Amorphous ice

Amorphous ice (non-crystalline ('vitreous') ice) is an amorphous solid form of water. Common ice is a crystalline material where the molecules are regularly arranged in a hexagonal lattice whereas amorphous ice is distinguished by a lack of long-range order in its molecular arrangement. Amorphous ice is produced either by rapid cooling of liquid water (so the molecules do not have enough time to form a crystal lattice) or by compressing ordinary ice at low temperatures. Amorphous ice (non-crystalline ('vitreous') ice) is an amorphous solid form of water. Common ice is a crystalline material where the molecules are regularly arranged in a hexagonal lattice whereas amorphous ice is distinguished by a lack of long-range order in its molecular arrangement. Amorphous ice is produced either by rapid cooling of liquid water (so the molecules do not have enough time to form a crystal lattice) or by compressing ordinary ice at low temperatures. Although almost all water ice on Earth is the familiar crystalline ice Ih, amorphous ice dominates in the depths of interstellar medium, making this likely the most common structure for H2O in the universe at large. Just as there are many different crystalline forms of ice (currently 17+ known), there are also different forms of amorphous ice, distinguished principally by their densities. The production of amorphous ice hinges on the fast rate of cooling. Liquid water must be cooled to its glass transition temperature (about 136 K or −137 °C) in milliseconds to prevent the spontaneous nucleation of crystals. This is analogous to the production of ice cream from heterogeneous ingredients, which must also be frozen quickly to prevent the growth of crystals in the mixture. Pressure is another important factor in the formation of amorphous ice, and changes in pressure may cause one form to convert into another. Chemicals known as cryoprotectants can be added to water, to lower its freezing point (like an antifreeze) and increase viscosity, which inhibits formation of crystals. Vitrification without addition of cryoprotectants can be achieved by very rapid cooling. These techniques are used in biology for cryopreservation of cells and tissues. Low-density amorphous ice, also called LDA, vapor-deposited amorphous water ice, amorphous solid water (ASW) or hyperquenched glassy water (HGW), is usually formed in the laboratory by a slow accumulation of water vapor molecules (physical vapor deposition) onto a very smooth metal crystal surface under 120 K. In outer space it is expected to be formed in a similar manner on a variety of cold substrates, such as dust particles. Melting past its glass transition temperature (Tg) between 120 and 140 K, LDA is more viscous than normal water. Recent studies have shown the viscous liquid stays in this alternative form of liquid water up to somewhere between 140 and 210 K, a temperature range that is also inhabited by ice Ic. LDA has a density of 0.94 g/cm3, less dense than the densest water (1.00 g/cm3 at 277 K), but denser than ordinary ice (ice Ih). Hyperquenched glassy water (HGW) is formed by spraying a fine mist of water droplets into a liquid such as propane around 80 K or by hyperquenching fine micrometer-sized droplets on a sample-holder kept at liquid nitrogen temperature, 77 K, in a vacuum. Cooling rates above 104 K/s are required to prevent crystallization of the droplets. At liquid nitrogen temperature, 77 K, HGW is kinetically stable and can be stored for many years.