The use of image analysis to characterize aggregates in a shear field

A means of morphological characterization of flocculated suspensions in a shear field is described. By use of image analysis combined with rheometry it was possible to measure the size and fractal dimension of solid aggregates in non-aqueous suspensions under variable shear stress. This enabled structural changes such as alignment and aggregate break-up to be observed and quantified. Studies of the aggregation kinetics were also performed by monitoring the size increase of aggregates, after the relaxation of an applied shear stress sufficient to disperse the aggregates. The technique is illustrated by measurements on lactose aggregates in 1,1,2-trichlorotrifluoroethane, a system relevant to the behaviour of inhalation aerosols. Aggregate size R was rapidly reduced by the application of shear stresses s up to 1 N m(-2) followed by a more gradual decrease at greater stresses. The average aggregate size showed good agreement with an empirical law of the form R similar to s(-1/3), which enabled estimates of the aggregate binding energy to be obtained. Boundary fractal dimension showed only a slight reduction from 1.4 to 1.3 over the same range of shear stress, indicating that the rugged aggregate structure and self similarity were largely preserved during disruption. Measurement of the formation kinetics and fractal dimension of aggregates showed a correlation between the resultant aggregate morphology and the formation rate which was in agreement with existing theories of fractal aggregation kinetics. (C) 1997 Elsevier Science B.V.