Translational and rotational Brownian motion in water

Fluid processes at high frequencies associated with fluid inertia, compressibility, and viscoelasticity contribute to the spectra of the random force- and velocity-autocorrelation functions of a Brownian particle immersed in water. In addition, the internal rotational degrees of freedom of the fluid particles also contribute to the structure of the correlation functions in both the high- and low-frequency regimes. As we show here, the relative importance of these various effects for translational and rotational Brownian motion depends on the size of the Brownian particle, and on the fluid parameters.