A Design for Mixing Using Bubbles in Branched Microfluidic Channels
"Design for mixing using bubbles in branched microfluidic channels"
Piotr Garstecki, Michael A. Fischbach, and George M. Whitesides
Applied Physics Letters 86(24) 244108 (2005)
Soft Matter Keywords
microfluidic, bubbles, laminar mixing, Peclet
This paper details experimental work and simple supporting theory regarding mixing in microfluidic channels. For most microfluidic systems, the Reynolds number remains small (less than 1000), so turbulence is absent and mixing only occurs via diffusion. Typical Peclet numbers in microfluidic channels are on the order of 1e5, indicating that mixing to homogeneity requires length scales on the order of 10 meters. These lengths are not easily achieved on microfluidic devices due to finite substrate limits for fabrication and large pressure drops in the long channels, so the authors propose a novel method of mixing that aid the diffusion process. Using bubbles to fold two liquid streams into one another, greater contact area between the fluids is created, aiding in diffusion.
Practical Application of Research
A drawback to continuous flow microfluidics has been the inability to homogeneously mix streams on-chip. This limits the application of continuous flow microfluidics, as it is difficult to introduce additional reagents or samples to a flow. This passive, on-chip mixing scheme open a new realm of experiments that can take advantage of introduction of precise amounts of fluid at a given spatial or temporal point in a flow.
Microfluidic Mixing Using Bubbles
As shown in Figure 1, the two liquids to be mixed are combined with a stream of air at a microfluidic flow focussing junction. The liquid streams pinch off air bubbles, which then flow downstream. This break up is mediated primarily by conservation of mass. As the thread of air enters the junction, it restricts flow of the two outer liquids. Pressure builds up in the outer liquid lines, causing the air/liquid interface to deform. Eventually, the interface detaches from the walls, becomes unstable, and breaks, forming a bubble .
Once in the device, sequential air bubbles alternate arms of the branched channels they flow into (see Figure 2). As an air bubble enters on branch, it causes the hydrodynamic resistance in that side of the branch to increase. This causes a greater percentage of the incoming flow to be diverted down the other branch. This leads to the next air bubble flow through the branch that does not contain the first bubble. As the authors mention in the paper, this is an additive effect, and will work for any number of droplets in a series of branches. The incoming droplet will end up being passively steered by the majority of the fluid streamlines to the channel with the smallest hydrodynamic resistance.
 P. Garstecki, H.A. Stone, and G.M. Whitesides, Physical Review Letters 94 164501 (2005)