Relation between the size of fog droplets and their contact angles with CR39 surfaces

The formation of fog on CR39 surfaces has been studied. Water droplets form fog coalesce with time, especially during the first 20 s at the beginning of the formation of fog. Consequently, their mean diameter increases. Formation of fog being related to the wettability of the surfaces, the latter has been increased by the implantation of Ar ions into CR39 surfaces under an oxygen partial pressure. A very wetting CR39 surface with advancing (ACA) and receding (RCA) contact angles below 5degrees has been obtained with an implantation dose of 1.28 x 10(17) Ar+ cm(-2). In this condition, no formation of fog was observed. Characterization using x-ray photoelectron spectroscopy has shown that the molecular structure of CR39 is strongly modified by Ar+ implantation, which would be responsible for the increase in wettability. Unfortunately, both ACA and RCA increase with time, which is called ageing, and the formation of fog is again observed. The diameter and concentration of water droplets forming fog have been plotted against the contact angle. These plots show that no formation of fog occurs for ACA < similar to40(+). Usually, the ACA reaches 40degrees after an ageing time of similar to200 h. A He pre-implantation with a dose of 5 x 10(15) He+ cm(-2) and an energy of 2 keV, sufficient to push the ions deeper than the Ar depth profile, delays the ageing effect in such a way that ACA congruent to 40degrees is reached after similar to2000 h and no formation of fog is observed during these first similar to2000 h.