Impact of inlet channel geometry on microfluidic drop formation

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Wiki entry by : Dongwoo Lee, AP225 Fall 2010.

Paper in this Wiki : A. R. Abate, A. Poitzsch, Y. Hwang, J. Lee, J. Czerwinska, and D. A. Weitz, Impact of inlet channel geometry on microfluidic drop formation, PHYSICAL REVIEW E 80, 026310 (2009)

Fig. 1 Schematics of drop makers with different inlet channel geometries (top row) and example images of drops formed by each device for the flow-rate ratio of 1:1 and different capillary number are shown in the lower rows. The scale bar is 100um. Here, o denotes oil and w denotes water. Also, the capillary numbers are shown in the left side of lower rows.
Fig. 2 (left) mean (a) drop volume and (b) standard deviation divided by the mean in the drop volume as a function of capillary number. Fig. 3 (right) Phase diagrams for stable drop formation as a function of capillary number and flow rate ratio.


The paper illustrates the impact of inlet channel geometry on microfluidic drop formation. With some experiments, it was found that in TJ and PJFF drop formation, the asymmetric injection of fluids leads to a stable drop formation at low capillary numbers, while monodisperse drop formation was made at high capillary numbers in FF drop formation as shown in the fig. 1. In the experiment, the emulsions formed consist of water drops in fluorocarbon oil stabilized by fluorosurfactant. figure 2 shows more quantitative comparison of the capillary effect on the drops for different geometries. In the figure, the authors did not include the unstable drops. Thus, we can clearly see the geometry and the capillary number effects on the stability of the drops. One interesting characteristic we can see in the plot is that as capillary number is increased, drop size is reduced. This is because shear stress in the nozzle increases. Fig. 3 shows the full phase space for stable drop formation for different channel geometries. Generally, the PJFF drop maker has a large region of stable drop formation as in the fig. 3(b) In FF2 drop maker, small stable drop formation region exists. However, the authors says that FF drop makers form monodisperse drops most rapidly, useful for applications that require large quantities of drops.

Soft Matter Discussion

This paper describes the effect of channel geometry and the capillary number on the stability and shape of drop formation. Given the fact that larger capillary number means larger viscosity effect compared with surface tension effect, one can expect that larger capillary number makes the drop smaller. This general pattern was shown in the result as shown in the fig. 1. However, above or below threshold, the drops lost their stability and this effect is due to the shape of the channel. This research is valuable since the results give the design considerations for micro fluidic channels which controls the stability or the size of drops. It would be great if the authors could explain the reason of the channel shape effect on the drop formation.