In the present study, we numerically demonstrate an approach for separation of micro and sub-micro diamagnetic particles in dual ferrofluid streams based on negative magnetophoresis. The dual streams are constructed by an intermediate sheath flow, after which the negative magnetophoretic force induced by an array of permanent magnets dominates the separation of diamagnetic particles. A simple and efficient numerical model is developed to calculate the motions of particles under the action of magnetic field and flow field. Effects of the average flow velocity, the ratio of sheath fluid flow to sample fluid flow, the number of the magnet pair as well as the position of magnet pair are investigated. The optimal parametric condition for complete separation is obtained through the parametric analysis, and the separation principle is further elucidated by the force analysis. The separation of smaller micro and sub-micro diamagnetic particles is finally demonstrated. This study provides an insight into the negative magnetophoretic phenomenon and guides the fabrication of feasible, low-cost diagnostic devices for sub-micro particle separation.
Abbreviations: SE, separation efficiencyColor online: See article online to view Figs. 1-4 in color.
The emergence of microfluidic droplets offers new opportunities to advance biomedical engineering, food production, and energy storage applications. These applications always involve complex fluids exhibiting obvious non-Newtonian behavior. Droplet generation has been extensively addressed, while the complete understanding of droplet generation in non-Newtonian fluid system is still nascent. Here, we present the study of non-Newtonian droplet generation in a flow-focusing microchannel. Polyethylene oxide aqueous solutions are used as the dispersed phase, while olive oil serves as the continuous phase to induce the generation. The molecular weight of polymer is constant while the concentrations are varied from dilute to semi-dilute regimes that are rarely explored in existing studies. The main features of non-Newtonian droplet generation are first identified, after which the concentration-dependent dripping to jetting transitions are clarified. The effects of shear thinning and elasticity on droplet generation are then separately investigated. We finally propose a scaling relation to predict the primary droplet size with the satellite droplets neglected. These results can not only extend the fundamental theory of droplet microfluidics but also facilitate the practical applications.
Fin efficiency, as a measure of the effectiveness of the heat transfer enhancement, is of great importance in studying the heat transfer performance of H-type finned tube banks. The fin efficiency of square fins is adopted by most researchers as an alternative to that of H-type fins, which can create certain errors in the fin efficiency of H-type fins. For this paper, the linear nomograms and fitting formulae of fin efficiency of H-type fins are obtained by the definition method of fin efficiency based on numerous numerical simulations, and the results calculated by this method are verified by experimental data. On this basis, the effects of three geometric parameters (slit width, fin height, and fin thickness) and two thermal parameters (surface heat transfer coefficient and fin thermal conductivity) on the fin efficiency of H-type fins are also investigated and compared to those of square fins. The results indicate that the fin efficiency of H-type fins increases with the increment of fin thickness and thermal conductivity, and decreases with the increase of slit width, fin height, and surface heat transfer coefficient. Accordingly, the linear nomograms and fitting formulae for the fin efficiency of H-type fins, which are well compatible with experimental data, can help to facilitate further theoretical research and engineering application.
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