The effect of shear on colloidal aggregation and gelation studied using small-angle light scattering

Mokhtari, Tahereh; Sorensen, C. M.; Cheng, Chungyin; Vigil, R. Dennis

doi:10.1016/j.jcis.2008.08.017

Cited by 13 publications

(30 citation statements)

References 56 publications

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“…Aggregation of colloidal nanoparticles is a nonequilibrium multiscale problem that is characterized by a wide range of length and time scales ranging from those associated with a monomer to superaggregates comprising tens of thousands of monomers. Usually colloidal aggregation in real physical systems occurs in the presence of external forces due to gravity [1,2] or shear flow [3][4][5][6][7][8][9][10][11]. This work is focused on the effects of shear.…”

Section: Introductionmentioning

confidence: 99%

“…Shear flow affects the size and structure of aggregates and the rate at which they are formed. A rich variety of phenomena are observed depending on the magnitude of the shear rate, the initiation time [11] (i.e., whether shear is applied after some aggregation has taken place, or right from the onset of aggregation), and the duration of time over which the system is subjected to shear.…”

Section: Introductionmentioning

confidence: 99%

“…These phenomena have been experimentally investigated by studying the influence of externally applied shear on the aggregation of latex nanoparticles [8,11]. If shear is applied after aggregates have already formed, it can change aggregate structure.…”

Section: Introductionmentioning

confidence: 99%

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Characterization of sheared colloidal aggregation using Langevin dynamics simulation

2014

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Aggregation of colloidal particles under shear is studied in model systems using a Langevin dynamics model with an improved interparticle interaction potential. In the absence of shear, aggregates that form are characterized by compact structure at small scales and ramified structure at larger scales. This confirms the structural crossover mechanism previously suggested by Sorensen and coworkers, that colloidal aggregation occurs due to monomer addition at small scales and due to cluster-cluster aggregation at large scales. The fractal dimension of nonsheared aggregates is scale-dependent. Smaller aggregates have a higher fractal dimension than larger ones, but the radius of gyration where this crossover occurs is independent of potential well depth for sufficiently deep wells. When these aggregates are subjected to shear they become anisotropic and form extended cigar-like structures. The size of sheared anisotropic aggregates in the direction perpendicular to the shear flow is limited by shear-induced breakage because the shear force dominates interparticle attraction for sufficiently large aggregates. Anisotropic aggregates are not completely characterized by a single radius of gyration, but rather by an inertia ellipsoid. Consequently the fractal dimension is no longer an adequate metric to properly characterize them, and to identify changes in their structure from their nonsheared isotropic counterparts. We introduce a new compactness-anisotropy analysis that characterizes the structure of anisotropic aggregates and allows us to distinguish between aggregates from sheared and nonsheared systems. Finally, using the ratio of interparticle force to the shear force f_{pot,sh} we are able to characterize different outcomes of sheared aggregation as a function of dimensionless well depth and Péclet number.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Characterization of sheared colloidal aggregation using Langevin dynamics simulation

2014

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“…The number of solute particles A and solvent molecules B in the MD were chosen to mimic a dilute system with volume fraction close to conditions in experiments. 16 The MD simulations are performed using the LAMMPS 38 software package. The MD simulations correspond to the NVT ensemble, which is appropriate for comparison with the constant-temperature LD simulations.…”

Section: Methodsmentioning

confidence: 99%

Coarse-Graining Approach to Infer Mesoscale Interaction Potentials from Atomistic Interactions for Aggregating Systems

Markutsya

Fox

Subramaniam

2012

Ind. Eng. Chem. Res.

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A coarse-graining (CG) approach is developed to infer mesoscale interaction potentials in aggregating systems, resulting in an improved potential of mean force for Langevin dynamics (LD) and Brownian dynamics (BD) simulations. Starting from the evolution equation for the solute pair correlation function, this semi-analytical CG approach identifies accurate modeling of the relative acceleration between solute particles in a solvent bath as a reliable route to predicting the time-evolving structural properties of nonequilibrium aggregating systems. Noting that the solute-solvent pair correlation function attains a steady state rapidly as compared to characteristic aggregation time scales, this CG approach derives the effective relative acceleration between a pair of solute particles in the presence of this steady solute-solvent pair correlation by formally integrating the solvent force on each solute particle. This results in an improved potential of mean force that explicitly depends on the solute-solute and solute-solvent pair potentials, with the capability of representing both solvophilic and solvophobic interactions that give rise to solvation forces. This approach overcomes the difficulty in specifying the LD/BD potential of mean force in aggregating systems where the solute pair correlation function evolves in time, and the Kirkwood formulaU(r) = −kBT ln g(r) that is applicable in equilibrium diffusion problems cannot be used. LD simulations are compared to molecular dynamics (MD) simulations for a model colloidal system interacting with Lennard-Jones pair potentials to develop and validate the improved potential of mean force. LD simulations using the improved potential of mean force predict a solute pair correlation function that is in excellent match with MD in all aggregation regimes, whereas using the unmodified MD solute-solute pair potential in LD results in a poor match in the reactionlimited aggregation regime. The improved potential also dramatically improves the predicted extent of aggregation and evolution of cluster size distributions that exhibit the same self-similar scaling found in MD. This technique of coarse-graining MD potentials to obtain an improved potential of mean force can be applied in a general multiscale framework for nonequilibrium systems where the evolution of aggregate structure is important. ABSTRACT: A coarse-graining (CG) approach is developed to infer mesoscale interaction potentials in aggregating systems, resulting in an improved potential of mean force for Langevin dynamics (LD) and Brownian dynamics (BD) simulations. Starting from the evolution equation for the solute pair correlation function, this semi-analytical CG approach identifies accurate modeling of the relative acceleration between solute particles in a solvent bath as a reliable route to predicting the time-evolving structural properties of nonequilibrium aggregating systems. Noting that the solute−solvent pair correlation function attains a steady state rapidly as compared to characteristic aggreg...

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“…is the elementary particle radius, k the Boltzmann constant, and T the absolute temperature. The same expression is also frequently used for aggregates by substituting the aggregate radius of gyration for d 027 . Péclet numbers above 4.4 10 5 are thus obtained for flocs remaining after the breaking procedure.…”

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confidence: 99%

Floc Cohesive Force in Reversible Aggregation: A Couette Laminar Flow Investigation

2010

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A simple theoretical model is proposed to describe the limiting size of aggregates attained at steady state under given shear conditions. The stable size is assumed to be the result of a dynamic equilibrium between simultaneous aggregate growth and breakup that are represented as first-order processes. The theory establishes that the evolution of steady-state aggregate size versus shear rate is written as the sum of two exponential laws. The validity of the model is verified by direct observation of the coagulation behavior of latex particles in the stagnant plane of a counter-rotating Couette reactor. The influence of latex elementary particle size, initial particle volume fraction, and inner gap spacing of Couette reactor, are investigated. In all cases, the model shows good agreement with the experimental results. Aggregate growth proceeds with a monomodal size distribution that exhibits a scaling behavior. Such monomodal distribution evolves toward broad and even bimodal steady-state distributions at both low and high shear rates, whereas a narrow monomodal pattern is observed at intermediate shear gradients. The aggregate cohesive force F(C) can be calculated from the critical shear rate of dislocation defined by the model. In contrast to the broadly accepted view that larger flocs should be more fragile than smaller aggregates, we find that F(C) scales as D(3/2) where D is the aggregate characteristic diameter. The latter relationship may be derived by assuming linear elasticity of aggregates.

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The effect of shear on colloidal aggregation and gelation studied using small-angle light scattering

Cited by 13 publications

References 56 publications

Characterization of sheared colloidal aggregation using Langevin dynamics simulation

Characterization of sheared colloidal aggregation using Langevin dynamics simulation

Coarse-Graining Approach to Infer Mesoscale Interaction Potentials from Atomistic Interactions for Aggregating Systems

Floc Cohesive Force in Reversible Aggregation: A Couette Laminar Flow Investigation

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