Lightning is a powerful display of atmospheric electricity, whose formation has received great attention but is still poorly understood. Secondary ice production (SIP) through particle breaking during collisions explains the mismatch between ice crystal concentration in natural clouds and the number concentration of primary nucleating ice particles, but not particle charging. This group investigated the effect of electric fields in phase transition using video techniques. In this work, we report two unprecedented observations made during ice formation from water vapor, evidencing new mechanisms for fast ice particle formation in the atmosphere. Images in the Figure were frequently observed and they provide evidence for: a) the applied electric field accelerates ice crystal formation, with needle habit and b) ice particle formation by a non-classical mechanism, reminiscent of the well-known spinodal decomposition. According to the classical nucleation theory, the rate of formation of new particles depends on the height of the energy barrier for the formation of critical nuclei that in turn depends on their surface tension. Excess electric charge on liquid water lowers its surface tension and we now propose that this also applies to ice. Thus, ice formation is necessarily enhanced when water (vapor, liquid, solid) acquires excess charge under a field. On the other hand, the observed needle shape is consistent with particle charging, allowing for diminished repulsion between charges at the crystal surface. Unfortunately, there is not yet a theory for spinodal decomposition under a field, but the particles formed are also needle-shaped, thus they also carry excess charge. Net charge determination was made on grown-up ice needles and dendrites allowing the calculation of the electric fields created by these. The calculated fields are in the 40 to 200 kV/m, encompassing the 102 kV/m figure obtained from field measurements.

Electric field effect on ice formation from vapor: new evidence for classical and non-classical nucleation

ACS Fall 2021

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