New Model Explains Unique Transport Dynamics of Complex Fluids

In complex fluids like cell nuclei, polymeric fluids, and supercooled water, molecules do not move in the same way as they do in simple liquids at room temperature. But current theories have been unsuccessful in explaining exactly how these fluids move. Scientists at Chung-Ang University proposed a theory to explain molecular movement in complex fluids.

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Chung-Ang University

Jan 21, 2020, 08:32 ET

SEOUL, South Korea, Jan. 21, 2020 /PRNewswire/ -- Particles in fluids are always moving and colliding with each other at a high frequency (a movement called thermal motion). Advanced techniques like super-resolution single particle tracking and molecular dynamics simulation have helped to monitor real-time trajectories of microscopic probe particles and molecules undergoing thermal motion. The observed motion is much more complicated than expected by classical theories or their modern generalizations. Moreover, this discrepancy increases with the complexity of the fluid system. While details of thermal motion vary with the type of fluid, some common features can be seen. What mathematical equation governs this thermal motion? How did their common features originate? These long-unexplained questions now demand answers.

Infographic: Molecular movement in complex fluids

In a study in the Proceedings of the National Academy of Sciences USA, researchers from Chung-Ang University have proposed a new model of the probabilistic molecular movement in complex fluids. Led by Prof Jaeyoung Sung, the scientists derived a general transport equation that explains, in a unified manner, thermal motion of particles in complex fluids.

Particle displacement in complex fluids does not assume a normal distribution (or Gaussian distribution); this deviation increases at short times but decreases at long times. Complex fluids commonly exhibit an unusual time dependence of the mean-square displacement, a measure of the space spanned by a randomly moving particle. In their study, Prof Sung's team used a unique theoretical approach to construct a quantitative model for a dynamical system interacting with complex environments. Prof Sung explained, "Constructing an accurate and explicit model of systems interacting with a complex environment is difficult because, often, we don't know enough about these environmental interactions. We can resolve this by employing a general and flexible model."

Interestingly, they showed that, the same equation could be derived from models in which fluid molecules obeyed Newton's equation of motion. By accurately explaining experimental data obtained for various systems, including supercooled water (water cooled below 0°C but not yet frozen) and colloidal beads moving on lipid tubes, they confirmed that their transport equation is universally applicable.

Prof Sung believes that this theory has opened new doors for various applications. He says, "Most biological processes involve transport processes in complex cell environments; batteries and solar cells operate through charge carrier transport in complex media. Our work could help scientists predict the dynamic behavior of these complex systems."

Reference
Title of original paper:	Transport dynamics of complex fluids
DOI:	10.1073/pnas.1900239116
Name of author (from Chung-Ang University):	Jaeyoung Sung
Affiliation:	Creative Research Initiative Center for Chemical Dynamics in Living Cells; Department of Chemistry at Chung-Ang University

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