In a study published in Optics Letters, a research group led by Prof. LI Wei from Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP), Chinese Academy of Sciences (CAS) has shown that the nano-mechanical dynamics of water surfaces can be measured quantitatively by observing the interplay between optics and fluid mechanics.

The researchers induced a pushing force for a wide annular Gaussian beam excitation of the thin-film regime of water using a Gaussian/annular laser beam of incidence near total internal reflection (TIR). It allowed to simultaneously observe and distinguish the two rivalling forms of momentum, Abraham's and Minkowski's, by measuring the nano-metric AW interface deformation induced by annular GB excitation. Abraham and Minkowski both are fundamental momentum yet have contrasting amplitude and direction inside a dielectric media.

Figure 1: Nanoscale surface dynamics of water thin film under annular Gaussian beam excitation.

The study is the first, to the best of the researchers' knowledge, to demonstrate the simultaneous measurement of both types of momentum emerging in a single setup with a single laser shot. The team utilized a time-resolved interferometric technique with high precision to measure the interface dynamics in thick and thin water films with different excitation methods. According to their experimental findings, the detection of momentum at the AW interface is influenced not just by the incident laser but also by the mechanics of the fluid. The resulting deformation of the surface is a multifaceted interplay between radiation pressure and fluid dynamics.

The experimental results are strongly backed by numerical simulations performed with realistic experimental parameters.

The findings of the study have significant potential applications in opto-fluidics, fluid droplets, and re-configurable lenses. The results help in the measurement of light momentum with utmost precision inside a dielectric medium, which can be beneficial for various fields.

The interplay between optics and fluid mechanics observed in the study highlights the importance of considering fluid mechanics when studying the interaction between light and matter.