Aggregation kinetics in two dimensions: Real experiments and computer simulations

Abstract

The aggregation of silanized glass spheres (75±5 μm diam) was studied experimentally at liquid–air (water–air, aqueous surfactant solution–air, and aqueous glycerol solution–air) interfaces from a kinetic point of view. The number, the size, and the polydispersity of clusters was investigated as a function of time. Particles having water contact angles of ≈30° (lower hydrophobic sample) and ≈82° (higher hydrophobic sample) were prepared and used in the aggregation experiments. In the early stage of aggregation the kinetics was found to be of the second order. The experiments revealed that the increasing particle hydrophobicity increased the rate constants in every case, which could be attributed to the increasing particle–particle attractions and the decreasing hydrodynamic resistance of particles (clusters) to motion. Moreover, the lower hydrophobicity of particles manifested itself in a more important polydispersity of clusters and an unexpected cross-over during the growth. The cluster–cluster aggregation was succeeded by a particle- (large) cluster aggregation after the first (initial) part of the growth. An off-lattice computer simulation of cluster-cluster aggregation, based on molecular dynamics, was also developed for the better understanding of the interfacial aggregation. The simulations supported well the conclusions derived from the real experiments, and gave an indispensable possibility for the study of the effect of single parameters on the complex phenomenon.

Dynamics of colloidal particles with capillary interactions

“…This latter feature is beyond the scope of our model and we are not in a position to judge the influence of the temperature inhomogeneities. We simply note that, for buoyant particles of size R ≈ 35 µm spread with an initial areal density corresponding to q ≈ 10 [48], Fig. 8 predicts indeed capillary-induced clustering and Fig.…”

“…3 of Ref. [48]). In summary, according to our estimates it seems to be possible to perform experiments within an appropriate range of controllable physical parameters which would promote the occurrence of the capillary-induced instability in a variety of conditions and which would allow the systematic study of its dynamical evolution.…”

“…The only quantitative experimental studies of the clustering instability we are aware of which do provide data amenable to comparison with our calculations can be found in Refs. [48,69]. According to the interpretation of those authors, temperature inhomogeneities at the interface set the particles in tomotion, the clustering of which is initially driven by capillary forces; further restructuring of the emerging formations inside the clusters involves short-ranged forces.…”

“…In this respect we are only aware of Ref. [48], where the clustering is studied experimentally as well as by means of moleculardynamics simulations and interpreted within the framework of a certain effective kinetics of aggregation. In an attempt to be as realistic as possible, this latter theoretical approach takes into account many effects simultaneously (capillary forces, DLVO interactions, solvation forces, fluid streaming by temperature inhomogeneities).…”

Collective dynamics of colloids at fluid interfaces

“…(1). We are aware, however, of only one systematic experimental study of this instability with colloidal monolayers [27], in which the long-ranged capillary interactions have been identified as the driving forces of particle aggregation.…”