## 2 Methodology
### Stereoscopic Particle Image Velocimetry  
For stereoscopic particle image velocimetry (SPIV) experimentation, the phantom was installed in a box that contained a pressurised water-glycerol fluid. The box was manufactured from 8 mm thick transparent acrylic sheet to facilitate optical access to the phantom. The sheets were laser cut and assembled. The water-glycerol ratio was 39:61 which has a viscosity 0.0117 Pa-s and a density of 1156 Kg/m\({}^{3}\) at 20 \({}^{\circ}\)C, respectively [17]. This viscosity is distinct from blood. However, it allows dynamic similarity by matching the Reynolds and Womersley numbers to the physiological conditions. Preserving the dynamic similarity, the Womersley and maximum Reynolds number were 18.52 and 1220, respectively. To match the Womersley number, an in vivo heart rate of 60 beats per min was used. The fluid was pressurised to 0.52 KPa to mimic the transmural pressure and surrounding tissue [44; 45]. The working fluid matched the refractive index of the phantom to eliminate optical distortion [24; 46; 47]. The refractive index was measured n = 1.417 using NAR-3T Abbe benchtop refractometer (ATAGO CO., LTD, To kyo, Japan). A water-glycerol solution was impregnated with Silver-Coated Hollow Glass Spheres particles (10 \(\upmu\)m). A physiological flowrate waveform from a magnetic resonance imaging (MRI) study [48] was scaled to enable dynamic similarity and was induced by a piston pump (Figure 3b). The piston pump was actuated by a stepper motor and was connected to a flow straightener and an electromagnetic flowmeter. The phantoms' outlets were connected to a header tank to mimic the impedance of the capillaries. A weir in the header tank controlled the outlet and back pressure (Figure 3a). Two TSI Powerview 4 MP LS PIV cameras (TSI Inc., Shoreview, MN, USA), 60 mm Nikon lenses, a 120 mJ/pulse Nd-YAG laser (New Wave Solo 120 XT), and a light sheet forming lens train were used for SPIV. The light sheet was 2 mm thick. Coupled images captured the movement of particles in the working fluid and provided data for the TSI INSIGHT 4G software to reconstruct the flow fields in both phantoms. Since the current work focuses on the phantom manufacturing process, the PIV measurement was done on the phantom symmetry plane, and the planar velocity components are presented. The images from the two cameras were rotated and averaged to yield planar velocity fields. The time delay between the laser pulses was set at 1100 us to capture the image pair. Two time-points were measured on the cardiac cycle; one at mid-acceleration and one at mid-deceleration. The analysis was phase locked with 20 pairs of images averaged at each phase. However, only the deceleration phase outcomes are presented since the reduction of the momentum is hypothesised to show clinically interesting flow separation. The images were captured with an _f_# of 11 for the lens aperture size. The laser power was modulated to achieve the best contrast in the seeding particles. The first phantom exhibited regions of significant light saturation and subsequent low contrast in the flow field when high laser powers were used. Hence, lower laser power was used to capture flow in the initial phantom. Viewing the pressure box from an angle may cause optical distortion across the images. To overcome this issue, the images were calibrated by placing a Cartesian grid calibration target plane in the pressure box filled with working fluid. Disparity correction was employed to correct the misalignment between the light sheet and calibration target plane. Average intensity background was subtracted from the images to enhance the signal-to-noise ratio. The images were interrogated using Recursive Nyquist window refinement with the start and final window dimensions of \(64\times 64\) and \(32\times 32\) pixels, respectively. Cross-correlation was used to calculate the displacement vector field. Fast Fourier transform was used to calculate the cross-correlation function and reduce the computational time. The PIV uncertainty was not measured.