In this article, we introduce the basic theory of how liquid flow is controlled at microscale with the help of a microfluidic precision pump. We explore the types of different microfluidic pump mechanisms, and then move onto the most efficient microfluidic pump in terms of stability & precision.
Movement of liquids is an important aspect of everyday life. Although, when it comes to the micro scale of liquid movement, the important factors to take into account change quite a bit. The impact of gravity becomes negligible, and the concept of laminar flow takes over.
Microfluidics is basically concerned with the movement of liquids from the micro to nanometer scale. The study of manipulation & control of fluids at a micro to nanometer scale is called Microfluidics.
To conduct a microfluidic experiment, a device containing micro-sized channels that can allow the flow of liquids is used. This device is known as a microfluidic device. A microfluidic pump facilitates the flow of liquid through a microfluidic device.
Microfluidics can be used for many fields of application from dynamic cell culture, food industry to Soft robotics. Find below an example of why microfluidics was chosen unlike other macroscale batch methods to generate droplets:
There are several ways to conduct the flow of liquids through this device. When it comes to the movement of fluids, the two major types of fluid-transfer pumps are positive displacement pumps & non-positive displacement (centrifugal) pumps.
There are various pumps that have been used over the years to facilitate the flow of liquids through a microfluidic system or device. In this article we focus on the widely employed 3 mechanisms/pumps:
A peristaltic pump exerts a mechanical action on a flexible tube that connects the fluid reservoir to the microfluidic device.This force helps the liquid to flow from the from the entrance of the tube to the exit.
This pump mechanism also used a mechanical action on a plunger that exerts force on a syringe containing the liquid.
Due to this motion, the fluid is able to move from the syringe nozzle, through the tubing, and into the microfluidic device. The movement of the plunger is controlled by an endless screw that can be tuned as required for a specific experiment.
The last, and most efficient microfluidic pump for use in all types of experiments is a pressure-driven flow control pump. This microfluidic pump is a type of pneumatic pump system developed by Elveflow.
It uses an external pressure source, that is connected to a sealed liquid reservoir. The pump exerts a gas pressure which moves the liquid through the tubing and into the microfluidic device. The system is designed around two pressure sources – the P out, and the pressure inside the sealed reservoir (P in). The direction & flow rate of the liquid is manipulated using the difference in pressure between P in & P out.
The flow rate of the liquid is directly proportional to the pressure difference. To calculate the flow rate of the liquid is performed with the help of this formula given below, that incorporates the use of microfluidic resistance (R) as the proportionality coefficient. The microfluidic resistance coefficient of a specific microfluidic system is calculated based on the device geometry, and properties of the liquid being used.
To calculate the flow rate or the microfluidic resistance in your system, please check our microfluidic calculator and the related application notes:
The type of flow can be considered as an important parameter while making the selection of a microfluidic pump. Given below are some important factors to be considered :
As explained in the previous section, the basic principle of a pressure-driven flow control pump is based on the pressure difference between the outlet pressure & inlet pressure (deltaP). The required pressure can be set based on the direction of flow required. e.g – to use a vacuum mechanism, the delta P can be set as negative, and the liquid will be sucked in the opposite direction.
The switching between push & pull of a sample liquid through a fluidic system is possible only with the help of a pressure-driven flow control pump. Peristaltic pumps & syringe pumps use a mechanical action, and implementation within a system that demands this type of flow is not possible.
Depending on the application and the resulting flow regime required, one may need to consider the use of millifluidics or nanofluidics instead of microfluidics.
Millifluidics refers to a system that utilizes flow through capillaries with an internal cross-section above 1mm. The use of millifluidics becomes limited when the value of Reynolds’s number reaches 1, at a point when turbulence of flow can no longer be neglected.
Nanofluidics is defined as the study of liquids and their behaviour when flowing through channels limited to the nanometer scale (typically 1-100 nm).
For more information, please refer to the full review about the flow control in microfluidic device.
A key factor to keep in mind is that the size of the channel is inversely proportional to the stability & accuracy of flow rate that is required. Therefore, the smaller the microfluidic device, the greater should be the stability and flow control efficiency.
Viscosity is the topical issue in every industrial processes from painting or adhesive iindustry to biotechnologies and for any operator trying to adjust a formulation to achieve the right final product properties at a high-throughout.
Higher viscosity results in higher difficulty to push the liquid through the device, for a single phase liquid. Indeed, the higher the viscosity, the greater the viscous forces at stake when the liquid is being moved for a steady force.
For fluid mixture, the problem is even tougher because by mixing fluids, the overall viscosity inscreases as per emulsion (liquid-liquid mixture) or foam (gas-liquid mixture). Microfluidics can process a limited range of viscosities up to a few Centipoise / MPa.s).
To discover more about how the viscosity can affect microfluidic resistance of your fluid in your microfluidic chip, please check the following application note and our microfluidic calculator designed to help you estimate the key parameters in your system.
The chemical properties of a solution containing single or several components can impact significantly the ability to move such solution through a microfluidic channel.
Indeed, properties such as the density, the pH of your solution or even the Zeta potential of a dispersion of nanoparticles in a liquid are parameters to take carefully into account while preparing your microfluidic setup.
The pH of the solution has to be checked prior to any experimentation because certain microfluidic device materials won’t resist very acidic or very basic solutions. Indeed, glass over PDMS device should be considered for such solution.
For solid-liquid mixture or dispersions such as particles dispersed into a liquid phase, the Zeta potential of the solution on top of the average size of your particle should be checked prior to any experimentation to prevent any clogging risks of your microfluidic device.
The first two microfluidic pumps detailed in the previous section – peristaltic pumps & syringe pumps are both easy to use. Both these pump systems are based on a mechanical action that drives the liquid through the device.
Various research & industrial applications of microfluidic systems rely on the flow rate stability & accuracy of the microfluidic pump. This is why it is well understood that pressure-driven flow control has emerged as the go-to microfluidic pump for industrial applications for physics, chemistry & biology.
Although, a mechanical motion results in hiccups & pulses in the liquid flow, which affects the performance of the microfluidic system in terms of response time and flow stability. The graphical representation of flow rate plotted against time for all three of these microfluidic pumps explains it very clearly.
For more information about how to implement pressure-driven flow controlled microfluidics, feel free to contact our team of experts!
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