A research team led by Prof. Li Chuanfeng from the University of Science and Technology of China (USTC) has achieved a breakthrough in quantum photonics. They developed an on-chip photonic simulator capable of simulating arbitrary-range coupled frequency lattices with gauge potential. This study was published in Physical Review Letters.
The quest for effective simulators that can replicate the dynamics of real systems has been a driving force in quantum physics. Photonic systems, with their ability to control properties like polarization and frequency, have emerged as versatile candidates for quantum simulation.
However, the challenge lies in creating frequency lattices that can simulate complex structures like atom chains and nanotubes, which are crucial for understanding low-dimensional materials.
To address this challenge, the team’s innovative approach involves the use of thin-film lithium niobate chips, which are particularly suited for creating lattices in the frequency domain due to their high electro-optic coefficient. By periodically modulating an on-chip resonator, the researchers observed band structures, a significant advancement as it allows for the simulation of structures with arbitrary-range coupling.
Remarkably, their method enabled coupling up to eight and nine times the lattice constant while reducing the required modulation frequency by over five orders of magnitude. This is achieved by including multiple lattice points within one resonant peak, which alleviates the difficulty of applying and detecting multiharmonic signals conventionally of ultrahigh frequency on chips.
In this study, the special focus on low-frequency radio-frequency modulation offers a high degree of flexibility in choosing lattice points and regulating compound interaction.
This approach significantly reduces the required frequencies by more than three orders of magnitude, translating to a reduction from near 100 GHz to around 10 MHz in their examples. This not only simplifies the design and fabrication challenges but also lessens the demands on source and measurement equipment.
This work not only greatly alleviates the difficulties posed by high frequencies in on-chip synthetic dimensions but also maintains the scalability of traditional implementation methods, allowing it to be extended to higher-dimensional models. It achieves high-dimensional and complex frequency synthetic dimensions on thin-film lithium niobate optical chips.
The reviewers highly praised the achievement, stating it “opens a new avenue within the area of studying synthetic dimensions on photonic chips.”
More information: Zhao-An Wang et al, On-Chip Photonic Simulating Band Structures toward Arbitrary-Range Coupled Frequency Lattices, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.233805
Long gone are the days where all our data could fit on a two-megabyte floppy disk. In today’s information-based society, the increasing volume of information being handled demands that we switch to memory options with the lowest power consumption and highest capacity possible.
Magnetoresistive Random Access Memory (MRAM) is part of the next generation of storage devices expected to meet these needs. Researchers at the Advanced Institute for Materials Research (WPI-AIMR) investigated a cobalt-manganese-iron alloy thin film that demonstrates a high perpendicular magnetic anisotropy (PMA)—key aspects for fabricating MRAM devices using spintronics.
The findings were published in Science and Technology of Advanced Materials on November 13, 2024.
“This is the first time a cobalt-manganese-iron alloy has strongly shown large PMA,” says Professor Shigemi Mizukami (Tohoku University),
“We previously discovered this alloy showed a high tunnel magnetoresistance (TMR) effect, but it is rare that an alloy potentially shows both together.” For example, iron-cobalt-boron alloys, which are conventionally used for MRAM, possess both traits, but their PMA is not strong enough.
MRAM devices use magnetic storage elements instead of an electric charge to store data, which gives it several advantages such as reduced power consumption. Ideally, alloys for MRAM devices have both a high TMR and PMA, which allow them to integrate a large number of bits with high capacity and high thermal stability.
In order to find new, alternative materials to solve the issues seen with currently used alloys, researchers at Tohoku University have investigated the PMA of cobalt-manganese-iron alloy thin films, which were shown to have high TMR in their previous research.
Remarkably, the alloy they produced was found to exhibit high PMA. They also demonstrated that the PMA in their multilayer films was large enough to be capable of its intended end purpose: large memory capacity for MRAM devices using a simulation.
The results of this research will offer a new candidate for memory materials, and contribute to the continuous development of novel spintronics memory devices, with the aim of creating a more sustainable society for everyone.
More information: Deepak Kumar et al, Metastable body-centered cubic CoMnFe alloy films with perpendicular magnetic anisotropy for spintronics memory, Science and Technology of Advanced Materials (2024). DOI: 10.1080/14686996.2024.2421746
Combining metallic glass with the Berreman mode of epsilon-near-zero (ENZ) thin films achieves a dual-function system for infrared camouflage and thermal management within an identical wavelength region of the atmospheric window. In recent research, metallic glasses were selected for their tunable optical properties, providing adjustable emissivity for versatile thermal camouflage while maintaining effective thermal management.
Thermal infrared camouflage aims to reduce the detectability of a target using thermal imaging devices. Given the typically high thermal emissivity in everyday environments, the thermal emissivity of the background environment must be considered. The conventional low-emissivity strategy for thermal camouflage is only effective for targets at extremely high temperatures, making it unsuitable for applications near room-to-medium-high temperature range (<350 °C).
In a study published in Materials Horizons, Professor Hsuen-Li Chen from the Department of Materials Science and Engineering at National Taiwan University led his research team in designing an innovative multilayer thin-film structure. This structure introduces metallic glass into infrared thermal camouflage technology, exploiting its adjustable emissivity to accommodate diverse infrared thermal camouflage scenarios.
Moreover, this is the first time combining metallic glass with the Berreman mode of epsilon-near-zero (ENZ) thin films.
In the long wave infrared (LWIR, 8–14 μm) regions, the small viewing angle exhibited the optical properties of metallic glasses. As the viewing angle increased, driven by the multiple Berreman modes of the ENZ thin films, it provided high thermal emissivity in transverse-magnetic (TM) polarization. It enabled thermal management without compromising the thermal camouflage performance.
The cooling power exhibited by ENZ thin films on metallic glass surpassed that of the conventional low-emissivity strategy for thermal camouflage by a factor of 1.79. Furthermore, the thermal images indicated over 97% similarity in thermal radiation between the target and background environments.
This presents new avenues for advancing infrared thermal camouflage technology.
More information: Pei-Chi Hsieh et al, Epsilon-near-zero thin films in a dual-functional system for thermal infrared camouflage and thermal management within the atmospheric window, Materials Horizons (2024). DOI: 10.1039/D4MH00711E
(A) Schematic illustration of PPT device consisting of a liquid medium, a lubricant layer, and an LMPs/P(VDF-TrFE) film sandwiched between top and bottom poly (methyl methacrylate) (PMMA) slides.(B) Photograph of the PPT platform containing a NIR laser light source and a portable PPT device with a large manipulation area of 12.5 cm2. Scale bar: 10 mm.(C) Schematic illustration of the PPT platform for object manipulation based on the photopyroelectric effect.(D) The output voltage of the PPT device upon exposure to NIR irradiation (power density: 100 mW mm−2, frequency: 0.5s ON and 5s OFF).(E) The voltage changes increase from 0.26 to 3.34 V with an increase in the laser power density from 2 to 111 mW mm−2.(F) The light-induced charge density of the PPT shows slight variation from 870 to 590 pC mm−2 by increasing the medium (silicone oil) thickness from 1 to 10 cm. Error bars are calculated from five independent measurements.(G) Manipulating 5-μm SiO2 particle, 1 pL water droplet, and 10 mL water droplet in a non-conductive medium (silicone oil, Video S2).(H) Manipulating a live medaka egg cell (1 mm diameter), and the time-lapse trajectory of 1-mm POM bead in the conductive medium of water. Credit: The Innovation (2024). DOI: 10.1016/j.xinn.2024.100742
Optical tweezers and related techniques provide extraordinary opportunities for research and applications in the physical, biological, and medical fields. However, certain requirements such as high-intensity laser beams, sophisticated electrode designs, additional electric sources, and low-conductive media, significantly impede their flexibility and adaptability, thus hindering their practical applications.
In a study published in The Innovation, a research team led by Dr. Du Xuemin from the Shenzhen Institutes of Advanced Technology (SIAT) of the Chinese Academy of Sciences reported a novel photopyroelectric tweezer (PPT) that combines the advantages of the light and electric fields. The PPT enables versatile manipulation in various working scenarios.
The proposed PPT consists of two key components, a near infrared (NIR) spectrum laser light source and a PPT device that includes a liquid medium and a photopyroelectric substrate.
The photopyroelectric substrate includes a superhydrophobic ferroelectric polymer layer made of Ga-In liquid metal microparticle-embedded poly (vinylidene fluoride-co-trifluoroethylene) (LMPs/P(VDF-TrFE)) composites, and a lubricant-infused slippery layer. The polymer layer generates real-time surface charges via the photopyroelectric effect, while the lubricant layer reduces motion resistance, suppresses contamination, and prevents charge screening by conductive media.
Owing to its rationally designed structure, the PPT efficiently and durably generates surface charges when exposed to low-intensity NIR (as low as ~ 8.3 mW mm-2) irradiation. This induces a strong driving force (up to ~ 4.6×10-5 N) without requiring high-intensity laser beam, complex electrode designs, and additional electric sources.
“The innovation lies in the rational design of the photopyroelectric substrate, which efficiently generates charges, and the lubricant layer that prevents charge screening by conductive media. “This design imparts unparalleled flexibility and adaptability for diverse object manipulation,” said Dr. Du.
The PPT can remotely and programmably manipulate objects of diverse materials (polymer, inorganic, and metal), phases (bubble, liquid, and solid), and geometries (sphere, cuboid, and wire). Moreover, it is adaptable to various media with wide-range conductivities (0.001 mS cm-1~ 91.0 mS cm-1) and is versatile for both portable macroscopic manipulation platforms and microscopic manipulation systems. It supports on-demand manipulating areas ranging from 5 μm to 2.5 mm, enabling cross-scale manipulations of solid objects, liquid droplets, and biological samples from single cell to cell assemblies.
The PPT proposed in this study offers a new tool for robotics, colloidal science, organoids, tissue engineering, and neuromodulation.
More information: Fang Wang et al, Photopyroelectric tweezers for versatile manipulation, The Innovation (2024). DOI: 10.1016/j.xinn.2024.100742
The impact of a drop on a solid surface is an important phenomenon that has various applications. Especially when the drop splashes, it can cause deterioration of printing and paint qualities, erosion, and propagation of airborne virus, among others. Therefore, it is important to observe and understand the characteristics of the splashing drops of different liquids.
However, the multiphase nature of the phenomenon causes difficulties for observation when it is performed merely using the naked eye. Although the recent development of artificial intelligence (AI) has shown promise in tackling this problem, AI models usually function as a black box, where the underlying decision-making process is unknown.
At the Tokyo University of Agriculture and Technology, a research team from the Department of Mechanical Systems Engineering has developed an explainable AI to observe and understand the splashing drops of different liquids from an AI perspective.
The research team led by Prof. Yoshiyuki Tagawa and Prof. Akinori Yamanaka, which includes Jingzu Yee (former assistant professor), Pradipto (former assistant professor), Shunsuke Kumagai (1st-year master’s student), and Daichi Igarashi (former master’s student), published their findings in Flow on December 20, 2024.
The research team adopted the architecture of a feedforward neural network to develop an AI model to classify videos of splashing and non-splashing drops that were recorded using a high-speed camera.
After training, the AI model has successfully classified videos of splashing and non-splashing drops with a success rate of 92% for low-viscosity liquid and 100% for high-viscosity liquid. Then, the researchers implemented their proposed method of visualizing the AI to analyze and interpret the classification process.
Their results show that the AI classifies splashing and non-splashing drops from the contour of the drop’s main body, the ejected droplets, and the thin sheet ejected from the side of the drop called lamella. Moreover, the proposed visualization method successfully determined which frame of the video has the most influence on the classification of the AI.
The results show that the differences between splashing and non-splashing drops of low-viscosity liquids are more obvious during the earlier stage of the impact, while it is more obvious during the later stage of the impact for high-viscosity liquids.
“Our newly proposed explainable AI method provides an alternative to the conventional investigation methods for drop impact research,” said Jingzu Yee, a former assistant professor at the Tokyo University of Agriculture and Technology.
“Our method reveals the fundamental aspects of drop impact, which can be leveraged to enable various devices and systems that will benefit humankind.”
More information: Jingzu Yee et al, Morphological evolution of splashing drop revealed by interpretation of explainable artificial intelligence, Flow (2024). DOI: 10.1017/flo.2024.28
Scientists from the National University of Singapore (NUS) have developed a highly effective and general molecular design that enables an enhancement in radioluminescence within organometallic scintillators by more than three orders of magnitude. This enhancement harnesses X-ray-induced triplet exciton recycling within lanthanide metal complexes.
Detection of ionizing radiation is crucial in diverse fields, such as medical radiography, environmental monitoring and astronomy. As a result, significant efforts have been dedicated to the development of luminescent materials that respond to X-rays.
However, current high-performance scintillators are almost exclusively limited to ceramic and perovskite materials, which face issues such as complex manufacturing processes, environmental toxicity, self-absorption and stability problems.
Organic phosphors present a promising alternative owing to their flexibility and cost-effectiveness. However, they are less efficient in X-ray detection because of weak X-ray absorption and limited use of molecular triplet excitons.
While halogen-doped organic phosphors and thermally activated delayed fluorescence molecules show potential, they require precise structural engineering and face absorption and reabsorption challenges, limiting their efficiency.
A research team led by Professor Liu Xiaogang from the Department of Chemistry at NUS, leveraged rare-earth X–ray absorption and ligand-mediated triplet exciton harvesting to overcome these challenges and significantly improved the performance of molecular scintillators.
The effective trapping of the energy dissipated during secondary X-ray relaxation via organic ligands led to a remarkable 1,300-fold increase in radioluminescence compared to lanthanide salts.
The study unveiled the role of triplet exciton recycling in determining scintillation efficiency, demonstrating that high photoluminescence quantum yield may not necessarily result in high scintillation efficiency.
The research was conducted in collaboration with Professor Yiming Wu from Xiamen University, China and Professor Xian Qin from Fujian Normal University, China.
The findings were published in the journal Nature Photonics.
Significantly, these organolanthanide compounds exhibit robust resistance to high-energy radiation and show scintillation efficiencies that surpassed those of well-known organic scintillators and inorganic LYSO:Ce crystals. Their performance was also comparable to those of CsI:Tl crystals.
By tailoring the metal centers and their coordination ligands, the researchers demonstrate the ability to achieve full-spectral X-ray scintillation from the ultraviolet to near-infrared range. Additionally, their methodology enables the fine-tuning of emission lifetimes, ranging from 50 nanoseconds to 900 microseconds.
These organolanthanide scintillators exhibit substantial Stokes shifts and offer the advantage of synthesis and processing at room temperature in solution form. Additionally, they demonstrate excellent solubility, stability, and flexibility, allowing molecular-level mixing for high-resolution radiographic imaging and potential applications in X-ray-mediated deep-tissue radiotherapy.
Prof Liu said, “The efficiency of triplet exciton recycling holds the key to better scintillation performance. These discoveries lend profound insights into X-ray-induced exciton migration dynamics and radioluminescence behavior, shaping the future of organic scintillators and their harnessing of high-energy X-ray quanta.
“The high stability of radioluminescence, large Stokes shift and full spectral tunability make organolanthanide molecules a promising platform for scintillation applications.”
More information: Jiahui Xu et al, Ultrabright molecular scintillators enabled by lanthanide-assisted near-unity triplet exciton recycling, Nature Photonics (2024). DOI: 10.1038/s41566-024-01586-w
A research group led by Prof. Yao Baoli and Dr. Xu Xiaohao from Xi’an Institute of Optics and Precision Mechanics (XIOPM) of the Chinese Academy of Sciences have revealed a full-gray optical trap in structured light, which is able to capture nanoparticles but appears at the region where the intensity is neither maximized nor minimized. The study is published in Physical Review A.
The optical trap is one of the greatest findings in optics and photonics. Since the pioneering work by Arthur Ashkin in the 1970s, the optical trap has been employed in a broad range of applications in life sciences, physics, and engineering. Akin to its thermal and acoustic counterparts, this trap is typically either bright or dark, located at the field intensity maxima or minima.
In this study, researchers developed a high-order multipole model for gradient forces based on multipole expansion theory. Through immersing the Si particles in the structured light with a petal-shaped field, they found that the high-order multipole gradient forces can trap Si particles at the optical intensity, which is neither maximized nor minimized.
Therefore, researchers demonstrated that there may exist an intermediate trapping state, which is referred to as the full-gray optical trapping. The origin of this novel trap can be traced back to the nonlocal pondermotive effect of the optical intensity gradient, which is achieved through the excitation of higher-order multipole Mie resonances in nanoparticles.
The full-gray trap underscores the impact of Mie responses on optomechanics, and will facilitate the development of nanoparticle cooling, patterning and ultra-sensitive sorting in the future.
More information: Yanan Zhang et al, Full-gray optical trapping by high-order multipole-resonant gradient forces in structured light, Physical Review A (2024). DOI: 10.1103/PhysRevA.110.063517
Experimental setup to couple MWs to N- 𝑉s using grape dimers. A stripped optical fiber with N- 𝑉 spins, cantilevered from a rod, lies between two grapes. The grapes were positioned on a platform with a vertical straight copper wire, equidistant from each grape. Credit: Fawaz, Nair, Volz
Macquarie University researchers have demonstrated how ordinary supermarket grapes can enhance the performance of quantum sensors, potentially leading to more efficient quantum technologies.
The study, published in Physical Review Applied on 20 December 2024, shows that pairs of grapes can create strong localized magnetic field hotspots of microwaves which are used in quantum sensing applications—a finding that could help develop more compact and cost-effective quantum devices.
“While previous studies looked at the electrical fields causing the plasma effect, we showed that grape pairs can also enhance magnetic fields, which are crucial for quantum sensing applications,” says lead author Ali Fawaz, a quantum physics Ph.D. candidate at Macquarie University.
The research builds on viral social media videos showing grapes creating plasma—glowing balls of electrically charged particles—in microwave ovens.
While previous studies focused on electric fields, the Macquarie team examined magnetic field effects crucial for quantum applications.
The team used specialized nano-diamonds containing nitrogen-vacancy centers—atomic-scale defects that act as quantum sensors. These defects (one of the many defects giving diamonds their color) behave like tiny magnets and can detect magnetic fields.
“Pure diamonds are colorless, but when certain atoms replace the carbon atoms, they can form so-called ‘defect’ centers with optical properties,” says study co-author Dr. Sarath Raman Nair, who is a lecturer in quantum technology at Macquarie University.
“The nitrogen-vacancy centers in the nanodiamonds we used in this study act like tiny magnets that we can use for quantum sensing.”
The team placed their quantum sensor—a diamond containing special atoms—on the tip of a thin glass fiber and positioned it between two grapes. By shining green laser light through the fiber, they could make these atoms glow red. The brightness of this red glow revealed the strength of the microwave field around the grapes.
“Using this technique, we found the magnetic field of the microwave radiation becomes twice as strong when we add the grapes,” says Fawaz.
Senior author Professor Thomas Volz, who heads the Quantum Materials and Applications Group at Macquarie’s School of Mathematical and Physical Sciences, says the findings unlock exciting possibilities for quantum technology miniaturization.
“This research opens up another avenue for exploring alternative microwave resonator designs for quantum technologies, potentially leading to more compact and efficient quantum sensing devices,” he says.
The size and shape of the grapes proved crucial to the experiment’s success. The team’s experiments relied on precisely sized grapes—each approximately 27 millimeters long—to concentrate microwave energy at approximately the right frequency of the diamond quantum sensor.
Quantum sensing devices traditionally use sapphire for this purpose. However, the Macquarie team theorized that water might work even better. This made grapes, which are mostly water enclosed in a thin skin, perfect for testing their theory.
“Water is actually better than sapphire at concentrating microwave energy, but it’s also less stable and loses more energy in the process. That’s our key challenge to solve,” says Fawaz.
Looking beyond grapes, the researchers are now developing more reliable materials that could harness water’s unique properties, bringing us closer to more efficient sensing devices.
More information: Ali Fawaz et al, Coupling nitrogen-vacancy center spins in diamond to a grape dimer, Physical Review Applied (2024). DOI: 10.1103/PhysRevApplied.22.064078
Bright, twisted light can be produced with technology similar to an Edison light bulb, researchers at the University of Michigan have shown. The finding adds nuance to fundamental physics while offering a new avenue for robotic vision systems and other applications for light that traces out a helix in space.
“It’s hard to generate enough brightness when producing twisted light with traditional ways like electron or photon luminescence,” said Jun Lu, an adjunct research investigator in chemical engineering at U-M and first author of the study on the cover of this week’s Science.
“We gradually noticed that we actually have a very old way to generate these photons—not relying on photon and electron excitations, but like the bulb Edison developed.”
Every object with any heat to it, including yourself, is constantly sending out photons (particles of light) in a spectrum tied to its temperature. When the object is the same temperature as its surroundings, it is also absorbing an equivalent amount of photons—this is idealized as “blackbody radiation” because the color black absorbs all photon frequencies.
While a tungsten lightbulb’s filament is much warmer than its surroundings, the law defining blackbody radiation—Planck’s law—offers a good approximation of the spectrum of photons it sends out. All together, the visible photons look like white light, but when you pass the light through a prism, you can see the rainbow of different photons within it.
This radiation is also why you show up brightly in a thermal image, but even room-temperature objects are constantly emitting and receiving blackbody photons, making them dimly visible as well.
Behind the bulb, a screen displays the temperature of the glowing filament. The wavelengths of light emitted by the filament depend on its temperature, and how well the filament twirls the light depends on how close the wavelengths are to the pitch of the filament’s twists. Credit: Brenda Ahearn/Michigan Engineering
Typically, the shape of the object emitting the radiation doesn’t get much consideration—for most purposes (as so often in physics), the object can be imagined as a sphere. But while shape doesn’t affect the spectrum of wavelengths of the different photons, it can affect a different property: their polarization.
Usually, photons from a blackbody source are randomly polarized—their waves may oscillate along any axis. The new study revealed that if the emitter was twisted at the micro or nanoscale, with the length of each twist similar to the wavelength of the emitted light, the blackbody radiation would be twisted too. The strength of the twisting in the light, or its elliptical polarization, depended on two main factors: how close the wavelength of the photon was to the length of each twist and the electronic properties of the material—nanocarbon or metal, in this case.
Twisted light is also called “chiral” because the clockwise and counterclockwise rotations are mirror images of one another. The study was undertaken to demonstrate the premise of a more applied project that the Michigan team would like to pursue: using chiral blackbody radiation to identify objects. They envision robots and self-driving cars that can see like mantis shrimp, differentiating among light waves with different directions of twirl and degrees of twistedness.
“The advancements in physics of blackbody radiation by chiral nanostructures is central to this study. Such emitters are everywhere around us,” said Nicholas Kotov, the Irving Langmuir Distinguished Professor of Chemical Sciences and Engineering, director of NSF Center of Complex Particles and Particle Systems (COMPASS) and corresponding author of the study.
“These findings, for example, could be important for an autonomous vehicle to tell the difference between a deer and a human, which emit light with similar wavelengths but different helicity because deer fur has a different curl from our fabric.”
Jun Lu examines the twisted filament glowing within the bulb. He, along with a team of U-M researchers, demonstrated for the first time that a twisted filament could produce twirling light waves. Credit: Brenda Ahearn/Michigan Engineering.
While brightness is the main advantage of this method for producing twisted light—up to 100 times brighter than other approaches—the light includes a broad spectrum of both wavelengths and twists. The team has ideas about how to address this, including exploring the possibility of building a laser that relies on twisted light-emitting structures.
Kotov also wants to explore further into the infrared spectrum. The peak wavelength of blackbody radiation at room temperature is roughly 10,000 nanometers or 0.01 millimeters.
“This is an area of the spectrum with a lot of noise, but it may be possible to enhance contrast through their elliptical polarization,” Kotov said.
Kotov is also the Joseph B. and Florence V. Cejka Professor of Engineering, a professor of macromolecular science and engineering and a member of U-M’s Biointerfaces Institute. Lu is an incoming assistant professor of chemistry and physics at the National University of Singapore.
The device was built in the COMPASS Lab located at the North Campus Research Complex of U-M and studied at the Michigan Center for Materials Characterization.
More information: Jun Lu et al, Bright, circularly polarized black-body radiation from twisted nanocarbon filaments, Science (2024). DOI: 10.1126/science.adq4068
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.”
