Indirect optical geometry measurement based on optical tweezers in transparent microchannels

This study introduces an indirect optical geometry measurement approach for transparent microchannels using optical tweezers to trap and manipulate a dielectric particle as a measuring probe. Traditional direct optical methods, such as white light interferometry and microscopy, face challenges in assembled microfluidic systems due to refractive index matching between the channel material and fluid, which reduces surface visibility. The proposed method overcomes these limitations by scanning along the axial direction with an optically trapped silica microsphere of 2 μm diameter as a probe and observing its displacement from the center of the trap, as it interacts with objects. The surface boundary is detected by fitting a piecewise linear model to the axial variations in the width of the probe's image. The experimental setup employs a 976 nm laser for trapping the probe particle, combined with high-resolution imaging, achieving sub-micrometer axial resolution and micrometer lateral resolution. Measurements of the height and cross-section of a microchannel and the surface of a silica microsphere of 20 μm in a microchannel demonstrate the technique's potential for characterizing complex microstructures inside microchannels. Thus, the proposed method enables non-contaminating, high-precision geometry assessment in lab-on-a-chip and microfluidic applications to improve device performance.