Trajectory-based force and viscosity measurements in optical tweezers with real-time integrated particle tracking

Quantitative reconstruction of particle trajectories from low-contrast optical tweezers videos remains a long-standing challenge, limiting precise force and viscosity measurements. Herein, we present an integrated detection and tracking framework that mitigates this limitation by combining dynamic background correction, mask-based enhancement, and robust trajectory association. This approach reliably extracts continuous trajectories of single and multiple particles under optically demanding conditions (e.g., low contrast and uneven illumination). By linking the reconstructed orbital motion in a ring-shaped trap formed by a perfect vortex beam to the equipartition theorem and to the Langevin equation with Stokes drag, we obtain a self-consistent estimate of the trap stiffness, as well as weak radial restoring and tangential driving forces. Furthermore, using water as a reference fluid, we apply a Stokes scaling to convert the measured azimuthal rotation frequency into a quantitative readout of the effective viscosity of aqueous and slightly thickened solutions. These results establish vortex-beam optical tweezers with real-time, integrated video-based particle tracking as a versatile platform for quantifying trap stiffness, optical forces, and effective viscosity, enabling new opportunities in optical trapping and microrheology in low-viscosity regimes.