Q: Why does my microfluidic flow-focusing device switch between large droplets and many small droplets?
I am using an MFFD to create hyaluronic acid microgels. While I have been able to get stable flow over the course of 4 hours, I seem to get alternating flow patterns that generate either many small droplets or one large droplet. Is this a consequence of using a syringe pump?
Continuous phase: Light mineral oil, span 80
Dispersed phase: Norbornene-Hyaluronic acid in PBS, DTT, I2959
Flow rates: 1ul/min for dispersed and 1.9ul/min for continuous
I have attached a sped-up video to demonstrate this:
A: Due to the pressure/flow rate fluctuations. Due to the mechanism of the stepper motor inside the syringe pump, there is always oscillation in the flow rate. Using a smaller syringe may help. But the best thing that you can do to get rid of the fluctuations is the compliance effect. If you use flexible tubing it can damp fluctuations.
A: I believe it is because of pressure instability.
1) For low flow rates such as the ones you have used – try using small diameter syringes. This creates enough piston travel distance per rotation of the axle of your pump.
2) Try initiating the flow with just the continuous phase. Once the flow is stable, then initiate the dispersed phase.
3) In any case, displacement pumps do have a degree of pulsation. You might want to consider continuous pumping options – such as pressure-driven pumps or a HPLC pump that can create a more stable flow.