Imagine a fluid that defies Newton's third law, pushing back harder than it's pushed, and spontaneously forming intricate patterns. This isn't science fiction, but the reality of "odd viscoelastic fluids," a new class of materials being explored by researchers at the University of Chicago. In a new study published in Physical Review Research, Carlos Floyd, a postdoctoral fellow working with the Suri Vaikuntanathan group, investigates these non-equilibrium materials, shedding light on their unique properties and potential implications for understanding biological systems.
Traditional viscoelastic fluids, like cornstarch mixed with water, exhibit both viscous (fluid-like) and elastic (solid-like) characteristics. However, they ultimately settle into an equilibrium state. Odd viscoelastic fluids, on the other hand, are inherently out-of-equilibrium, driven by active "particles" within them that consume energy and generate forces.
As Floyd explains, "Odd viscoelasticity is a generalization of standard theories of viscoelasticity to encompass fluids whose particles are active, meaning that they can each access an energy source and use it to do mechanical work." This active nature leads to striking behaviors, including instabilities that grow over time.
Motivated by a desire to understand active biological materials like developing tissues and the cytoskeleton, Floyd states: "Our main interest is in biological materials, such as tissue layers in developing organisms and the cytoskeletal cortex that lies underneath the plasma membrane of cells," says Floyd. "Both of these examples are viscoelastic materials and are also active, since chemical reactions at small scales give rise to local mechanical forces that are out of equilibrium."
A key finding is the discovery of a pattern-forming instability in these fluids, manifesting as arrays of swirling vortices, similar to patterns seen in biological systems. Floyd highlights the significance of this discovery: "Various instabilities in biological materials play an important role, as they are often the mechanism that gives rise to symmetry breaking and patterning during development."
He explains that unlike conventional instabilities that rely on chemical gradients, the instability observed in their simulations is purely mechanical, offering a new perspective on pattern formation in biological systems capable of simulating the unique properties of odd viscoelastic fluids. They then embarked on a challenging analytical journey to understand its underlying mechanisms. This involved "laborintensive paper and pencil work" to derive a mathematical prediction for the instability threshold, which ultimately agreed well with their simulation results.
Furthermore, the research explored the role of compressibility in odd viscoelastic fluids. While water is essentially incompressible, biological materials like cell tissues, composed of deformable cells, are better modeled as compressible fluids. Their findings revealed that compressibility significantly influences the nature of the instability, providing a more comprehensive and biologically relevant description of these systems.
This research opens up exciting new avenues for understanding biological processes and represents a significant step forward in this emerging field. Floyd and his team are particularly enthusiastic about the possibility of further describing biological materials. As research continues, we can anticipate further insights into the fascinating world of odd viscoelasticity and its implications for both biology and materials science.
Citation - Floyd, C.; Dinner, A. R.; Vaikuntanathan, S. Pattern Formation in Odd Viscoelastic Fluids. Physical Review Research 2024, 6 (3). DOI:10.1103/physrevresearch.6.033100.
Funding - This work was mainly supported by funds from DOE BES Grant No. DE-SC0019765 (C.F. and S.V.).
A.R.D. acknowledges support from the University of Chicago Materials Research Science and Engineering Center, which is funded by the National Science Foundation under Award No. DMR-2011854. C.F. acknowledges support from the University of Chicago through a Chicago Center for Theoretical Chemistry Fellowship. The authors acknowledge the University of Chicago’s Research Computing Center for computing resources