large dynamic consequences.

All the qualitative features we observed in the evolution of the SSFs for samples A–H are also seen in the SSFs for samples I–K at the higher volume fraction of $\phi\approx 0.64$, Fig. \[fig:sqHi\]. However, the effects are significantly less obvious, largely because the range of polymer concentration is now much smaller and $\phi$ is higher. At low $q$, the values of $S(q_{\text{min}})$ are lower than those of similar polymer concentration but lower $\phi$ (C–E) (Fig. \[fig:sqLo\](b)). This is because at higher volume fraction, a tight local clustering of some particles does not create so much room elsewhere — there is less space for developing heterogeneities.

Dynamic structure factor {#sec:DSF}
------------------------

Our goal is to study how the polymer-induced depletion attraction affects the particle dynamics. But the presence of the polymer influences the dynamics in another, essentially trivial, manner — by increasing the effective viscosity of the medium in which the particles diffuse from that of the pure solvent, $\eta_0$, to that of a polymer solution, $\eta_r \eta_0$ at concentration [$c_p^{\text{free}}$]{}. To determine $\eta_r$, we measured the viscosity of pure polymer solutions with a miniature suspended-level viscometer, Fig. \[fig:viscosity\], and used a quadratic fit to the data 