Colloids In Polymer Solution Research Articles

Drag Force for Asymmetrically Grafted Colloids in Polymer Solutions

We consider the situation in which a colloidal particle modifies locally the solvent leading to a spatially dependent viscosity. This situation is typical for colloidal particles in crowded environment, for example DNA-grafted particles in a polymer solution, or a hot particle which implies a temperature gradient to a viscous liquid. By means of suitable approximations we calculate the dependence of the friction force on the profile of the local viscosity. Our results show that in the case of axially symmetric viscosity profile the friction force is sensitive to the anisotropy of the viscous profile whereas it is not sensitive to for-ahead asymmetries. Our results are crucial for active microrheology measurements where tracer particles are pulled through complex fluids.

Sterically stabilized lock and key colloids: A self-consistent field theory study

A self-consistent field theory study of lock and key type interactions between sterically stabilized colloids in polymer solution is performed. Both the key particle and the lock cavity are assumed to have cylindrical shape and their surfaces are uniformly grafted with polymer chains. The lock-key potential of mean force is computed for various model parameters, such as length of free and grafted chains, lock and key size matching, free chain volume fraction, grafting density, and various enthalpic interactions present in the system. The lock-key interaction is found to be highly tunable, which is important in the rapidly developing field of particle self-assembly.

Phase Behaviors of Colloids in Polymer Solutions

A density functional theory (DFT) based on the weighted density has been developed for studying the polymer-induced depletion interaction and phase behaviors of colloids in polymer solutions. The depletion potentials between two colloids induced by the polymer fluids and the colloid-colloid second virial coecients have been calculated. The calculated depletion potentials are compared with the simulation results. The phase diagram shows that short chains can give a more stable dispersion for colloids than long chains. The large colloid/polymer size ratio develops strong attractive interactions. This leads to a broadening of the coexistence region between the solid and the fluid phases of the colloids in a polymer solution.

One-dimensional diffusion of colloids in polymer solutions

For colloid-polymer systems confined in one-dimensional channels, the diffusion of the colloidal particles is obtained by tracking individual particles using enhanced video microscopy and digital image analysis. For short times, the diffusion is normal, of the Fickian type, with mean-squared displacement varying linearly with time. For long times, however, the mean-squared displacement is found to increase more slowly with time, being proportional to the square root of time, in agreement with the theoretical prediction for diffusion of hard rods in one dimension in which mutual crossing of the particles cannot take place. The crossover from short-time to long-time diffusion is observed and is found to depend on the colloid and polymer concentrations. Unexpectedly, for small polymer-to-colloid size ratios, it is the polymer rather than the colloid concentration which has a leading effect on the colloid diffusion.

Two-dimensional diffusion of colloids in polymer solutions

Enhanced digital videomicroscopy is used to study the two-dimensional diffusion of colloidal polystyrene spheres of diameter 0.99 µm in confined aqueous solutions of polymer PEO (polyethyleneoxide) of molecular weight 8 × 106. Diffusion is found to obey the Brownian motion law of linearity of the mean square displacement with time, with a constant appropriate to two dimensions. The diffusivity is determined for various polymer concentrations ranging from zero to those corresponding to dilute and semi-dilute polymer solutions. An abrupt slope change occurs near the polymer concentration of 0.002 wt-%, which lies below the concentration at which the polymer chains begin to overlap.