Changes in the hydrophilicity of the surface of PADC under ionizing radiations: Anomalous increase of the contact angle of water droplets under protons

The contact angle of water droplets decreases above certain absorbed doses in the case of heavy ions (He, C and Kr ions). Similarly, monotonic decreases are seen due to exposures to corona discharge and UV lights. The present trends indicate that the hydrophilicity of PADC surface increases by the contribution of hydroxyl groups. In comparison, the contact angle increases due to proton irradiations up to 1.3x105 Gy. Then it begins to decrease monotonically with increasing the absorbed dose. The present anomalous trend is discussed based on the combination of the track overlapping model and the Young's formula.