MICRORHEOLOGY OF LOW-CONCENTRATION COLLOIDAL GELS - COEXISTENCE OF SOLID AND FLUID LAYERS

The analysis of the experimental curve of the viscosity of 2.5 % collo idal silica gel as a function of the shear rate epsilon (the value of epsilon is defined with a viscometer for the faces of the gel sample) led to the conclusion that the flow state of the gel should be inhomog eneous (over the section of the sample) within a certain range of vari ation of epsilon having a width of nine orders of magnitude. In this r ange, the gel should consist of ''solid'' and ''fluid'' layers with a viscosity of layers of the first type nine orders of magnitude higher than the viscosity of layers of the second type. A change in epsilon i n this range should be only be accompanied by a change in the ratio of the total thickness of layers of the first type to the total thicknes s of layers of the second type. The shear rates in the layers of each type should not vary. This picture of flow is obtained from the requir ement of the minimum rate of energy dissipation and is in agreement wi th the experimentally measured gel viscosity as a function of the shea r rate epsilon.