Shear-Relaxation Processes in Liquids

Abstract

Both cooperative behavior, involving breakup and re-formation of molecular aggregates, and environmental dissimilarities, attributed to incompleteness of short-range order, have been proposed earlier by two of the authors as possible mechanisms giving rise to the distribution of shear-relaxation times exhibited by most liquids. Experimental evidence is presented here giving qualitative indication that environmental-dissimilarity effects indeed can be operative in a liquid together with cooperative phenomena. Shear-relaxation characteristics are studied at a number of temperatures for a host liquid, hexachlorobiphenyl, having methanol and toluene in various concentrations as separate impurity liquids. Small amounts of the additive liquids are found to lessen the decay rate of cooperative behavior with increasing temperature, a result attributed to both degraded cooperative behavior and an environmental-dissimilarity mechanism. Although cooperative behavior predominates throughout the temperature and concentration range investigated, in the low-temperature region the impurity additive primarily decomposes aggregates of cooperative molecules, whereas at higher temperatures the impurity markedly enhances environmental dissimilarities and the distribution function widens with increasing (small) concentration. Toluene impurity additive displays greater degradation effect on cooperative behavior as well as greater enhancement of environmental dissimilarities than does methanol. Critical solution temperatures are noted as a matter of interest, and a method is presented for determining analytically the distribution of relaxation times associated with our experimentally determined components of shear modulus.