1 Introduction

The problem of defining the gel state is one that recurs throughout the literature of colloid science. Flory1 recalled the quotation attributed to D. Jordan Lloyd in a 1920's text-book: "the colloidal condition, the gel, is one which is easier to recognize than to define". And as recently as the previous food colloids conference in Wageningen, the same issue of the relationship of colloidal aggregation to gel formation was assessed and reviewed.2

Bungenberg de Jong, in 1949,3 defined a gel as a colloidal system of solid character in which the colloidal particles somehow constitute a coherent structure, the latter interpenetrated by a liquid system. Phenomenologically this definition has connotations of the concepts of percolation. Modern terminology describes the gel as a viscoelastic solid, i.e., a system which, depending on the circumstances, can flow like a viscous liquid or behave as an elastic solid. It is the coherent structure or network that gives rise to this characteristic behaviour, the ability to respond to the applied mechanical force in the rheometer. The problem in the study of gelation kinetics is recognizing when this load-bearing network has been created. Implicitly, in considering gels resulting from an aggregation reaction beginning with monomers, this is tantamount to asking when a container-spanning cluster is first formed. The availability of such experimental information allows the testing of various models of the aggregation...