December 20, 2024 by Ingrid Fadelli , Phys.org
Collected at: https://phys.org/news/2024-12-particles-3d-gels-denser-porous.html
Colloidal gels are complex systems made up of microscopic particles dispersed in a liquid, ultimately producing a semi-solid network. These materials have unique and advantageous properties that can be tuned using external forces, which have been the focus of various physics studies.
Researchers at University of Copenhagen in Denmark and the UGC-DAE Consortium for Scientific Research in India recently ran simulations and performed analyses aimed at understanding how the injection of active particles, such as swimming bacteria, would influence colloidal gels.
Their paper, published in Physical Review Letters, shows that active particles can influence the structure of 3D colloidal gels, kneading them into porous and denser structures.
“Traditionally, much of physics focuses on systems that evolve toward their most stable or ‘favorable’ state, referred to as equilibrium,” Kristian Thijssen, senior author of the paper, told Phys.org.
“For instance, a gas or liquid that spreads evenly to fill its container is considered to be in equilibrium. However, in the physical world we inhabit, many systems do not reach equilibrium within the timescales of practical interest, or they remain continually energized in some way.”
An example of systems that remain continually energized to some extent is glass. The arrangement of particles is known to prevent the material from relaxing into its most thermodynamically stable state, which translates into a high sensitivity to its formation history.
“This is evident in glassblowing, where the process of shaping the material directly influences its internal structure,” explained Thijssen. “Colloidal gels, which consist of networks of particles with large voids, exhibit similar behavior. Their structure is not only influenced by their initial formation but also by the forces exerted on them.”
An emerging research field, known as active matter, has been trying to understand how living systems behave as far-from-equilibrium systems. This entails studying the behavior of living organisms, such as bacteria, when they are introduced into various environments.
These organisms introduce energy into their surroundings, by moving or swimming with the energy they acquire from food or other energy sources. This injection of energy prevents a system from reaching a state of equilibrium, continuously influencing their behavior.
“In our research, we sought to investigate what occurs when these two systems combine,” said Thijssen. “Specifically, we explored the dynamics of a gel, which is normally dependent on its history, when subjected to active particles that locally inject energy into their surroundings.”
Thijssen and his colleagues initially predicted that active particles would simply compress a gel into a more compact state, as this is what was observed in two-dimensional (2D) systems. Surprisingly, however, they found that their effect on 3D colloidal gels was far more intriguing.
“Instead of merely compacting the gel, the active particles reorganized the gel into a denser structure while preserving sufficient pathways for particle movement,” said Thijssen. “In this way, the gel is adapted to facilitate the transport of the active particles, resulting in a dynamic and efficient structure that continuously evolves as the active particles interact with it.”
To investigate the effects of injected active particles on 3D gels, the researchers ran a series of computer simulations using the open-source platform LAMMPS, which modeled the dynamics of gel particles and active particles. To simulate the gel particles, they used a model known as “short-range sticky potential” that captures the formation of colloidal gels.
“When colloidal particles are mixed with smaller particles in a liquid, the polymers around the colloids tend to spread evenly throughout the fluid,” said Thijssen.
“However, when two colloidal particles approach each other closely, the polymers can no longer fit between them, leading to a repulsive force that pushes the particles together. This results in attractive forces strong enough to drive the formation of a gel structure.”
To simulate the active particles, the team drew inspiration from a model describing the behavior of swimming bacteria called active Brownian particles (ABPs). These particles are known to self-propel in one direction, which they periodically change, mimicking the ‘run-and-tumble’ motion of bacteria.
“To understand how the gel responds to these active particles, we applied a technique called topological data analysis (TDA),” explained Thijssen.
“Although TDA has been used in other fields, it has not been widely applied to gels or active matter systems. TDA allows us to analyze the gel’s structure based on its topology, or overall shape. For example, a sphere would be classified as a single connected component, a ring would have one hole, and a shell would have a cavity in the center.”
Using this technique, the researchers characterized the structure of the colloidal gel in ways that unveiled crucial mechanical properties. They particularly focused on the connections between the empty spaces within a gel, which active particles use to move through the material.
“This connectivity is crucial because the active particles can alter the gel’s structure, creating more accessible pathways for movement,” said Thijssen.
The simulations and analyses carried out by the researchers yielded very interesting results. Firstly, they revealed that when injected with active particles, 3D colloidal gels restructure themselves into more compact and energetically favorable structures, while retaining several spaces that the particles can traverse.
This adaptation was only identifiable using TDA, thus demonstrating the potential of this analytical tool. In this case, TDA allowed the researchers to unveil the dynamic adaptation of colloidal gels in response to the movement of active particles.
“Our study demonstrates how Topological Data Analysis (TDA) can be leveraged to quantify gel structures,” said Thijssen. “This innovative approach offers new insights into the mechanical properties of gels and other porous materials, which have long posed challenges to comprehensive understanding.”
This recent work also demonstrates that there is a fundamental topological difference between 2D and 3D systems in adaptable materials. In 2D materials, empty regions can only form enclosed spaces that trap any particles within them.
In 3D systems, on the other hand, empty regions form both enclosed and interconnected spaces, which allow particles to move freely through networks of spaces.
“This distinction has profound implications for understanding the behavior of porous media—beyond just gels—in response to reconfigurations driven by living organisms,” said Thijssen.
“By bridging this gap, our work paves the way for more accurate models and predictions of how a diverse range of materials—ranging from biological tissues to engineered systems—respond to dynamic changes in their environments.”
This study could soon pave the way for further investigations focusing on the impact of active particles on both colloidal gels and other porous materials. In their next studies, the team plan to build on their findings to carry out additional simulations and analysis that integrate models of other materials or more complex living organisms.
“In this project, we used relatively simple active particles as models for living organisms,” said Thijssen. “However, in densely packed living systems—such as swarming bacteria or flocks of birds—collective motion often emerges from the interactions between individual agents. This motion is a defining characteristic of active systems, but it is also strongly influenced by the surrounding environment.”
A further interesting aspect of the evolution of porous media observed by the researchers is that it could also produce feedback loops. In other words, the motion of the active particles could adjust in response to the evolving porous structures, which could produce dynamic interactions with even more complex outcomes.
“Exploring these feedback mechanisms is a promising direction for future research,” added Thijssen.
“Understanding these dynamics could have practical applications in areas such as regulating bacterial movement to enhance biodegradation, preventing contamination in industrial piping systems, or managing bacterial infections by disrupting their ability to penetrate mucosal membranes.”
More information: Martin Cramer Pedersen et al, Active Particles Knead Three-Dimensional Gels into Porous Structures, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.228301. On arXiv: arxiv.org/html/2404.07767v1
Journal information: Physical Review Letters , arXiv
Leave a Reply