Preference for ice nucleation at the three-phase contact line on rough surfaces
Authors
Raymond A Shaw — Michigan Technological University
Colin Gurganus — Michigan Technological University
Joseph Charnawskas — Stony Brook University
Alex Kostinski — Michigan Technological University
Category
Ice Nucleation and Cloud Phase
Description
Three modes of nucleation. Top: A schematic of the droplet-fiber geometry. A 30 microliter droplet with a contact
angle of approximately 90 degrees rests on a siliconized glass slide that is cooled from below. An optical fiber, partially immersed within the droplet, can act as a heterogeneous nucleation catalyst. Three possibilities for nucleation then arise: on the substrate (red), on the immersed fiber (green) and at the fiber contact lines (blue). Bottom: By imaging the crystallization at 5 kHz we pinpoint the nucleation site (boxed area in film strips). Film strips here represent each of the three nucleation modes. Every 15th frame is shown resulting in a 3 ms spacing.
Nucleation of ice is a highly uncertain aspect of the microphysics of mixed phase clouds. One reason for the uncertainty is the poor understanding and quantification of the different modes of nucleation, especially contact nucleation. While nucleation of solids in supercooled liquids is ubiquitous, surface nucleation, the tendency for freezing to begin preferentially at the liquid-gas interface, has remained puzzling. Furthermore, in the presence of foreign catalysts the associated heterogeneous nucleation has been observed to prefer the three-phase contact line (triple line), especially for small particles and rough surfaces. Motivated by the conjectured importance of roughness and the contact line, we have searched for evidence of a shift to surface nucleation as the characteristic roughness length scale is decreased. Two plausible length scales associated with heterogeneous nucleation, the critical radius for a nucleation seed, and the length scale at which linear and surface energies are comparable, yield a range from micrometers to nanometers. We have performed experiments that show, using high speed imaging of the transient freezing process in supercooled water, that nano-scale texture causes a shift in the nucleation to the three-phase contact line, while micro-scale texture does not. Both the mean and variance of the freezing temperature are observed to increase, also pointing to the importance of nanotexture given that variances of independent causes add. The possibility of a transition or optimal length scale has implications for the e effectiveness of nucleation catalysts, including formation of ice in atmospheric clouds. If confirmed in subsequent studies, this will provide a physical parameter suitable for quantification of ice nucleation efficiency in the immersion and contact modes.
