How to choose and adapt the microfluidic resistance of your setup?

Each element of a microfluidic circuit offers some resistance to the flow which is translated into a drop of pressure. The resistance to flow through a cylindrical microfluidic channel depends on several factors (described by Poiseuille) as follows:

R = (8 L η) / (π r4)

With r the channel internal radius (m), L the channel length (m), and η the viscosity (Pa.s). 

A proper conditioning of the setup is needed in order to avoid poor setup performance. Without this tuning, your sensor values will most likely be not stable enough, making the flow control unstable, slow, or even over-reactive. This tuning is done by adding microfluidic resistance (1/16’’ OD PEEK tubing) to the fluidic path. As a result, the tuning consists in selecting a diameter and a length for the resistance tubing.

The choice of a specific resistance tubing will depend on the overall resistance of the system (microfluidic chip, length of tubing, viscosity of the fluid…). As a result, if the fluidic path is modified (microfluidic system, addition of a valve, working medium…), you need to check the suitability of the resistance chosen. 

Elveflow provides a set of standard resistances that are designed to match most application cases (see Fig.1).

Figure 1 - Flow resistance kit

If you are working with aqueous solutions, we share a table (see Table 1) with indicative resistance tubing’s length and diameter, depending on the type of flow rate sensor and the pressure range used. Please note that the values in this table are estimated values and will need to be adapted depending on your experimental conditions. Here, the values correspond to a 0 – 2 bar pressure channel and using a 1/16’’ OD (800µm ID) PTFE tubing.

Table 1 - Standard resistances table when working with a 0 - 2bar pressure channel and using a 1/16'' OD PTFE tubing

Fine-tuning of your microfluidic resistance

In your experimental set-up, it is important that the resistance should be placed after the flow sensor, in order to keep the flow stable in the flow sensor.

Figure 2 - Flow resistance tubing installation for MFS-4 and MFS-5

Figure 3 - Flow resistance tubing installation for MFS-1; MFS-2 and MFS-3

Now, you need to check if you chose the right resistance for your setup. To do so, on the ESI interface, in regulator mode, check that the resistance chosen fits the flow rate range you want to work with by setting the maximum pressure of your channel range. The general idea to keep in mind for successful operation is to match the pressure range with the flow rate range. By carrying out this check, you can ensure that you are using 100% of your system's performance. 

For example, working with a 0-2 bar pressure channel and a MFS4, set in small increments the pressure at 2 bar and check that the flow range displayed is around 1000 µL/min. The aim is to get as close to it as possible. However, in the vast majority of cases, an approximation will be sufficient to enable experiments to be carried out under good conditions.  

N.B: Beyond the working flow rate range of the sensor, the measured values are not accurate anymore and may even decrease, while the actual flow rate value increases. If you are in this situation, get back to lower flow rates compatible with your sensor specifications. You may also need to increase the microfluidic resistance of your set-up.

Figure 4 - Flow measurement above the working flow range of the sensor (MFS-4) 

Tips:

• If the flow rate is too low, your system is too resistive, you should either increase the inner diameter of the resistance or decrease the tubing length. In some cases, your system might not need any microfluidic resistance tubing or you might need to switch your entire microfluidic tubing. For example, switching a 1/16” OD tubing to a 1/8” OD (2.4 mm ID) tubing.  

Figure 5 – Configuration with a too low flow rate. The system is too resistive. The maximum pressure of the channel corresponds to 20% of the maximum flow rate measurable by the sensor (MFS- 4).    

Figure 6 - Matching the range between pressure and flow rate. The maximum pressure of the channel corresponds to the maximum flow rate measurable by the sensor (MFS-4).  

• If the flow rate is too high, your system is not resistive enough and your flow rate will be instable. In this case, you should either decrease the inner diameter of the resistance or increase the resistance tubing length. In some specific set-up, you might need to switch the entire microfluidic tubing to a smaller internal diameter one. For example, switching a 1/16” OD tubing (800 µm ID) to a 1/32” OD (300 µm ID) tubing.

Figure 7 - Configuration with a too high flow rate. The system is not resistive enough and the flow rate is above the working flow range of the sensor (MFS-4).