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 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

The combination of metal films and soft substrate can give a tremendous chance to designing flexible electronics for various applications. In the paper, the authors investigated the mechanism of large elongation of Au film on a PDMS substrate for over 30% elongation. The soft matter under the metal film allows the film to deflect without large stress concentration. Given this fact, future works can be done for finding out the optimized Young's modulus of the soft matter for larger elongation of the film. Softer is better in terms of conformation to the metal film but rigid is better in terms of maintaining the shape of the structure. Also, for the real application of the metal film / polymer structure to flexible electronics, future works should be done for faster cyclic experiments and this will improve the reliability of the structure. One question that I have for this work is that how the thickness of the Au film effect on the elongation of the structure. Very thin ductile metal film can become brittle when its thickness is very small. In the paper, Au film was just 25nm. This can limit the movement of the dislocation and decrease the plasticity of the material. I expect that once the optimized thickness of the Au film is figured out, the structure would show much better performance.