Microscopic shadow imaging technique
The size of the microbubbles was measured using a microscopic shadow imaging technique. As indicated in Figure 2a, the sketch of the imaging system shows a CMOS-Camera (Nova S12 Photron LaVision) with a resolution of 1024 by 1024 pixels, a Zeiss stereo microscope (SteREO Discovery V12 Zeiss LaVision), a flow test section, and an LED light (VI - Strobe LED 120W V2 Lavision Germany). The MBs could stagnate on the top wall of the test section due to the buoyancy effect during the experiments when the test section is placed horizontally, which would block the camera. Therefore, the flow test section was placed vertically in the experiments at the lens’s focal length, 8 mm, and the Zeiss stereo microscope and CMOS camera were placed in a horizontal configuration. An LED light was fixed behind the flow test section, and a Fresnel lens collated the light source to illuminate the test section. The optical platform was equipped with a motorized x-y table, and adjustment of the focal length was controlled by Davis software (Lavision Germany) and a joystick.
The flow test section was made into a flat structure so that less light reflection would occur, which was constructed with two transparent Plexiglass (PMMA) with dimensions of 100 × 100 × 1 mm (Length × Width ×Thickness) and a stainless steel plate of the same size with a flow channel in the middle. The length and width of the flow channel are designed to be 2 mm and 50 mm, respectively, and manufactured by laser cutting. To better conform to the operation of the TCD, an inlet hose with a diameter close to the 18 G venous retention needle was used (17) . The details of the flow test section are depicted in Figure 2b. Finally, the third outlet of the three-way stopcock was connected to the flow test section inlet.