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.