The new findings, which appear in the journal Science Advances, will pave the way for cheaper, lighter devices that use less battery power, and which can function in harsh or hazardous environments. Andrea Alù, founding director of the CUNY ASRC Photonics Initiative and Professor of Physics at The Graduate Center, CUNY, and postdoctoral research associate Xiang Ni were authors on the paper, together with Amir Ardabi and Michael Leamy from Georgia Tech.

The field of topology examines properties of an object that are not affected by continuous deformations. In a topological insulator, electrical currents can flow along the object’s boundaries, and this flow is resistant to being interrupted by the object’s imperfections. Recent progress in the field of metamaterials has extended these features to control the propagation of sound and light following similar principles.

In particular, previous work from the labs of Alù and City College of New York Physics Professor Alexander Khanikaev used geometrical asymmetries to create topological order in 3D-printed acoustic metamaterials. In these objects, sound waves were shown to be confined to travel along the object’s edges and around sharp corners, but with a significant drawback: These waves weren’t fully constrained — they could travel either forward or backward with the same properties. This effect inherently limited the overall robustness of this approach to topological order for sound. Certain types of disorder or imperfections would indeed reflect backwards the sound propagating along the boundaries of the object.

This latest experiment overcomes this challenge, showing that time-reversal symmetry breaking, rather than geometrical asymmetries, can be also used to induce topological order. Using this method, sound propagation becomes truly unidirectional, and strongly robust to disorder and imperfections

“The result is a breakthrough for topological physics, as we have been able to show topological order emerging from time variations, which is different, and more advantageous, than the large body of work on topological acoustics based on geometrical asymmetries,” Alù said. “Previous approaches inherently required the presence of a backward channel through which sound could be reflected, which inherently limited their topological protection. With time modulations we can suppress backward propagation and provide strong topological protection.”

The researchers designed a device made of an array of circular piezoelectric resonators arranged in repeating hexagons, like a honeycomb lattice, and bonded to a thin disk of polylactic acid. They then connected this to external circuits, which provide a time-modulated signal that breaks time-reversal symmetry.

As a bonus, their design allows for programmability. This means they can guide waves along a variety of different reconfigurable paths, with minimal loss. Ultrasound imaging, sonar, and electronic systems that use surface acoustic wave technology could all benefit from this advance, Alù said.

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The results of this research were published today (July 17) in Science Advances.

With an enhanced ability to repel droplets, this design could be applied to personal protective equipment (PPE) to better resist virus-laden particles, such as COVID-19, among other applications.

“For the past few decades, conventionally designed water repellent surfaces have usually been based on plants, like lotus leaves,” said Lin Wang, a doctoral student in the Department of Materials Science and Engineering at Penn State and the lead author of the paper.

Classical engineering theories have used this approach to create superhydrophobic, or water repellent, surfaces. Traditionally, they are manufactured with low solid fraction textures, which maintain an extremely thin layer of air above a low density of microscopic, hair-like nanostructures, which the researchers liken to an air hockey table.

“The reasoning is if the droplet or object is floating on top of that air, it won’t become stuck to the surface,” said Tak-Sing Wong, the Wormley Early Career Professor of Engineering, associate professor of mechanical and biomedical engineering and Wang’s adviser.

Since it works effectively, human-made coatings tend to mimic the low density of these nanostructures.

However, this paper details an entirely different approach. When examining surfaces like the eye of a mosquito, body of a springtail or the wing of a cicada under high resolution electron microscopes, Wang found that the nanoscopic hairs on those surfaces are more densely packed, referred to in engineering as high solid fraction textures. Upon further exploration, this significant departure from plants’ structure may imbue additional water repelling benefits.

“Imagine if you had a high density of these nanostructures on a surface,” Wang said. “It could be possible to maintain the stability of the air layer from higher impact forces.”

This could also mean the more densely packed structures may be able to repel liquid that is moving at a higher speed, such as raindrops.

While the design concept is new to humans, the researchers theorize this nanostructure boosts the insect’s resiliency in its natural environment.

“For these insect surfaces, repelling water droplets is a matter of life and death. The impact force of raindrops is enough to carry them to the ground and kill them,” Wang said. “So, it is really important for them to stay dry, and we figured out how.”

With this knowledge gleaned from nature, the researchers hope to apply this design principle to create next generation coatings. By developing a water repellent surface that can withstand faster moving and higher impact droplets, the applications are abundant.

From small, flying robotic vehicles, such as the drones that Amazon hopes to deliver packages with, to commercial airliners, a coating that can emulate these insect surfaces could provide increased efficiency and safety.

However, in light of the COVID-19 pandemic, researchers have since realized this knowledge could have an additional impact on human health.

“We hope, when developed, this coating could be used for PPE. For example, if someone sneezes around a face shield, those are high velocity droplets. With a traditional coating, those particles could stick to the surface of the PPE,” Wong said. “However, if the design principles detailed in this paper were adopted successfully, it would have the ability to repel those droplets much better and potentially keep the surface germ-free.”

As seen in this work, the Wong Laboratory for Nature Inspired Engineering draws insights from biological phenomena to make humanity’s innovations better and more effective.

“While we didn’t imagine that application at the beginning of this project, COVID-19 made us think about how we can use this design principle to benefit more people,” Wong said. “It’s up to us as engineers to take these discoveries and apply them in a meaningful way.”

The next step for this work will be developing a large scale, cost effective method that can manufacture a coating to mimic these properties.

“In the past, we didn’t have an effective surface that could repel high speed water droplets,” Wong said. “But the insects told us how. There are so many examples like this in nature; we just need to be learning from them.”

This research was funded by the National Science Foundation, the PPG Foundation, the Wormley Family Early Career Professorship, the Humanitarian Materials Initiative Award sponsored by Covestro and the Materials Research Institute at Penn State. Additional contributors include Ruoxi Wang, an undergraduate alumna, and Jing Wang, a doctoral graduate, both in the Department of Mechanical Engineering.

Batteries using more abundant multivalent metals could revolutionize energy storage. Researchers review the current state of multivalent metal-ion battery research and provide a roadmap for future work in Nature Energy, reporting that the top candidates — using magnesium, calcium, zinc and aluminum — all have great promise, but also steep challenges to meet practical demands.

“In this review, we clarify the key strengths as well as common misconceptions of multivalent metal-based batteries,” said Yan Yao, Cullen College of Engineering associate professor of electrical and computer engineering at the University of Houston and co-corresponding author of the paper. “Multivalent metal-ion batteries are better viewed as alternative solutions for large-scale energy storage than as a direct competitor to lithium-based batteries in the race toward ever-rising energy density targets.”

Researchers also examined the growth behavior of metal anodes. While magnesium is a promising material, Yao said it is important to note that it is not guaranteed to plate universally in dendrite-free morphology. “It only does so in selected electrolyte solutions where there are no side reactions, the active metal surface is free of passivation, and the coulombic efficiency of magnesium plating?stripping is close to unity,” he said.

Yao is also a principal investigator with the Texas Center for Superconductivity at UH (TcSUH).

Yanliang “Leonard” Liang, a research assistant professor in the Department of Electrical and Computer Engineering at UH and co-first author, said the paper’s review of existing cathode materials also offers new insights. “We also discuss design strategies to enable genuine multivalent metal-ion-based energy storage materials with competitive performance,” he said.

Researchers’ key points include:

• Concerns about the availability and cost of lithium have prompted interest in alternative battery technologies that use more abundant elements with the potential for higher energy density and enhanced safety. Prime candidates include magnesium, calcium, zinc and aluminum, all of which are known as multivalent metals, or metals with multiple valence electrons to give
• These multivalent metal-ion batteries share many similarities in working principles with lithium-ion batteries, suggesting they could be swiftly adopted by industry
• Previous assessments of the anticipated energy density of these batteries often considered only the multivalent metal anode — merely one of the two electrodes in a battery — which tends to reach misleading conclusions. The researchers have provided a reassessment of the energy density based on both anode and cathode, aiming to better position these batteries in the energy storage landscape
• Direct use of metals as anodes is an important aspect for the safety and energy density promises of these batteries, but there are uncertainties surrounding the viability of these anodes
• Electrolyte solutions and understanding of the associated interfacial phenomena are improving but still far from established
• And finding good cathode materials requires considerations that are uncommon in traditional battery studies. The ion storage mechanism of multivalent battery cathodes is much more complicated than its lithium-ion counterpart.
• Misinterpretation of the cathode chemistry is surprisingly widespread in the literature as a result

The researchers also issued a list of recommendations to ensure future research is squarely directed at improving the batteries, including:

• Gaining a better understanding of the growth behavior of the metal anodes, a critical step in realizing the purported safety promises over lithium-ion batteries
• Practices to properly assess the compatibility of metal anodes with electrolyte solutions and the effectiveness of protection coatings
• Procedures and techniques to correctly interpret the ion storage mechanism at the cathode
• Approaches to design better cathode materials

A table comparing the state-of-the-art components — metal anode, electrolyte and cathode materials — for batteries based on each of the multivalent metal elements that could potentially replace lithium found that although some are further along than others, none of the options are ready for commercialization.

Yao’s research group has focused on materials chemistry and design of magnesium and other multivalent metal batteries; he has published widely in top journals.

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“These trial results show that vaporized cannabis appears to be generally safe,” said Gupta, a professor of medicine on the faculty of UCI’s Center for the Study of Cannabis. “They also suggest that sickle cell patients may be able to mitigate their pain with cannabis — and that cannabis might help society address the public health crisis related to opioids. Of course, we still need larger studies with more participants to give us a better picture of how cannabis could benefit people with chronic pain.”

Opioids are currently the primary treatment for the chronic and acute pain caused by sickle cell disease. But the rise in opioid-associated deaths has prompted physicians to prescribe them less frequently, leaving sickle cell patients with fewer options.

The double-blind, placebo-controlled, randomized trial was the first to employ such gold-standard methods to assess cannabis’s potential for pain alleviation in people with sickle cell disease. The cannabis used in the trial was obtained from the National Institute on Drug Abuse — part of the National Institutes of Health — and contained equal parts of THC and CBD.

“Pain causes many people to turn to cannabis and is, in fact, the top reason that people cite for seeking cannabis from dispensaries,” Gupta said. “We don’t know if all forms of cannabis products will have a similar effect on chronic pain. Vaporized cannabis, which we employed, may be safer than other forms because lower amounts reach the body’s circulation. This trial opens the door for testing different forms of medical cannabis to treat chronic pain.”

Twenty-three patients with sickle cell disease-related pain completed the trial, inhaling vaporized cannabis or a vaporized placebo during two five-day inpatient sessions that were separated by at least 30 days. This allowed them to act as their own control group.

Researchers assessed participants’ pain levels throughout the treatment period and found that the effectiveness of cannabis appeared to increase over time. As the five-day study period progressed, subjects reported that pain interfered less and less with activities, including walking and sleeping, and there was a statistically significant drop in how much pain affected their mood. Although pain levels were generally lower in patients given cannabis than in those given the placebo, the difference was not statistically significant.

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