Colloidal Fractal Aggregates

Abstract

The size of a solid particle is known to play a very important role in both natural and technical phenomena. There are various reasons for this observation. The simplest arises from the fact that as the particle is reduced, the magnitude of surface area per gram increases. For example, if a cube of a solid of side 1 cm weighs 1 g, then the surface area = 6 cm². Now, if we subdivide this solid into cubes of 1 mm (e.g., by grinding), then the volume of each cube is 0.1³ cm³. The number of small cubes is 1/0.001 = 1000. The surface area of each of the smaller cubes is 6(0.1²) = 6(0.01) cm². Thus, the surface area of the smaller cubes is 1000(6)(0.01) = 60 cm². This gives an increase in surface area by a factor of 10 when each side of the cube is reduced by a factor of 10. It is easily seen that in the case of even smaller cubes, this increase would be tremendous. The magnitude of surface area per gram for such powders as active charcoal is ca. 1000 m²/g. This enormous surface area gives very special properties to such powders (e.g., adsorption of pollutants from drinking water). Further, in many cases when the particles are not smooth, the edges and corners will give rise to special characteristics. Another important parameter that is related to particle size is light reflection. As the size decreases, the degree of light reflection changes, such that in some cases one sees a bluish tinge.