Temporal Super Resolution X-ray Particle Velocimetry for Vitrifying Flow

Vitrifying fluids exhibit creeping flow, but are challenging to analyze due to significant temperature and viscosity gradients, and phase change which may occur in part of the fluid. Vitrification is of interest due to the potential for enabling indefinite cryopreservation of biological tissue and cells. The fluid is typically an aqueous solution with high concentrations of cryoprotective agents that penetrate biological tissue and reduce the critical cooling and warming rates necessary to avoid ice formation. During the rapid cryogenic cooling process isochoric vitrification in thick-walled metal confinement can be monitored via x-ray photon counting for imaging. We have experimentally demonstrated the feasibility of ice and cavity free isochoric vitrification in Ali et al. (2024), and now are utilizing X-ray particle velocimetry (XPV) to study previously optically inaccessible vitrifying flows. However, as rapid cooling rates are necessary to inhibit ice formation, large number of radiographs equiangularly acquired over 180 or 360 degrees, for parallel and cone beam geometries, respectively that are traditionally required for XPV are not feasible. We, therefore, propose a new temporal super resolution approach to XPV for studying vitrification. The particle tracking is achieved using a small number of radiographs and a region based convolutional neural network (R-CNN) to identify and locate the 2D projection coordinates of the tracer particles, then solve a system of linear equations that translate the projection coordinates to world coordinates. The experimental data is utilized to quantitatively characterize vitrifying flows in an unprecedented detail and is subsequently compared to computational predictions by Rabin (2021).