Thin-film dynamics unveils interplay between light momentum and fluid mechanics.

We quantitatively measure the nanomechanical dynamics of a water surface excited by the radiation pressure of a Gaussian/annular laser beam of incidence near total internal reflection (TIR). Notably, the radiation pressure near TIR allowed us to induce a pushing force (Abraham's momentum of light) for a wide annular Gaussian beam excitation of the thin-film regime of water, which, to the best of our knowledge, has never been observed with nanometric precision previously. Our finding suggests that the observation of either/both Abraham's and Minkowski's theories can be witnessed by the interplay between optics and fluid mechanics. Furthermore, we demonstrate the first, to the best of our knowledge, simultaneous measurement of Abraham's and Minkowski's momenta emerging in a single setup with a single laser shot. Our experimental results are strongly backed by numerical simulations performed with realistic experimental parameters and offer a broad range of light applications in optofluidics and light-actuated micromechanics.