4 research outputs found
Mobility of Nanoparticles in Semidilute Polyelectrolyte Solutions
We
measure the mobility of nanoparticles at low concentrations
in non-Newtonian semidilute aqueous solutions of high-molecular-weight
polyelectrolyte polymers. Using optical microscopy and particle tracking
algorithms, we image and track hydrophilic polystyrene nanoparticles
of diameter 400 nm moving in aqueous solutions of partially hydrolyzed
polyacrylamide of molecular weight 8 000 000 Da and
concentration of 0.042î—¸4.2 g/L. The effective diffusivity of
the nanoparticles in the semidilute polymer solutions, extracted from
the long-time limit of the mean-squared displacement using the Stokes–Einstein
relation, is greater than that calculated from the zero-shear-rate
viscosity measured using bulk rheology. For concentrations <i>c</i> > 0.42 g/L, the mean-square displacements (MSD) of
particles
measured as a function of lag time revealed that the particle dynamics
are subdiffusive at short time scales and are Fickian on long time
scales. The time scale for the crossover from subdiffusive to Fickian
dynamics increases with increasing polymer concentration; moreover,
it is longer than the relaxation time scale for polymer blobs and
shorter than that for the chain. Our results suggest that the nanoparticle
dynamics are coupled to those of the polymers on a length scale intermediate
between the blob size and the end-to-end distance of the polymer
Diffusive Dynamics of Nanoparticles in Arrays of Nanoposts
The diffusive dynamics of dilute dispersions of nanoparticles of diameter 200–400 nm were studied in microfabricated arrays of nanoposts using differential dynamic microscopy and single particle tracking. Posts of diameter 500 nm and height 10 μm were spaced by 1.2–10 μm on a square lattice. As the spacing between posts was decreased, the dynamics of the nanoparticles slowed. Moreover, the dynamics at all length scales were best represented by a stretched exponential rather than a simple exponential. Both the relative diffusivity and the stretching exponent decreased linearly with increased confinement and, equivalently, with decreased void volume. The slowing of the overall diffusive dynamics and the broadening distribution of nanoparticle displacements with increased confinement are consistent with the onset of dynamic heterogeneity and the approach to vitrification