Abstract
One approach to achieve a homogeneous mixture in microfluidic systems in the quickest time and shortest possible length is to employ electroosmotic flow characteristics with heterogeneous surface properties. Mixing using electroosmotic flow inside microchannels with homogeneous walls is done primarily under the influence of molecular diffusion, which is not strong enough to mix the fluids thoroughly. However, surface chemistry technology can help create desired patterns on microchannel walls to generate significant rotational currents and improve mixing efficiency remarkably. This study analyzes the function of a heterogeneous zeta-potential patch located on a microchannel wall in creating mixing inside a microchannel affected by electroosmotic flow and determines the optimal length to achieve the desired mixing rate. The approximate Helmholtz–Smoluchowski model is suggested to reduce computational costs and simplify the solving process. The results show that the heterogeneity length and location of the zeta-potential patch affect the final mixing proficiency. It was also observed that the slip coefficient on the wall has a more significant effect than the Reynolds number change on improving the mixing efficiency of electroosmotic micromixers, benefiting the heterogeneous distribution of zeta-potential. In addition, using a channel with a heterogeneous zeta-potential patch covered by a slip surface did not lead to an adequate mixing in low Reynolds numbers. Therefore, a homogeneous channel without any heterogeneity would be a priority in such a range of Reynolds numbers. However, increasing the Reynolds number and the presence of a slip coefficient on the heterogeneous channel wall enhances the mixing efficiency relative to the homogeneous one. It should be noted, though, that increasing the slip coefficient will make the mixing efficiency decrease sharply in any situation, especially in high Reynolds numbers.
Highlights
Microtools and microfluidics have been used in many engineering fields in recent years [1,2,3,4], and the fluid flow pattern and how it is mixed has received much attention [5,6,7].The most critical point about mixing on a micron-scale is that the cause of this phenomenon is molecular diffusion, which is an inherently slow process [8,9]
Turbulence and creating disturbances can dramatically increase the mixing rate in macro-scale flows; creating turbulent flows in microchannels requires a high-pressure drop within the microchannel [10,11,12]
The results of applying the of the heterogeneity patch on the wall are discussed using the introduced concentr slip coefficient on the wall through the entropy mixing index employing the full numerical criterion and the H–S approximate solution
Summary
Microtools and microfluidics have been used in many engineering fields in recent years [1,2,3,4], and the fluid flow pattern and how it is mixed has received much attention [5,6,7].The most critical point about mixing on a micron-scale is that the cause of this phenomenon is molecular diffusion, which is an inherently slow process [8,9]. Turbulence and creating disturbances can dramatically increase the mixing rate in macro-scale flows; creating turbulent flows in microchannels requires a high-pressure drop within the microchannel [10,11,12]. The flow inside the straight or even curved microchannels is in the laminar flow range in most practical applications. For this reason, mixing enhancement inside the microchannel through the generation of turbulence does not seem reasonable. Mixing inside a microchannel can be done in two ways: passive or active, each of which has proposed different techniques [13,14,15].
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