Description
Viscoelastic fluids exhibit a broad range of behaviors that can be traced back to the coupling between viscous and elastic stresses that is not found in Newtonian fluids, such as pure water[1]. Viscoelasticity is the consequence of adding a small amount of large polymers to water. The result is turbulence-like fluctuations at small Reynolds numbers (Re<1) which in turn leads to changes in flow resistance. The flow properties of the viscoelastic fluid is a function of polymer content, making it highly relevant to perform rheological analysis of fluids in a broad range of application areas.
What limits the use of rheology as a tool for process monitoring, quality control and diagnostics is that standard rheometers are typically bulky, expensive and complicated to operate. They are therefore often found in dedicated laboratories leading to a time delay between sample collection and answer. Especially for medicine, the requirement for relatively large sample volumes limits their applicability and creates discomfort for the patients during sample collection. Highly trained staff are needed for their operation.
We offer a simple solution that addresses these concerns with conventional rheometers and that builds on our past work on viscoelastic fluctuations in microchannels[2, 3]. A rectifier is made by connecting four fluidic diodes[3] in a microfluidic device, just like the well-known electrical rectifier. We leverage the fact that the fluidic diodes only work for viscoelastic fluids and that their flow properties depend on the polymer composition of the fluid. We short-circuit the diode bridge with an observation channel where we detect the resulting unidirectional flow for viscoelastic fluids and zero flow for Newtonian fluids using particle tracking. Similarly, we monitor the total flow rate as a function of applied pressure in the inlet channel, to derive the viscosity. We show different responses for different concentrations and molecular masses of viscoelastic solutions of polyethylene oxide, DNA, hyaluronic acid, mucin, nanocellulose, at minute volumes of down to 10 microliter and concentrations down to 0.04%, something that is very challenging if not impossible using standard rheological methods.
In industry, we envision our simple device to open up for comprehensive rheology process monitoring and for quality control. In medicine, we envision not only to broader clinical use, with relevance for osteoarthritis (synovial fluid), but also home-use by patients, e.g. those in need of continuous monitoring with relevance for cystic fibrosis (sputum) and xerostomia (saliva). On the long term, the diode bridge be might be useful as a pump driven by ambient vibrations in wearable fluidics, in analogy to piezoelectric energy harvesting applications.
1. Datta, S.S., et al., Perspectives on viscoelastic flow instabilities and elastic turbulence. Physical Review Fluids, 2022. 7(8): p. 080701.
2. Ström, O.E., J.P. Beech, and J.O. Tegenfeldt, Short and long-range cyclic patterns in flows of DNA solutions in microfluidic obstacle arrays. Lab on a Chip, 2023. 23(7): p. 1779-1793.
3. Beech, J.P., O.E. Ström, E. Turato, and J.O. Tegenfeldt, Using symmetry to control viscoelastic waves in pillar arrays. RSC Advances, 2023. 13(45): p. 31497-31506.