Optomechanical micro-rheology of complex fluids at ultra-high frequency.

We present an optomechanical method for locally measuring the rheological properties of complex fluids in the ultra-high frequency range (UHF). A mechanical disk of microscale volume is used as an oscillating probe that monitors a liquid at rest, while the oscillation is optomechanically transduced. An analytical model for fluid-structure interactions is used to deduce the rheological properties of the liquid. This method is calibrated on liquid water, which despite pronounced compressibility effects remains Newtonian over the explored range. In contrast, liquid 1-decanol exhibits a non-Newtonian behavior, with a frequency-dependent viscosity showing two relaxation times of 797 and 151 picoseconds, associated to supramolecular and intramolecular processes. A shear elastic response appears at the highest frequencies, whose value allows determining the volume of a single liquid molecule. UHF optomechanical micro-rheology provides direct mechanical access to the fast molecular dynamics in a liquid, in a quantitative manner and within a sub-millisecond measurement time.