What is the best method to produce uniform droplets in microchannels? Pneumatic pressure, syringe pumps? Is pressure control really better than syringe pumps? If yes, why? Please help me to understand better.
Hello Claus,
driving a droplet generator with pressure-controlled pumps seems to be the 'better' solution to keep flow absolutely constant. Having said this, I think it strongly depends on your samples. If you have samples with changing viscosity, it is a pain to make sure that your flowrates are what you want them to be (imagine mixing two aqueous phases with different viscosities in a defined ratio before making the droplets.) Another thing to consider is deadvolume: when you introduce something quite expensive pressure controlled systems tend to result in quite a loss of sample-liquid unused.
With syringe pumps you always know exactly the flowrate you deliver, no matter which viscosity. - and by keeping the tubing length between the syringe and the chip as low as possible you can minimize deadvolume. You can make syringe pumps more precise (and prevent 'pulsing' of flow, when you use 1) incompressible glass syringes (which makes the system more 'responsive'), and 2) syringes with a really small diameter. Then the effects of the 'steps' of the stepper motor inside the pump will not show in your experiments even when you use really small flowrates.
Hi Claus,
I found that a German company has found a very neat solution to producing uniformly sized micro droplets - they are called GESIM and make a variety of small volume handling products. For the production of nano-sized droplets they are using piezoelectricity:
http://www.gesim.de/en/dispensers/dispensers.html
hello claus,
I think if you are able to optimize the speed and pressure of microfluidic channel then you can use both of them it doesnt matter you will get uniform droplets.
Microfluidic T junctions and flow focusing devices can be fed by both pressure or flow controlled systems, you will obtain droplets but this dynamic equilibrium can vary and change the size distribution. If you are looking for a really precise size distribution maybe have a look at the work of Dan Angelescu on liquid–gas phase transition (http://pubs.rsc.org/en/Content/ArticleLanding/2013/RA/C2RA23090A#!divAbstract).
Also, satellite droplets that occurs during breakup can show high monodispersity.
I agree, I also don't see why either volume or pressure driven flow should be better, but maybe I am missing something... However, flow control in small channels is much easier and more stable with pressure driven flow. This may result in generally more uniform droplets (I invented it so I must(!) like it ;-) if the instability is not chaotic. Thanks for the hint with the Gesim system. This looks very interesting and it is so close to my place (Leipzig). Here are some theoretical hints about it: http://homepage.usask.ca/~mmg864/paper/RC/RC-499-2.pdf. The Sang et al. method is also very interesting and I will read it more in detail soon. In fact there is some information about droplet formation in http://www.wiley-vch.de/books/sample/3527315500_c01.pdf. However, things appear not to be clear to me. Does anybody know a conclusive text on this issue? I would be grateful for a link or a publication. I also find following excellent article interesting: http://www.off-ladhyx.polytechnique.fr/people/baroud/reprints/bgd.pdf but it doesn't seem to present a conclusive explanation to droplet formation, they write: "Finally, pressure vs. flow rate driving can also lead to subtly different flow patterns. For all of these reasons, knowledge of particular flow situations cannot readily be used to predict the behaviour when some of the parameters are changed." Seems to be a far richer world than I ever expected...
Looking forward to more answers, Cheers Claus
A comparison of pressure versus flow-rate-driven pumping in flow focusing : in
http://www.coulomb.univ-montp2.fr/perso/manouk.abkarian/Manouk_Abkarian_Homepage/Publications_files/Ward_Electrophoresis_26_2005.pdf
I think both methods would work well to obtain reasonably monodisperse droplets. Syringe pumps may have the advantage that you have better control over your flow rate. Pressure controllers will give you faster response times so that "steady-state" is reached quicker, esspecially if you are operating in a high fluidic resistance system.
Hi. Using siringe pump a flexibility of flow rate regulation is very wide. However it is very difficult to avoid problrms with plastic sirnges diameter variation (along their length), uniformity of helical transmission of piston motion and hoses (capilaries) vibration.
Pneumatically driven flows are much easier for stabilisation, however their flexibility of flows regulation is limited due to limitation of accessible pressure sources. Additionally, pressure stability is increasing with pressure drop kept between pressure regulator and liquid reservoir. Sharp changes of flow rates in such system are impossible.
Also there is a third problem appearing in just filled systems. This problem is associated with solution of some solvents in microfluidic chip material. However so precise flow control is very rearly demanded.
Considering droplets formation I prefer flow focusing circuit than t-junction one. It gives wider flexibility of the length of formed droplets. However in both systems it is quite easy to keep droplet volume variation better than 1 %.
I have had experience with both syringe pumps and pressure control. As many have stated, there does not appear to be much difference in the final output, but I can make the following general comments:
1. Syringe based flow is easier to set up and get going, especially where one is working with new designs and the flow parameters are not yet known.
2. Pressure based systems are easier to scale, as fluids can be stored in reservoirs, whereas syringe volumes are typically limited.
Hello Claus,
driving a droplet generator with pressure-controlled pumps seems to be the 'better' solution to keep flow absolutely constant. Having said this, I think it strongly depends on your samples. If you have samples with changing viscosity, it is a pain to make sure that your flowrates are what you want them to be (imagine mixing two aqueous phases with different viscosities in a defined ratio before making the droplets.) Another thing to consider is deadvolume: when you introduce something quite expensive pressure controlled systems tend to result in quite a loss of sample-liquid unused.
With syringe pumps you always know exactly the flowrate you deliver, no matter which viscosity. - and by keeping the tubing length between the syringe and the chip as low as possible you can minimize deadvolume. You can make syringe pumps more precise (and prevent 'pulsing' of flow, when you use 1) incompressible glass syringes (which makes the system more 'responsive'), and 2) syringes with a really small diameter. Then the effects of the 'steps' of the stepper motor inside the pump will not show in your experiments even when you use really small flowrates.
Dear Martin,
indeed you are right. I added a syring pump simulator giving the response time as a function of channel, tubing and syringe size: http://biophysical-tools.de/p2cs-syringe-pump-comparison-calculator/
The slowing down of the reaction time is dramatic for microchannels, however, for large channels the difference is of less importance.
We (collaborators and me) currently are investigating the possibilities of droplet formation, we keep you informed...
Cheers. Claus
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