Ice crystals exhibit diverse shapes, from hexagonal prisms to flat plates and columns. The reason for this structural variation has been a subject of scientific inquiry.
Historically, researchers hypothesized a connection to Michael Faraday's 'premelting film' concept, which posits a microscopically thin liquid water layer on ice below its melting point. However, the existence and thickness of this premelting film have generated significant scientific debate due to contradictory experimental evidence.
Luis MacDowell from Universidad Complutense de Madrid addressed this contention in The Journal of Chemical Physics.
MacDowell's research focused on the phase diagram of ice, specifically the triple point where ice, liquid water, and vapor coexist in equilibrium. Using computer simulations, he observed a nanometer-thin film at the ice surface at this triple point. He suggested that discrepancies in experimental measurements, which often report thicker films, likely stem from studies unintentionally conducted slightly away from equilibrium conditions.
The liquid film's thickness near equilibrium is limited due to water's unique density properties, where solid ice is an energetically preferred state over liquid water.
By integrating theories from various physics disciplines, MacDowell provided an explanation for observations of liquid droplets condensing on the film and theorized that changes in the premelting film's thickness at the ice surface are linked to the different shapes of snow crystals. These changes represent surface phase transitions, where each transition alters the properties and growth rates of the crystal faces, leading to distinct crystal geometries.
MacDowell anticipates that these theories can be applied to fields such as atmospheric physics and friction science, potentially aiding the understanding of phenomena like ice skating. Future research will explore the influence of friction and impurities on film thickness.