Microfluidics and Mixing - Introduction

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Mixing of fluids is easy at the everyday scaleMixing of fluids is easy at the everyday scale

Some of the physical phenomena, that we frequently encounter in our daily life operates by unusual rules at the microsystem level. In this article, we look at one specific example: the mixing of liquids.

Mixing fluids, at the scale that we encounter in our daily life, is rather straight forward - Pour milk into your coffee and the mixing is immediate or, at most, requires a few swirls from a spoon. In sharp contrast, it is notoriously hard to mix solutions in microchannels, as the physical phenomena that homogenize fluids in a common-sized container (your coffee cup for example) are ineffective at the microscopic level.

Fluid molecules in microfluidic channels follow parallel streamlines (Photograph source: REF 1)Fluid molecules in microfluidic channels follow parallel streamlines (Photograph source: REF 1)

Under typical operating conditions, flows in micro-channels are laminar. The chaotic fluctuations of velocity that tend to homogenize fluids in turbulent flows are absent; rather, fluids in laminar flows follow parallel streamline, which remain parallel even in meandering channels.

Two fluids flowing in a standard microchannel will not mix with each other, except via diffusion at their common interface. Although the dimensions involved are characteristically very short, the diffusion process is typically extremely slow compared with the flow of material along the microfluidic channel principal axis. In other words - in continuous flow systems - the fluids will most likely exit the microfluidic device before they mix in any substantial way.

 

 

 

Various designs - passive and active - have been proposed to address this microfluidic mixing challenge. Some of the most common approaches are discussed in the articles below.

 

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- Microfluidics Mixing Using Ridges & Chevrons

- Mixing and Microfluidc Using Arrays of Shaped Posts

 

 

REeferences:

  1. “Microfluidic Sorting System Based on Waveguide Integration and Diode Laser Bar Trapping”, Robert W. Applegate, Jeff Squier, Tor Vestad, John Oakey, David W. M. Marr, Ph. Bado, Mark A. Dugan, Ali A. Said, OSA Biomedical Optics, Fort Lauderdale (FL), presentation TuI49, (2006)