Microfabricated integrated circuits revolutionized computation by vastly reducing the space, labor, and time required for calculations. Microfluidic systems hold similar promise for the large-scale automation of chemistry and biology, suggesting the possibility of numerous experiments performed rapidly and in parallel, while consuming little reagent. While it is too early to tell whether such a vision will be realized, significant progress has been achieved, and various applications of significant scientific and practical interest have been developed. Here a review of the physics of small volumes (nanoliters) of fluids is presented, as parametrized by a series of dimensionless numbers expressing the relative importance of various physical phenomena. Specifically, this review explores the Reynolds number Re, addressing inertial effects; the Peclet number Pe, which concerns convective and diffusive transport; the capillary number Ca expressing the importance of interfacial tension; the Deborah, Weissenberg, and elasticity numbers De, Wi, and El, describing elastic effects due to deformable microstructural elements like polymers; the Grashof and Rayleigh numbers Gr and Ra, describing density-driven flows; and the Knudsen number, describing the importance of noncontinuum molecular effects. Furthermore, the long-range nature of viscous flows and the small device dimensions inherent in microfluidics mean that the influence of boundaries is typically significant. A variety of strategies have been developed to manipulate fluids by exploiting boundary effects; among these are electrokinetic effects, acoustic streaming, and fluid-structure interactions. The goal is to describe the physics behind the rich variety of fluid phenomena occurring on the nanoliter scale using simple scaling arguments, with the hopes of developing an intuitive sense for this occasionally counterintuitive world.

/pdf/microfluidics-fluid-physics-at-the-nanoliter-scale-4bpahtd8qa.pdf

Microfluidics: Fluid physics at the nanoliter scale

This review reports the progress on the recent development of micromixers. The review first presents the different micromixer types and designs. Micromixers in this review are categorized as passive micromixers and active micromixers. Due to the simple fabrication technology and the easy implementation in a complex microfluidic system, passive micromixers will be the focus of this review. Next, the review discusses the operation points of the micromixers based on characteristic dimensionless numbers such as Reynolds number Re, Peclet number Pe, and in dynamic cases the Strouhal number St. The fabrication technologies for different mixer types are also analysed. Quantification techniques for evaluation of the performance of micromixers are discussed. Finally, the review addresses typical applications of micromixers.

https://iopscience.iop.org/article/10.1088/0960-1317/15/2/R01/pdf

Micromixers?a review

http://hg272.persiangig.com/che/Most%20Populare%20Papers/Micromixers%E2%80%94a%20review%20on%20passive%20and%20active%20mixing%20principles.pdf

Micromixers—a review on passive and active mixing principles

The fine chemicals and pharmaceutical industries are transforming how their products are manufactured, where economically favorable, from traditional batchwise processes to continuous flow. This evolution is impacting synthetic chemistry on all scales-from the laboratory to full production. This Review discusses the relative merits of batch and micro flow reactors for performing synthetic chemistry in the laboratory.

Deciding whether to go with the flow: evaluating the merits of flow reactors for synthesis.

This letter describes a method for generating monodisperse gaseous bubbles in a microfluidic flow-focusing device. The bubbles can be obtained in a range of diameters from 10 to 1000μm. The volume Vb of the bubbles scales with the flow rate q and the viscosity μ of the liquid, and the pressure p of the gas stream as Vb∝p∕qμ. This method allows simultaneous, independent control of the size of the individual bubbles and volume fraction of the dispersed phase. Under appropriate conditions, bubbles self-assemble into highly ordered, flowing lattices. Structures of these lattices can be adjusted dynamically by changing the flow parameters.

Formation of monodisperse bubbles in a microfluidic flow-focusing device

A mixing technique based on the principle of bubble-induced acoustic microstreaming was developed. The mixer consists of a piezoelectric disk that is attached to a reaction chamber, which is designed in such a way that a set of air bubbles with desirable size is trapped in the solution. Fluidic experiments showed that air bubbles resting on a solid surface and set into vibration by the sound field generated steady circulatory flows, resulting in global convection flows and thus rapid mixing. The time to fully mix a 22 μL chamber is significantly reduced from hours (for a pure diffusion-based mixing) to tens of seconds. Numerical simulations showed that the induced flowfield and thus degree of mixing strongly depend on bubble positions. Optimal simulated mixing results were obtained for staggered bubble distribution that minimizes the number of internal flow stagnation regions. Immunomagnetic cell capture experiments showed that acoustic microstreaming provided efficient mixing
of bacterial cell (Esherichia coli K12) matrix suspended in blood with magnetic capture beads, resulting in highly effective immunomagnetic cell capture. Bacterial viability assay experiments showed that acoustic microstreaming has a relatively low shear strain field since the blood cells and bacteria remained intact after mixing. Acoustic microstreaming has many advantages over most existing chamber micromixing techniques, including simple apparatus, ease of implementation, low power consumption (2 mW), and low cost.

Maciej Z. Pindera

Papers

Bubble-induced acoustic micromixing

Accuracy of 1D microvascular flow models in the limit of low Reynolds numbers