Introduction to digital microfluidic using pressure driven flow

Making droplets in a capillary or in a microfluidic device often meets two problems:

Droplets need an equilibration time to become stable in size.

Droplets size oscillates depending on the imposed flow rate and the materials used.

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:

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Microfluidic droplets generation by Elveflow pressure source. Demonstration of time response

Fast response of droplets size to pressure driven flow instruction change

Six seconds after pressure instruction change (Pwater from 80mbar to 90 mbar), droplets sizes have changed and form another homogenous train of drops:

dropplet in microfluidic device

Why droplet size and speed oscillate when using a syringe pump to control flow?

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.

Why does droplets generation need some equilibration time to become stable when changing flow rate of the syringe pump?

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 fluidic low-pass filter.

Form’s authors: F. Bertholle, G. Velvé Casquillas

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