Fully vectorial, high precision, and high spatial resolution flow velocity measurement in microfluidic chips using a back-focal-plane optical tweezer

Accurately monitoring the flow velocity within microfluidic chips is critical for many multidisciplinary research areas. Existing flow velocity measurement approaches, such as particle imaging and tracking velocimetry, are challenging to resolve three-dimensional flow velocities, and the measurement accuracy is affected by the particle random Brownian motion. Optical tweezer velocimetry can overcome the influence of Brownian motion at low flow rates, while at present it is strictly limited to in-plane measurement. Here we report for the first time a fully vectorial flow velocity measurement technique using a back-focal-plane single-beam optical tweezer. The technique can resolve the flow velocity distribution in microfluidic chips with tens of nanometers spatial resolution over all dimensions, along with a flow velocity measurement accuracy ∼5 times higher than the present techniques. The effects of trapping parameters on the dynamic range and sensitivity of the flow velocity measurement are investigated. The measured flow velocity distribution in both conical and Z-shaped microfluidic chips is in excellent agreement with numerical simulations, demonstrating the feasibility of the technique for resolving complex flow fields in microfluidic devices.