# Evolution of Water-in-Oil Droplets in T-Junction Microchannel by Micro-PIV
## 3 Results and Discussion
### Internal Velocity Profile of Generated Water Droplets in Offset T-function Microchannel  
Figures 5-8 show pre-processing images, vector analysis and also velocity magnitude analysis performed by a micro-PIV technique via MATLAB software for the droplet flow and slug flow pattern. As the water is in the dispersed phase, the hydrophobic coating promotes these dispersed flows in which the water phase must wet the wall of the microchannel [46]. Figure 4: The motion of water droplets after 364 ms within radiuses of (**a**) 400 \(\upmu\)m, (**b**) 500 \(\upmu\)m, and (**c**) 750 \(\upmu\)m offset T-junction microchannels. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Inlet and Outlet Radiuses of** & **Theoretical Velocity of Water** & \multirow{2}{*}{
\begin{tabular}{c} **Experimental Velocity of** \\ **Water Droplets (mm/s)** \\ \end{tabular} } \\
**Offset T-Junction** & **Droplets Based on Reynolds** & **Water Droplets (mm/s)** \\
**Micro-Channel (\(\upmu\)m)** & **Number (mm/s)** & \\ \hline
400 & 0.322 & 0.330 \\
500 & 0.206 & 0.266 \\
750 & 0.092 & 0.096 \\
1000 & 0.051 & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Theoretical and experimental velocity of water droplets. Figure 5: Pre-processing image, vector and velocity magnitude for the offset T-junction microchannel with a radius of 400 \(\upmu\)m at (i) 91 ms, (ii) 273 ms, (iii) 455 ms, (iv) 636 ms, (v) 818 ms and (vi) 1000 ms.  
Figure 6: Pre-processing image, vector and velocity magnitude for the offset T-junction microchannel with a radius of 500 \(\upmu\)m at (i) 91 ms, (ii) 273 ms, (iii) 455 ms, (iv) 636 ms, (v) 818 ms and (vi) 1000 ms.  
Figure 7: Pre-processing image, vector and velocity magnitude for the offset T-junction microchannel with a radius of 750 \(\upmu\)m at (i) 91 ms, (ii) 273 ms, (iii) 455 ms, (iv) 636 ms, (v) 818 ms and (vi) 1000 ms.  
Figure 8: Pre-processing image, vector and velocity magnitude for the offset T-junction microchannel with a radius of 1000 \(\upmu\)m at (i) 91 ms, (ii) 273 ms, (iii) 455 ms, (iv) 636 ms, (v) 818 ms and (vi) 1000 ms. The pattern of the flow involving droplets as shown in Figures 5-7 can be characterized by the sub-channel-sized droplets [34]. The flow within these droplets is guided forward at the interfaces to the right end of the microchannels, where the water droplets are bypassed by the continuous oil phase. The formation of such droplets, having diameters less than the channel diameter, are generated at low dispersed and high continuous phase flow rates. In such conditions, the inertial force input at the higher flow rate of the continuous phase is high enough to break down the dispersed phase into smaller droplets with lesser resistance posed by the dispersed phase at a lower flow rate. Meanwhile, for the slug flow pattern, which is likely to be the flow behaviour for the microchannel with a radius of 1000 \(\upmu\)m (Figure 8), the slug occupied almost the whole cross-section of the microchannel with a very thin layer of the oil phase between the slug and the wall of the offset T-junction microchannel. This behaviour fit well in [PERSON], [PERSON] [34] generalized flow regime map. It is evident from this discussion that changing the channel size has a significant impact on the slug size. In contrast to the smaller offset T-junction microchannel, bigger channels produce larger slugs. All of the microchannels exhibit same slug size behaviours in relation to flow velocity [32]. However, the shear stress is not significant in this regime, and the interfacial tension, along with the pressure gradient, results in the breakup of the dispersed phase into slug. The interfacial tension could not support and endure the pressure gap between inside and outside the interface at the droplet's tail [28]. This differential pressure increased prior to the neck regime, squeezing the interface and, consequently, forced the rear tip or interface to break, resulting in the formation of a droplet. Resultantly, the slug growth leads to the obstruction to the continuous phase flow [34].