Schematic illustration of the template-directed eutectic solidification process. Liquid (gold) AgCl (cyan)-KCl (black) eutectic system solidifies through the pillar template. Credit: The Grainger College of Engineering at University of Illinois Urbana-Champaign

Self-assembled solidifying eutectic materials directed by a template with miniature features demonstrate unique microstructures and patterns as a result of diffusion and thermal gradients caused by the template. Despite the template trying to force the material to solidify into a regular pattern, when the template carries a lot of heat it also can interfere with the solidification process and cause disorder in the long-range pattern.

Researchers at the University of Illinois Urbana-Champaign and the University of Michigan Ann Arbor have developed a template material that carries almost no heat and therefore stops heat transfer between the template material itself and the solidifying eutectic material. They accomplished this by forming the template from a material with very low thermal conductivity, ultimately resulting in highly organized self-assembled microstructures.

The results of this research were recently published in the journal Advanced Materials.

“The key novelty of this research is that we carefully controlled the flow of heat. By controlling the flow of heat, the pattern becomes far better and more regular than before because we’re controlling more of the parameters. Previously, the template controlled the flow of atoms, but the heat flows were uncontrolled,” says Paul Braun, a professor of materials science and engineering and director of the Materials Research Laboratory, who led this research along with postdoctoral researcher Sung Bum Kang.

Eutectic materials are a homogeneous mixture that have a melting point that is lower than the melting point of either constituent. Common examples of eutectic systems include solder (a mixture of lead and tin) and mixtures of salt (sodium chloride) and water. When eutectic mixtures are cooled from the liquid phase, they separate into two materials that form a pattern at the solidifying front.

The material doesn’t separate into just two large layers. Instead, it forms structures including a multi-layered structure (lamellar), like a tiered cake, a rod-like structure or even more complex structures. The resulting microstructure of the material, however, is only well-ordered over short distances. Instabilities that arise in the self-assembly process lead to defects in the microstructure and affect the properties of the resulting solid material. For many applications, such as optics or mechanics, very good order over long distances is required.

The solidification process can be controlled by a template consisting of pillars that act as barriers to the movement of atoms and molecules. This forces the structure to form a more regular pattern when it solidifies. But the issue, Braun explains, is that the pillars carry a lot of heat, and instead of having a flat, solidifying front, the shape of the front becomes complex. This leads to irregular patterns and long-range disorder.

“We figured out how to make the pillars so that they were really good insulators,” Braun says. “So all of the heat is only flowing through the material that’s solidifying. The template is now only acting as a barrier to the flow of atoms, but almost no heat is moving between the solidifying material and the template.”

The researchers explored template materials with lower thermal conductivities than the eutectic system and found that low thermal conductivity template material resulted in highly organized microstructures with long-range order. Specifically, they used porous silicon (essentially a silicon foam) that is at least 100 times less thermally conductive than crystalline silicon. The template material’s low thermal conductivity minimizes the flow of heat in the “wrong” direction.

“The thermal conductivity of the template is a critical factor in determining the rate of heat transfer during the solidification process,” Kang says. “The porous silicon we used for the templates has a low thermal conductivity and led to about 99% uniformity of the unit cells of the structure.”

In comparison, with higher thermal conductivity crystalline silicon pillars, the expected pattern is only present in 50% of the unit cells.

“This means we can design eutectic materials with highly predictable and consistent properties. This level of control is crucial for applications where uniformity directly impacts performance,” Kang says.

llustration of the uncertainty of Earth’s orbit 56 million years ago due to a potential past passage of the Sun-like star HD7977 2.8 million years ago. Each point’s distance from the center corresponds to the degree of ellipticity of Earth’s orbit, and the angle corresponds to the direction pointing to Earth’s perihelion, or closest approach distance to the Sun. 100 different simulations (each with a unique color) are sampled every 1,000 years for 600,000 years to construct this figure. Every simulation is consistent with the modern Solar System’s conditions, and the differences in orbital predictions are primarily due to orbital chaos and the past encounter with HD 7977. Credit: N. Kaib/PSI.

Stars that pass by our solar system have altered the long-term orbital evolution of planets, including Earth, and, by extension, modified our climate.

“Perturbations—a minor deviation in the course of a celestial body, caused by the gravitational attraction of a neighboring body—from passing stars alter the long-term orbital evolution of the sun’s planets, including Earth,” said Nathan A. Kaib, Senior Scientist at the Planetary Science Institute and lead author of “Passing Stars as an Important Driver of Paleoclimate and the solar system’s Orbital Evolution” that appears in The Astrophysical Journal Letters. Sean Raymond at the Laboratoire d’Astrophysique de Bordeaux also contributed to this work.

“One reason this is important is because the geologic record shows that changes in the Earth’s orbital eccentricity accompany fluctuations in the Earth’s climate. If we want to best search for the causes of ancient climate anomalies, it is important to have an idea of what Earth’s orbit looked like during those episodes,” Kaib said.

“One example of such an episode is the Paleocene-Eocene Thermal Maximum 56 million years ago, where the Earth’s temperature rose 5-8 degrees centigrade. It has already been proposed that Earth’s orbital eccentricity was notably high during this event, but our results show that passing stars make detailed predictions of Earth’s past orbital evolution at this time highly uncertain, and a broader spectrum of orbital behavior is possible than previously thought.”

Simulations (run backward) are used to predict the past orbital evolution of the Earth and the sun’s other planets. Analogous to weather forecasting, this technique gets less accurate as you extend it to longer times because of the exponential growth of uncertainties. Previously, the effects of stars passing near the sun were not considered in these “backward forecasts.”

As the sun and other stars orbit the center of the Milky Way, they inevitably can pass near one another, sometimes within tens of thousands of au, 1 au being the distance from the Earth to the sun. These events are called stellar encounters. For instance, a star passes within 50,000 au of the sun every 1 million years on average, and a star passes within 10,000 au of the sun every 20 million years on average. This study’s simulations include these types of events, whereas most prior similar simulations do not.

One major reason the Earth’s orbital eccentricity fluctuates over time is because it receives regular perturbations from the giant planets of our solar system (Jupiter, Saturn, Uranus, and Neptune). As stars pass near our solar system, they perturb the giant planet’s orbits, which consequently then alters the orbital trajectory of the Earth. Thus, the giant planets serve as a link between the Earth and passing stars.

Kaib said that when simulations include stellar passages, we find that orbital uncertainties grow even faster, and the time horizon beyond which these backward simulations’ predictions become unreliable is more recent than thought.

This means two things: There are past epochs in Earth’s history where our confidence in what Earth’s orbit looked like (for example, its eccentricity or degree of circularity) has been too high, and the real orbital state is not known, and the effects of passing stars make regimes of orbital evolution (extended periods of particularly high or low eccentricity) possible that past models did not predict.

“Given these results, we have also identified one known recent stellar passage, the sun-like star HD 7977, which occurred 2.8 million years ago, that is potentially powerful enough to alter simulations’ predictions of what Earth’s orbit was like beyond approximately 50 million years ago,” Kaib said.

The current observational uncertainty of HD 7977’s closest encounter distance is large, however, ranging from 4,000 au to 31,000 au. “For larger encounter distances, HD 7977 would not have a significant impact on Earth’s encounter distance. Near the smaller end of the range, however, it would likely alter our predictions of Earth’s past orbit,” Kaib said.

SETI ellipsoid. Credit: Zayna Sheikh

A team of researchers from the SETI Institute, Berkeley SETI Research Center and the University of Washington reported an exciting development for the field of astrophysics and the search for extraterrestrial intelligence (SETI), using observations from the Transiting Exoplanet Survey Satellite (TESS) mission to monitor the SETI Ellipsoid, a method for identifying potential signals from advanced civilizations in the cosmos.

The SETI Ellipsoid is a strategic approach for selecting potential technosignature candidates based on the hypothesis that extraterrestrial civilizations, upon observing significant galactic events such as supernova 1987A, might use these occurrences as a focal point to emit synchronized signals to announce their presence.

In this work, researchers show that the SETI Ellipsoid method can leverage continuous, wide-field sky surveys, significantly enhancing our ability to detect these potential signals. By compensating for the uncertainties in the estimated time-of-arrival of such signals using observations that span up to a year, the team implements the SETI Ellipsoid strategy in an innovative way using state-of-the-arc technology.

“New surveys of the sky provide groundbreaking opportunities to search for technosignatures coordinated with supernovae.” said co-author Bárbara Cabrales.

“The typical timing uncertainties involved are a couple of months, so we want to cover our bases by finding targets that are well-documented over the course of about a year. In addition to that, it’s important to have as many observations as possible for each target of interest so that we can determine what looks like normal behavior and what might look like a potential technosignature.”

In examining data from the TESS continuous viewing zone, covering 5% of all TESS data from the first three years of its mission, researchers utilized the advanced 3D location data from Gaia Early Data Release 3. This analysis identified 32 prime targets within the SETI Ellipsoid in the southern TESS continuous viewing zone, all with uncertainties refined to better than 0.5 light-years.

While the initial examination of TESS light curves during the Ellipsoid crossing event revealed no anomalies, the groundwork laid by this initiative paves the way for expanding the search to other surveys, a broader array of targets, and exploring diverse potential signal types.

Applying the SETI Ellipsoid technique to scrutinize large archival databases signifies a monumental step forward in the search for technosignatures. Utilizing Gaia’s highly precise distance estimates, the study demonstrates the feasibility of cross-matching these distances with other time-domain surveys like TESS to enhance monitoring and anomaly detection capabilities in SETI research.

The SETI Ellipsoid method, combined with Gaia’s distance measurements, offers a robust and adaptable framework for future SETI searches. Researchers can retrospectively apply it to sift through archival data for potential signals, proactively select targets, and schedule future monitoring campaigns.

“As Dr. Jill Tarter often points out, SETI searches are like looking for a needle in a 9-D haystack,” said co-author Dr. Sofia Sheikh. “Any technique that can help us prioritize where to look, such as the SETI Ellipsoid, could potentially give us a shortcut to the most promising parts of the haystack. This work is the first step in searching those newly-highlighted parts of parameter space, and is an exciting precedent for upcoming large survey projects like LSST.”

The research is published in The Astronomical Journal.

Meghan Huber wearing the hip exoskeleton with Mark Price and Banu Abdikadirova (credit: Derrick Zellmann). Credit: Derrick Zellmann

More than 80% of stroke survivors experience walking difficulty, significantly impacting their daily lives, independence, and overall quality of life. Now, new research from the University of Massachusetts Amherst pushes forward the bounds of stroke recovery with a unique robotic hip exoskeleton designed as a training tool to improve walking function.

This invites the possibility of new therapies that are more accessible and easier to translate from practice to daily life compared to current rehabilitation methods.

Following a stroke, people often experience walking asymmetry, where one step is shorter than the other. The study, published in IEEE Transactions on Neural Systems and Rehabilitation Engineering, reveals that the robotic hip exoskeleton has the potential to effectively train individuals to modify their walking asymmetry, presenting a promising avenue for stroke rehabilitation.

The approach employed by the robotic exoskeleton is inspired by split-belt treadmills, which are specialized machines with two side-by-side belts moving at different speeds. Prior research has shown that repeated training on a split-belt treadmill can reduce walking asymmetry in stroke patients.

Wouter Hoogkamer, assistant professor of kinesiology and author of the paper, has spent the last decade studying split-belt treadmills. “Split-belt treadmill training is designed to exaggerate a stroke patient’s walking asymmetry by running the belts under each foot at different speeds. Over time, the nervous system adapts, such that when the belts are set to the same speed, they walk more symmetrically.”

Unfortunately, there are limits to the benefits gained from treadmill-based training methods. “What is learned on a treadmill does not completely transfer to overground contexts,” says Banu Abdikadirova, mechanical and industrial engineering doctoral candidate and lead study author. “This is because walking on a treadmill is not exactly the same as walking overground.”

“The ultimate goal of gait rehabilitation is not to improve walking on a treadmill—it is to improve locomotor function overground,” says Meghan Huber, assistant professor of mechanical and industrial engineering and senior author on the paper. “With this in mind, our focus is to develop methods of gait rehabilitation that translate to functional improvements in real-world contexts.”

This proof-of-concept study showed that applying resistive forces about one hip joint and assistive forces about the other with their exoskeleton mimicked the effects of split-belt treadmill training in neurologically intact individuals.

Now that the research team has proven that the exoskeleton can alter gait asymmetry, they are eager to move their research into overground contexts that are more akin to the real world.

“Because our exoskeleton is portable, it can be used during overground walking,” says Mark Price, a postdoctoral researcher in mechanical and industrial engineering and kinesiology and author on the paper. “We can build upon the successes of split-belt treadmill training with this device to enhance the accessibility of gait training and enhance the transfer of training benefits into everyday walking contexts.”

The researchers also plan to expand their work by measuring the neural changes caused by walking with the exoskeleton and testing this new method on stroke survivors.

“A portable exoskeleton offers numerous clinical benefits,” says Abdikadirova. “Such a device can be seamlessly integrated into the daily lives of chronic stroke survivors, offering an accessible way to increase training time, which is critical for improving walking. It can also be used during early intervention in hospitals for improved functional outcomes.”

The robotic hip exoskeleton is just one of the innovative devices designed to study and enhance gait function developed by the collaborative team of undergraduate students, graduate students, and postdoctoral researchers from the Human Robot Systems Lab, led by Huber, and the Integrative Locomotion Lab, led by Hoogkamer.

“It is inspiring to witness the innovations that emerge when individuals from diverse backgrounds unite under a shared mission,” says Huber. “Only through this type of cross-disciplinary research can we engineer technologies that can have a meaningful impact on people’s lives.”