What is the reason for the increase in surface tension of molten copper/cadmium with increase in temperature?
First, it looks like surface tension of, say, molten cadmium increases with temperature only in a limited temperature range above the melting point (D. W. G. WHITE, METALLURGICAL TRANSACTIONS, v.3, p.1936, JULY 1972). It is explained in the same article that, due to a thermodynamical equality, the temperature gradient of surface tension is related to the difference of entropy on the surface and in the bulk, which is related to atomic order and is determined, among other things, by mutual attraction of atoms. Therefore, specifics of the attraction potential define the temperature dependence of surface tension. Calculations taking account of these specifics seem to yield results in agreement with experiments. The above is just my poor digest, so you may wish to look at the article and the references there.