incorporating into said fluid a trisubstituted cyclic compound, of which {HEAD ~ the ring} is substituted by {TAIL ~ one or two X}—Am—Yn groups and wherein the remaining one or two substituents are —X Z groups, wherein the substituted ring is a 1,3,5 substituted cyclohexane, the compound being non symmetrical in that at least one of said —X Z groups is free of an Am moiety, wherein 