 Solution

Under a step stress, the Maxwell spring extends immediately by ${\sigma }_0/G_0$, while the dashpot adds ${\sigma }_{0}t/\eta$. Hence $J(t)=G_0^{-1}+t/\eta$, and the microrheology of a Maxwell fluid relation givesThis idealized overdamped relation describes an elastic plateau followed by long-time diffusion. Its nonzero extrapolated intercept is not the actual displacement variance at exactly zero lag, which is zero. Inertia or fast solvent relaxation must resolve the initial rise; those processes were omitted from the ideal spring-dashpot model.