# The Design and Investigation of Hybrid a Microfluidic Micromizer
## 5 Conclusions  
This paper investigated the performance of a microfluidic micromixer using a hybrid actuation approach, such as active (acoustic) and passive (inclusion of micropillars in the microfluidics chamber). The simulations were performed for four different micromixer designs such as circular-shaped micropillars, hexagonal-shaped micropillars, blade-shaped micropillars, and without micropillars. Additionally, different micropillar gaps such as 0.150 mm, 0.175 mm, 200 mm, 225 mm, and 250 mm were also used to investigate mixing performance. The detailed conclusions drawn from this study are described below.  
1. The performance of microfluidics micromixers is greatly affected by the inclusion of micropillars compared to the case with no micropillars. The presence of micropillars in the microfluidic chamber increases the microvortices and strengthens the acoustic field.
2. Similarly, the shapes of micropillars are also impacted by the flow characteristics and the mixing performance of the microfluidic micromixer. Various micropillar shapes such as circular, hexagonal, and blade were investigated and compared to the case with no micropillars. The inclusion of blade-shaped micropillars delivers the best outcomes compared to the other shapes.
3. The performance of the microfluidic micromixer was also investigated with different micropillar gaps and found that the mixing performance increased with the decreasing of micropillar gaps. The reason behind this increment in performance is due to larger surface areas and more boundaries to generate higher microvortices and strengthen the acoustic field near the micropillar walls. The maximum performance was achieved with a micropillar gap of 0.150 mm.
4. It was also investigated that mixing performance and flow characteristics are significantly affected by altering the Reynolds numbers. It was revealed that the mixing performance increased with decreasing Reynolds numbers. The maximum and minimum values of mixing performance were achieved at Reynolds numbers of 2 and 12, respectively.
5. The maximum and minimum magnitude values of 0.97 and 0.70 for the mixing index are achieved, respectively, with the inclusion of blade-shaped micropillars and a 0.150 mm micropillar gap.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Flow Rates/Re** & **Mixing Index** & **References** \\ \hline \(7.5\times 10^{-3}\) to \(150\times 10^{-2}\) & 0.96 & [52] \\ 0.1 to 100 & 0.85 & [22] \\ \(\leq\)60 \(\mu\)L/min & 0.93 & [23] \\ 2 mL/min & 0.89 & [40] \\ 2–12 & 0.97 & Present Study \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparison study with the current literature.  
On the basis of the current research work, further work is required for the optimisation of hybrid microfluidics micromisers based on various micropillar arrangements. In addition, an analysis of the acoustic structure with different materials and geometrical parameters is also required to examine the structural stability and structural strength of the micropillars.  
Conceptualization, M.W. and G.J.; Methodology, A.P.; Software, M.W. and G.J.; Validation, G.J., V.N. and A.P.; Formal analysis, V.N. and A.P.; Investigation, M.W., G.J. and A.P.; Writing--original draft, M.W.; Writing--review & editing, G.J. and A.P. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are available in the article.  
The authors declare no conflicts of interest.