The flow rate in every fluidic system can be computed with the following equation:
∆P=Q×Rh
Where: ∆P is the pressure difference between the inlet and the outlet of the system, Q is the flow rate throughout the system, Rh is the fluidic resistance of the system.
This equation is analog to the one that applies for electronic systems
∆U=i×RΩ
Calculations for the equivalent resistance with resistors in series or in parallel apply similarly for fluidics and electronics. Kirchhoff’s law for circuit calculations is also similar in both domains.
Let’s take the example of a microfluidic chip with channels that are 500 µm wide, 70 µm high and 2 cm long. The fluidic resistance is quite small and implies an approximate pressure of 1 mbar for a flow rate of 5 µL/min. A user wanting to control the flow rate with an OB1 MK3+ 200 mbar and a flow-sensor MFS 2 will meet several difficulties:
– The resolution of 0.1 mbar of the OB1 200 mbar will only allow steps of 0.5 µL/min. The precision of the flow sensor becomes inadequate
– Flow rate instabilities may appear due to its sensitivity to small pressure variations
– The flow sensor will saturate at low pressure/high flow rate, leaving the major part of the pressure range unusable. The performance regarding the usable pressure range will be degraded.
Regarding the former example, it is easy to match the instrument performances to the setup by adding a flow restrictor in series with the microfluidic chip.
Increasing the resistance of the system enables decreasing its sensitivity; hence increasing its stability.
Nevertheless, the maximum reachable flow rate becomes smaller, which sometimes can be a limitation for flushing the system.
Flow restrictors consist of capillaries with different diameters. Their resistance depends on their inner diameter (ID), their length and the viscosity of the liquid that flows through them.
The flow restrictor should be connected directly on the reservoir. Due to the connector size, the flow restrictor must be longer than 10 cm. If it is long enough, the flow restrictor can be directly connected to the flow sensor or other instrument. Otherwise, it can be connected to an intermediate capillary adapted junction (see picture).
The following table gives the resistance of different flow restrictors for different liquids. A selection of Flow resistance kits can be found here.
Note that these values are given for a 1 cm-long flow restrictor. Since the resistance is proportional to the capillary length, you can easily adjust the length to reach the desired fluidic resistance.
With water, flow restrictors with an ID of 64 µm and a length of 10 cm will give a resistance of 38 mbar/(µL/min). A pressure drop of 38 mbar across the restrictor will result in a flow rate of 1 µL/min.
To reach the best flow control, the maximum pressure of your OB1 must match the maximum flow rate of your flow sensor, e.g. an OB1 200 mbar for a 7 µL/min flow sensor gives an ideal resistance of 28 mbar/(µL/min).
In the former example, the user cannot flush the system with a flow rate higher than 7 µL/min, which can be limiting for transient phases. If the user wants to reach a max flow rate of 20 µL/min, a resistance of 10 mbar/(µL/min) is more adapted.
In order to find the best compromise, Elveflow recommends starting with flow restrictors exhibiting a resistance which is equal to 25% of the OB1 range divided by the desired flow rate.
For an OB1 200 mbar and a desired flow rate of 5 µl/min, use a resistance of 50/5=10 mbar/(µL/min).
Elveflow also offers a broad range of flow resistances kits that are easy and quick to connect and offer excellent chemical compatibility for your flow restrictors:
Installation could not be easier, since these kits contains enough tubing to make about three to fifteen flow resistances, along with all necessary fittings, and a microfluidic tubing cutter.
The following table indicates the tubing length and flow resistance kit reference to be used based on the typical working pressure and the Elveflow flow sensor used.
Important Note : This table is not intended to provide absolute values for a particular setup and should only be seen as a beginners’ guide. The microfluidic resistance should be refined based on each setup characteristics. The particular conditions of your use and application of our products are beyond our control. Only a test in the specific conditions of your application will determine the appropriateness of a flow resistance size, which remains a hypothesis among other parameters (e.g. biophysical model chosen, length of fluidic channel, pressure source, chip height, etc …). To go even further in understanding the hydraulic resistance in microchannels, we highly recommend the following article on flow restrictors: Reexamination of Hagen-Poiseuille flow: Shape dependence of the hydraulic resistance in microchannels – NA Mortensen, F Okkels, H Bruus – Physical Review E 71 (5), 057301.
Email* Message
Newsletter subscription I hereby agree than Elveflow uses my personal data
Do you want tips on how to best set up your microfluidic experiment? Do you need inspiration or a different angle to take on your specific problem? Well, we probably have an application note just for you, feel free to check them out!
Learn how to set up your development environment for Elveflow products with this comprehensive tutorial.
This user guide will show you how to run microfluidic colocalization studies of single molecule spectroscopy.
Study the impact of molecular transport on cell cultures with a cross flow membrane chip and microfluidic instruments.
Precise liquid injection system for manipulation of small volumes of fluids using the MUX distribution and the MUX recirculation valve.
This application note explains how to set up a robust and reproducible microfluidic platform for liposomes assembly with improved encapsulation efficiency and reduced polydispersity in size.
Single-wall carbon nanotubes (SWCNTs) are considered as quasi 1-dimensional (1D) carbon nanostructures, which are known for their outstanding anisotropic electronic, mechanical, thermal and optical properties.
This application note describes how to combine and synchronise liquid perfusion and imaging using an Olympus spinning disc confocal microscope together with an Elveflow pressure-driven flow controlled microfluidic system.
Mixing is a crucial step for several microfluidic applications like chemical synthesis, clinical diagnostics, sequencing and synthesis of nucleic acids
This application note describes how microfluidic can be employed as a nanoparticle generator based on the example of PLGA bead generation.
Learn how to perform PLGA nanoparticle preparation with Elveflow instruments and a microfluidic chip
This application note explores the basic principle of pneumatic pumps and a flow controller based on the basic principle of pneumatic pumps, known as pressure driven flow control. It also demonstrates the applications of pressure driven flow control in a range of industrial & research fields.
The application note describes how to convert various units of shear stress and/or pressure from one to another: shear stress conversion from Pascal, atmosphere, and N/m²...!
The application note describes how to convert various units of viscosity from one to another: viscosity conversion from Poise, Pa.s, Dyn.s/cm²...
Elveflow's developped a microfluidic resistance calculator dependent upon the microfluidic system from the flow rate, the device, the tubing to your fluid properties.
Get a quote
Name*
Email*
Message
Newsletter subscription
We will answer within 24 hours
By filling in your info you accept that we use your data.
Collaborations
Need customer support?
Serial Number of your product
Support Type AdviceHardware SupportSoftware Support
Subject*
I hereby agree that Elveflow uses my personal data Newsletter subscription
How can we help you?
Message I hereby agree that Elveflow uses my personal data Newsletter subscription