Micromixing and microchannel design: Vortex shape and entropy

Nature Chemistry, 2013

We consider a microfluidic mixer based on hydrodynamic focusing, which is used to initiate the folding process of individual proteins. The folding process is initiated by quickly diluting a local denaturant concentration, and we define mixing time as the time advecting proteins experience a specified to achieve a local drop in denaturant concentration. In previous work, we presented a minimization of mixing time which considered optimal geometry and flow conditions, and achieved a design with a predicted mixing time of 0.10 µs. The aim of the current paper is twofold. First, we explore the sensitivity of mixing time to key geometric and flow parameters. In particular, we study the angle between inlets, the shape of the channel intersections, channel widths, mixer depth, mixer symmetry, inlet velocities, working fluid physical properties, and denaturant concentration thresholds. Second, we analyze the uniformity of mixing times as a function of inlet flow streamlines. We find the shape of the intersection, channel width, inlet velocity ratio, and asymmetries have strong effects on mixing time; while inlet angles, mixer depth, fluid properties, and concentration thresholds have weaker effects. Also, the uniformity of the mixing time is preserved for most of the inlet flow and distances of down to within about 0.4 µm of the mixer wall. We offer these analyses of sensitivities to imperfections in mixer geometry and flow conditions as a guide to experimental efforts which aim to fabricate and use these types of mixers. Our study also highlights key issues and provides a guide to the optimization and practical design of other microfluidic devices dependent on both geometry and flow conditions.

Sensors and Actuators B: Chemical, 2006

This study presents a theoretical and experimental investigation on convective micro mixing in different mixing structures and their combinations. Different mixing elements have been integrated on a silicon chip to achieve a device for a high mass flow above 20 kgih. These test structures were fabricated and characterized concerning their flow behavior and mixing performance. Flow measurements with pH neutralisation and indication by Bromothymol Blue confirm the numerical simulations of the flow characteristics and mixing behaviour. The integral mixing quality in the micro mixer is measured with the iodide-iodate-reaction (Villermaux-Dushman) and shows excellent values for high Re numbers. This opens the potential to use microstructures for new applications in the production of chemicals.

Chemical Engineering and Processing: Process Intensification, 2011

Continuous flow microreactors can greatly improve the safety and product yields of processes in the pharmaceutical and fine chemical industry by overcoming many of the drawbacks of traditional batch and semi-batch stirred reactors. This study compares on a common scale the pressure drop and mixing performance of different size commercial microreactor plates composed of a tangential, SZ-shaped or caterpillar mixer followed by a rectangular serpentine main channel. The pressure drop was fitted to a friction factor model, which suggests that the mixing zone had significant chaotic secondary flow patterns, whereas the main channel did not. Moreover, the mixing zone was the main contributor to the overall pressure drop. Mixing performance was then characterized using competitive parallel reactions. Upon the formation of chaotic secondary flows, typically due to the interactions of artificially induced vortices, the mixer performance was found to be independent of geometry for a given energy dissipation rate. However, the mixer geometry will affect the critical Reynolds number that induces chaotic advection and changes the mixing time scale.

2012

Microfluidics is changing the way modern biology is performed and is becoming a key technology in the field of micro arrays, DNA sequencing, and Lab on a Chip applications. Microsystems, being compact in size, disposable, and ensuring high speed of analysis using decreased sample volumes, allow to replace large-scale conventional laboratory instrumentation with miniaturized devices, reducing hardware costs, and assuring low reagent consumption and faster analysis. At the microscale mixing of species becomes crucial to i) improve the effectiveness of and ii) speed up chemical reactions, but it is often critical to be achieved, since microfluidics is characterized mainly by very low Reynolds flows, and cannot take advantage of turbulence in order to enhance mixing. Hence, given that diffusiondriven mixing in very low Reynolds number flow regimes is characterized by long time scales, methods for enhancing the rate of the mixing process are essential in microfluidics. In order to enhance mixing, several techniques have been developed. In general, mixing strategies can be classified as either active or passive, according to the operational mechanism. Active mixers employ external forces in order to perform mixing, so that actuation system must be embedded into the microchips. On the contrary, passive mixers avoid resorting to external electrical or mechanical sources by exploiting characteristics of specific flow fields in microchannel geometries to mix species, offering the advantage to be easy to be produced and integrated. In this work, a survey of the passive micromixing solutions currently adopted is presented. In detail, the most widely used microchannel geometries and the metrics used to quantify mixing effectiveness in microfluidic applications are discussed.