Learning how to calculate viscosity and select a syringe pump for highly-viscous fluid applications.
Liquids with contrasting viscosities. Liquid (left) has high viscosity. Liquid (right) has high viscosity.
Topics Covered:
- What is viscosity
- Considering viscosity
- Which syringe pump can infuse viscous liquids
- Challenges working with viscous liquids in microfluidics
Why Viscosity of a Liquid Matters
- Knowing the properties of a liquid is key when choosing an application.
- Selecting the appropriate instruments and accessories for a liquid ensures for a more accurate infusion rate.
- Comprehending how a fluid behaves under multiple conditions gives insight to programming pump parameters.
One of the fundamental properties of any liquid is viscosity. Viscosity plays an significant role in how a fluid will act and flow in different environments. It is important to concurrently consider all factors of a liquid and pump set up for medical and research processes. In fact, the viscosity of your liquid will help determine what type of pump, syringe, and tubing or needle you’ll need. Furthermore, certain fluid viscosities can cause a wide array of issues during an infusion, but there are a good range of application opportunities for them as well. Nevertheless, selecting an improper syringe pump or program to dispense a high or low viscous fluid could modify the pump performance, data results, or waste resources. The following article briefly discusses viscosity of fluids and how this aspect impacts selecting the most appropriate syringe pump accessories for scientific applications.
What is Viscosity?
Firstly, the factors that determine viscosity must be defined to understand the property of viscosity. Viscosity, also known as resistance to flow, is the relationship between the shear stress and shear rate of a liquid. Shear stress is caused by friction amongst a liquid’s particles and is explained as a force per unit area. On the other hand, the shear rate of a fluid is calculated as the difference in velocity between the two surfaces holding the liquid, divided by the distance between them. These two variables provide the equation for calculating a fluid’s viscosity (viscosity = shear stress/shear rate).
Considering Viscosity
Similar to the friction created between solids passing one another, fluid viscosity determines the force or energy required to produce fluid flow. Fluid viscosity correlates linearly with resistance to flow; therefore, a high-viscosity liquid, like molasses, reduces the flow rate and requires more energy. In contrast, a fluid with low shear stress and force, such as water, results in a high shear rate and velocity. In microfluidic applications, viscosity impacts microfluidic dynamics and is essential to consider in the experimental configuration.
Which Syringe Pump can Infuse Viscous Liquids
High-pressure syringe pump for highly-viscous fluids.
In general, syringe pumps work best with fluids that are similar to or slightly higher than the viscosity of water and can be adapted to handle a broad scope of applications. In contrast, centrifugal pumps operate best with low-viscosity fluids, syringe pumps function at lower speeds and use less energy. The main advantage of syringe pumps is due to the capability to precisely control the flow rate across microchannels independently of fluid friction. Several varieties of syringe pumps for infusing liquids exist. It is best to choose a syringe pump based on the liquid and application. There are high-pressure syringe pumps designed for dosing viscous solutions, and semi-solid material injections against pressure.
Challenges Working with Viscous Fluids in Microfluidics
Using fluids that are more viscous than water can pose several issues in microfluidic applications, particularly if elastic materials are used:
- Pumping viscous fluids can require an extreme amount of pressure, which can distort microfluidic tubing over time leading to inconsistent flow and leakage.
- Channel deformation occurs if the applied pressure exceeds the stiffness of the material and can introduce a variety of transient effects that may pose significant consequences to microfluidic applications, such as the precise metering of microvolumes of fluid.
- Although modifications in tube length to surface area can help alleviate pressure, highly viscous fluids often exceed the capability of microfluidic systems.
Markedly, viscosity is an important factor in the selection of valves, filters, syringes, and tubing material.
Syringe pumps for thick liquids are precision machined with chemically treated metal components capable of meeting the requirements of most microfluidic applications. For example, there are high-pressure syringe pumps specifically designed for dosing viscous solutions and can handle liquids thicker than 250,000 cP, which is close the thickness of peanut butter.
The time required to stabilize the effect of pulsation at low flow rates is another issue concerning the effects of viscosity on syringe pumps. Increased viscousness is a result of increased molecular friction since it can lead to fluctuations in steady-state flow.
To minimize this effect, there are a range of pulseless syringe pumps that infuse and withdraw solutions at reproducible flow rates via a cutting-edge micro stepper motor, alleviating flow rate variability. For example, some syringe pumps have the capability of operating with a step resolution as low as 0.0938 µm/step.
If you’re looking for a syringe pump for viscous liquids, Chemyx Inc. recommends the Fusion 6000X. The 6000X is a high-pressure syringe pump for highly-viscous fluids, and it can be temperature controlled.
Conclusion
Overall, fluid viscosity has a significant effect on the experimental setup of syringe pumps and microfluidic systems. The ability of syringe pumps to regulate continuous and pulseless flow with precision infusion can pose several challenges as the viscosity of a fluid increases. As a result, additional considerations about fluid choice in syringe pump configurations include flow rate requirements, abrasive and corrosive characteristics of the fluid, and particle size in the fluid.
Lastly, a proper pump selection can be made once the fluid characterization and pump operating parameters are defined.
References
American Institute of Chemical Engineers. Positive Displacement Pumps: A Guide to Performance Evaluation. John Wiley & Sons, 2010.
Banas, Timothy. “How to Calculate Viscosity.” Sciencing, 6 Dec. 2020, sciencing.com/calculate-viscosity-6403093.html.
Girdhar, P. (2008). Performance Evaluation of Pumps and Compressors. 4-10.
Levick, J R. “Haemodynamics: Pressure, flow and resistance.” An Introduction to Cardiovascular Physiology, 1991, pp. 90–116, https://doi.org/10.1016/b978-0-7506-1028-5.50010-x.
Penn State University. “What Is Fluid Mechanics?” What Is Fluid Mechanics?, https://www.me.psu.edu/cimbala/Learning/Fluid/Introductory/what_is_fluid_mechanics.htm
Perry, S., Higdon, J., Kenis, P. (2010) Design rules for pumping and metering of highly viscous fluids in microfluidics. Lab Chip. 10(22), 3112–3124.
Sloop GD, De Mast Q, Pop G, Weidman JJ, St Cyr JA. The Role of Blood Viscosity in Infectious Diseases. Cureus. 2020 Feb 24;12(2):e7090. doi: 10.7759/cureus.7090. PMID: 32226691; PMCID: PMC7096068.
