Abstract
We performed total internal reflection microscopy (TIRM) experiments to determine the depletion potentials between probe spheres and a flat glass wall which are induced by long and thin, rod-shaped colloids (fd-virus), and probe the spatially resolved dynamics of the probe spheres. The dynamic information from the same raw TIRM intensity time traces is extracted in three different ways: by determining the spatially averaged diffusion constant of the probe sphere normal to the wall, by measuring the position dependence of the diffusion coefficient, and by measuring the particle's local drift velocity. Up to a concentration of about 6 times the overlap concentration of the rod-like colloids, the spatially averaged diffusion coefficient and the amplitude of the depletion potential are in surprisingly good agreement with theoretical predictions in which mutual interactions between the rods are neglected, that is, where the concentration is less than the overlap concentration. On increasing the depletant content even further, however, both the static and the averaged dynamic quantities begin to deviate from such theoretical predictions. In particular we find large deviations from the prediction by Mao, Cates, and Lekkerkerker [J. Chem. Phys., 1997, 106, 3721] based on the third order virial expansion for the rod concentration. It is shown that there are significant inaccuracies in TIRM measurements of diffusion coefficients due to the limited time range in which the mean squared displacements vary linearly in time, whereas mean displacements give much more accurate information concerning the probe sphere dynamics.
Highlights
Whenever a particle suspension consists of more than one colloidal component, the static properties of the suspension cannot be described anymore on the basis of DLVO-type pair interaction potentials alone
The static interaction potentials between a wall and probe spheres of different size dispersed in suspensions with increasing concentrations of the rod-shaped depletant fd-virus were measured by total internal reflection microscopy (TIRM)
This mismatch is most probably due to the quite large inaccuracy in determining the diffusion coefficients, due to the very short time range over which the correlation function varies linearly in time, there seems to be a correlation with the observation from static data where, in the same range of probe sphere size and virus concentrations, the apparent amplitude of the depletion potential is much deeper than expected from the theoretical prediction
Summary
Total internal reflection microscopy (TIRM)[17] has proven to be an ideal method for the measurement of depletion interactions between a probe sphere and a flat glass wall.[6,7,18,19,20,21,22,23,24] In our previous contribution[16] we used TIRM to show that depletion potentials induced by the rod-like fd-virus follow the classical Asakura–Oosawa (AO) predictions at depletant concentrations and size ratios at which this theory is expected to fail. Measurements of the near wall diffusion coefficients were mainly reported as an independent method to determine the particle separation distance from the wall This method is based on the fact that the particle mobility close to a wall becomes anisotropic and position dependent due to the hydrodynamic interaction with the wall.[25,26] The feasibility of this approach has been debated,[27,28,29,30,31] mainly because of the fact that the Brownian motion of the probe spheres is overlaid with a drift term which is caused by the forces on the probe particle due its direct interaction with the wall as well as gravity. In this paper we are tackling this problem again, and we will show that measurements of the particle’s local drift velocity are significantly more reliable than the measurement of the local diffusion constant This suggests a new approach to the analysis of dynamic information, inherent to TIRM data
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