Stokesian Dynamics Simulations of Neutral and Charged Colloids
G. Nägele in collaboration with A.J. Banchio (University of Cordoba, Argentina):
The dynamics of suspensions of charged colloidal particles is of fundamental interest in soft matter science, surface chemistry and food science. Charged colloidal particles interact with each other directly by means of a screened electrosteric repulsion, and through solvent-medi- ated hydrodynamic interactions (HIs). These interactions cause challenging problems in the theory and computer simulation of the colloid dynamics.
We have made a comprehensive accelerated Stokesian Dynamics (ASD) simulation study on the influence of electrosteric and hydrody- namic interactions in dense suspensions of charged and neutral colloidal spheres [1,2].
Numerous short-time properties have been computed, including the high-frequency vis- cosity, wavenumber-dependent diffusion function, hydrodynamic function, and short- time self-diffusion coefficients. The simula- tions have been performed using the model of dressed spherical macroions interacting by an effective pair potential of screened Coulomb type. A large variety of systems with differing particle concentrations, charges and added salt concentrations have been considered, spanning the range from hard-sphere systems (HS) to low-salinity suspensions (CS). Through comparison with the simulation data, the applicability and level of accuracy of analytical methods and ad-hoc concepts have been tested, which are frequently applied in the description of the colloid dynamics. Our ASD
study provides a detailed database on the effect of many-body HIs on colloidal short-time dynamics.
For an example of this study, ASD simulation results are shown for the high-fre- quency-limiting viscosity, η∞, of neutral and charged spheres, the latter under salt-free con- ditions, as a function of the colloid volume fraction φ (from [1]). The small viscosity dif- ference between charged and neutral particles is in accord with the experimental findings. In a second example, we depict ASD results for the static structure factor, S(q), normalized short- time diffusion function, D(q), and hydrody- namic function H(q) = S(q)×D(q)/D0 of a salt-
free suspension of charged particles, as functions of wavenumber q times the particle radius a.
The dynamic quantities are scaled by the short-time self-diffusion coefficient Ds. The dashed vertical lines mark the wavenumber qs where S(qs) = 1 (from [2]).
A detailed comparison of Synchrotron and DLS data on short-time diffusion properties of charged colloids and proteins with theory/simulation is included in [3-6] and [7], respectively.
References:
[1] A.J. Banchio and G. Nägele, J. Chem. Phys. 128, 104903 (2008).
[2] A.J. Banchio, M.G. McPhie and G. Nägele, J. Phys.: Condensed Matter 20, 404231 (2008).
[3] J. Gapinski, A. Patkowski, A.J. Banchio, J. Buitenhuis, P. Holmqvist, M.P. Lettinga, G.
Meier and G. Nägele, J. Chem. Phys. 130, 084503 (2009).
[4] A. Patkowski, J. Gapinski, A. Fluerasu, P. Holmqvist, G. Meier, M.P. Lettinga, and G.
Nägele, Acta Physica Polonica A 114, 339 (2008).
[5] J. Gapinski, A. Patkowski, A.J. Banchio, P. Holmqvist, G. Meier, M.P. Lettinga and G.
Nägele, J. Chem. Phys. 126, 104905 (2007).
[6] A.J. Banchio, J. Gapinski, A. Patkowski, W. Häußler, A. Fluerasu, S. Sacanna, P.
Holmqvist, G. Meier, M.P. Lettinga and G. Nägele, Phys. Rev. Lett. 96, 138303 (2006).
[7] J. Gapinski, A. Wilk, A. Patkowski, W. Häußler, A.J. Banchio, R. Pecora and G. Nägele, J. Chem. Phys. 123, 054708 (2005).
