Hey !
I found this really interesting article ( Article Monodispersed microfluidic droplet generation by shear focus...
) that explains the utility of a expansion design following the nozzle to create a velocity gradient and forces the breakup of the inner flow to occur at the same point.However, they added expansions of the continuous phase channels before the intersection, calling them "pressure reservoirs" and mentioned that it " stabilizes the water and oil phases" (see Fig 2 in the paper).
Is there a mechanic explanation for this stabilization, or a paper on this ? I couldn't find any other mentions of these pressure reservoirs in other papers, and I would like to have a more mechanistic explanation before adding them to my designs.
Does someone have an idea about the justification of these pressure reservoirs, or experience using them compared to more standard designs ?
Many thanks for your help,
Best wishes,
Romain
Disclaimer: not my area of expertise.
The single phase, stationary CFD sims are woefully inadequate to explain what's going on.
Based on Fig 5, it looks like the primary water channel is pushing forward (thread growth) and creating back pressure on the oil stream. Oil flow at the water channel inlet would be slow, instead it's feeding directly at the orifice (only point of escape) and compressing the water at the nozzle. The time lag here creates a small thread of water that went past the nozzle as the oil compresses, and that is the droplet that pinches off. Unfortunately (I imagine), as it pinches, the oil compression force drops because there is more room for oil flow. This would destabilize the compression mechanism, and the string of water between the water inlet and the nozzle falls apart, giving those small satellite droplets. If I were designing this, I would want the pinching to occur as close to the water inlet as possible, so the back end of the water string stays coalesced with the inlet stream. I.e., I would guess that including this pressure reservoir is not a good idea.
Frankly, I'm not sure how the downstream geometry affects this at all. Did I miss a real world comparison to alternative geometries? If anything, I would be worried about flow recirculations at the outlet of the triangular geometry creating back pressure effects that vary with total flow rate. Not a big fan of unpredictable dynamics, dead volumes, and would use a continuous, smooth bubble geometry to guide the flow... If it's even necessary.
Hope that helped, Romain.
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