) tested 189 drops and found that there is no preference to form the ice germ at the 3-phase boundary (surfacedroplet-air) or 3-phase contact line over the 2-phase contact area. Gurganus et al (2013) confirmed their previous observations with an improved version of their experimental setup (i.e. a side view of the tested droplets was possible with a second high-speed camera).…”

“…Recently, Gurganus et al (2011) and Gurganus et al (2013) reported experimental results which contradict the previously mentioned mechanisms. Using an improved and modified version of a cold stage (Suzuki et al, 2007) to avoid the point-like contact (i.e., the contact between the drop and the IN) and to minimize the temperature variation on the water drop surface, the preferred location to nucleate an ice crystal was investigated on silicon wafers.…”

Ice nucleation behaviour on sol–gel coatings with different surface energy and roughness

“…There have been quite some debates about the initiation site of nucleation. Some authors [48][49][50] suggested that the nucleation preferentially starts at the three-phase contact line, while others 51,52 claimed that it starts randomly at the whole solid-liquid interface. The controversy might be caused by different test environment or Such a nucleation rate is plotted in Fig.…”

Comparing contact and immersion freezing from continuous flow diffusion chambers

“…Suzuki et al (2007) found from their experiments with water droplets on silicon surfaces coated with various silanes that the temperature at which nucleation occurs at a contact line depends on the contact angle between water and the substrate. On the other hand, Gurganus et al (2011Gurganus et al ( , 2013 investigated the freezing of droplets deposited on clean and coated silicon wafers and did not observe any preference of nucleation at the contact line. The same group also studied this phenomenon on catalyst substrates with imposed surface structures and found that the preferred nucleation site was the contact line in the case of nanoscale texture but not for microscale texture (Gurganus et al, 2014).…”