Determination of the Gel Point from the Fractal Dimension
The gel point is an important parameter for understanding the structure of particulate aggregates in solution and is defined as the minimum solids volume fraction at which a suspension of particulate aggregates or flocs in solution form a continuous network. At solids volume fractions lower than the gel point, aggregates are isolated and liable to settle under the influence of gravity. At higher solids concentrations particulate suspensions have a physical strength that increases with solids volume fraction, which is quantified in shear as the shear yield stress and in compression as the compressive yield stress.
Accurate measurement of the gel point is very difficult due to the fact that a suspension at the gel point has no inherent strength. Extrapolation of data from multiple equilibrium batch settling tests is a simple method that is often applied to estimate the gel point. Analysis shows that the average solids volume fraction within a distribution of aggregates can be determined from the particle size distribution, aggregate size distribution and fractal dimension. Application of this analysis to a flocculated alumina suspension has revealed a packing factor that describes the relationship between the average solids volume fraction within a distribution of aggregates and the gel point.