The Effects of Surface Curvature and Temperature on Charge Transfer During Ice‐Ice Collisions

Repetitive collisions of sublimating ice surfaces, created by freezing liquid water, produced charge separation Q of a magnitude (pC) comparable to earlier results of vapor‐grown ice crystals repetitively collided in a saturated environment. Those results had been explained by a theory of charge distribution in a quasi‐liquid surface layer quasi‐liquid layer (QLL) mediated at the moment of impact by collisional melting. The present results are from long‐duration contacts (15 ms) with transfer of kinetic energy estimated to be a factor of 104 greater and charging rate 98% slower. They do not support an assumption in the proposed mechanism of collisional melting and are contrary to the basic assumptions of that theory: that significant Q occurs only in growing surfaces (i.e., in a supersaturated condition that implies the presence of liquid water) that were originally grown from vapor, and for which the contact is less than 1 ms. The results are opposite to a rule that in the case of differing surfaces (vapor‐grown and rimed), the greater sublimating surface would charge negatively. Here, for surfaces of frozen water, the surface further from equilibrium becomes positively charged. The results are not inconsistent with recent reports of simulations that suggest that the thickness of a QLL in sublimating surfaces may not be less than the equilibrium value. Apparently, for differing ice surfaces, Q is of the order of 1 fC, whereas for collisions of similar ices on metal substrates (whether both vapor‐deposited, or both frozen liquid water), Q is of the order of 1 pC. Plain Language Summary Results of charge separation between two pieces of ice in repeated collisions give results of similar magnitude (picoCoulombs) to an earlier experiment but of polarity which can be explained in terms of the difference in curvature (and inferred relative thicknesses of a quasi‐liquid surface layer) of the two pieces (in contrast to the earlier work which collided two similar pieces). These results are distinguishable from those which are the basis of a theory based on collisions between vapor‐grown crystals and rimed ice. Key Points Charge transfer between ice surfaces is from a curved surface to a flat surface in undersaturated and near‐saturation environments Conditions for a quasi‐liquid layer (QLL) disappear at contact; a new QLL must be established about any bridging spicules Flow in the QLL is parallel (not transverse) to the QLL‐bulk interface