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If you have ever brewed a cup of black tea with hard water you will be familiar with the oily film that can form on the surface of the tea after just a few minutes.
Known as “tea scum” the film consists of calcium carbonate crystals within an organic matrix. Yet it can be easily broken apart with a quick stir of a teaspoon.
They did so by first sprinkling graphite powder into a water tank. Thanks to capillary forces, the particles gradually clump together to form rafts. The researchers then generated waves in the tank that broke apart the rafts and filmed the process with a camera.
Through these experiments and theoretical modelling, they found that the rafts break up when diagonal cracks form at thte raft’s centre. This causes them to fracture into larger chunks before the waves eventually eroded them away.
They found that the polygonal shapes created when the rafts split up is the same as that seen in ice floes.
Despite the visual similarities, however, sea ice and tea scum break up through different physical mechanisms. While ice is brittle, bending and snapping under the weight of crushing waves, the graphite rafts come apart when the viscous stress exerted by the waves overcome the capillary forces that hold the individual particles together.
Buoyed by their findings, the researchers now plan to use their model to explain the behaviour of other thin biofilms, such as pond scum.
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Matter and antimatter Artist’s impression of positronium being instantaneously cooled in a vacuum by a series of laser pulses with rapidly varying wavelengths. (Courtesy: 2024 Yoshioka et al./CC-BY-ND)
Researchers at the University of Tokyo have published a paper in the journal Nature that describes a new laser technique that is capable of cooling a gas of positronium atoms to temperatures as low as 1 K. Written by Kosuke Yoshioka and colleagues at the University of Tokyo, the paper follows on from a publication earlier this year from the AEgIS team at CERN, who described how a different laser technique was used to cool positronium to 170 K.
Positronium comprises a single electron bound to its antimatter counterpart, the positron. Although electrons and positrons will ultimately annihilate each other, they can briefly bind together to form an exotic atom. Electrons and positrons are fundamental particles that are nearly point like, so positronium provides a very simple atomic system for experimental study. Indeed, this simplicity means that precision studies of positronium could reveal new physics beyond the Standard Model.
Quantum electrodynamics
One area of interest is the precise measurement of the energy required to excite positronium from its ground state to its first excited state. Such measurements could enable more rigorous experimental tests of quantum electrodynamics (QED). While QED has been confirmed to extraordinary precision, any tiny deviations could reveal new physics.
An important barrier to making precision measurements is the inherent motion of positronium atoms. “This large randomness of motion in positronium is caused by its short lifetime of 142 ns, combined with its small mass − 1000 times lighter than a hydrogen atom,” Yoshioka explains. “This makes precise studies challenging.”
In 1988, two researchers at Lawrence Livermore National Laboratory in the US published a theoretical exploration of how the challenge could be overcome by using laser cooling to slow positronium atoms to very low speeds. Laser cooling is routinely used to cool conventional atoms and involves having the atoms absorb photons and then re-emitting the photons in random directions.
Chirped pulse train
Building on this early work, Yoshioka’s team has developed new laser system that is ideal for cooling positronium. Yoshioka explains that in the Tokyo setup, “the laser emits a chirped pulse train, with the frequency increasing at 500 GHz/μs, and lasting 100 ns. Unlike previous demonstrations, our approach is optimized to cool positronium to ultralow velocities.”
In a chirped pulse, the frequency of the laser light increases over the duration of the pulse. It allows the cooling system to respond to the slowing of the atoms by keeping the photon absorption on resonance.
Using this technique, Yoshioka’s team successfully cooled positronium atoms to temperatures around 1 K, all within just 100 ns. “This temperature is significantly lower than previously achieved, and simulations suggested that an even lower temperature in the 10 mK regime could be realized via a coherent mechanism,” Yoshioka says. Although the team’s current approach is still some distance from achieving this “recoil limit” temperature, the success of their initial demonstration has given them confidence that further improvements could bring them closer to this goal.
“This breakthrough could potentially lead to stringent tests of particle physics theories and investigations into matter-antimatter asymmetry,” Yoshioka predicts. “That might allow us to uncover major mysteries in physics, such as the reason why antimatter is almost absent in our universe.”
I am fortunate to have several different roles, and problem-solving is a skill I use in each. As physicists, we’re constantly solving problems in different ways, and, as researchers, we are always trying to question the unknown. To understand the physical world more, we need to be curious and willing to reformulate our questions when they are challenged.
Researchers need to keep asking ‘Why?’ Trying to understand a problem or challenge – listening and considering other views – is essential.
In everyday work such as administration, research, teaching and mentoring, I also find that thinking outside the box is a winner when it comes to problem solving. I try not to just go along with whatever the team or the group is thinking. Instead, I try to consider different points of view. Researchers need to keep asking ‘Why?’ Trying to understand a problem or challenge – listening and considering other views – is essential.
Another critical skill I use is communication. In my work, I need to be able to listen, speak and write a lot. It could be to convey why our research is important and why it should be funded. It could be to craft new policies, mediate conflict or share research findings clearly with colleagues, students, managers and members of the public. So communication is definitely key.
What do you like best and least about your job?
I graduated about 30 years ago and, during that time, the things I like best or least have never stayed the same. At the moment, the best part of my job is working with research students – not just at master’s and PhD level, but final-year undergraduates who might be getting hands-on experience in a lab for the first time. There’s great satisfaction and a sense of “job well done” whenever I demonstrate a concept they’ve known for several years but have never “seen” in action. When they shout “Ah, I get it!”, it’s a great feeling. It’s also really rewarding to receive similar reactions from my education and public engagement work, such as when I visit primary and secondary schools.
At the moment, my least favourite part of my job is the lack of time. I’m not very good at time management, and I find it hard to say “no” to people in need, especially if I know how to help them. It’s difficult to juggle work, mentoring, volunteering activities and home life. During the COVID-19 pandemic, I realized that taking time off to pursue a hobby is vital – not only for my wellbeing but also to give me clarity in decision making.
What do you know today that you wish you knew when you were starting out in your career?
I wish I had realized the important of mentorship sooner. Throughout my career, I’ve had people who’ve supported me along the way. It might just have been a brief conversation in the corridor, help with a grant application or a serendipitous chat at a conference, although at other times it might have been through in-depth discussion of my work. I only started to regard the help as “mentorship” when I did a leadership course that included mentor/mentee training. Looking back, those encounters really boosted my confidence and helped me make rational choices.
There are so many opportunities to meet people in your field and people are always happy to share their experiences
Once you realize what mentors can do, you can plan to speak to people strategically. These conversations can help you make decisions and introduce you to new contacts. They can also help you understand what career paths are available – it’s okay to take your time to explore career options or even to change direction. Students and young professionals should also engage with professional societies, such as the Institute of Physics. There are so many opportunities to meet people in your field and people are always happy to share their experiences. We need to come out of our “shy” shells and talk to people, no matter how senior and famous they are. That’s certainly the message I’d have given myself 30 years ago.
Connexion
