Fig. 3.

Probing the microviscosity of PEG solutions with FCS. (A) Diffusion of a particle through a solution of crowders with different sizes, with the respective thickness of the depletion layer, δ (red dashed circles). With increasing size of the depletion layer, the effect of the bulk viscosity of the solution on the local diffusion of the particle decreases. (B) Relation between relative bulk viscosity and microviscosity, quantified via the translational diffusion coefficient of NCBD according to ${\eta }_{\text{micro}}/{\eta }_{\text{s}}=D_0/D_1$ for solutions containing different concentrations and sizes of PEG (color scale above), where η_micro is the microscopic viscosity, η_s the neat solvent viscosity, and D₀ the diffusion coefficient in the absence and D₁ in the presence of crowder. The black dashed line shows the behavior expected from the Stokes–Einstein equation and the bulk viscosity. (C) Microviscosity analyzed with a model using solvent viscosity within the depletion layer and bulk viscosity outside (Eq. 5). A global fit of $D_0/D_1={\eta }_{\text{micro}}/{\eta }_{\text{s}}$ across all PEG sizes and concentrations yields R = 1.8 ± 0.1 nm for NCBD.