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Interaction of light with fluid produces many competing phenomena at the nanoscale, which are less well understood due to the lack of picometer precision in measuring optofluidic deformation. Here, we employ a microliter sessile fluid drop as a self-stabilized laser microinterferometer and resolve its nanoscale interface dynamics, with precisions of about 600 pm in real-time and 20 pm with a modulated beam, below the thermal limit. For evaporating droplets having various absorbance values, we isolate a nanodimple due to laser heating from the nanobump induced by Minkowski's optical momentum transfer. We model the dimple as resulting from a negative surface-tension thermal gradient induced by nonuniform local temperature variation, which we resolved with unprecedented 600 nK precision, besides detecting pN level radiation pressure force at the transparent fluid interface. These signatures are generic for a wide variety of fluids including Au-nanoparticle suspension, olive oil, glycerine, and biofluids such as egg-white and human saliva. Our study opens a route to achieve picometer precision with tiny fluid samples for intriguing applications.

Precision measurement on momentum transfer between light and fluid interface has many implications including resolving the intriguing nature of photons momentum in a medium. For example, the existence of Abraham pressure of light under specific experimental configuration and the predictions of Chau-Amperian formalism of optical momentum for TE and TM polarizations remain untested. Here, we quantitatively and cleanly measure nanomehanical dynamics of water surface excited by radiation pressure of a laser beam. We systematically scanned wide range of experimental parameters including long exposure times, angle of incidence, spot size and laser polarization, and used two independent pump-probe techniques to validate a nano- bump on the water surface under all the tested conditions, in quantitative agreement with the Minkowski’s momentum of light. With careful experiments, we demonstrate advantages and limitations of nanometer resolved optical probing techniques and narrow down actual manifestation of optical momentum in a medium.

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Although, many conventional approaches have been used to measure viscosity of fluids, most methods do not allow non-contact, rapid measurements on small sample volume and have universal applicability to all fluids. Here, we demonstrate a simple yet universal viscometer, as proposed by Stokes more than a century ago, exploiting damping of capillary waves generated electrically and probed optically with sub-nanoscale precision. Using a low electric field local actuation of fluids we generate quasi-monochromatic propagating capillary waves and employ a pair of single-lens based compact interferometers to measure attenuation of capillary waves in real-time. Our setup allows rapid measurement of viscosity of a wide variety of polar, non-polar, transparent, opaque, thin or thick fluids having viscosity values varying over four orders of magnitude from $$10^{0}{-}10^{4}~\text{mPa} \, \text{s}$$ 10 0 - 10 4 mPa s . Furthermore, we discuss two additional damping mechanisms for nanomechanical capillary waves caused by bottom friction and top nano-layer appearing in micro-litre droplets. Such self-stabilized droplets when coupled with precision interferometers form interesting microscopic platform for picomechanical optofluidics for fundamental, industrial and medical applications.