Making droplets in a capillary or in a microfluidic device is often met with two problems:
These effects are due to the combination between the flow rates’ driven flow and the elasticity of materials (tubing, syringe, device…). Those problems can be overcome by using pressure driven flow instead of syringe pumps, since pressure driven flows do not oscillate and shift almost instantaneously.
Most of the researchers actually use syringe pumps since pressure regulators need the installation of a compressed air line or an air compressor. For this reason, we now propose intuitive and stand-alone instruments which both generate pressure and regulate it with a precision as low as 100µbar.
As an example, the media below shows a simple droplets generator setup using pressure driven flow.
Six seconds after pressure instruction change (Pwater from 80mbar to 90 mbar), droplets sizes have changed and form another homogenous trail of drops:
The step motor which allows the rotation of the infinite screw of the syringe pump produces periodic vibrations. Those vibrations are transmitted to the syringe piston translation and then generate periodic variations of the flow rate. Those periodic variations of the flow rate induce variations in size and displacement speed of the droplets.
Contrary to syringe pumps, pressure generators do not generate vibrations since pressure is regulated through a pressure controller.
When changing the flow rate of your syringe pump, the pressure in the device slowly changes. The increase of pressure is partly absorbed by deformation of the fluidic system (syringe, tube, device…). Then, the flow rate in the capillary stabilizes only after complete deformation of the fluidic system. Depending on the system elasticity and the flow rate, the stabilization can take several minutes or hours.
With the pressure driven flow, the pressure almost instantaneously changes all over the fluidic system leading to almost instantaneous stabilization of the flow rate and droplet uniformity.
Another solution to decrease the stabilization time of the flow when using syringe pumps is to decrease the elasticity of the fluidic system. For example you can use a glass syringe and a glass capillary. This method reduces the stabilization time of the flow rate since the fluidic system has a lower elasticity; but increases the oscillating effect of the syringe pump on the flow rate since the fluidic system do not act anymore as a Syringe Pump Flow Stabilizer Microfluidic Kit.
Authors: F. Bertholle, G. Velvé Casquillas
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