Impulse Response Function for Brownian Motion

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.