Shape-dependent aggregation behaviour

So far, all theoretical models are based on surfactant solutions with a distribution of surfactant molecules as monomers. One of the specific properties of surfactants is that they form aggregates once a certain concentration, the critical micelle concentration CMC, is reached. The shape and size of such aggregates are different and depend on the structure and chain length of the molecules. At higher concentrations, far beyond the CMC, the phase behaviour is often complex giving rise to novel physical properties (Hoffmann 1990). 

Nevertheless the use of dielectric materials obtained by conductive filler dispersion (carbon black, graphite fibres, metallic powders) is limited. As a matter of a fact material performances are dependent on the filler content as well as particle aggregation phenomena. These composites require a high level of reproducibility and their behaviour is linked to the control of electronic inter-particular transfer. The measured parameter (complex permittivity) depends on the texture of the percolation aggregates and consequently on the processing conditions. The percolation threshold (the particle concentration, after which particles are in contact and the electrical current exists) depends on the particle shape (sphere, plates or fibres). 

Most dusts and freeze-dried material are of irregular shape and may be aggregated into loose clumps. The behaviour of such dusts when airborne depends entirely upon the mobility or settling velocity of the particles and not on their apparent microscopic size. A clump of dust may appear to be 15 pm in diameter, but if its structure is loose it may only sediment with the same speed as a 7 pm diameter sphere. When it is inhaled into the lung it will be deposited in the same manner as if it were a 7 pm sphere. 

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