Approximation for modelling electro-osmotic mixing in the boundary layer of membrane systems

Numerical Analysis of the Heterogeneity Effect on Electroosmotic Micromixers Based on the Standard Deviation of Concentration and Mixing Entropy Index

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.

Characterizing the roles of pretreatment methods for model suspensions in the membrane fouling process: The case of yeast and kaolin

Model suspensions are important substances for characterizing fouling processes to study the mechanism of membrane fouling, thus, choosing a suitable standard suspension is critical for researches on membrane fouling processes. The pretreatment methods of model suspensions have long been neglected, which can alter the physical and chemical properties and affect fouling results. This study compared four different pretreatment methods (centrifuging and drying, magnetic stirring, ultrasonic processing, and dissolving in buffer solution) and found effects on size distribution, dispersion stability, pH variation over time, and fouling properties. Among the characterization of model suspensions, different pretreatment methods led to various changes in physical and chemical properties in this study. Membrane filtration experiments showed that these four pretreatments changed the rate of membrane fouling. The results of the analysis of filtration flux for suspensions indicated that for ultrasonic processing of all suspensions and magnetic stirring of kaolin suspensions, pore blocking was the main mechanism, while cake formation was dominant for the others. Therefore, special attention should be placed on different pretreatment methods and selecting appropriate model foulants. Finally, some guidance on selecting appropriate model foulants was given in the study.

A Review of CFD Modelling and Performance Metrics for Osmotic Membrane Processes

Simulation via Computational Fluid Dynamics (CFD) offers a convenient way for visualising hydrodynamics and mass transport in spacer-filled membrane channels, facilitating further developments in spiral wound membrane (SWM) modules for desalination processes. This paper provides a review on the use of CFD modelling for the development of novel spacers used in the SWM modules for three types of osmotic membrane processes: reverse osmosis (RO), forward osmosis (FO) and pressure retarded osmosis (PRO). Currently, the modelling of mass transfer and fouling for complex spacer geometries is still limited. Compared with RO, CFD modelling for PRO is very rare owing to the relative infancy of this osmotically driven membrane process. Despite the rising popularity of multi-scale modelling of osmotic membrane processes, CFD can only be used for predicting process performance in the absence of fouling. This paper also reviews the most common metrics used for evaluating membrane module performance at the small and large scales.