Structure transition in cluster-cluster aggregation under external fields

The structure transition in cluster-cluster aggregation (CCA) under different external electric fields has been investigated by computer simulations. The aggregates are generated from off-lattice CCA models involving the field-induced dipolar interactions and temperatures by means of a Metropolis algorithm. When the parameter K increases from 0 to infinity, the clusters gradually change from a diffusion-limited CCA to a chainlike pattern, where K stands for the relative strength of the field-induced dipolar interaction with thermal energy. The relation of fractal dimension D(f) to parameter K can be approximately expressed as D(f)=D(E)+(D(DLCA)-D(E))e(-betaK) with beta=0.64, where D(E) and D(DLCA) stand for the fractal dimensions of the CCA when K-->infinity and K-->0, respectively. This structure transition is a transition between a prototype disorder structure and a relative order one. The transition is attributed to the variation of the dominating interaction of systems from thermal disorder to field-induced dipolar interaction with K rising.