Motivated from the central role of the mean-square displacement and its second time-derivative -- that is the velocity autocorrelation function $\left\langle v(0)v(t)\right\rangle=\frac{ 1}{ 2} \frac{ \mathrm{ d}^{ 2} \left\langle \Delta r^{ 2} (t)\right\rangle}{ \mathrm{ d}t^{ 2}} $ in the description of Brownian motion, we revisit the physical meaning of the first time-derivative of the mean-square displacement of Brownian particles. By employing a rheological analogue for Brownian motion, we show that the time-derivative of the mean-square displacement $\frac{ \mathrm{ d}\left\langle \Delta r^{ 2} (t) \right\rangle}{ \mathrm{ d}t}$ of Brownian microspheres with mass $m$ and radius $R$ immersed in any linear, isotropic viscoelastic material is identical to $\frac{ N K_B T}{ 3 \pi R}h(t)$, where $h(t)$ is the impulse response function of a rheological network that is a parallel connection of the linear viscoelastic material with an inerter with distributed inertance $m_R=\frac{ m}{ 6 \pi R}$. The impulse response function $h(t)=\frac{ 3\pi R}{ N K_B T}\frac{ \mathrm{ d}\left\langle \Delta r^{ 2} (t) \right\rangle}{ \mathrm{ d}t}$ of the viscoelastic material-inerter parallel connection derived in this paper at the stress-strain level of the rheological analogue is essentially the response function $\chi(t)=\frac{ h(t)}{ 6\pi R}$ of the Brownian particles expressed at the force-displacement level by Nishi \textit{ et al.} (2018). By employing the viscoelastic material-inerter rheological analogue we derive the mean-square displacement and its time-derivatives of Brownian particles immersed in a viscoelastic material described with a Maxwell element connected in parallel with a dashpot which captures the high-frequency viscous behavior and we show that for Brownian motion in such fluid-like soft matter the impulse response function, $h(t)$ maintains a finite constant value in the long term.
Categories: cond-mat.soft cond-mat.stat-mech