High-resolution single-camera measurement of near-wall film flow at spray cleaning process using a double-helix point spread function

Abstract Film flows play a key role in spray cleaning, microfluidics, and biomedical devices. They spread liquids across surfaces and generate shear stress that drives surface interactions. Accurate film flow data are essential for industrial spray cleaning, precise fluid dosing in lab-on-a-chip devices and studies of cell responses to shear. The interaction between the film flow and the surface can be characterized by the wall shear stress, which can be determined by the near-wall flow profile. However, determining the flow profile experimentally in the typical millimeter thick film remains challenging, which, on the one hand, is due to the limited optical access and, on the other hand, the required spatial resolution in the micrometer range. To address this challenge, we present a single-camera particle tracking velocimetry (PTV) that incorporates a double-helix point spread function (DH-PSF) to determine the axial position of tracer particles. Each particle is rendered as a dual-lobed image, the orientation of which encodes its axial depth, hence enabling a full three-dimensional (3D), three-component (3D-3C) measurement. The setup achieves 7.3 µm spatial resolution for films up to 1 mm thick, enabling near-wall flow characterization and direct wall shear stress estimation. Experiments resolve velocities up to 25 mm/s and quantify wall shear stress profiles, validating near-wall accuracy. Experiments reveal a reciprocal dependence of the wall shear stress with the radial distance, which agrees with a theoretical momentum balance model. This system provides a compact, versatile tool for optimizing spray cleaning, probing cell-shear studies and advancing microfluidic design.