# Evolution of Water-in-Oil Droplets in T-Junction Microchannel by Micro-PIV
## Materials and Methods
### Experimental Setup and Micro-PIV Processing  
The schematic diagram of a microfluidic channel and the associated optical imaging measurement system is shown in Figure 2 along with the fluid driving mechanism. A digital video microscope (Model: MD500, AmScope, Irvine, CA, USA) having 4\(\times\) magnification was used to capture the droplet behaviours focused at the junction of the microfluidic channel, with an image resolution of 800 \(\times\) 600 pixels. A two-phase stepper motor (Model: NEMA 17, StepperOnline Inc., New York, NY, USA) driven lead screw was used to drive the syringes containing the different solutions. The syringe with 10 mL nominal capacity \(\pm\)4% at room temperature was used for the setup. An Arduino Uno microcontroller was programmed to control stepper motor powered by a linear DC supply (Model: GPS-3030D, Good Will Instrument Co., Ltd., New Taipei City, Taiwan). Table 1 summarizes the input parameter setting maintained for all the experiments, including the setting of the microscope and initial flow rate. Figure 1: Fabricated offset T-junction microchannel with an inlet and outlet radius of 750 \(\upmu\)m.  
The experiments originally used the Magnaglo(r) 14A fluorescent magnetic powder (mean particle size of 6 \(\upmu\)m, manufactured by Magnaflux, Glenview, IL, USA) suspended in Carrier II (density of 802.837 kg/m\({}^{3}\), dynamic viscosity of 0.0026 Pa\(\cdot\)s at 38 \({}^{\circ}\)C, manufactured by Magnaflux, Glenview, IL, USA) at the dispersed phase. However, due to its fast settling in the oil, the preliminary experiments were unsuccessful. Hence, food-grade palm olein, having a density of 917 kg/m\({}^{3}\) and viscosity of 7.97 \(\times\) 10\({}^{-2}\) Pa\(\cdot\)s [35], and distilled water (density of 997 kg/m\({}^{3}\), viscosity of 8.90 \(\times\) 10\({}^{-4}\) Pa\(\cdot\)s at ambient temperature) seeded with polystyrene microspheres particles (manufactured by Thermo Scientific(tm) 4210A, San Francisco, CA, USA) were injected into inlet A and inlet B, respectively, for different radiuses of offset T-junction microchannels. Food-grade palm olein was chosen for this study because it is one of the most commonly used oils in the household and food industries [36], and also has a high stability index [36] in the emulsification process. The suspension containing seeding particles (Table 2) and distilled water had a ratio of 1:4 for each experiment. The food-grade palm olein and the seeding particles were assumed to have the same density and dynamic viscosity as distilled water. These particles do not affect the palm olein because they are suspended in water. Due to the particles' small size, and minimal density difference between the particles (1050 kg/m\({}^{3}\)) and water (997 kg/m\({}^{3}\)), they are assumed to faithfully follow the flow of water [18; 37]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Exposure time & 3.906 ms \\ Colour temperature & 5547 \\ Frame rate & 21.8 f/s \\ Resolution & 800 \(\times\) 600 pixels \\ Motor step & 5 \\ Flow rate of liquid at the inlets & 0.16 mm\({}^{3}\)/s \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Controlled parameters. Figure 2: Schematic diagram of experimental setup. As the densities of the two fluids (food-grade olein and distilled water) were different, the properties of the distilled water were taken as the reference. By adopting time-resolved as the image sequencing style in micro-PIV for MATLAB software [39; 40], the flow patterns of the fluids were obtained. The region of interest was set to the whole area of the 800 \(\times\) 600 pixels frame. The physical dimension of the region is 1420 \(\times\) 1060 \(\upmu\)m\({}^{2}\) (see Figure 1). Therefore, the resolution is 1.77 \(\upmu\)m \(\times\) 1.77 \(\upmu\)m per pixel, respectively. For image pre-processing, contrast limited adaptive histogram equalization (CLAHE) was applied to the images for visibility enhancement [41].