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New Physics at Play: Physicists Discover a New Force Acting on Water Droplets Moving Over Superhydrophobic Surfaces – SciTechDaily

Assistant Physics Professor Matilda Backholm’s unique micropipette force sensor technology probes the tiny forces acting between a superhydrophobic material and a water droplet. Credit: Matilda Backholm/Aalto University

Researchers at Aalto University have discovered a new force acting on water droplets moving over superhydrophobic surfaces like black silicon by adapting a novel force measurement technique to uncover the previously unidentified physics at play. This force, identified as air-shearing, challenges previous understandings and suggests modifications in the design of these surfaces to reduce drag, potentially improving their efficiency and application in various fields.

Microscopic chasms forming a sea of conical jagged peaks stipple the surface of a material called black silicon. While it’s commonly found in solar cell tech, black silicon also moonlights as a tool for studying the physics of how water droplets behave.

Black silicon is a superhydrophobic material, meaning it repels water. Due to water’s unique surface tension properties, droplets glide across textured materials like black silicon by riding on a thin air-film gap trapped beneath. This works great when the droplets move slowly—they slip and slide without a hitch.

But when the droplet moves faster, some unknown force seems to tug on its underbelly. This has stumped physicists, but now a team of researchers from Aalto University and ESPCI Paris have an explanation, and they’ve got the numbers to back it up.

Aalto University Assistant Professor Matilda Backholm is the first author of the paper that details these findings, published on April 15 in the journal Proceedings of the National Academy of Sciences. She conducted this during her time as a postdoctoral researcher in Professor Robin Ras’s Soft Matter and Wetting group in the Department of Applied Physics.

The Aalto University research group’s solution to the air-shearing force was to build pillars on the black silicon surface, which are then etched to have similarly textured caps. Credit: Maja Vuckovac/Aalto University

“When observing water-surface interactions, there are typically three forces at play: contact-line friction, viscous losses, and air resistance. However, there is a fourth force that arises from the movement of droplets on highly slippery surfaces like black silicon. This movement actually creates a shearing effect on the air trapped beneath, resulting in a drag-like force on the droplet itself. This shearing force has never been explained before, and we are the first to identify it,” Backholm says.

The complex interactions of fluid and soft matter physics prove challenging to simplify into cut-and-dried formulae. But Backholm has managed to develop a technology to measure these tiny forces, explain how the force works, and finally provide the solution for eliminating the drag force altogether. Hook, line, and sinker.

Air-Shearing Effect

Creating better superhydrophobic surfaces would make the world’s transportation systems more aerodynamic, medical devices more sterile, and generally improve the slipperiness of anything requiring a liquid-repellent surface.

Black silicon exploits the specific surface tension of water to minimize the contact between the droplet and the surface. Cones etched onto the substrate make the water droplets glide on an air-film gap, known as a plastron. But in a counterintuitive twist, the very mechanism that enables hydrophobic surfaces to deflect water droplets also leads to the shearing effect outlined in Backholm’s paper.

Water Droplet Probed by Micropipette Force Sensor

A water droplet is probed with a micropipette force sensor. Credit: Matilda Backholm/Aalto University

“The field has been making these ultra slippery surfaces by reducing the length scale of the cones to make them smaller and more plentiful. But no one has stopped to realize, “Hey, we’re actually working against ourselves here.” In actuality, etching shorter cones onto the black silicon surface leads to a greater air-shearing effect,” Backholm says.

Other researchers have noted the existence of this force but have not been able to explain it. Backholm’s findings prompt a reconsideration of the way that ultra slippery surfaces are designed. Her team’s workaround was to add taller cones with textured caps onto the black silicon surface to further minimize the total contact surface area of the droplets.

“This work builds upon the wealth of expertise from the Soft Matter and Wetting research group on the subject of superhydrophobic surfaces. Rarely does the opportunity emerge to fully explain the subtleties of the microscopic forces involved in wetting dynamics, but this paper accomplishes just that,” says Ras.

Specialized Measurement Technique

Backholm adapted a unique micropipette measurement technique to gauge the forces acting against the water droplets. She is an expert on these micropipette force sensors, having used them to measure the growth dynamics of plant roots, the swimming behavior of mesoscopic shrimp swarms, and now in observing the forces in moving water droplets.

Through arduous fine-tuning, she was able to use this technique to make the breakthrough in identifying the shearing effect. Backholm oscillated the droplet and probe to detect the subtle forces tugging beneath.

“We have also ruled out the possibility that there are any other forces at play at the contact line by running these same tests on carbonated droplets. Those droplets constantly off-gas carbon dioxide, causing them to levitate slightly above the surfaces they sit on. Even still, the shearing effect was measured at certain velocities, ultimately confirming that this force acts independently of its contact with the black silicon surface,” Backholm says.

Backholm expects these findings will further enable physicists and engineers to develop hydrophobic surfaces with better performance.

Backholm now leads the Living Matter research group at the Department of Applied Physics.

Reference: “Toward vanishing droplet friction on repellent surfaces” by Matilda Backholm, Tytti Kärki, Heikki A. Nurmi, Maja Vuckovac, Valtteri Turkki, Sakari Lepikko, Ville Jokinen, David Quéré, Jaakko V. I. Timonen and Robin H. A. Ras, 15 April 2024, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2315214121

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NASA prepping for mid-May Arctic Circle balloon flights – UPI News

April 30 (UPI) — NASA officials are planning to launch the space agency’s long-duration balloon campaign from Sweden to gather scientific data to assist with several projects starting in mid-May.

The National Aeronautics and Space Administration’s balloon program is partnering with the Swedish Space Corporation to launch the balloons capable of operating in near-space some 150,000 feet above the Earth, NASA announced Tuesday.

“Our partnership with the Swedish Space Corporation is valuable to NASA and the scientific community by allowing us to use their high-quality facilities at Esrange,” Andrew Hamilton, acting director of NASA’s balloon program said Tuesday.

Esrange is located in a large and unpopulated area of Sweden that gets 24 hours of sunlight during the summer months.

Hamilton said high-altitude balloons will not lose gas needed to keep them aloft because of the constant sunlight during the summer months in the polar regions.

“The location of the launch range and the stratospheric winds allow for excellent flight conditions to gather many days of scientific data as the balloons traverse from Sweden to northern Canada,” Hamilton explained.

The balloon program has four primary missions this summer.

One mission uses a superconducting magnet to measure the flux of high-energy cosmic ray isotopes to unexplored energies while helping determine the age of the Milky Way galaxy’s cosmic rays.

A second mission will use high-resolution imaging equipment to record X-rays from energetic electron microbursts in the polar atmosphere.

The mission requires a balloon measuring 60 million cubic feet to fly higher than 150,000 feet.

A third mission is focused on the Sun and will use high-resolution images to observe spectro-polarimetry of its layers called the solar photosphere, cromosphere and other active regions of the Sun.

The images will help measure temperature, magnetic field and velocities associated with those layers.

The fourth mission will use a telescope to observe the sources of several black holes and neutron stars in the Milky Way to help learn how they accelerate electrons and emit X-rays.

The fourth mission will include two smaller payloads to complete two additional scientific missions.

One will use an infrared channeled spectro-polarimeter to measure the tops of clouds and the size and shape of ice particles that help scientists better understand weather while improving climate models.

Another will help NASA develop sensor technologies that measure ultraviolet UVA, UVB and UVC wavelengths and ozone concentrations.

NASA says its high-altitude balloons can lift up to 8,000 pounds and are quick, cost-effective ways to test, track and recover scientific experiments for NASA and universities located around the world.

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Social worker to launch book – The Herald

Social worker to launch book

Arts Reporter

Desmond Chikamhi, a Mandela Washington fellow and development practitioner, is set to launch his first book titled “Yes You Can” on Friday at Cresta Jameson Hotel in Harare from 5pm to 9pm.

The World Life Changers founder and executive director, who has been involved in community development since 2018, said the book is designed to open readers’ minds to a world of possibilities in spite of their circumstances.

“The book carries some invaluable wisdom nuggets for a victorious life and my hope is that it will unleash a culture of excellence and productivity amongst our people.

“It is a landmark motivational publication designed to equip and empower readers to chase their dreams and bring them to fruition.

“It is an antidote to the lethargy, demotivation and fear,” Chikamhi said.

The Zaka-born community builder, who grew up under challenging circumstances, said the wisdom in the book is derived from his difficult personal experiences.

“The goal is to demonstrate that impossibilities only exist in our minds. Once we reconfigure our minds to operate in a positive way, nothing is impossible. Life is there for the taking for anyone who can dash and grab it with both hands.

“The book launch is, therefore, going to be a life celebration as we share ideas and life nuggets on how to get ahead and stay ahead. Its all about daring each other not only to dream, but to also see our dreams materialising,” Chikamhi said.

The event is set to be curated by creative agency Esteem Communications while various influencers, corporate leaders and creatives are expected to attend the event.

“We have enlisted the services of Esteem Communications to help us curate the launch and all is in place for an uplifting life experience.

Music and entertainment will be provided by Sabastian Magacha, Mavery very, Loice Manzunzu, Fire Colloso and Simbarashe Mparira,” Chikamhi said.

Chikamhi, who has implemented a number of life changing community development projects around Zimbabwe, said working with vulnerable communities to change their lives was the biggest inspiration behind the book.

“Working with communities and getting to change their lives through our organisation was the biggest inspiration behind this book. My eyes were opened to possibilities and I want fellow Zimbabweans and readers across the world to see things the same way.

During Cyclone Idai, we assisted over 10 000 families in Chimanimani under one of our Disaster and Relief Programmes, where we respond to natural disasters that befall our vulnerable communities. We donated non-food items like crutches, wheelchairs, blankets and clothes worth thousands, with support from United Methodist Church in the United Kingdom,” Chikamhi said.

Various guests among them academics, politicians, company executives and creatives are expected to attend the event.

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Breaking news. – The Herald

New gold-backed ZiG aims for economic independence

Marshall Ndlela Correspondent

ZIMBABWE’S financial landscape is undergoing a significant transformation.

After years of relying on the US dollar, the nation’s banks have begun issuing new Zimbabwean notes called the Zimbabwe Gold (ZiG).

This strategic shift marks a pivotal moment in the country’ economic history, potentially redefining its path towards self-sufficiency and stability.

A nation’s decision to issue its own currency is often driven by a deep yearning for economic independence and control over its financial destiny.

A sovereign currency empowers a country to tailor its monetary policies to address its unique economic needs and aspirations.

This flexibility allows Zimbabwe to manage interest rates, influence inflation, and stimulate domestic growth through strategic fiscal measures.

Imagine Zimbabwe being able to adjust interest rates to encourage borrowing for business expansion or lower them to ease the burden on consumers, all without relying on external forces.

The adoption of a national currency presents several advantages for Zimbabwe.

Perhaps the most significant benefit lies in the new-found ability to influence the domestic economy.

Zimbabwe can now set its own interest rates, a crucial tool for stimulating investment and economic activity.

Additionally, managing the money supply allows for greater control over inflation, a critical factor in ensuring price stability and protecting the value of citizens’ savings.

A national currency empowers Zimbabwe to manage its exchange rates strategically.

By manipulating the exchange rate, the country can potentially make its exports more competitive in the global marketplace, attracting foreign investment and increasing its export earnings.

This could lead to a significant boost in economic activity and job creation.

Beyond the tangible economic benefits, a national currency serves as a powerful symbol of sovereignty and national identity.

It fosters a sense of unity and belonging among Zimbabweans, representing their nation’s unique place in the global economic landscape.

The path towards de-dollarisation is not without its hurdles.

A major concern lies in the potential for economic instability during the transition period.

Zimbabwe’s citizens and businesses may be hesitant to trust and readily accept the new currency, especially if they have witnessed past episodes of hyperinflation and devaluation with previous local currencies.

Building trust and confidence in the new system is paramount. The good thing is that de-dollarisation will happen in 2030 – by that time, the ZiG will have cemented its position as a currency of choice for Zimbabweans.

The success of the new Zimbabwean currency also hinges on the Government’s commitment to fiscal discipline.

Resisting the urge to print excessive amounts of money, a tactic that can lead to hyperinflation, is crucial.

Establishing a sound economic foundation with credible monetary policies and sufficient reserves to back the new currency will be essential for ensuring its long-term viability.

Without a doubt, Zimbabwe’s decision to reintroduce its own currency represents a bold step towards economic self-determination. While the advantages of a sovereign currency are undeniable, the road ahead is paved with challenges.

The nation’s success hinges on its ability to navigate the complexities of de-dollarisation with prudence, transparency, and a commitment to sound economic management.

Marshall Ndlela is a Zimbabwean economist based in Melbourne, Australia.

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