Mechanism for the Efficient Homogeneous Nucleation of Ice in a Weakly Ionized, Ultracold Plasma

Abstract

It is proposed that the rapid observed homogeneous nucleation of ice dust in a cold, weakly ionized plasma depends on the formation of hydroxide (OH−) by fast electrons impacting water molecules. These OH− ions attract neutral water molecules because of the high dipole moment of the water molecules and so hydrates of the form (OH)−(H2O) n are formed. The hydrates continuously grow in the cold environment to become macroscopic ice grains. These ice grains are negatively charged as a result of electron impact and so continue to attract water molecules. Because hydroxide is a negative ion, unlike positive ions, it does not suffer recombination loss from collision with plasma electrons. Recombination with positive ions is minimal because positive ions are few in number (weak ionization) and slow-moving as result of being in thermal equilibrium with the cold background gas.

Phase and Morphology of Water-ice Grains Formed in a Cryogenic Laboratory Plasma

“…This mechanism predicts that nucleation will cease at low pressure and at high RF power, in which electrons are accelerated by the RF electric field to energies above the electron affinity of OH − , and knock electrons off the negative ions before ice nuclei can form around them; this agrees with observations reported by Chai & Bellan (2015) in their similar cryogenic plasma experiment. Chai & Bellan (2015) were also able to form ice grains with other polar molecules (acetone, methanol) but were unable to form ice grains from nonpolar molecules (CO 2 ), suggesting that the dipole moment is instrumental in the nucleation process, a prediction of the ion-catalyst theory by Bellan (2022).…”

“…Gas-phase ice nucleation has been traditionally disregarded as inefficient based on arguments from classical nucleation theory (CNT). However, the validity of CNT has been questioned for application to ice nucleation in this context, as it requires extrapolating surface tension from the liquid to the solid phase (Makkonen 2012), assumes thermodynamic equilibrium when it does not apply (Rapp & Thomas 2006), and does not consider the molecular mechanics of the nucleation process (Bellan 2022). An efficient alternative to the nucleation described by CNT is ion-catalyzed nucleation, in which ions attract the dipole moment of water molecules to form thermodynamically stable nuclei onto which more vapor can condense.…”

“…A study by Bellan (2022) investigated this mechanism for the conditions of this experiment and showed that OH − ions created by dissociation of water vapor by energetic electron impacts were the best candidates to attract water molecules and efficiently form OH − (H 2 O) n clusters that then serve as nuclei for ice grain production. This mechanism predicts that nucleation will cease at low pressure and at high RF power, in which electrons are accelerated by the RF electric field to energies above the electron affinity of OH − , and knock electrons off the negative ions before ice nuclei can form around them; this agrees with observations reported by Chai & Bellan (2015) in their similar cryogenic plasma experiment.…”

“…Grains acquire electric charge from this ambient plasma, as first noted by Spitzer (1941), because electrons have a higher collision frequency with the grains than do ions and so are collected by grains more frequently. The charge can affect ice grain formation (Bellan 2022), agglomeration (Ivlev et al 2015), accretion of water molecules (Bellan 2020), and dynamics (Mitchell et al 2006;Mendis & Horányi 2013). Ice grain nucleation has traditionally been assumed to be heterogeneous, i.e., the grain forms as an ice coating on a non-ice nucleus, which is typically carbon or silicate.…”