Understanding active matter unlocks new material properties

An engineering collaboration has yielded new insights into modeling and understanding the inner workings of active-matter systems. Though ubiquitous throughout natural (and, in some cases, synthetic) systems, “active matter” is a relatively new categorization of matter that describes particles with the ability to convert stored or ambient energy into motion.

When introduced to a system, active matter harvests energy and begins to move. The term refers to systems that can be either biological or artificial, ranging from the self-organizing components of a living cell to synthetic colloids that react to light with movement. When added to an ordinary material, active particles can change the properties and behavior of that material.

“Any time you have an active fluid, you have a dial that you can use to change its transport properties,” said Jerry Wang, an assistant professor of civil and environmental engineering. “What’s exciting about active particles is that you can change the transport properties of an existing fluid simply by adding an active component and adjusting its level of activity.”

This offers the versatility in design to fine-tune the properties of existing fluids, ultimately increasing their performance and range of applications. However, researchers must first be able to define the properties of active fluids and predict their behavior as energy is added or removed, ideally without exhaustive testing in a lab setup.

Wang and Arman Ghaffarizadeh, a mechanical engineering Ph.D. student, set out to define predictive and theoretically grounded techniques for describing transport in these active-matter systems. They performed a series of simulations of a model active-matter system, studying its properties as the number of active particles and their level of activity were changed. The relationship they observed between fluid structure and transport in their active-matter system closely resembled a similar relationship for inactive matter, called “excess entropy scaling.”

Excess entropy is a measure of the difference between the entropy of a system and its maximum possible entropy, in the state of an ideal gas. Excess entropy scaling describes an exponential relationship between the excess entropy of a system and the system’s diffusivity, a critical quantity for fluids engineering that measures how quickly particles wander within a system through small random movements.

With Wang and Ghaffarizadeh’s model, researchers would only need to know some basic thermodynamic details about a system and its constituent makeup to determine its transport properties.

This research could save researchers significant time and effort as they explore applications for active matter. For example, heat transfer fluids move heat from one area of a system to another, and include substances like coolants and a variety of oils. The addition of active matter could alter properties like viscosity or thermal diffusion within these common compounds, allowing for improved performance and versatility.

The findings were published in The Journal of Physical Chemistry Letters. 

Media contact: 
Dan Carroll, dccarrol@andrew.cmu.edu