Particle Tracking Velocimetry With Dynamic Aberration Correction For 3D Flow Measurements Through Fluctuating Phase Boundaries

Imaging based flow measurements allow the observation of complex three-dimensional flows with a high temporal resolution. However, at least one optical access with a good imaging quality is needed. In fluids with a dynamic phase boundary, often the only optical access is the dynamic surface (e. g. droplet on opaque surface). In this case, temporally varying aberrations are introduced that can increase the measurement uncertainty significantly. Additionally, many flows in technical systems are three-dimensional and turbulent so that volumetric measurements are desirable. In this contribution, we present the first 3D-3C flow measurement system with dynamic aberration correction and a single optical access. By using a deformable mirror with 69 actuators, a laser guide star and a wavefront sensor, the dynamic aberrations that are introduced by the fluctuating phase boundary can be corrected. By introducing a double-helix point spread function (DH-PSF) with a spatial light modulator (SLM) we achieve three-dimensional flow measurements with only one camera. For demonstration purposes, the flow in a microchannel was measured through an oscillating puddle-like droplet. By applying the adaptive-optics system, the measurement uncertainty could be lowered by 58 % relative to measurements through a static droplet. The technique has the potential to allow three-dimensional flow measurements in droplets on opaque surfaces (e. g. membrane of fuel cell). Thus, new insights about the movement of droplets (e. g. in fuel cells) are made possible.