Microfluidics Mixing Using Ridges & Chevrons

by

This blog is based on material published by the Whitesides group (Harvard University) and the Ligler group (Naval Research Laboratory and North Carolina State). See References in text.

Mixing liquids in microfluidic channels is difficult. A mixing concept, originally demonstrated by Whiteside [1], is of special interest, as it is broadly applicable.

The basic idea behind this elegant approach is that diffusion alone can provide good mixing between two liquids if:

  • -          the effective diffusion length (i.e. the distance from the interface between the two liquids to the microfluidic boundary) is very short, and
  • -          the interface (i.e. the contact surface between the two liquids) is extensive.

 

One could implement this concept using a microfluidic channel that is very narrow and long. However, in most cases, there are practical considerations (prevention of blockage, minimum flow requirement, etc.) that preclude drastically reducing the microfluidic channel width. The solution proposed by Whiteside takes this shortcoming into consideration.

Whiteside showed that, using small angled features, such as slots, ridges, or chevrons, one can generate transverse flow motions that reduce the diffusion length and increase the contact surface between the liquids.

Flow over a grooveFlow over a grooveLet’s look in more detail how this type of passive microfluidic mixer works. We start by looking at a simple case where one places angled grooves (or ridges) in one of the microfluidic channel surfaces (typically, but not necessarily, in the floor [2]).

By applying a (longitudinal) pressure gradient one pushes the fluid down the microfluidic channel. When the fluid encounters an angled groove, a transverse flow component is generated, as there is less resistance to the flow in the direction parallel to the groove. As a result the fluid will start to follow helical streamlines, slowly rotating along the longitudinal axis of the channe

 

 

 

 

 

Chevrons facilitate mixing of liquids in microfluidc channelsChevrons facilitate mixing of liquids in microfluidc channelsThis mixing process can be further enhanced by replacing the angled groves with an asymmetrical chevron (or herringbone) pattern. If two fluids are present, they end up being interlayered, which reduces the diffusion length.

The resulting mixed flow is not chaotic in the sense generally associated with turbulent flows. In fact, it is deterministic: the mixing introduced by a given sequence of grooves or chevrons can be reversed using an inverse sequence, as has been beautifully demonstrated by the Ligler group [3, 4].

Even better mixing is obtained by periodically reversing the chevron asymmetry. These patterns of chevrons can be repeated until an arbitrarily high degree of mixing is achieved.

 

 

RELATED ARTICLES

- Microfluidics and Mixing - Introduction 

- Mixing and Microfluidc Using Arrays of Shaped Posts

 

REeferences:

  1. 1) “Chaotic Mixer for Microchannel”, Abraham D. Stroock, Stephan K. W. Dertinger, Armand Ajdari, Igor Mezic, Howard A. Stone, George M. Whitesides, Science, 647 (2002).
  2. 2) “A microfluidic mixer with grooves placed on the top and bottom of the channel”, Peter B. Howell et al., Lab Chip, 524–530, (2005).
  3. 3) “Dynamic reversibility of hydrodynamic focusing for recycling sheath fluid”, Nastaran Hashemi, Peter B. Howell, Jr., Jeffrey S. Erickson, Joel P. Golden and Frances S. Ligler, Lab Chip, 1952–1959, (2010).
  4. 4) “A combinatorial approach to microfluidic mixing “,Howell, P.B., D.R. Mott, F.S. Ligler, J.P. Golden, C.R. Kaplan, and E.S. Oran, 18, 115019(1) – 115019(7). (2008).