Thermal Conductivity Measurement for Optimized Liquid Pumping
This article is based on a case study we conducted on droplet generation using the mp6 micropump. Since then, we have developed a successor to it: The Bartels Pump | BP7. Everything demonstrated in this article is possible using the BP7, if not easier.
Identifying a liquid or mixture through methods like measuring thermal conductivity is important in microfluidics for a variety of reasons:
- The performance of a micropump is viscosity dependent.
- An exact degree of mixing of two liquids should be controlled.
- The whole process depends on the fluid currently used.
Membrane-based micropumps, like the mp6 micropump, rely on two key parameters: frequency—the rate at which the membrane is actuated per second—and driving voltage, which governs the membrane’s stroke. These parameters must be precisely adjusted to correspond with both the liquid’s viscosity and the desired flow rate.
To address this challenge, our partner Sensirion has developed an innovative solution: an in-line, in-situ thermal conductivity sensor that integrates seamlessly into our system. This article illustrates how the mp6 micropump, in conjunction with Sensirion’s SLF3C-1300F sensor, can effectively identify liquids by measuring their thermal conductivity.
By leveraging this capability, we can automate pump parameter adjustments, significantly enhancing efficiency. This automation proves particularly advantageous in applications where the same pump manages multiple liquids, allowing for quick adaptations without manual intervention.
Fundamentals of thermal conductivity
Thermal conductivity is a fundamental property that describes a material’s ability to conduct heat. More specifically, it defines the rate at which heat flows through a material per unit area, given a unit temperature gradient. This property measures how efficiently heat transfers through a material.
In our case study, Sensirion sensors played a key role as flow-controlling elements. We utilized the SLF3C-1300F sensor, which is calibrated for both water and Isopropanol. The sensor can measure flows of up to 40 ml/min, with an impressive turndown ratio of 1:200 (the ratio of the lowest to highest flow rate) and a fast response time of less than 20 ms.
In addition to measuring flow rate, the SLF3C-1300F sensor provides an important feature: media recognition through the measurement of a liquid’s thermal conductivity.
At zero flow, the sensor’s heater-sensor element examines the thermal behavior of the liquid. This data allows the system to calculate its thermal conductivity, with the results delivered to the user in arbitrary units (a.u.). These units range from 100 a.u. for air to 10,000 a.u. for water, providing an accurate assessment of the liquid inside the sensor.
Utilizing the BP7 Micropump
At Bartels Mikrotechnik, the Bartels Pump | BP7 micropump serves as an essential component in microfluidic systems. This positive displacement membrane pump utilizes piezo actuators to efficiently handle both liquids and gases.
One of the key strengths of the BP7 micropump is its versatility. It offers a dynamic flow rate ranging from 0 to 14,000 μl/min for liquids and up to 35,000 μl/min for gases, making it adaptable to a variety of experimental setups.
In addition to its wide flow range, the pump operates at pressures of up to 500 mbar for water and 140 mbar for air, ensuring high precision in even the most demanding microfluidic applications.
Working Principle and Performance
The working principle of the mp6 micropump is based on the piezoelectric effect. Here, a piezo actuator applies a consistent force on the fluid. However, the pump’s performance varies when handling fluids with differing rheological properties—such as density or viscosity—even when operating under the same parameters.
When the density of a liquid changes, the mass of a defined portion of that liquid shifts as well. Since the piezo actuator provides only a constant force, increasing the pump frequency leads to higher acceleration. Consequently, the force of inertia also rises.
As a result, the constant force from the piezo actuator can become overwhelmed, reducing the pump’s performance even though the pump parameters have been increased.
Viscosity and Frequency Balance
To put it simply, viscosity refers to how easy or difficult it is to move the layers within a liquid. This means that the more viscous a liquid is, the harder it becomes to move those layers—or, in general, to move the liquid at all.
As you can see in the diagram above:
The relationship between viscosity and frequency plays an essential role in optimizing performance: lower viscosity requires higher frequencies for peak efficiency, while higher viscosity benefits from lower frequencies.
Therefore, it is often necessary to know which fluid is meant to be pumped to adjust the mp6 micropump to its optimal working point. In an ideal case, one would use a viscosity sensor.
As this would require an additional sensor, we use the SLF3C-1300F sensor which, beside the already used flow rate, also provides the thermal conductivity as output value as described above. This we can use to distinguish between different media and use a look-up table for the viscosity in order to adjust the pump parameters.
Experimental setup
Before conducting the experiment, we determined the thermal conductivity of the different media we aimed to distinguish. The results showed that water produced an output of approximately 10,000 a.u., while our water/glycerin mixture gave a reading of around 7,500 a.u.
By identifying these distinct values, we can easily differentiate between the two liquids during the experiment.
In our experiment, we now use the mp6 micropump to pump water. As you can see, that with a low frequency, we achieve a low pump rate.
Next, we stop the flow and take a thermal conductivity reading using the SLF3C-1300F sensor. The sensor provides a value of 10,500 a.u.. Considering that temperature compensation was not implemented in this experiment, this reading is close enough to the calibrated value for water (10,000 a.u.) to confidently identify the liquid in the pump as water.
By relying on this data, we can reasonably conclude the fluid in the system is indeed water, despite the absence of temperature adjustments.
As the next step, we apply a much higher frequency, but the pump rate remains low. This outcome indicates that the frequency is too high for the system. The water cannot keep up with the rapid pump strokes. As a result, we lower the actuation frequency slightly and, in doing so, achieve the maximum pump rate. This frequency could be stored in a look-up table as the optimal actuation frequency for liquids primarily containing water.
Switching to Glycerin/Water Mixture
Afterward, we switch to a different liquid: a mixture of glycerin and water with a viscosity of 18 mPas. Using the same actuation frequency that worked for water, we observe a very low pump rate.
This makes sense, as the viscosity of this mixture is significantly different from that of water. To optimize the pump rate, we increase the frequency. However, this further decreases the pump rate, suggesting that the optimal frequency must be lower. Using the SLF3C-1300F sensor, we take a thermal conductivity reading and find a value of 7,500 a.u., confirming the presence of the glycerin/water mixture in the system.
With this data, we can clearly differentiate between the two liquids—water and the water/glycerin mixture. By lowering the actuating frequency, we identify the maximum pump rate at 50 Hz, thus determining the optimal actuation frequency for both water and the glycerin mixture.
Measuring the thermal conductivity of the fluid to set the perfect frequency for the optimal flow rate:
| Fluid | Viscosity | Thermal Conductivity value measured | Frequency |
|---|---|---|---|
| Water | 1 cP | 10500 | ~100 Hz |
| Water Glycerin mixture | 18 cP | 7500 | ~50 Hz |
You can see the full experiment about determining thermal conductivity of liquids in a microfluidic system in the video below:
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More InformationResults: Thermal Conductivity as the Key to Precise Fluid Management
The performance of the mp6 micropump by Bartels Mikrotechnik significantly relies on the viscosity of the fluids it handles.
By utilizing the SLF3C-1300F sensor from Sensirion, which effectively measures the thermal conductivity of liquids, we can differentiate between various fluids. This capability allows us to assign specific thermal conductivities to each liquid. Consequently, we can adjust the micropump to achieve the optimal working point tailored for a specific fluid.
In summary, the SLF3C-1300F provides an effective solution, enabling smart and in-situ reactions to rheological changes within a fluidic system.
Looking ahead, the SLF3C-1300F will allow us to measure a critical property of liquids: their thermal conductivity. This functionality will also facilitate the analysis of mixtures and their concentrations.
If this article sparked your interest in micro pumps, visit our shop or contact us via our contact form so we can help you integrate micro pumps perfectly into your system.