PERIODIC AND APERIODIC PATTERNS IN NONEQUILIBRIUM PBI2 PBF2 PRECIPITATE SYSTEMS/

Precipitate systems display a variety of beautiful patterns. Of partic ular interest are those patterns formed by more than one insoluble sal t. Guided by a prior theoretical model, we present here experiments on two precipitate patterning using PbI2 and PbF2 as the precipitating s alts, in agar gel medium. Throughout the study, a solution of Pb2+ is allowed to diffuse into a homogeneous solution of the two halides and not the reverse. Two major categories of patterns are obtained: period ic, in which the band locations follow a rigorous spacing law, and ape riodic, where the bands occupy erratic spatial positions. The former p atterns are obtained at large values of the mean concentration differe nce Delta [=(Delta(I) + Delta(F))/2, where Delta(I) = [Pb2+](0) - 1/2[ I-](0) and similarly for Delta(F)], while the latter are formed at rel atively low values of Delta. The periodic patterns show two different types of behaviour. When Delta(I) = Delta(F), one type of bands contai ning the two precipitates is obtained, obeying the spacing law rigorou sly. When Delta(I) not equal Delta(F), two types of bands are observed with two distinct spacing laws. A chemical analysis of the bands show s that in both cases, the latter consist of a complete overlap of PbI2 and PbF2 (i.e. no alternation) consistent with the initial ratio of c oncentrations (of I- to F-). Different observations favour the mechani sm of post-nucleation patterning. Some special features indicate the r ole of the kinetics of particle growth in deciding whether we obtain o ne or two types of bands (and hence one or two spacing laws).