On the colour of thermal noise in fluids

In the paper by Franosch et al., Nature 478, 85 (2011), the positional fluctuations of Brownian microspheres in fluids were studied by confining the particles in an optical trap. Experimental access to short timescales has revealed a resonance peak in the spectrum of these fluctuations, in contrast to the commonly assumed overdamped motion. This work is also interesting as the first measurement of the "colour" of thermal noise driving the Brownian particles through collisions with the fluid molecules. The obtained results are described by the hydrodynamic theory of the Brownian motion in harmonic potentials. In the present work we show that the correlation properties of the thermal noise significantly differ from those determined in the discussed work.