Sketch of shear banding (top left) and vorticity banding (top right) as proposed by [18]. For shear banding, the rheological curve 𝜏 (𝛾˙) is single-valued but non-monotonic (bottom left). For vortex banding it is the 𝛾˙(𝜏) curve which is single valued and non-monotonic (bottom right). Credit: The European Physical Journal E (2024). DOI: 10.1140/epje/s10189-024-00444-5
Water and oil, and some other simple fluids, respond in the same way to all levels of shear stress. These are termed Newtonian fluids, and their viscosity is constant for all stresses although it will vary with temperature. Under different stresses and pressure gradients, other non-Newtonian fluids exhibit patterns of behavior that are much more complex.
Researchers Laurent Talon and Dominique Salin from Université Paris-Sacly, Paris, France have now shown that under certain circumstances, cornstarch suspensions can display a banding pattern with alternating regions of high and low viscosity. This work has been published in The European Physical Journal E.
Non-Newtonian fluids may exhibit shear thinning, where the viscosity decreases with stress; common examples include ketchups and sauces that can appear almost solid-like at rest. The reverse is shear thickening, in which viscosity increases with stress. Some suspensions exhibit a property called discontinuous shear thickening (DST).
“At low shear stress [these fluids] behave like Newtonian fluids, but at a certain stress value the viscosity increases very steeply,” explains Talon.
In 2014, Matthew Wyart of New York University, NY, U.S., and Michael Cates of the University of Edinburgh, Scotland, proposed a similar but even more counter-intuitive and interesting model: a so-called “S-shaped” rheology where the viscosity of a fluid first increases with increasing stress and then decreases.
Talon and Salin set out to investigate the plausibility of this simulated rheology using a suspension of cornstarch in a straight, cylindrical capillary tube. They observed the expected non-monotonic relationship between pressure and flow rate, but not exactly as predicted: the flow rate initially increased with pressure but then suddenly decreased.
“Assuming that the Wyart-Cates model is essentially correct, one solution that would match what we observed could be a ‘rheological segregation’ or ‘streamwise banding’ in the tube, in which some regions have a high viscosity and others a lower one,” explains Talon. “We are continuing to investigate the validity of this proposal, both experimentally and using numerical simulations.”
More information: L. Talon et al, On pressure-driven Poiseuille flow with non-monotonic rheology, The European Physical Journal E (2024). DOI: 10.1140/epje/s10189-024-00444-5
The unidirectional diffractive processor transmits high-quality images in the forward propagation direction, represented with the blue line, from field of view (FOV) A to FOV B, while effectively blocking the image formation in the backward propagation direction, represented with the brown line, from FOV B to FOV A. Credit: Ozcan Lab, UCLA
A team of researchers from the University of California, Los Angeles (UCLA) has unveiled a new development in optical imaging technology that could significantly enhance visual information processing and communication systems.
The work is published in the journal Advanced Photonics Nexus.
The new system, based on partially coherent unidirectional imaging, offers a compact, efficient solution for transmitting visual data in one direction while blocking transmission in the opposite direction.
This innovative technology, led by Professor Aydogan Ozcan and his interdisciplinary team, is designed to selectively transmit high-quality images in one direction, from field-of-view A to field-of-view B, while deliberately distorting images when viewed from the reverse direction, B to A.
This asymmetric image transmission could have broad implications for fields like privacy protection, augmented reality, and optical communications, offering new capabilities for managing how visual optical information is processed and transmitted.
Unidirectional imaging under partially coherent light
The new system addresses a challenge in optical engineering: how to control light transmission to enable clear imaging in one direction while blocking it in the reverse.
Previous solutions for unidirectional wave transmission have often relied on complex methods such as temporal modulation, nonlinear materials, or high-power beams under fully coherent illumination, which limit their practical applications.
In contrast, this UCLA innovation leverages partially coherent light to achieve high image quality and power efficiency in the forward direction (A to B), while intentionally introducing distortion and reduced power efficiency in the reverse direction (B to A).
“We engineered a set of spatially optimized diffractive layers that interact with partially coherent light in a way that promotes this asymmetric transmission,” explains Dr. Ozcan. “This system can work efficiently with common illumination sources like LEDs, making it adaptable for a variety of practical applications.”
Conceptual illustration of the technology. Credit: UCLA Engineering Institute for Technology Advancement
Leveraging deep learning for enhanced optical design
A key aspect of this development is the use of deep learning to physically design the diffractive layers that make up the unidirectional imaging system. The UCLA team optimized these layers for partially coherent light with a phase correlation length greater than 1.5 times the wavelength of the light.
This careful optimization ensures that the system provides reliable unidirectional image transmission, even when the light source has varying coherence properties. Each imager is compact, axially spanning less than 75 wavelengths, and features a polarization-independent design.
The deep learning algorithms used in the design process help ensure that the system maintains high diffraction efficiency in the forward direction while suppressing image formation in the reverse.
The researchers demonstrated that their system performs consistently across different image datasets and illumination conditions, showing resilience to changes in the light’s coherence properties. “The ability of our system to generalize across different types of input images and light properties is one of its exciting features,” says Dr. Ozcan.
Looking ahead, the researchers plan to extend the unidirectional imager to different parts of the spectrum, including infrared and visible ranges, and to explore various kinds of illumination sources.
These advancements could push the boundaries of imaging and sensing, unlocking new applications and innovations. In privacy protection, for example, the technology could be used to prevent sensitive information from being visible from unintended perspectives. Similarly, augmented and virtual reality systems could use this technology to control how information is displayed to different viewers.
“This technology has the potential to impact multiple fields where controlling the flow of visual information is critical,” adds Dr. Ozcan. “Its compact design and compatibility with widely available light sources make it especially promising for integration into existing systems.”
This research was conducted by an interdisciplinary team from UCLA’s Department of Electrical and Computer Engineering and California NanoSystems Institute (CNSI).
More information: Guangdong Ma et al, Unidirectional imaging with partially coherent light, Advanced Photonics Nexus (2024). DOI: 10.1117/1.APN.3.6.066008
Schematic drawing of the experimental setup. Credit: arXiv (2024). DOI: 10.48550/arxiv.2410.11367
A team of engineers and physicists affiliated with a host of institutions across Japan, working at the Japan Proton Accelerator Research Complex, has demonstrated acceleration of positive muons from thermal energy to 100 keV—the first time muons have been accelerated in a stable way. The group has published a paper describing their work on the arXiv preprint server.
Muons are sub-atomic particles similar to electrons. The main difference is their mass; a muon is 200 times heavier than an electron. They are also much shorter lived. Physicists have for many years wanted to build a muon collider to conduct new types of physics research, such as experiments that go beyond the standard model.
Unfortunately, such efforts have been held back by the extremely short muon lifespan—approximately 2 microseconds—after which they decay to electrons and neutrinos. Making things even more difficult is their tendency to zip around haphazardly, which makes forming them into a single beam extremely challenging. In this new effort, the research team has overcome such obstacles using a new technique.
The team started by shooting positively charged muons into a specially designed silica-based aerogel, similar to that used for thermal insulation applications. As the muons struck the electrons in the aerogel, muoniums μ+e− (an exotic atom consisting of a positive muon and an electron) were formed. The research team then fired a laser at them to remove their electrons, which forced them to revert back to positive muons, but with greatly diminished speed.
The following step involved guiding the slowed muons into a radio-frequency cavity, where an electric field accelerated them to a final energy of 100 keV, achieving approximately 4% of the speed of light.
The research team acknowledges that despite their achievement, building a working muon collider is still a distant goal. And while their technique might play a role in such a development, there are still problems that must be worked out, such as how to scale an apparatus to a usable size.
More information: S. Aritome et al, Acceleration of positive muons by a radio-frequency cavity, arXiv (2024). DOI: 10.48550/arxiv.2410.11367
The BASE-STEP transportable trap system, lifted by crane through the AD hall before being loaded onto a truck. The team monitored all the parameters during transport. Credit: CERN
Antimatter might sound like something out of science fiction, but at the CERN Antiproton Decelerator (AD), scientists produce and trap antiprotons every day. The BASE experiment can even contain them for more than a year—an impressive feat considering that antimatter and matter annihilate upon contact.
The CERN AD hall is the only place in the world where scientists are able to store and study antiprotons. But this is something that scientists working on the BASE experiment hope to change one day with their subproject BASE-STEP: an apparatus designed to store and transport antimatter.
Most recently, the team of scientists and engineers took an important step towards this goal by transporting a cloud of 70 protons in a truck across CERN’s main site.
“If you can do it with protons, it will also work with antiprotons,” said Christian Smorra, the leader of BASE-STEP. “The only difference is that you need a much better vacuum chamber for the antiprotons.”
This is the first time that loose particles have been transported in a reusable trap that scientists can then open in a new location and then transfer the contents into another experiment. The end goal is to create an antiproton-delivery service from CERN to experiments located at other laboratories.
Antimatter is a naturally occurring class of particles that is almost identical to ordinary matter except that the charges and magnetic properties are reversed. This has baffled scientists for decades, because according to the laws of physics, the Big Bang should have produced equal amounts of matter and antimatter. These equal-but-opposite particles would have quickly annihilated each other, leaving a simmering but empty universe. Physicists suspect that there are hidden differences that can explain why matter survived and antimatter all but disappeared.
The BASE experiment aims to answer this question by precisely measuring the properties of antiprotons, such as their intrinsic magnetic moment, and then comparing these measurements with those taken with protons. However, the precision the experiment can achieve is limited by its location.
“The accelerator equipment in the AD hall generates magnetic field fluctuations that limit how far we can push our precision measurements,” said BASE spokesperson Stefan Ulmer. “If we want to get an even deeper understanding of the fundamental properties of antiprotons, we need to move out.”
This is where BASE-STEP comes in. The goal is to trap antiprotons and then transfer them to a facility where scientists can study them with a greater precision. To be able to do this, they need a device that is small enough to be loaded onto a truck and can resist the bumps and vibrations that are inevitable during ground transport.
The transportable trap being carefully loaded in the truck before going for a road trip across CERN’s main site. Credit: CERN
The current apparatus—which includes a superconducting magnet, cryogenic cooling, power reserves, and a vacuum chamber that traps the particles using magnetic and electric fields—weighs 1,000 kilograms and needs two cranes to be lifted out of the experimental hall and onto the truck. Even though it weighs a ton, BASE-STEP is much more compact than any existing system used to study antimatter. For example, it has a footprint that is five times smaller than the original BASE experiment, as it must be narrow enough to fit through ordinary laboratory doors.
During the rehearsal, the scientists used trapped protons as a stand-in for antiprotons. Protons are a key ingredient of every atom, the simplest of which is hydrogen (one proton and one electron.) But storing protons as loose particles and then moving them onto a truck is a challenge because any tiny disturbance will draw the unbonded protons back into an atomic nucleus.
“When it’s transported by road, our trap system is exposed to acceleration and vibrations, and laboratory experiments are usually not designed for this,” Smorra said. “We needed to build a trap system that is robust enough to withstand these forces, and we have now put this to a real test for the first time.”
However, Smorra noted that the biggest potential hurdle isn’t currently the bumpiness of the road but traffic jams.
“If the transport takes too long, we will run out of helium at some point,” he said. Liquid helium keeps the trap’s superconducting magnet at a temperature below 8.2 Kelvin: its maximum operating temperature. If the drive takes too long, the magnetic field will be lost and the trapped particles will be released and vanish as soon as they touch ordinary matter.
“Eventually, we want to be able to transport antimatter to our dedicated precision laboratories at the Heinrich Heine University in Düsseldorf, which will allow us to study antimatter with at least 100-fold improved precision,” Smorra said. “In the longer term, we want to transport it to any laboratory in Europe. This means that we need to have a power generator on the truck. We are currently investigating this possibility.”
After this successful test, which included ample monitoring and data-taking, the team plans to refine its procedure with the goal of transporting antimatter next year.
“This is a totally new technology that will open the door for new possibilities of study, not only with antiprotons but also with other exotic particles, such as ultra-highly-charged ions,” Ulmer said.
Another experiment, PUMA, is preparing a transportable trap. Next year, it plans to transport antiprotons 600 meters from the ADH hall to CERN’s ISOLDE facility in order to use them to study the properties and structure of exotic atomic nuclei.
The fully assembled prototype high-beta 650-megahertz cryomodule. Four of these will make up the final stage in Fermilab’s new linear accelerator. Credit: Saravan Chandrasekaran, Fermilab
Patience and complexity are the hallmarks of fundamental scientific research. Work at the Department of Energy (DOE) Office of Science takes time.
Case in point: Technical staff at the DOE’s Fermi National Accelerator Laboratory have built a prototype of a superconducting cryomodule for the Proton Improvement Plan II (PIP-II) project.
Four of these 39-foot-long vessels, which weigh an astonishing 27,500 pounds each, will be responsible for accelerating hydrogen ions to more than 80% of the speed of light. Ultimately, the cryomodules will comprise the last section of the new linear accelerator, or linac, that will drive Fermilab’s accelerator complex.
Physicists like to accelerate particles to higher and higher energies. The higher the energy, the more finely penetrating and discriminating a particle probe can be. That increased precision allows scientists to study the tiniest of structures.
There are many benefits of faster and faster accelerators. To name a few: destroying cancer cells; revealing the structure of proteins and viruses; creating vaccines and new drugs; and advancing our knowledge of the origins of our universe.
For the PIP-II linac, each superconducting cryomodule vessel will contain a chain of devices called “cavities” at its core. These cavities look like oversized soda cans stacked end-to-end. They’re made of pure niobium, a superconducting material. Electricity flows through the superconducting material with no energy loss when the niobium is kept well below the average temperature of outer space.
Note the snippet “cryo” in the word cryomodule, meaning involving or producing cold. Especially extreme cold. In order to reach superconducting state, the cavities need to be kept at super-cold temperatures, hovering around absolute zero.
To keep things cool, the team fills the inside of the vessel with liquid helium. The vessel has many layers of insulation to protect the cavities from outside temperatures that are too warm.
Once the prototype is functioning properly, four of the modules will be assembled to build out the last section of Fermilab’s new linear accelerator.
Here’s how the journey will unfold. The superconducting cryomodules will power beams of hydrogen anions, which are hydrogen atoms made up of one proton and two electrons, instead of the usual one proton and one electron.
The beams will reach a final energy of 800 million electronvolts, or MeV, before they exit the accelerator.
From there, the beam will transfer to the upgraded Booster and Main Injector accelerators. There it will gain more energy before being turned into neutrinos.
The machine will then send these neutrinos on a 1,300-kilometer journey (800 miles) through Earth to the Deep Underground Neutrino Experiment (DUNE) at the Long Baseline Neutrino Facility in Lead, South Dakota.
The team is now making sure that all the preparations have paid off as the modules are tested at Fermilab’s Cryomodule Test Facility. This will reveal how well the modules function after practice shipments between Fermilab and the United Kingdom.
The final modules will be built by PIP-II’s partners around the world. Three will come together at Daresbury Laboratory, run by the Science and Technology Facilities Council of United Kingdom Research and Innovation, and shipped to Fermilab.
The fourth will be assembled at Fermilab using components provided by the Raja Rammana Center for Advanced Technology of India’s Department of Atomic Energy.
International partners from India, Italy, France, Poland and the United Kingdom are contributing to many aspects of the PIP-II project.
All of this work is done as part of the PIP-II project, an essential enhancement to the Fermilab accelerator complex. PIP-II will provide neutrinos for DUNE scientists to study.
In parallel, the high-power proton beams delivered by the PIP-II accelerator will enable muon-based experiments to search for new particles and forces at unprecedented levels of precision. The diverse physics program is powering new discoveries for decades to come.
Picture of a trapped-ion quantum computer on which the experiment was conducted. Credit: IonQ
In the context of quantum mechanics and information, “magic” is a key property of quantum states that describes the extent to which they deviate from so-called stabilizer states. Stabilizer states are a class of states that can be effectively simulated on classical computers.
Magic in quantum states is crucial to the realization of universal and fault-tolerant quantum computing via simple gate operations. Gaining insight about the mechanisms behind this property could help engineers to effectively create it and leverage it, thus potentially enabling the development of better performing quantum computers.
Researchers at University of Maryland and NIST, IonQ Inc. and the Duke Quantum Center recently showed that a random stabilizer code (i.e., a code designed to protect quantum information from errors) presents vastly different behavior with regards to magic when exposed to coherent errors.
Their observations, outlined in a paper published in Nature Physics, could broaden the understanding of how magic states originate, which could facilitate the generation of these states in quantum computing systems.
“Even though superposition and entanglement are the terms people most often associate with quantum computers, it turns out they aren’t enough to make quantum computers more powerful than classical computers,” Pradeep Niroula, co-author of the paper, told Phys.org.
“To attain a quantum advantage over traditional or classical computers, you need one another ingredient called ‘magic’ or ‘non-stabilizer-ness.’ If your quantum system has no ‘magic,’ it can be simulated by a classical computer, making the quantum computer unnecessary. It is only when your system has a lot of magic that you go beyond what’s possible with a classical computer.”
For error-resistant quantum computers, creating superpositions or entanglement between states is relatively easy. In contrast, adding magic to the state or dislocating them further from easy-to-simulate stabilizer states is expected to be highly challenging.
“In the literature of quantum information, you often encounter terms like ‘magic state distillation’ or ‘magic state cultivation,’ which refer to pretty arduous processes to create special quantum states with magic that the quantum computer can make use of,” said Niroula.
“Prior to this paper, we had written a paper that observed a similar phase transition in entanglement, in which we had observed phases where measurements of a quantum system preserved or destroyed entanglement depending on how frequent they are.”
While there is an extensive amount of literature focusing on the realization of entanglement in error-corrected quantum computing systems, the underpinnings of magic states remain less understood.
The main goal of the recent study by Niroula and his colleagues was to determine whether a similar phase transition as that previously observed for entanglement also exists for magic. The existence of such a transition may hint at the existence of a more general theory that is applicable to different quantum properties, including both entanglement and magic.
A) The circuit model for used in the study. Coherent error is used to tune magic on a random stabilizer code. B) A schematic illustration of how magic is created and destroyed from the circuit. The coherent errors dislocate a quantum state away from stabilizer states which are easy to represent and simulate. The final measurements sometimes destroy the injected magic, revert the states back to stabilizer states, and sometimes leave the magic intact. C) The phase diagram of magic. Credit: Niroula et al.
“A general feature of such phase transitions is that it involves two competing forces or processes,” explained Niroula. “One of these creates the resource and one which destroys it—tuning the relative strength or proportion of those processes seems to reveal such transitions.
“In the case of entanglement, a quantum gate acting between two qubits tends to produce entanglement between them, whereas a measurement of one of those qubits tends to destroy the entanglement. Now if you had a quantum circuit with many gates, you can randomly add measurements in the circuit and control the spread of entanglement in the system.”
Past studies focusing on entanglement in quantum circuits have established that if there are too few measurements in a quantum circuit, the entire quantum system becomes entangled. In contrast, if there are too many measurements, entanglement is suppressed and thus minimal. Moreover, if one gradually increases the density of measurements in a system, the entanglement will rapidly shift from high to almost null.
“Measurements destroy magic too, but to be able to controllably add magic to the system, you need to be able to do small rotations of the qubit,” said Niroula. “So, the two competing forces here are ‘how much you measure’ and ‘how much you rotate the qubits.’ What we observed is that at a fixed rate of measurement, you can tune your rotation angle and go from a phase where you have a lot of magic to a phase where you have no magic.”
As part of their study, Niroula and his colleagues first ran a series of numerical simulations, which offered a strong indication that a phase transition in magic did in fact take place. Encouraged by these findings, they then set out to test their hypothesis in an experimental setting, using real quantum circuits.
“In our experiment, we observed the signature of the phase transition even in a noisy machine,” said Niroula “Our work thus uncovered a phase transition in magic.
“Earlier works have uncovered other kinds of transitions in entanglement and in charges etc. and this raises the questions: what other resources might exhibit similar transitions? Do they all belong to some universal type of transition? Are they all distinct or are they all related somehow? Also importantly, what does the presence of phase transition teach us about building noise-resilient quantum computers?”
The findings gathered by this team of researchers open new avenues for research focusing on resources in error-corrected quantum computing systems. Future studies could, for instance, explore other properties and resources that exhibit a phase transition resembling those observed for entanglement and magic.
“Magic states are important for error-correction,” added Niroula. “Our work gives us some insights on when we can concentrate magic and when we can suppress it. One avenue that would be interesting to explore is to see if we can use our experiment as a ‘magic state factory’ where you are producing good magic states for consumption by the quantum computer.
“Currently, there is a lot of interest in the field in demonstrating the primitives or the building blocks of error-correction, and our work could be a part of that.”
More information: Pradeep Niroula et al, Phase transition in magic with random quantum circuits, Nature Physics (2024). DOI: 10.1038/s41567-024-02637-3.
Researchers modeled the trail of nicotine particles that are released from the mouth, nose, and cigarette. Credit: Younes Bakhshan
While many smoking rooms in U.S. airports have closed in recent years, they are still common in other airports around the world. These lounges can be ventilated, but how much does it actually help the dispersion of smoke?
Research published in Physics of Fluids shows that not all standing positions in airport smoking lounges are created equal.
Researchers from the University of Hormozgan in Iran studied nicotine particles in a simulated airport smoking room and found that the thermal environment and positioning of smokers influenced how particles settle in the room.
Additionally, smokers seated farther from ventilation inlets experienced the lowest levels of pollution in the room.
“We expected that people who are standing in the corners would report the same amount of particles settling on their body,” author Younes Bakhshan said. “But according to the numbers that we determined, the wave created by the ventilation in the room is not the same every time.”
The researchers created a smoking room using computational models and placed heated and unheated manikins in the room to simulate smokers. They also modeled the ventilation system with three exhaust air diffusers.
The manikin smokers “exhaled” cigarette smoke through their mouths and noses, and the flow of the particles was modeled and observed. They found that over time, as the concentration of particles decreases in the air, the particles settling on the smokers increases.
“According to the results, body heat causes more absorption of cigarette pollution,” Bakhshan said. “We suggest that if people have to smoke in the room, empty places are the best to choose.”
The results gave insight into improving ventilation in smoking lounges.
“According to previous research, displacement ventilation system is the best for a smoking room,” Bakhshan said. “But if we want to optimize the HVAC system, we suggest that the exhaust should be installed on the wall in addition to the vents placed on the ceiling.”
Next, the researchers want to take a step beyond measuring particle dispersion to particle reduction.
“We believe that smokers who go into the smoking room for the sake of others’ health should be also protected from the harmful effects of secondhand smoke,” Bakhshan said.
More information: Numerical simulation of particles distribution of environmental tobacco smoke and its concentration in the smoking room of Shiraz airport, Physics of Fluids (2024). DOI: 10.1063/5.0223568
Since the first X-ray image of a comet was reported using an X-ray telescope in 1996, the investigation of charge exchange in collisions between highly charged ions and atoms or molecules has emerged as a hot research topic.
Astrophysicists require more atomic data to model observed X-ray spectra. Traditionally, the charge exchange is assumed to follow statistical rules regarding the total spin quantum number. These assumptions of pure spin statistics are of fundamental importance across various fields.
However, a new study published in Physical Review Letters on October 22 has challenged the assumptions by providing direct evidence of the breakdown of spin statistics in ion-atom charge exchange collisions. This study was led by scientists from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS).
The experiment was performed at the low-energy setups of the Heavy Ion Research Facility in Lanzhou, employing the high-resolution reaction microscope, which is characterized by high precision, sensitivity and detection efficiency. The neutral helium was used as a target in collisions with C3+ions in the experiment.
“The C3+ ion is a good candidate for this study because it has no long-lived excited states and is always in its ground state in the collision region. Using the reaction microscope, we can easily determine the atomic states at the moment of electron captured in collisions, overcoming the difficulties encountered in previous experiments. Thus, it is relatively easier to accurately analyze the underlying mechanisms,” said Prof. Zhu Xiaolong from IMP, the first author of this study.
Through experimental and theoretical approaches, scientists directly measured spin-resolved cross section ratios, as a probe of spin statistics, which demonstrated the breakdown of spin statistics assumptions at high impact energies where they are traditionally expected to be valid.
“The novel finding raises intriguing questions both in understanding the electronic dynamics during such fast collisional processes and in exploring quantum manipulation of atomic and molecular reactivity,” said Prof. Ma Xinwen from IMP, one of the corresponding authors of this study.
More information: XiaoLong Zhu et al, Direct Evidence of Breakdown of Spin Statistics in Ion-Atom Charge Exchange Collisions, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.173002
Active droplet electrotaxis in a microchannel, with an increasing electric field. Credit: Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.158301
In a new study in Physical Review Letters, scientists have demonstrated a method to control artificial microswimmers using electric fields and fluid flow. These microscopic droplets could pave the way for targeted drug delivery and microrobotics.
In the natural world, biological swimmers, like algae and bacteria, can change their direction of movement (or swimming) in response to an external stimulus, like light or electricity. The ability of biological swimmers to change directions in response to electrical fields is known as electrotaxis.
Artificial swimmers that can respond to external stimuli can be extremely helpful for targeted drug delivery applications. In this study, researchers chose to model artificial swimmers that respond to electric fields.
Phys.org spoke to the co-authors of the paper: Ranabir Dey, an Assistant Professor at the Indian Institute of Technology Hyderabad; and Corinna Maaß, an Associate Professor at the University of Twente. Both were formerly at the Max Planck Institute for Dynamics and Self-Organization Göttingen, where the study germinated.
Speaking of their motivation behind the study, Prof. Dey said, “The physics driving active, intrinsic motion is fascinatingly rich and different from the one governing passive, externally driven matter, and we find many complex, even counterintuitive phenomena.”
Prof. Maaß added, “Discovering the working principle behind such effects in a simple model system can help us understand and control far more complicated, even biological systems.”
Artificial swimmers
Artificial swimmers mainly belong to two categories, active colloids (also known as Janus particles) and active droplets. They are called “active” because they move in response to a stimulus.
Janus particles, named after the two-faced Roman god Janus, have two distinct surfaces with different chemical or physical properties. The design allows these surfaces to have an asymmetry for self-propulsion. For example, one side might attract water while the other repels it.
However, Janus particles require specialized materials, external stimuli to move, and asymmetry complications. They can be challenging to study and work with.
Active droplets, on the other hand, are much simpler in structure. They are oil-based droplets suspended in an aqueous solution. They do not require external stimuli to self-propel, instead relying on internal reactions.
External stimuli like electric fields can be used to change their motion, making them very useful in confined environments like microchannels, which are narrow channels often used in lab-on-a-chip devices and microfluidic systems.
Electrotaxis in artificial swimmers is understudied, especially in confined spaces involving flowing fluids (like microchannels). Electrotaxis offers advantages over other taxis, such as the ability to be instantly turned on and off, adjusting the swimmers’ motion for direction and speed, and it can also be scaled to operate over short and long distances.
Biological swimmers respond naturally to electric fields generated by potential differences across cellular boundaries or tissue structure. However, artificial swimmers don’t, and must be engineered to do so.
Active droplets in microchannels
The researchers aimed to study how active droplets respond to external electric fields in confined microchannels.
“Swimmers have to communicate with the world outside their local environment via interactions with the system boundaries. Imagine guiding a swimmer along a channel—one might want to avoid the swimmer crashing into or adhering to the walls, reorienting it in a specific direction, or staying in a specific area,” explained Prof. Maaß.
Prof. Dey added, “This can be engineered for a wide range of swimmers by choosing appropriate values for an externally applied flow and electric field in the channel.”
The researchers used oil droplets containing a compound called CB15 (commonly used for active droplet studies) mixed in with a surfactant. These droplets were placed in microchannels, with electrodes placed at the ends to apply electric fields. The radius of these droplets was roughly 21 micrometers.
Along with the electric field, the researchers could also control the fluid flow, i.e., the pressure for more comprehensive control. The voltage varied up to 30 volts.
To analyze the trajectories of the active droplets, the researchers used video tracking and particle image velocimetry, which can measure the velocities in fluid flows.
Additionally, they developed a hydrodynamic model incorporating the droplet’s surface charge, movement direction, flow interactions, and electric field orientation to predict electrotactic dynamics.
Controlling flow and electric fields
The experiment found that the droplets showed a range of responses to the varying electric field. The researchers observed that the active droplets perform U-turns when the electric field opposes their motion. They also noted that the velocity of the droplets increases with the strength of the electric field.
By controlling the electric field in conjunction with the flow, the researchers could direct the precise motion of the droplets. This is known as electrorheotaxis.
When the electric field opposed the flow of the droplets, their oscillatory motion was reduced, and the researchers were able to achieve stable centerline swimming.
When the electric field aligned with the flow of the droplets, the researchers were able to maintain upstream swimming with modified oscillations. At high voltages, this switched to downstream swimming, following the wall of the microchannel.
The hydrodynamic modeling revealed the reason behind the motion of the droplets in the electric field. They found that these droplets carry an inherent electric charge, which affects their movement when exposed to an electric field.
They further found that the channel walls also played a role in affecting the droplets’ movement, due to their interactions with the surrounding fluid dynamics. The observed data aligned well with the predictions made by the researchers’ hydrodynamic model.
“We demonstrated that tuning two parameters (flow and electric field) gives access to a distinct number of motility states, encompassing upstream oscillation, wall and centerline motion, and motion reversal (U-turns),” said Prof. Dey.
Potential for more
The study demonstrates that simple droplets can mimic complex biological behaviors, making it a very promising avenue for biomedical applications.
Electric field and pressure-driven flow are readily available methods, which makes this application extremely appealing.
Discussing potential applications, Prof. Maaß said, “Since these guidance principles apply to any swimmer with surface charges in a narrow environment, they could be used to guide motile cells in medical applications, lab-on-a-chip or bioreactor scenarios, and in the design of motile carriers, such as microreactors or intelligent sensors.”
More information: Carola M. Buness et al, Electrotaxis of Self-Propelling Artificial Swimmers in Microchannels, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.158301
(a) an illustration of the environment where the countoscope operates, (b) an imaginary two-dimensional box with 54 particles inside, and (c) a plot of the prototype particle number fluctuations over time, from simulations. Credit: Physical Review X (2024). DOI: 10.1103/PhysRevX.14.041016
A team of scientists have invented a new technique to determine the dynamics of microscopic interacting particles by using image recognition to count the number of particles in an imaginary box. By changing the size of the observation box, such counting enables the study of the dynamics of the collective system, even for a dense group of particles suspended in a fluid.
Their work has been published in Physical Review X.
For over a century, scientists of all kinds have sought to exploit counts of particles, such as molecules undergoing Browning motion in a liquid, something scientists in many disciplines would like to know, from biology studying cells to chemists studying molecules to physics.
A useful way to characterize this motion is via the “diffusion constant,” which describes how fast the average particle in the fluid moves. This number can be calculated by following an individual particle as it randomly walks through the fluid. The diffusion constant is then half the proportionality constant between the average displacement and time.
To address this limitation, Sophia Marbach of Sorbonne Université in Paris and her colleagues invented a technique they call the “countoscope.” It uses image recognition software to count the number of particles in an imaginary box in the sample, which can be in the thousands.
The system of particles could be a colloid—particles suspended in a liquid—or cellular organisms, or even artificial. The number of particles in these boxes—finite observation volumes—can change as particles move into or out of the field of view, much like they do in a microscope. The user can select the size of the countoscope box desired in order to study the particles’ dynamics at larger or smaller scales.
But following particle paths and displacements can be difficult, if not impossible, if there are a large number of particles and/or they are indistinguishable.
To address this, the group developed an equation that instead used fluctuating particle counts in the boxes, which can also be used to calculate the diffusion constant and to infer the dynamic properties of the interacting particle suspensions. That constant can then be deduced simply by counting and calculating.
The group tested their technique on a two-dimensional layer of 2.8-micron diameter plastic spheres in a cell filled with water. Using this artificial colloidal system, they choose square boxes with sides from 4- to 32-microns long. The boxes were imaged by a custom-built inverted microscope. Their software then counted, box by box, the number of particles in each box.
With this data they could calculate the mean change in particle number relative to the first box, which they found increased as the square root of time. By this methodology, their value for the diffusion constant matched that obtained from more traditional methods that reconstruct particle trajectories.
When they increased the number of particles in their simulated colloid, particles diffused away from their starting points, as was expected. Their method still worked, but they began to see the formation of temporary bunches of particles, about 10 or so, in their prototype setup. This was something not seen in traditional studies, simply because tracking only a single particle at a time cannot reveal bunches.
While the particles did not interact in their prototype colloid, real world experiments usually cannot be approximated as a noninteracting system. Unlike less dense systems (specified by the “packing fraction” of the spheres), the team found that significant deviations from their mathematical expressions took place at high packing fractions.
This was due to interactions between particles, and they were able to modify their analysis when both hydrodynamic and/or steric factors complicated the system. (Hydrodynamic effects are those induced by the particles’ movement through the fluid, and steric effects arise from the spatial arrangement of the particles.)
In fact, a new length scale appeared in their analysis, characterizing a transition between hyperuniform-like particle behavior and collective states.
The groups believe their methodology can be extended. “We trust our analytical approach can be extended to 3D [three-dimensions], to solids or crystals,” they wrote in their paper.
“We definitely have received interest in use by other scientists,” said Marbach. “It’s such an easy thing to do actually that some colleagues just tried it on their own data and could see similar or different things depending on the system they were investigating.”
She continued, “Many scientists would like to use the framework to investigate very diverse systems beyond colloids: microalgae, bacteria, active colloids, colloidal glasses, molecules, etc.,” she said.
She said there are many directions for future research—to improve the countoscope technique, expand it and generalize it to “include the possibility of probing different dynamical features beyond diffusion. For instance, in microalgae/bacteria/active colloids, we need to know how to resolve active swimming velocities.”
More information: Eleanor K. R. Mackay et al, The Countoscope: Measuring Self and Collective Dynamics without Trajectories, Physical Review X (2024). DOI: 10.1103/PhysRevX.14.041016
