Triboelectrification between polycrystalline and monocrystalline ice

The triboelectrification of ice was measured using polycrystalline and monocrystalline ice samples. The resulting electrification could not be explained using the temperature gradient theory; instead, the following results were found: (1) The polycrystalline sample was always charged positively when put in contact with the monocrystalline sample, regardless of temperature difference of 1°C between them. (2) If the rubbing point on a sample recrystallizes into a polycrystal of smaller grain due to excessive rubbing, this sample becomes positively charged compared to the other sample. (3) The electrical potential increases as the grain size decreases.