What is Viscosity?
The viscosity of a fluid is the measure of its resistance to flow and often refers to the thickness of a fluid.5 Water, for example, has a lower viscosity than molasses and flows more freely. At a molecular level, viscosity is a result of friction between molecules in a fluid.
Similar to friction between moving solids, fluid viscosity determines the energy or force required to produce flow.8 Fluid viscosity is the force per unit area and calculated by measuring the amount of time it takes for a sphere to fall a given distance through a liquid.4
In microfluidic applications, viscosity is an important consideration in the experimental configuration. Fluid viscosity correlates linearly with resistance to flow; therefore, high viscosity liquids reduce the flow rate and require more energy.3
Viscosity when Using Syringe Pumps
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 wide range of applications. In contrast with centrifugal pumps, which operate best with low viscosity fluids, syringe pumps function at lower speeds and use less energy.7
The main advantage of syringe pumps is due to the capability to precisely control the flow rate across microchannels independently of fluid friction. Chemyx has developed a variety of syringe pump systems to choose from based on the applications of viscosity.
Challenges with Viscous Fluids
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.6
- Although modifications in tube length to surface area can help alleviate pressure, highly viscous fluids often exceed the capability of microfluidic systems.
Viscosity is therefore an important factor in the selection of valves, filters, syringe, and tubing material.
Chemyx syringe pumps are precision machined with chemically treated metal components capable of meeting the requirements of most microfluidic applications.1 For example, the Fusion 6000 high-pressure syringe pump is specifically designed for dosing viscous solutions and can handle liquids thicker than 250,000 cP, which is the viscosity of peanut butter.2,8
Another issue concerning the effects of viscosity on syringe pumps is the time required to stabilize the effect of pulsation at low flow rates. Increased viscosity is a result of increased molecular friction, since it can lead to fluctuations in steady-state flow.6
To minimize this effect, Chemyx has developed a range of pulseless syringe pumps which infuse and withdraw solutions at reproducible flow rates via a cutting-edge micro stepper motor, alleviating flow rate variability.1 For example, the Chemyx Fusion 6000 has the capability of operating with a step resolution as low as 0.0938 µm/step.2
Overall, fluid viscosity has a significant effect on the experimental setup of 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 pertaining to fluid choice in syringe pump configurations include flow rate requirements, abrasive and corrosive characteristics of the fluid, and particle size in the fluid.
Once the fluid characterization and pump operating parameters are defined, proper pump selection can be made.
- Fusion 200 Two-Channel Syringe Pump. (2017). Retrieved from https://www.chemyx.com/syringe-pumps/fusion-200/
- Fusion 6000 Syringe Pump. (2017). Retrieved from https://www.chemyx.com/syringe-pumps/fusion-6000/
- Girdhar, P. (2008). Performance Evaluation of Pumps and Compressors. 4-10.
- How to Calculate Viscosity. (2017). Retrieved from https://sciencing.com/calculate-viscosity-6403093.html
- Merriam-Webster ‘s Dictionary. (2017). Retrieved from https://www.merriam-webster.com/dictionary/viscosity
- 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.
- Positive displacement pumps: a guide to performance evaluation. (2007). Hoboken, NJ: John Wiley & Sons.
- Viscosity Tables. (2017). Retrieved from http://www.vp-scientific.com/Viscosity_Tables.htm
- Volk, M. (2014). Pump characteristics and applications. Boca Ratón, FL: CRC Press.