Can electrostatic stresses affect charged water structures in weakly ionized plasmas?

Abstract

This theoretical and numerical study investigates the impact of electrostatic stresses on the shape of charged water structures (grains) in weakly ionized plasmas. We developed an analytic model to predict the conditions under which a grain in a plasma is deformed. We find that electrostatic stresses can overcome the opposing surface tension stresses on nanometer-scale grains, causing initially spherical clusters to elongate and become ellipsoidal. The exact size limit of the grain for which electrostatic stress will dominate depends on the floating potential, surface tension, and local radius of curvature. Clusters larger than this limit are not affected by electrostatic stresses due to an insufficient number of electrons on the surface. The model is compared to Molecular Dynamics (MD) simulations performed with a calculated solvated electron potential on initially spherical grains of 2.5 nm radius charged with 0.5 to 1% electrons. We find excellent agreement between MD simulations and the analytic theory. We also carried out Quantum Mechanics (QM) computations showing that the surface tension increases with decreasing size of the water molecule cluster and increases even more with the addition of solvated electrons. This increase in surface tension can hinder the elongation of the grains. Our QM computations also show that on the nanosecond time scale, the binding force of electrons to water molecule clusters is stronger than the electrostatic repulsion between adjacent electrons, and thus the cluster behaves as an insulator. However, consideration of the very small conductivity of ice shows that on time scales of a fraction of a second, ice clusters behave as conductors, so their surface may be considered to be at an equipotential.