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Interactions between colloidal particles are strongly affected by the particle surface chemistry and composition of the liquid phase. Further complexity is introduced when particles are exposed to shear flow, often leading to broad variation of the final properties of formed clusters. Here we discover a new dynamical effect arising in shear-induced aggregation where repeated aggregation and breakup events cause the particle surface roughness to irreversibly increase with time, thus decreasing the bond adhesive energy and the resistance of the aggregates to breakup. This leads to a pronounced overshoot in the time evolution of the aggregate size, which can only be explained with the proposed mechanism. This is demonstrated by good agreement between time evolution of measured light-scattering data and those calculated with a population-balance model taking into account the increase in the primary particle nanoroughness caused by repeated breakup events resulting in the decrease of bond adhesive energy as a function of time. Thus, the proposed model is able to reproduce the overshoot phenomenon by taking into account the physicochemical parameters, such as pH, till now not considered in the literature. Overall, this new effect could be exploited in the future to achieve better control over the flow-induced assembly of nanoparticles.

The scattering structure factor of a colloidal gel in a q range of 5 orders of magnitude has been determined by combining static light scattering, small-angle light scattering, and neutron scattering measurements. It exhibits simultaneously two types of structure information:  a mass fractal scaling within the clusters that constitute the gel and a surface fractal scaling for length scales larger than that of the clusters. Such scattering behavior can be well interpreted by the pair-correlation function proposed in the literature to model an ideal structure constituted of mass fractal objects inside surface fractal objects.

The rotational diffusion behavior of fractal clusters generated through an off-lattice cluster−cluster aggregation algorithm in both diffusion-limited cluster aggregation and reaction-limited cluster aggregation conditions is investigated. The extended Kirkwood−Riseman theory (Garcia de la Torre et al., Macromolecules, 1987) is used to estimate the cluster rotational diffusion tensor. The three eigenvalues of this tensor, which correspond to the three main rotational diffusivity values of the cluster, have been computed for each generated cluster. Once the eigenvalues have been sorted in ascending order, each of them has been averaged over several thousands of clusters. It is found that one of the three main average rotational diffusivities is substantially larger than the other two, indicating significant anisotropy of fractal clusters. Moreover, a rotational hydrodynamic radius Rh,r has been determined on the basis of the mean value of the three average rotational diffusivities, which is about 25% larger than the mean translational hydrodynamic radius Rh calculated through the same Kirkwood−Riseman theory. Finally, the obtained Rh,r values have been applied to interpret dynamic light scattering data from aggregating colloidal systems and to investigate the reliability of the assumption, Rh = Rh,r, typically made in the literature.

We investigate experimentally the structural properties of colloidal gels, formed under both diffusion-limited and reaction-limited aggregation conditions, using light scattering measurements and compare the results with the literature Monte Carlo (MC) simulations. The scattering structure factors have been measured for the two classes of gels in the range of the particle volume fractions between 0.02 and 0.07. From these, the corresponding fractal dimension values have been estimated. These have been found to be in good agreement with those estimated from the structure factors computed from MC simulated gels. On the basis of our previous research (Lattuada et al. Langmuir 2003, 19, 6312), this confirms that the scattering structure factor of a gel provides erroneously a small fractal dimension value, which decreases as the particle volume fraction increases. Furthermore, it is observed that the average size of the fractal clusters is larger in real gels than in simulated gels.

The effect of the small difference in densities between dispersion medium and polymer particles on the kinetics of fast or diffusion-limited cluster aggregation (DLCA) has been investigated experimentally using small-angle light scattering (SALS). The density of the dispersion medium was tuned by varying the volume fraction of D2O from 0 to 80% so as to investigate the effect of both aggregates sedimentation and buoyancy. It is found that the time evolution of the average radius of gyration, 〈Rg〉 determined by SALS, initially follows the usual DLCA kinetics, i.e., a power law. However, when 〈Rg〉 reaches a certain value, the growth of 〈Rg〉 starts to accelerate. The onset of such acceleration shifts to larger 〈Rg〉 values for smaller density differences between the dispersion medium and the colloidal particles, thus indicating that the acceleration results from the sedimentation or buoyancy of the aggregates that reach a certain size. This is confirmed by the measured dependence of the scattering intensity at zero angle and obscuration on 〈Rg〉, which deviate from the theoretical predictions when 〈Rg〉 reaches a certain value. On the other hand, the effect of the small difference in densities on the structure of aggregates is insignificant, as indicated by the fact that the values of the fractal dimension, df, estimated from the average structure factors measured in the presence or absence of sedimentation (or buoyancy), are practically identical. Moreover, four different techniques for estimating df have been proposed and compared, and a substantially small value for df (=1.63) has been observed in all investigated DLCA systems.

Gels are complex structures that can be described in terms of the fractal dimension, df, of the clusters that constitute them. Classical techniques, based on the structure factor obtained through scattering measurements, provide erroneous values of df, which differ from the values estimated from the corresponding particle-density correlation function. The source of this error is identified in this work, and a procedure to get the correct value of df from experimental scattering data is indicated.