Photonic nanojet-enhanced microcone-shaped fiber-optic tweezers for broad-range microscopic viscosity sensing

The viscosity coefficient is a fundamental parameter in fluid dynamics and soft matter physics; however, conventional measurement techniques, including falling-ball, capillary flow, and vibrational methods, generally require large, homogeneous samples and are incompatible with microscale or optically scattering systems. Here, we report a single-fiber optical tweezers platform that utilizes a microcone-shaped fiber probe for stable, non-contact manipulation of microscale particles and quantitative viscosity probing. The tapered fiber tip, fabricated via controlled chemical etching and optimized through numerical simulations, promotes photonic nanojet formation at the fiber output, resulting in enhanced optical gradient forces and robust single-beam trapping. Experimental measurements demonstrate efficient trapping of 6 μm polystyrene microspheres, achieving a maximum trapping force of 24.9 pN and a trapping efficiency of 46.7%, in excellent agreement with theoretical predictions, thereby validating the underlying optical force model. In addition, the same platform was employed for in situ viscosity extraction from glycerol–water mixtures and whole blood. The measured values deviate by less than 4% from commercial viscometers across a broad dynamic range extending to 6.12 mPa s. These results demonstrate that nanojet-enhanced optical forces provide a compact, energy-efficient route for probing fundamental fluid properties at the microscale, while offering insight into light–matter interactions in complex fluidic environments.