Microfluidics is a multidisciplinary field that refers to the behaviour, control, and the manipulation of fluids that are constrained to a small scale and volume. This field involves engineering, physics, chemistry, biotechnology, nanotechnology, and biochemistry. It finds application in the development of inkjet printheads, DNA chips, and lab-on-chip technology, etc.
Microfluidics technology has several advantages such as precise control over process parameters, excellent resolution and better sensitivity in separation and detection, lower reagent and sample consumption, reduced analysis time, portability, and scalability.
Because of these advantages, microfluidic devices are getting greater recognition in fields of applications like chemical synthesis, environmental monitoring, synthesis of nanoparticles, drug development, and biomedical diagnosis.
In order to mix the sample and reagents of microfluidic devices, a micromixer is used. Because of the importance and various applications of microfluidic devices, an efficient micromixer is also very important.
Micromixers depend on the phenomenon of lamination and chaotic advection, where the interfacial area is enlarged, and the distance across which diffusion occurs is reduced, or by encouraging the transversal flows that will assist in the stretch, folding, and breaking/splitting of fluids layers.
There are two distinct methods of mixing – passive and active.
In the case of passive mixing, geometrical structures of the channel are modified to promote breaking up, stretching, folding, twisting, bending, etc. with the help of enhanced mixing. Active mixing on the other hand refers to the use of external agents such as magnetic, electrical, thermal, and acoustic fields to develop perturbations in the inside of the flow domain.
Even though active mixing is more efficient than passive mixing, it is not preferred because it has limited adoption due to its complexity in fabrication and complex integration within the microfluidic chip. Also, temperature rise in active mixers can damage biological molecules because of which they cannot be used in medical and biological applications. Passive micromixers on the other hand have several advantages over active micromixers in that they are easy and straightforward to design, require lower cost, have simple fabrication, and have better integrability.
An array of geometrical structures such as groves, obstacles, baffles, crossing channels, and curved channels have been used by researchers. However, designs based on grooves, obstacles, and baffles caused deformation of the mixing species, which make them unfavourable for handling larger particles and biomolecules. Hence the use of curved and spiral channels is more popular for the design of micromixers. This structure helps in obtaining transverse flows to improve the mixing. It also takes advantage of the secondary flows, also called Dean vortices, generated in the channel transverse sections due to the effect of centrifugal instabilities on the fluids while they followed a curved path.
From several studies, it has been found that a simple curved channel shows better homogeneity. Asymmetrical transversal flows leading to chaotic advection in simple serpentine channels were found to show better mixing.
In this study conducted by Dr. Wasim Raza and Prof. Abdus Samad from the Department of Ocean Engineering, Indian Institute of Technology (IIT) Madras, Chennai, India, and Dr. Nazrul Islam from the Department of Mechanical Engineering, King Abdulaziz University, Jeddah 21589, Saudi Arabia, two novel bi-layer curved serpentine (BCS1 and BCS2) micromixers having asymmetrical cross-sections were proposed. The novelty of this research lies in the design concept. Here varying the radii and alteration of the direction of turns of the curved channel in mixing units, thus generating asymmetrical vortices was done.
The central idea of these designs was to generate chaotic advection through asymmetry of vortices along the flow path beside the Dean vortices in the transversal direction inside the curved microchannels. Chaotic advection causes stretching, reorientation, and folding of the fluid interfaces.
In the BCS1 micromixer, the curved channel radii in both the layers are unequal, while in the BCS2 micromixer, the curve direction and the mean radius of the curved channel change alternately at each half mixing unit length in both the layers.
The two chaotic serpentine micromixers, BCS1 and BCS2 were compared with a simple serpentine (SS) micromixer. BCS1 micromixer was found to exhibit the best mixing. BCS1 and BCS2 micromixers showed significantly higher mixing indices than the simple serpentine (SS) micromixer.
The optimum mixing cost, efficient mixing over an extensive flow range, and easy to fabricate geometry of the proposed micromixers make these a viable choice to be integrated into microfluidic systems and a standalone device for mixing in biochemical and micro-chemical high throughput applications.
Dr. Mubashshir Ahmad Ansari, Professor from the Department of Mechanical Engineering, Aligarh Muslim University, Aligarh, India, acknowledged the importance of the work done by the authors by giving the following comments: “The paper entitled “Design and analysis of a novel bi-layer curved serpentine chaotic micromixer for efficient mixing” provides an innovative design to enhance the mixing while keeping the pressure drop to a minimum value. As discussed in the paper, mixing of fluid samples is an important and challenging aspect of microfluidics applications. They have extensively analyzed the mixing performance over a range of flow conditions that will cater to the demands of wide-ranging fields of interest such as chemical synthesis and biological analysis. The proposed method of enhancing the mixing using Dean vortices and keeping the flow area constant would cause a lower pressure drop and shear stress. They have used equal-width channels in a two-layer structure to keep the flow area constant. I appreciate the authors for their effort in sharing this design and analysis, which will help the researchers focus on developing low-pressure drop micromixers. The suggested micromixer structure will allow effective mass and heat transmission in a miniaturized and compact environment, serving as a tool to meet the process-intensification goals.”
Article by Akshay Anantharaman
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