Sewer Top Desublimation

February 17, 2009


Photographer: Rob Sheridan
Summary Author: Rob Sheridan

Phase transitions are common in nature, but usually go step-wise, from gas to liquid to solid, or solid to liquid to gas. For example, an ice cube melts in your glass, then evaporates (solid to liquid to gas). In some circumstances the middle step is skipped, and a solid goes directly to a gas (sublimation) or a gas goes directly to a solid (desublimation). These events, sublimation and desublimation, are unusual because they can only occur under highly specific circumstances. In sublimation, molecules locked in a stable crystalline structure are freed to their gas phase, requiring the input of energy (an endothermic reaction). Desublimation (sometimes called deposition) brings energized gas phase molecules together in a highly ordered crystalline form, releasing energy (an exothermic reaction).

When gaseous water (water vapor) is cooled below its freezing point in sub-freezing air (super-cooled), desublimation by crystallization may occur on any available nucleus, such as particles of dust. This is how snow crystals form high in the atmosphere. A variety of surfaces, such as salt particles, can also serve as a nucleus for crystallization. Desublimation by crystallization on a surface is the principle behind thin-film vapor deposition processes in the manufacture of microelectronics.

In this photo, taken on a very cold day when water vapor was seen emerging from sewer drains, the gaseous water became super-cooled (cooled below is freezing point). The cold metal grate over the sewer served as a nucleating surface, prompting desublimation of the super-cooled water vapor as it emerged from the sewer. Instant crystallization of the gaseous water molecules occurred, without an intermediate liquid water phase. Beautiful crystal growth was the result. Beauty is everywhere, if you look for it! Photos taken on the morning of January 17, 2009.