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The internal structure of clusters formed by colloidal heteroaggregation of particles with opposite signs of charge is studied by means of computer simulations. Every particle is surrounded by a layer of particles of opposite sign, a second neighbors shell of particles mainly with the same sign, a third one of opposite sign, etc. As the distance from the particle increases, the system becomes more homogeneous and no difference between the numbers of particles with similar or opposite signs of charge can be noticed for distances larger than ten times the particle radius. For low ionic concentrations the local environment of particles is formed by quasi-straight branches, where the sign of charge alternates, and at high concentrations the structure of the cluster is typical of DLCA and the alternation is restricted to very short distances. However, this effect is not responsible for the low fractal dimensions observed in charge heteroaggregates.

We investigate the displacements of a probe particle inside a glass, when a strong external force is applied to the probe (active nonlinear microrheology). Calculations within mode coupling theory are presented for glasses of hard spheres and compared to Langevin and Brownian dynamics simulations. Under not too strong forces where the probe remains trapped, the probe density distribution becomes anisotropic. It is shifted towards the direction of the force, develops an enhanced tail in that direction (signalled by a positive skewness), and exhibits different variances along and perpendicular to the force direction. A simple model of an harmonically trapped probe rationalizes the low force limit, with strong strain softening setting in at forces of the order of a few thermal energies per particle radius.

We present simulations of the aging of a quasi-hard-sphere glass, with Newtonian and Brownian microscopic dynamics. The system is equilibrated at the desired density (above the glass transition in hard spheres) with short-range attractions, which are removed at t = 0. The structural part of the decay of the density correlation function can be time rescaled to collapse onto a master function independent of the waiting time, t(w), and the timescale follows a power law with t(w), with exponent z ∼ 0.89; the non-ergodicity parameter is larger than that of the glass transition point (the localization length is smaller) and oscillates in harmony with S(q). The aging with both microscopic dynamics is identical, except for a scale factor from the age in Newtonian to the age in Brownian dynamics. This factor is approximately the same as that which scales the α-decay of the correlation function in fluids close to the glass transition.

We analyze the nonlinear active microrheology of dense colloidal suspensions using a schematic model of mode-coupling theory. The model describes the strongly nonlinear behavior of the microscopic friction coefficient as a function of applied external force in terms of a delocalization transition. To probe this regime, we have performed Brownian dynamics simulations of a system of quasi-hard spheres. We also analyze experimental data on hard-sphere-like colloidal suspensions [Habdas et al., Europhys. Lett., 2004, 67, 477]. The behavior at very large forces is addressed specifically.

The effective interaction between two colloidal particles in a bath of monovalent co- and counterions is studied by means of lattice Monte Carlo simulations with the primitive model. The internal electrostatic energy as a function of the colloid distance is studied fixing the position of the colloids. The free energy of the whole system is obtained introducing a bias parabolic potential, that allows us to sample efficiently small separations between the colloidal particles. For small charges, both the internal and free energy increase when the colloids approach each other, resulting in an effective repulsion driven by the electrostatic repulsion. When the colloidal charge is large enough, on the other hand, the colloid-ion coupling is strong enough to form double layers. The internal energy in this case decreases upon approaching the colloids because more ions enter the double layer. This attractive contribution to the interaction between the colloids is stronger for larger charges and larger ionic concentrations. However, the total free energy increases due to the loss of ionic entropy, and resulting finally in a repulsive interaction potential driven by the entropic contributions. The loss of ionic entropy can be almost quantitatively reproduced with the ideal contribution, the same level of approximation as the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The overall behavior is captured by the DLVO theory qualitatively, and a comparison is made with the functional form predicted by the theory, showing moderate agreement.

We analyze the slow, glassy structural relaxation as measured through collective and tagged-particle density correlation functions obtained from Brownian dynamics simulations for a polydisperse system of quasi-hard spheres in the framework of the mode-coupling theory of the glass transition (MCT). Asymptotic analyses show good agreement for the collective dynamics when polydispersity effects are taken into account in a multi-component calculation, but qualitative disagreement at small $q$ when the system is treated as effectively monodisperse. The origin of the different small-$q$ behaviour is attributed to the interplay between interdiffusion processes and structural relaxation. Numerical solutions of the MCT equations are obtained taking properly binned partial static structure factors from the simulations as input. Accounting for a shift in the critical density, the collective density correlation functions are well described by the theory at all densities investigated in the simulations, with quantitative agreement best around the maxima of the static structure factor, and worst around its minima. A parameter-free comparison of the tagged-particle dynamics however reveals large quantiative errors for small wave numbers that are connected to the well-known decoupling of self-diffusion from structural relaxation and to dynamical heterogeneities. While deviations from MCT behaviour are clearly seen in the tagged-particle quantities for densities close to and on the liquid side of the MCT glass transition, no such deviations are seen in the collective dynamics., Comment: 23 pages, 26 figures

We report simulation and experimental evidence of multiple cluster-formation mechanisms arising in coagulation in a binary mixture composed of oppositely charged colloids with large asymmetry in the relative particle concentration. These mechanisms can be clearly distinguished from one another as they cause the appearance of several maxima in the cluster-size distribution curves at well-separated time scales. We show that this behavior is caused by the fact that clusters of a given size can have different internal compositions. Moreover, the peaks collapse into a single maximum when electrostatic interactions are screened, which is a confirmation that this effect is very sensitive to the strength and range of the interparticle electrostatic interactions. The experimental confirmation of the simulation results represents the first evidence of non bell-shaped cluster-size distribution curves. 4.457 JCR (2010) Q1, 28/127 Chemistry, physical, 23/225 Materials science, multidisciplinary, 7/80 Physics, multidisciplinary, 6/79 Polymer science

The strength of the cage of neighbours in a quasi‐hard spheres colloidal system is measured by means of active microrheology up to densities close to the glass transition. An external parabolic trap is imposed to one single particle in the system, and its dynamics is analysed. The long time limit of the mean squared displacement provides information about the strength of the cage of the particle (due to the external force and neighbours), and the temperature. Assuming that both strengths are additive, the strength of the cage of neighbours is calculated. The power spectrum (Laplace transform) of the mean squared displacement is also presented.

A novel particulate membrane, comprised of a confined fluid of colloidal hard spheres, is presented and studied by means of simulations. Using a fluid of smaller hard spheres as feed, the transport properties of the membrane are studied as a function of the volume fractions of both the feed solution and membrane and the size ratio between both types of particles. Our simulations show that the fluid in the membrane is compressed to the permeate side due to the pressure of the feed. This effect controls the permeability behavior of the membrane: impermeable when the feed pressure is too low, or when the pressure is high enough to induce crystallization of the membrane fluid. Thus, the permeability first increases and then decreases, upon increasing the feed concentration. Finally we focus in systems with high concentrations of the feed and membrane fluids, where completely impermeable membranes are obtained only when the feed spheres are big enough (sigma(f)>0.38sigma(m)).

We present a first-principles theory for the active nonlinear microrheology of colloidal model systems: for constant external force on a spherical probe particle embedded in a dense host dispersion, neglecting hydrodynamic interactions, we derive an exact expression for the friction. Within mode-coupling theory (MCT), we discuss the threshold external force needed to delocalize the probe from a host glass, and its relation to strong nonlinear velocity-force curves in a host fluid. Experimental microrheology data and simulations, which we performed, are explained with a simplified model.

Using Newtonian and Brownian dynamics simulations, the structural and transport properties of hard and soft spheres have been studied. The soft spheres were modeled using inverse power potentials ($V\sim r^{-n}$, with $1/n$ the potential softness). Although the pressure, diffusion coefficient and viscosity depend at constant density on the particle softness up to extremely high values of $n$, we show that scaling the density with the freezing point for every system effectively collapses these parameters for $n\geq 18$ (including hard spheres), for large densities. At the freezing points, the long range structure of all systems is identical, when the distance is measured in units of the interparticle distance, but differences appear at short distances (due to the different shape of the interaction potential). This translates into differences at short times in the velocity and stress autocorrelation functions, although they concur to give the same value of the corresponding transport coefficient (for the same density to freezing ratio); the microscopic dynamics also affects the short time behaviour of the correlation functions and absolute values of the transport coefficients, but the same scaling with the freezing density works for Newtonian or Brownian dynamics. For hard spheres, the short time behaviour of the stress autocorrelation function has been studied in detail, confirming quantitatively the theoretical forms derived for it., Comment: 8 pages, 10 figures

The aggregate structure and long-time kinetics of aggregation between oppositely charged particles is studied. Experimental and simulation results show that heteroaggregation produces more branched aggregates than diffusive aggregation. The kinetics is shown to slow down as time proceeds. Both results are connected by considering the friction of the aggregates with the suspension medium.

We have studied the link between the kinetics of clustering and the phase behavior of dilute colloids with short range attractions of moderate strength. This was done by means of computer simulations and a theoretical kinetic model originally developed to deal with reversible colloidal aggregation. Three different regions of the phase diagram were accessed. For weak attractions, a gas phase of small clusters in equilibrium forms in the system. For intermediate attractions, the system undergoes liquid-gas separation, which is signatured by the formation of a few large droplike aggregates, a gas phase of small clusters, and an overall kinetics where a few seeds succeed in explosively growing at long times, after a lag time. Finally, for very strong attractions, fractal unbreakable clusters form and grow following DLCA-like (diffusion limited cluster aggregation) kinetics; liquid-gas separation is prevented by the strength of the bonds, which do not allow restructuration. Good qualitative and quantitative agreement is found between the dynamic simulations and the kinetic model in all the three regions.

We report a numerical investigation of the visco-elastic behavior in models for steric repulsive and short-range attractive colloidal suspensions, along different paths in the attraction-strength vs packing fraction plane. More specifically, we study the behavior of the viscosity (and its frequency dependence) on approaching the repulsive glass, the attractive glass and in the re-entrant region where viscosity shows a non monotonic behavior on increasing attraction strength. On approaching the glass lines, the increase of the viscosity is consistent with a power-law divergence with the same exponent and critical packing fraction previously obtained for the divergence of the density fluctuations. Based on mode-coupling calculations, we associate the increase of the viscosity with specific contributions from different length scales. We also show that the results are independent on the microscopic dynamics by comparing newtonian and brownian simulations for the same model. Finally we evaluate the Stokes-Einstein relation approaching both glass transitions, finding a clear breakdown which is particularly strong for the case of the attractive glass., Comment: 12 pages; sent to J. Chem. Phys

We report on the low temperature behavior of the colloidal electrolyte by means of molecular dynamics simulations, where the electrostatic interactions were modeled using effective screened interactions. As in previous works, we have found a region of gas-liquid coexistence located in the low $T$--low $\ensuremath{\rho}$ region. At temperatures much lower than the critical one, the system cannot reach equilibrium, that is, the gas-liquid transition is arrested. Two different mechanisms have been identified to cause arrest: crowding at intermediate $T$ values, associated with the crossing point between the binodal and the glass line, and a gelationlike arrest at very low $T$. To test the latter, the dynamics of the colloidal electrolyte near this crossing point has been computed and compared to the universal predictions of the ideal mode-coupling theory. As in other glass-forming liquids, we found good agreement between this mean field theory and the dynamics of this complex system, although it fails just at the transition. Interestingly, in this region we found that the dynamics of this system is driven mainly by the steric interactions, showing all the typical properties of a repulsive colloidal glass. Finally, the isodiffusivity lines show that in this system with short-range attractions, there is no reentrant glass phenomenon as opposed to monocomponent attractive systems.

A colloidal system of spheres interacting with both a deep and narrow attractive potential and a shallow long-ranged barrier exhibits a prepeak in the static structure factor. This peak can be related to an additional mesoscopic length scale of clusters and/or voids in the system. Simulation studies of this system have revealed that it vitrifies upon increasing the attraction into a gel-like solid at intermediate densities. The dynamics at the mesoscopic length scale corresponding to the prepeak represents the slowest mode in the system. Using mode coupling theory with all input directly taken from simulations, we reveal the mechanism for glassy arrest in the system at 40% packing fraction. The effects of the low-q peak and of polydispersity are considered in detail. We demonstrate that the local formation of physical bonds is the process whose slowing down causes arrest. It remains largely unaffected by the large-scale heterogeneities, and sets the clock for the slow cluster mode. Results from mode-coupling theory without adjustable parameters agree semi-quantitatively with the local density correlators but overestimate the lifetime of the mesoscopic structure (voids)., Comment: 10 pages, 8 figures

Aging in an attraction-driven colloidal glass is studied by computer simulations. The system is equilibrated without attraction and instantaneously ``quenched'', at constant colloid volume fraction, to one of two states beyond the glass transition; one is close to the transition, and the other one deep in the glass. The evolution of structural properties shows that bonds form in the system, increasing the local density, creating density deficits (holes) elsewhere. This process slows down with the time elapsed since the quench. As a consequence of bond formation, there is a slowing down of the dynamics, as measured by the mean squared displacement and the density, bond, and environment correlation functions. The density correlations can be time-rescaled to collapse their long time (structural) decay. The time scale for structural relaxation shows for both quenches a super-linear dependence on waiting time; it grows faster than the bond lifetime, showing the collective origin of the transition. At long waiting times and high attraction strength, we observe {\rem completely} arrested dynamics for more than three decades in time, although individual bonds are not permanent on this time scale. The localization length decreases as the state moves deeper in the glass; the non-ergodicity parameter oscillates in phase with the structure factor. Our main results are obtained for systems with a barrier in the pair potential that inhibits phase separation. However, when this barrier is removed for the case of a deep quench, we find changes in the static structure but almost none in the dynamics. Hence our results for the aging behavior remain relevant to experiments in which the glass transition competes with phase separation., 12 pages, 15 figures

The structure factor, S(q), of a system composed of a 1:1 mixture of oppositely charged colloids undergoing heteroaggregation is studied by Browninan dynamics simulations. A peak develops in S(q) at low wave vectors, which can be scaled for different times to overlap using the scaling of spinodal decomposition, as shown for DLCA. The same master function is obtained for different interaction ranges. The origin of the peak can be traced back to a depletion layer of clusters surrounding every aggregate. At those long distances, cluster-cluster interaction is negligible and the aggregation is diffusion limitted, as deduced from the evolution of peak position, and the S(q) scaling at different interaction ranges. The interaction is, nevertheless, strong enough to affect the internal cluster structure.

The liquid-gas transition of an electroneutral mixture of oppositely charged colloids, studied by Monte Carlo simulations, is found in the low temperature -- low density region. The critical temperature shows a non-monotonous behavior as a function of the interaction range, $\kappa^{-1}$, with a maximum at $\kappa \sigma \approx 10$, implying an island of coexistence in the $\kappa$-$\rho$ plane. The system is arranged in such a way that each particle is surrounded by shells of particles with alternating charge. In contrast with the electrolyte primitive model, both neutral and charged clusters are obtained in the vapor phase, Comment: 6 pages

We present simulation results addressing the dynamics of a colloidal system with attractive interactions close to gelation. Our interaction also has a soft, long range repulsive barrier which suppresses liquid-gas type phase separation at long wavelengths. The new results presented here lend further weight to an intriguing picture emerging from our previous simulation work on the same system. Whereas mode coupling theory (MCT) offers quantitatively good results for the decay of correlators, closer inspection of the dynamics reveals a bimodal population of fast and slow particles with a very long exchange timescale. This population split represents a particular form of dynamic heterogeneity (DH). Although DH is usually associated with activated hopping and/or facilitated dynamics in glasses, the form of DH observed here may be more collective in character and associated with static (i.e., structural) heterogeneity., Comment: 12 pages, 12 figures

The dynamical heterogeneities (DH) in non-ergodic states of an attractive colloidal glass are studied, as a function of the waiting time. Whereas the fluid states close to vitrify showed strong DH, the distribution of squared displacements of the glassy states studied here only present a tail of particles with increased mobility for the lower attraction strength at short waiting times. These particles are in the surface of the percolating cluster that comprises all of the particles, reminiscent of the fastest particles in the fluid. The quench deeper into the attractive glass is dynamically more homogeneous, in agreement with repulsive glasses (i.e. Lennard-Jones glass)., Comment: Proceedings of 5th IDMRCS - Lille 2005

The equilibrium phase diagram of a 1:1 symmetrical mixture composed of oppositely charged colloids is calculated using Monte Carlo simulations. We model the system by the DLVO effective interaction potential. The phase diagram is similar to that of its atomic analog (the ionic fluid), where a liquid-gas first order transition emerges in the low $T-\rho$ regions being stable with respect to crystallization. As in the ionic fluids, we have found two different crystals: at high $T$ the fluid crystallizes in a FCC lattice, whereas at low $T$, the liquid coexists with a BCC crystal. The region of gas-liquid stability is observed to be narrower as the interaction range is diminished., Comment: Proceedings of Quitel-2005 (Margarita, Venezuela)

The predictions of the mode-coupling theory of the glass transition (MCT) for the tagged-particle density-correlation functions and the mean-squared displacement curves are compared quantitatively and in detail to results from Newtonian- and Brownian-dynamics simulations of a polydisperse quasi-hard-sphere system close to the glass transition. After correcting for a 17% error in the dynamical length scale and for a smaller error in the transition density, good agreement is found over a wide range of wave numbers and up to five orders of magnitude in time. Deviations are found at the highest densities studied, and for small wave vectors and the mean-squared displacement. Possible error sources not related to MCT are discussed in detail, thereby identifying more clearly the issues arising from the MCT approximation itself. The range of applicability of MCT for the different types of short-time dynamics is established through asymptotic analyses of the relaxation curves, examining the wave-number and density-dependent characteristic parameters. Approximations made in the description of the equilibrium static structure are shown to have a remarkable effect on the predicted numerical value for the glass-transition density. Effects of small polydispersity are also investigated, and shown to be negligible., Comment: 20 pages, 23 figures

We present some recent theory and simulation results addressing the phenomena of colloidal gelation at both high and low volume fractions, in the presence of short-range attractive interactions. We discuss the ability of mode-coupling theory and its adaptations to address situations with strong heterogeneity in density and/or dynamics. We include a discussion of the effect of attractions on the shear-thinning and yield behaviour under flow., Comment: 17 pages, 6 figures

Experiments, theory, and simulation were used to study glass formation in a simple model system composed of hard spheres with short-range attraction (“sticky hard spheres”). The experiments, using well-characterized colloids, revealed a reentrant glass transition line. Mode-coupling theory calculations and molecular dynamics simulations suggest that the reentrance is due to the existence of two qualitatively different glassy states: one dominated by repulsion (with structural arrest due to caging) and the other by attraction (with structural arrest due to bonding). This picture is consistent with a study of the particle dynamics in the colloid using dynamic light scattering.

Using computer simulations, we identify the mechanisms causing aggregation and structural arrest of colloidal suspensions interacting with a short-ranged attraction at moderate and high densities. Two different non-ergodicity transitions are observed. As the density is increased, a glass transition takes place, driven by excluded volume effects. In contrast, at moderate densities, gelation is approached as the strength of the attraction increases. At high density and interaction strength, both transitions merge, and a logarithmic decay in the correlation function is observed. All of these features are correctly predicted by mode coupling theory.

Colloidal stability of two latexes with variable charge surface groups has been studied at a range of pH. Experimental log W vs log[electrolyte] plots were fitted using the DLVO theory, and values for the diffuse potential, psi(d), and the Hamaker constant, A, were obtained. Reasonable tendency was found for the diffuse potential of both latexes, decreasing when the surface charge decreased. However, the Hamaker constant, which was expected to be constant over the range of pH covered, also showed a decreasing behavior with decreasing charge. Several theories for the electric repulsive potential have been used to try to explain this paradoxing trend, although none of them was successful. Finally, two explanations are proposed, one based on coion adsorption and the other on the hairy layer model, although the first one seems to be the most probable on the basis of thermodynamics considerations. Copyright 1999 Academic Press.

The use of computational techniques in magnetorheology is not new. I general, these approaches assume dipolar magnetic interactions, hard sphere repulsions, and no-slip conditions. In this contribution we focus on the dynamics of the equilibrium state in the presence of uniaxial DC fields. To achieve this goal we make use of Brownian Dynamic Simulations. We highlight the importance of the Brownian forces versus magnetic dipolar interaction in the range of low magnetic field strengths. We monitor the formation of columnar structures and their dynamics, in competition with the Brownian motion, until a hexatic crystal phase appears at high field strengths for monodisperse systems. The shear viscosity is computed from the Einstein relation and eventually compared with experimental data at very low-shear rates. A reasonably good agreement between both data sets is observed.

The rheological behaviour of unsheared magnetorheological fluids is studied using Brownian dynamics simulations and experiments. In the simulations, we use monodisperse and polydisperse systems, and study the structure formation and the stress autocorrelation function that provide the shear moduli and shear viscosity of the system. Whereas the monodisperse system crystallizes, as identified by the pair distribution function, polydispersity hinders crystallization and allows a comparison with the experiments. These are performed with carbonyl iron particles in different Newtonian solvents (silicone oils and glucose syrup). Special attention is paid to the equilibration of the samples. A rescaling of the viscosity is introduced that collapses data from different systems and shear rates, leaving solely the dependence on the external magnetic field. The simulation data can be collapsed onto the same curve if the magnetic field is also rescaled, due to the approximations involved. The master curve shows the expected quadratic dependence on the external field. The shear moduli from simulations and experiments agree qualitatively; both moduli develop a shoulder at low frequencies, indicating a slow mechanism of stress relaxation connected to structural relaxation.

We present experiments and computer simulations of a tracer (or probe) particle trapped with optical tweezers and dragged at constant speed through a bath of effectively hard colloids with approximately the same size as the probe. The results are analyzed taking the single-particle case and assuming effective parameters for the bath. The effective microscopic friction coefficient and effective temperature of the tracer are obtained. At high probe velocities, the experimental microviscosity compares well with the viscosity from bulk rheology, whereas a correction due to hydrodynamic interactions (absent in the simulations) is necessary to collapse the simulation data. Surprisingly, agreement is found without any need of hydrodynamic corrections at small probe velocities. The dynamics of the tracer inside the trap shows, both in the simulations and experiments, a fast relaxation due to solvent friction and a slow one caused by the collisions with other particles. The latter is less effective in dissipating the energy introduced by the moving trap and causes increasing fluctuations in the tracer motion, reflected as higher effective temperature.

The phase behaviour of a colloid-polymer mixture is studied at very low colloid concentrations (below 0.5%). The size ratio between the polymer and the colloidal particles is around 0.09, so that the colloids experience short-range attractions. At these low volume fractions, fluid-crystal coexistence is found at moderate polymer concentrations and the kinetics of crystallization are analyzed by turbidimetry. At higher polymer concentrations, clustering of the particles occurs, but some of them remain in a diluted, gas, phase composed mainly of single particles. These states do not correspond to vapor-liquid coexistence, as shown by studying the density of the gas phase. Strong interactions induce flocculation of the system, producing fractal aggregates with dimension df ≈ 1.8, compatible with diffusion limited cluster aggregation (DLCA). These results are discussed in connection with the phase diagram of colloid-polymer mixtures, obtained at much higher colloid concentrations. For low colloid volume fractions, below ∼0.05%, no clustering of the particles is observed for any polymer concentration.

We present simulation results addressing the phenomena of colloidal gelation induced by attractive interactions. The liquid-gas transition is prevented by the glass arrest at high enough attraction strength, resulting in a colloidal gel. The dynamics of the system is controlled by the glass, with little effect of the liquid-gas transition. When the system separates in a liquid and vapor phases, even if the denser phase enters the non-ergodic region, the vapor phase enables the structural relaxation of the system as a whole., Comment: Proceedings of the glass conference in Pisa (September 06)

Dynamical heterogeneities in a colloidal fluid close to gelation are studied by means of computer simulations. A clear distinction between some fast particles and the rest, slow ones, is observed, yielding a picture of the gel composed by two populations with different mobilities. Analyzing the statics and dynamics of both sets of particles, it is shown that the slow particles form a network of stuck particles, whereas the fast ones are able to move over long distances. Correlation functions show that the environment of the fast particles relaxes much faster than that of the slow ones, but at short times the bonds between fast particles are longer lived due to the flexibility of their structure. No string-like motion is observed for the fast particles, but they occupy preferential sites in the surface of the structure formed by the slow ones.

A model for describing heteroaggregation kinetics has been developed and tested with experiments in a colloidal system composed of positively and negatively charged particles. At first order, the model reduces to the classical Hogg, Healy, and Fuerstenau approximation. Static light scattering was used to monitor the aggregations. The dependence of the initial aggregation rate on the relative particle fraction and on the surface charges was studied and compared with the theoretical predictions. The experimental results indicated that first-order approximation cannot be used to describe the kinetics. However, good agreement was found when second order was introduced in the theoretical model.

The influence of steric interactions in the initial stages of aggregation kinetics in colloidal systems has been studied by simulation and experiments. A simulation model has been proposed to study the initial stages of aggregation under short range repulsive potentials. A polystyrene latex was used as model colloid and the steric interaction was provided by adsorption of a nonionic surfactant. Depending on the range or strength of the interactions, sensitive or insensitive systems to electrolyte concentration can be observed. At low κa, the long-range electrostatic repulsion dominates the system behavior, stabilizing the colloidal system. In conditions of screened electrostatic potential, particle collision is the result of a competition between van der Waals attraction and steric repulsion, leading to a decrease in the dimer formation constant as the range or strength of the steric interaction increases. The steric interaction energy has been included in the theoretical calculation of the aggregation rate constant by means of Fuchs’ stability factor. Good agreement between simulation, experimental, and theoretical calculations was obtained.