Viscosity and rigidity in suspensions of fine particles. I–Aqueous suspensions

Abstract

The variation of the coefficient of viscosity of many colloidal liquids with the rate of shear has been extensively studied and discussed, chiefly in connection with substances—such as gelatin or rubber—which are soluble or dispersible in the liquids in which they are examined. The observation of Humphrey and Hatschek that a similar variation is shown by suspensions of starch grains in a liquid exercising no solvent or dispersing action on them suggests that a common factor may operate in both instances, and it appeared possible that an examination of systems of the second type, in which the nature and condition of the physical units of the dispersed phase are more open to investigation, might give some information about one or more of the factors which cause viscosity to vary. The systems available for this purpose are (1) suspensions of electrically charged particles in an aqueous liquid and (2) suspensions, like those studied by Humphrey and Hatschek, of electrically uncharged particles in neutral organic liquids. The present communication deals with the former. The viscosity of systems of tills type has been measured from time to time for specific purposes; thus Odén found a linear relation between viscosity and concentration in sulphur sols, in agreement with Einstein's equation, but showed also that sols with smaller particles possessed a higher viscosity than those of the same total concentration with larger particles. Ishiyaka, Gann, and Fernau and Pauli used measurements of viscosity as an index of the degree of coagulation readied by sols of metal oxides, and in particular Ishiyaka observed that stable sols of aluminium hydroxide showed the same coefficient of viscosity whether measured by the oscillating disc or the capillary viscometer method, but that in coagulating sols the former gave higher viscosities than the latter, a result which he attributed to the different rates of shear incidental to the two methods.