Rotational dynamics of charged colloidal spheres: Role of particle interactions

Koenderink, Gijsberta H.; Lettinga, M. Pavlik; Philipse, Albert P.
October 2002
Journal of Chemical Physics;10/22/2002, Vol. 117 Issue 16, p7751
Academic Journal
Time-resolved phosphorescence anisotropy (TPA) is used to measure the short-time rotational diffusion coefficient D[sup r][sub s](φ) of charged tracer spheres as a function of the volume fraction φ of like-charged colloidal host spheres in nonaqueous solvents. Sphere interactions are varied from long-range repulsive to short-range attractive by changing the ionic strength and the solvent composition. It is shown that D[sup r][sub s](φ) is very sensitive to details of the interaction near contact, in agreement with theory. In contrast, the low-shear viscosity η[sub L](φ) of the host dispersions is mostly controlled by the tail of the interaction potential. We discuss the applicability of Stokes-EinsteinDebye scaling D[sup r][sub s](φ) ∝ 1 / η[sub L] (φ), and D[sup r][sub s](φ) ∝ 1 / η[sub infin;](φ), where η[sub infin;] is the high-frequency-limiting viscosity. Scaling with η[sub L](φ) fails at high particle and low salt concentrations, while scaling with η[sub infin;] is fairly good, in particular when an apparent nonstick boundary condition is imposed on the friction factor. We conclude that TPA is well suited for use as a microrheological technique.


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