Effect of Gravity and Electrolyte Concentration on the Fractal Structure of Colloidal Aggregates

The new method developed by Li et al. was used to calculate the energy between two colloidal particles, which could break through the limit that the surface potential to be taken as a constant as electrolyte concentration changes in the classic DLVO theory. Also in this research, both the Boltzmann theory of kinetic energy of colloidal particles and Monte Carlo method were used to simulate the movement of colloidal particles, and the inelastic collision theory was used to solve the problem of effective collision probability. By improving the DDA model, the relationship between the cohesion efficiency and the electrolyte concentration in gravity field was established successfully. The results showed that: (1) the curves of the fractal dimension changed with electrolyte concentration as gravity field presence was quite different from that as the gravity field absence. The curves were “L”-shaped as the gravity field absence; while, as the gravity field presence, the curves were “S”-shaped. (2) As the gravity field presence, the slow aggregating process could be divided into two sections: the sensitive and non-sensitive sections to electrolyte concentration. In the sensitive section, an inflexion of electrolyte concentration was found. (3) As the gravity field absence, the fractal dimension of aggregates was 1.86±0.01 for different sizes of colloidal particles as the aggregating process was fast under a higher electrolyte concentration condition. Comparatively, for a slow aggregating process of low electrolyte concentration, the fractal dimension increased to 2.01±0.02. However, under the same low electrolyte concentration, the fractal dimension of aggregates approached 3 as the gravity field presence.