Tiny forces: measuring the effect of swimming bacteria. (Courtesy: Rhett S Miller / UC Regents)
A tiny “force probe” that can measure sub-piconewton forces when inserted directly into liquid media has been created by researchers in the US. The team says that it used the probe to detect the tiny forces associated with swimming bacteria and heart-muscle cells. The researchers suggest that the technique could be used to create miniature stethoscopes. A leading biophysicist, however, says more work must be done on characterizing the device before he is convinced of its efficacy.
Sensing and manipulating tiny forces is crucial to numerous areas of science. Scientists have therefore developed several techniques to do this – including the atomic force microscope (AFM). An AFM uses a very sharp tip attached to a flexible cantilever. The tip pushes against or pulls an object, while measuring the forces involved. This involves measuring the cantilever deflection – usually by reflecting light from the cantilever. Although the tip itself can be as small as one atom, the rest of the measurement system is much larger and this can make it difficult to map the forces in a tiny object such as a living cell.
Leaking light
Now, Donald Sirbuly of the University of California, San Diego, and colleagues have taken a different approach by detecting the forces on tiny optical fibres. Their probe comprises tin-dioxide fibres around 100 times thinner than a human hair that are coated with the highly compressible polymer polyethylene glycol. They then deposited gold nanoparticles on the polymer layer. When white light travels down a fibre, some of the electromagnetic energy leaks laterally. This “evanescent” light couples to the gold nanoparticles and then scatters into the surroundings.
The coupling between each nanoparticle and the waveguide – and therefore the strength of the scattering – is extremely sensitive to the positions of the nanoparticles. “Any time a force or sound wave hits these particles, they move,” explains Sirbuly, “and we can track that simply by looking at the optical scattering signals.”
The researchers calibrated their “nanofibre optic force transducer” (NOFT) by pressing on it with an AFM and measuring how the scattering signals varied with applied force. They reckon it is sensitive to forces as small as 160 fN – which they say is at least 10 times smaller than the sensitivity of an AFM.
Tiny stethoscope
To test the NOFT, they tried to measure the forces at work in a solution of living bacteria – finding them significantly greater than in a similar solution of dead bacteria. Then, they placed the device about 100 μm from mouse heart cells in a dish and resolved beating frequencies between 1–3 Hz. The researchers now want to explore the signals from other types of tissue: “The idea of having a really small stethoscope is certainly interesting,” Sirbuly says: “It would be interesting to see if we can detect differences in the acoustic signatures of bio-organisms.” On the more fundamental side, he says NOFTs might be useful to measure the mechanical signals cells produce as they undergo changes to, for example, diseased states.
Biophysicist Vincent Croquette of Ecole Normale Supérieure in Paris agrees that NOFTs could have potential where very small force sensors are required, but believes the paper describing the probe does not properly demonstrate this. He notes a detector’s sensitivity is defined by the smallest signal distinguishable from noise, and, without the noise spectrum of the probe, he says the figure of 160 fN is difficult to interpret.
He also says the NOFT needs proper calibration: “The classic test is to pull on a DNA molecule, which has a force-extension curve that is extremely well known,” he says. “So it’s perfectly suited to test a sensor for forces in the range of 200 fN. Everyone calibrates sensors with a DNA molecule these days, because it’s so reproducible it’s become a standard. Why don’t they choose a classical, canonical example to specify their sensitivity?” He adds: “They have done a good job of making a sensor, but a pretty bad job of showing that their sensor is good.”
Four years ago, theoretical physicists proposed a new quantum-communication scheme with a striking feature: it did not require the transmission of any physical particles. The research raised eyebrows, but now a team of physicists in China claims it has demonstrated that the “counterfactual” scheme works. The group built an optical apparatus that it says can transfer a simple image while sending (almost) no photons in the process.
The theoretical proposal was put forward by scientists at Texas A&M University (TAMU) in the US and the King Abdulaziz City for Science and Technology (KACST) in Saudi Arabia. It is based on the phenomenon of wave–particle duality. Specifically, it uses the fact that the presence of an object blocking an arm of an interferometer can be inferred by virtue of its collapsing the wavefunction of an interrogating photon – even though it has no physical contact with the photon. The work also relies on what is known as the quantum Zeno effect, which stipulates that an ongoing series of weak measurements will stifle the quantum-mechanical evolution of a particle and almost certainly cause it to remain in its initial state.
The communication protocol is defined in terms of two characters Alice and Bob – and it is Bob who sends the message. Alice fires single photons at a chain of interferometers, created by a series of beam splitters and mirrors. At the output of the final interferometer, photons end up in one of two detectors monitored by Alice. Bob, meanwhile, can choose whether or not to switch on a measuring device in the right-hand arm of each interferometer.
Left or right
If Bob switches on his devices, he forces the photon injected by Alice to behave as a particle and therefore follow a definite path – going either left or right – through each interferometer. But since the beam splitters are highly reflective, and photons are always reflected to the left, Bob – employing the quantum Zeno effect – causes the photon to remain in the left-hand channel as it travels through the apparatus and as such triggers Alice’s right-hand detector. But if Bob instead switches his devices off, the photon’s wavefunction is allowed to evolve and the photon instead ends up in the left-hand detector.
Intriguingly, therefore, Alice learns of Bob’s decision – whether or not to turn on the devices – even though no photon passes between them. In neither case does Bob’s equipment interact with a photon. As such, Bob can send Alice a message by using the states “on” and “off” to represent the ones and zeros of a binary code, even though he sends no physical particle to Alice.
The counterfactual protocol put forward by the TAMU-KACST group, which is led by TAMU’s Suhail Zubairy, was actually slightly more complicated. It involved the addition of an extra chain of interferometers in the right-hand arm of each existing interferometer. This was done to make sure that any photons that enter the communication channel between Alice and Bob are lost.
Eve the eavesdropper
That fix clearly didn’t satisfy everyone. After Zubairy and colleagues had published their research in Physical Review Letters, Lev Vaidman of Tel-Aviv University in Israel sent a comment to the journal arguing that photons would not pass between Alice and Bob only when Bob switches his devices on. With the devices off, reckoned Vaidman, a weak measurement would in fact reveal photons to be present in the channel. Saying that Zubairy’s group has a “naive classical approach to the past of the photons”, Vaidman adds that the misconception could allow an eavesdropper (Eve) to uncover part of the message being transmitted.
Notwithstanding the debate that ensued, Jian-Wei Pan of the University of Science and Technology of China in Hefei and team set about building an experiment to put the protocol to the test. As they point out in a paper describing the work in the Proceedings of the National Academy of Sciences, a completely counterfactual scheme would require an infinite number of interferometers, which is clearly not practical. So instead they used a simplified design – employing just two interferometers (one each for the external and internal chains) and sending each photon back and forth multiple times, thanks to the use of nanosecond timing and phase stabilization.
Pan and colleagues transmitted a 100 × 100 monochrome bit map of a Chinese knot. After five hours of painstakingly transmitting each of the 10,000 bits multiple times (to overcome channel loss), the researchers were able to clearly reproduce the image, successfully transmitting the correct bit value – black or white – 87% of the time. Comparing that figure with the rate at which photons erroneously leaked through the communication channel – just 1.4% – they conclude that they had indeed sent the information counterfactually. In other words, the vast majority of the transmitted bits, they say, were not associated with the passage of any physical particle.
Imaging delicate art
Despite their positive results, the Chinese researchers say that further experiments are necessary. Among the possible tests that could be carried out, they say, are weak measurements at the output of each inner interferometer to establish whether photons are in fact leaking through the communication channel. The researchers do not explicitly discuss the possibility of developing a practical ultra-secure communication scheme on the back of their work, but they do raise the possibility of “counterfactual imaging”. Involving an array of optical switches that are used to send data counterfactually to a camera, the technique, they suggest, could prove handy in imaging delicate pieces of ancient art that cannot be exposed to direct light.
As to exactly what is physically transmitting information from Bob to Alice, if not particles, that remains an open question. Hatim Salih, who was lead author on the theory paper and is now at the UK’s University of York, is convinced that the culprit must be the photon’s wavefunction. As such, he argues, the research would help settle a decades-old debate among physicists about the reality of the wavefunction. It must be real, says Salih, who is also co-founder of QuBet, a UK-based quantum technology company.
Prize winning: Sandra Faber is renowned for her work on galaxies. (Courtesy: Gruber Foundation)
Sandra Faber has won the 2017 Gruber Foundation Cosmology Prize for her significant contributions to the modern understanding of galaxies and dark matter. Worth $500,000, the prestigious prize was established in 2000 and honours scientists whose discoveries have led to fundamental advancements in cosmology. Faber holds emeritus positions at the University of California, Santa Cruz and the University of California Observatories, and has made many groundbreaking discoveries over the course of her four-decade career. For example, in 1979 she presented a comprehensive review for the evidence of dark matter that is now considered the turning point of the field. Her later theory of how cold dark matter could explain the structure and behaviour of galaxies now underpins modern understanding of galaxy formation. Faber also discovered that every large galaxy has a supermassive black hole at its centre and played a major role in the development of the 10 m Keck telescope in Hawaii and the Wide-Field Camera for the Hubble Space Telescope. The Gruber Foundation will present Faber with the prize money and a gold laureate medal during a ceremony in the autumn. She joins an elite group of winners including the Laser Interferometer Gravitational Wave (LIGO) scientists who made the first detection of gravitational waves and 2006 Nobel Prize winner John Mather for confirming the universe began with a hot Big Bang. Faber is only the third woman to be a named recipient of the award out of 33 winners in total (not including research groups) and was chosen by a male-only advisory board.
Magnetic-monopole transformation seen in ultracold gas
Poles apart: artist’s impression of the transformation of a quantum-mechanical monopole into a Dirac monopole. (Courtesy: Heikka Valja)
The transformation of a quantum monopole into a Dirac monopole has been observed for the first time by physicists at Amherst College in the US and Aalto University in Finland. Magnetic monopoles – entities that possess only a north or a south magnetic pole – were predicted 80 years ago by Paul Dirac. While isolated monopoles have never been seen, physicists have been able to create several different collective excitations in condensed-matter systems that resemble monopoles. Now, a team led by David Hall and Mikko Möttönen has used a Bose–Einstein condensate (BEC) of ultracold rubidium atoms to first create an excitation called a quantum monopole, which takes the form of a topological point defect. The quantum monopole exists in a non-magnetized state of the BEC, but then the team applies a magnetic field to the BEC, causing it to become magnetized. This causes the destruction of the quantum monopole, which is then reborn as a Dirac monopole – an excitation that more closely resembles Dirac’s original particle. “I was jumping in the air when I saw for the first time that we get a Dirac monopole from the decay,” says Möttönen. “This discovery nicely ties together the monopoles we have been producing over the years.” The research is described in Physical Review X.
Polarized gamma rays could shed light on vacuum scattering
A new way of observing how photons scatter from virtual particles has been proposed by James Koga and Takehito Hayakawa at the National Institutes for Quantum and Radiological Science and Technology in Japan. The pair looked at an effect called Delbrück scattering, whereby photons interact with virtual electron–positron pairs in the presence of the electric field of an atomic nucleus. The effect was first observed in the 1970s, but has proven very difficult to study in isolation because it occurs alongside three other scattering processes that contribute to the elastic scattering of photons from nuclei. Now, Koga and Hayakawa have done calculations that suggest that for certain scattering angles the Delbrück scattering of polarized gamma rays will be 100 times stronger than the three other processes. A potential experiment would involve firing a beam of gamma rays with energy of about 1.1 MeV at a tin target – and Koga and Hayakawa say that this could be done at the Extreme Light Infrastructure – Nuclear Physics (ELI-NP) facility that is being built in Romania. Writing in Physical Review Letters, the pair say that an experiment running at ELI-NP for 76 days could characterize Delbrück scattering to within 1% accuracy. This would provide a new test of quantum electrodynamics and might even reveal new physics beyond the Standard Model. ELI-NP will be fully operational in 2019 and Physics World visited recently to find out how construction is progressing: see “Visiting the most powerful laser in the world“.
From mobile phones to smart watches, the sophistication and popularity of portable electronic devices have increased rapidly over the last decade. Despite revolutionary advances, we still need to plug in and recharge our devices regularly. Researchers in China, the US and Taiwan, reporting in the journal ACS Nano, have developed a portable charger—made partially from paper—which harvests and stores energy from body movement.
Zhong Lin Wang, Chenguo Hu and colleagues came up with the idea for a self-charging power device for small portable electronics such as watches and hearing aids. Electronics on this scale require little power, and typically run on batteries which need to be recharged or replaced. Wang’s team have found a way to reduce our reliance on the wall socket by instead using the energy produced by the user’s body movements such as from physical activities like walking or running.
The chargers make use of triboelectric nanogenerators (TENGs). TENGs have attracted attention in recent years due to their efficiency, high output performance and low cost. They are also lightweight, making them an attractive option for portable power devices. TENGs work by generating a charge caused by friction between different materials. TENG-based self-charger power units (SCPUs) were initially developed as a lightweight and flexible textile for self-powered portable electronics. So far, a drawback to this technology has been the weight—the device substrate has typically been made from acrylic which limits the specific mass/volume charge output of the device.
Here, the researchers have taken the TENG-based SCPU technology and developed an ultra-lightweight, rhombic-shaped cut-paper device measuring just a few inches long. The portable power device can fit into a conventional wallet, and can be charged to 1 volt in just a few minutes. The device comprises different material layers, and charges through the application and release of pressure on the device. It is made by combining a paper-based TENG (PC-TENG) with a paper-based supercapacitor (P-SC).
For the P-SC, precisely cut sandpaper is coated with gold by physical vapour deposition before a layer of graphite is deposited as the active material. A wetted separator is then sandwiched between two of the prepared graphite surfaces. The PC-TENG also uses paper as the substrate, with one side adhering to a nanostructured fluorinated ethylene propylene (FEP) thin film after a layer of Gold is deposited on both sides of the substrate.
The device has been successfully demonstrated as a power source for electric watches, temperature sensors and a remote control. The researchers suggest that there is potential for the device to be used as a self-charging power unit in medical applications. Full details of the research are reported in ACS Nano.
Caroline Herschel has enjoyed fluctuating fame since the day in 1786 when she discovered her first comet. In The Comet Sweeper: Caroline Herschel’s Astronomical Ambition, author Claire Brock examines the reasons for that as well as the circumstances of Herschel’s life, which were not straightforward.
As the youngest daughter in a large family, Herschel’s mother earmarked her early on for a life of domestic servitude, to save paying another servant. This conflicted with – and perhaps even caused – Herschel’s own ambition to earn enough to support herself. Brock argues that this remained Herschel’s primary ambition for much of her life, though the means by which she tried to earn her keep changed a few times. There was a brief flirtation with millinery and a much longer, not unsuccessful, musical career in Bath before her brother William’s interest in astronomy stopped being a side project and he co-opted Caroline as his assistant.
Brock quotes extensively from Herschel’s letters and memoirs, revealing a woman who often came across as bitter about her lot in life – particularly her dependence on William. But Brock argues that while Herschel perhaps never did love astronomy, she certainly had ambition to make real contributions to it, for the sake of science as well as her own personal advancement. In 1787 she was granted her own salary by King George III, thus becoming the first woman to earn her living from astronomy and achieving her life’s ambition.
Brock also quotes from other contemporary accounts, particularly those by women, to give a deeper flavour of the life Herschel lived. But most of all, she emphasizes Herschel’s drive to always improve herself, to self-educate in every spare moment. Brock paints a rounded portrait of a woman too-often reduced to a side note in her brother’s biography.
Robert Kephart of Fermilab speaking about the “beam business” at IPAC17.
By Margaret Harris at the International Particle Accelerator Conference in Copenhagen
Normally, you’d expect a particle-accelerator conference to focus on research – either the fundamental research done at accelerator facilities around the world, or the applied research required to get such facilities up and running in the first place. And for the most part, that has been absolutely true of the 8th International Particle Accelerator Conference (IPAC), which is taking place this week on the outskirts of Copenhagen, Denmark.
On Tuesday, however, the conference organizers dedicated a session to the ways that accelerator science engages with industry. In a two-hour series of talks, audience members heard from speakers as varied as Bjerne Clausen, CEO of the Danish chemical technologies firm Haldor Topsoe; Bob Kephart, director of the Fermilab-affiliated Illinois Accelerator Research Center (IARC); and Giovanni Anelli, who leads the Knowledge Transfer group at CERN.
Apart from our home planet of Earth, the red planet is the most visited planet in our solar system. It is not surprising then, that humans have long been interested in and intrigued by Mars, both scientifically and culturally. In 4th Rock from the Sun: the Story of Mars, author Nicky Jenner explores all these aspects of one of our nearest neighbours, going into the planet’s evolution, its geology and its moons, as well covering our robotic explorations of Mars and plans for humans to visit it in the near future. Despite a somewhat banal beginning, Jenner picks up the pace in her opening chapter as she tries to deduce our fascination with the planet, giving the reader a good description of what it would be like to traverse the Martian surface, before describing why, in fact, Mars would make for a rather boring and inhospitable holiday destination. Although most of us will be aware of the rather cold and varying temperatures on Mars, it may come as a surprise to find out that standing on the Martian surface would put your feet tens of degrees warmer than your head. The next few chapters are also interesting – Jenner digs into how human beings have anthropomorphized the planet; the fact that its red hue is particularly eye-catching; the planet’s apparent switch in direction (retrograde motion); and the fact that many respectable scientists were, at one point, convinced that Mars harboured advanced life forms. Other chapters talk about Mars’ moons Phobos and Deimos; “robot cars” or rovers and their exploration of the dusty planet; and the realities of a manned mission to Mars. In a particularly strong chapter, Jenner discusses the “massive Mars problem” – the issue of how Mars’ size has thrown off our theories for how terrestrial planets form. Although the book is somewhat haphazard in its flow, and Jenner occasionally repeats herself, 4th Rock from the Sun is both a useful and enjoyable read, especially for those interested in the planet’s cultural significance as much as the science. Grab a copy to catch up on all things Martian, especially if you plan on visiting the red planet anytime soon.
After being found guilty of heresy by the Catholic Church, Galileo Galilei was infamously placed under house arrest for the last nine years of his life. But he was far from idle during this time, writing one of the foundational works of modern science, Discourses and Mathematical Demonstrations Relating to Two New Sciences. The text includes a discussion of why it would be impossible to scale up an animal, a tree or a building to infinity. Galileo phrased it as a question of geometry – assuming a fixed shape for an object, its volume will increase at a much faster rate than its area. In practical terms, as an animal grows in size, its weight increases faster than the corresponding strength of its limbs, until the animal collapses under the force of its own weight. That’s why there could never be an animal the size of Godzilla, or Hollywood’s latest incarnation of King Kong.
In other words, there are very real constraints on how large a complex organism can grow. This is the essence of all modern-day scaling laws, and the subject of Geoffrey West’s provocative new book, Scale: the Universal Laws of Life and Death in Organisms, Cities and Companies. A physicist by training, West is a pioneer in the field of complexity science, and former director of the prestigious Santa Fe Institute in the US. Scale is the culmination of years of interdisciplinary research geared toward answering one fundamental question: could there be just a few simple rules that all complex organisms obey, whether they are animals, corporations or cities?
West clearly thinks the answer is yes, in the form of a handful of inter-related scaling laws. As evidence, he points to three simple graphs: one plots the number of heartbeats in an animal’s lifetime versus the weight; another plots metabolic rate versus rate in various animals; and the third plots the net assets and income of publicly traded companies versus number of employees. All three show strong scaling behaviour. In fact, West makes the bold claim that just by knowing the size of a mammal, he can use scaling laws to determine how much food it requires, its heart rate, life span, even the radius of its aorta, among other measurable characteristics.
The essence of any complex adaptive system – a person, a company or a city – is that there are many small interacting components within a network that iteratively follow very simple rules. Over time, complex behaviour emerges in the system, usually in an unpredictable way. Such networks can be observed all around us, and West maintains that they are the mechanism by which nature distributes energy and materials.
Within this framework, a company is much like a living mammal, consuming energy and resources to transform them into something useful – it has a metabolism, if you will. So what happens to that company as you scale it up in size? Common sense might dictate that doubling in size would require a doubling of resources, but that is not what West found when he analysed the data. An animal that is twice the size of another only needs 75% more food and energy per day, and the same goes for a company that is twice the size of another. It’s an example of sublinear scaling – and it’s the reason companies, like living organisms, have a finite life span. They grow rapidly when they are young, but growth gradually slows as they mature, until they “die” via bankruptcy, mergers or acquisitions.
Cities behave very differently, according to West. They show the same sublinear scaling when it comes to infrastructure: the bigger the city, the more efficient the distribution of its roads, cables, gas stations, power lines, railways and other infrastructure, so the fewer of those a city needs.
But the essence of any city is its people, interacting and collaborating with each other to innovate and create wealth. The socioeconomic aspects of cities – wages, number of patents, wealth, not to mention negative aspects such as crime, pollution and disease – exhibit what West terms superlinear scaling. Cities also become more diverse as they grow, while companies become more homogenized and risk-averse, making them less robust when the inevitable catastrophic fluctuation hits. Cities therefore rarely die, even after a catastrophic event. The Japanese city of Hiroshima thrives today despite the devastation wreaked on it by the atomic bomb in 1945.
Of course, there is a catch to West’s theory: such unbounded growth is ultimately unsustainable. It’s the Godzilla problem with a twist. Such a system will keep growing to infinity, requiring infinite resources, and that is just not possible in the real world. The key is innovation via disruptive technologies, for instance. A major paradigm shift will essentially reset the system, staving off collapse. But those shifts must occur at an ever-accelerating pace. There may have been thousands of years between the Stone, Bronze and Iron Ages, West writes, but only two decades between the computer age and the dawn of the information age. He likens it to having to jump around on a series of accelerating treadmills at an ever-increasing rate. And he predicts we’re due another major shift in the next 20–30 years.
Physicists are infamous, especially among statisticians, for seeing power laws everywhere, but West has constructed a rigorous and convincing case for his thesis, in clear and engaging prose. Alas, he frequently repeats himself, and he can’t quite shake off the awkward academese. (I lost count of how many times the reader is told that something will be discussed in more detail later.) That said, given the sheer scope of his subject, perhaps it’s not a bad idea to hammer the central message home several times.
Reading Scale, one’s thoughts inevitably turn to death – what Steve Jobs once called “life’s change agent”. By West’s scaling calculations, human beings in the early 21st century have roughly three billion heartbeats in a life span. How can we best make use of that time? Perhaps this is why West invokes the famous chess-playing scene from Ingmar Bergman’s classic film The Seventh Seal, where Death asks Antonius if he ever stops questioning. Antonius answers, “No, I never stop.” As West concludes, “And neither should we.” Only by constantly asking questions about how the world works, can we hope to ensure our continued survival.
Ocean-going ships face a constant struggle. In order to maintain their motion, they must continuously overcome the drag of the water that surrounds them. When one considers that marine shipping accounts for 4% of all fossil-fuel use, a similar percentage of climate-change-causing emissions and more particulate pollution than all of the world’s cars combined, it is clear that reducing this drag by even a small fraction would bring considerable benefits. Since the drag consists mostly of friction between the skin of the moving hull and the stationary water around it, lubricating this surface to reduce frictional motion would be a big help in reducing total drag.
We usually think of lubricants as being liquids, such as oil, but when friction occurs between a solid and a liquid, gas is the only real option for lubrication. For example, a torpedo can “fly” underwater, reaching otherwise unimaginable speeds, if a large pocket of water vapour engulfs its entire body via a method known as supercavitation. Also, blowing air bubbles onto the bottom side of a ship’s hull would allow the ship to move faster at a given propelling power.
You might ask, then, why we do not have gas-lubricated boats around us already. The problem is that unlike a liquid lubricant on a solid surface, a gas lubricant on a solid surface in a liquid (such as air in water) will leave the surface rather than staying on it. And unfortunately, providing a continuous supply of a rapidly disappearing gas consumes a lot of energy, which tends to cancel out the energy saved through lubrication, limiting the overall benefit.
Superhydrophobicity to the rescue
This frustration helps to explain why superhydrophobic (SHPo) surfaces were so exciting when, in the early 2000s, researchers began considering their applications for drag reduction. A SHPo surface is one that repels water much more strongly than usual. For example, a Teflon surface will repel water, forming a contact angle of around 110° between the water droplet and the surface. However, when such a naturally water-repellent material is roughened, water will sit on top of the roughness as if levitated by the air in the rough surface, with contact angles increasing to more than 150° (figure 1). As a result, the water will bead up and roll straight off when the surface is tilted. This “lotus leaf” effect has been a very popular topic in science and engineering for the past two decades, and thousands of images and online videos vividly demonstrate its intriguing properties.
If SHPo surfaces repel water so well, they must reduce the drag of water – or so the thinking went. Returning to the gas lubrication process discussed above, it was speculated that gases would persist on the SHPo surface, and thus finally make it possible to lubricate water friction with a gas layer that would not dissipate. Yet despite this logical expectation, and a torrent of research activities worldwide over the last 15 years, so far no publication has reported a successful demonstration of superhydrophobicity reducing the drag on a boat in ocean water. This article discusses why this is so, and whether there is a light at the end of this tunnel.
To identify the problems, let’s break down the issues. First, is it at least theoretically possible to obtain an appreciable drag reduction using a SHPo surface for real applications, such as a boat? Second, would a SHPo surface really save us from having to constantly supply a gaseous lubricant? Third, would it be economical to produce and implement such a SHPo surface for practical applications?
1 Water repelling The surface of a hydrophobic material (left) becomes superhydrophobic when roughened, as water sits on the top of the roughness (right).
Enough drag reduction?
Scientifically, there is no doubt that a layer of air (known as a “plastron”) on the SHPo surface in water will lubricate the motion and help reduce the drag. The real question is, just how much reduction are we talking about? If it is more than 10%, for example, that would be meaningful in practice. If the reduction is below 1%, however, we would not expect much impact in the real world even though it would still be scientifically interesting.
Despite the excitement generated by the idea’s scientific merit, and some encouraging early experimental results, it took nearly 10 years for the research community to understand how much drag reduction would be possible, and on what kinds of SHPo surfaces. Looking back, a few reasons for the slow advance are apparent. First, hydrophobicity is related to drag reduction, but not directly. Since the dynamics of bulk water and droplets are fundamentally different, a surface more favourable for droplet rolling is not necessarily more slippery to water continuously flowing by. The underlying, and still widespread, notion – that if a SHPo is very repellent to water, then it must be also very slippery to water flowing on it – is now known to be flawed.
Second, there was some confusion between drag reduction and the “slipperiness” of the surface. These concepts are linked, but they are not the same thing: the amount of drag reduction is determined not only by how slippery a surface is, but also by the flow system where the surface is employed. This means that for a given drag-reducing surface, one may obtain 50% drag reduction in a microscopic channel but not even 0.1% reduction on a boat. This mix-up made it difficult to objectively compare one SHPo surface with another in terms of their ability to reduce drag.
A third source of delays was related to measurement. Some early work on SHPo surfaces reported fantastically large reductions in drag – reductions that we now know were impossible. In many cases, the errors seem to have come from the challenge of measuring drag reduction accurately. While the observed trends may have been correct, the actual amount of reduction was simply wrong. These early, incorrect experimental data probably slowed down the establishment of the knowledge base for SHPo drag reduction.
Today, the research community uses an objective measure called “slip length” to describe the slipperiness of a given SHPo surface. We also understand how much drag reduction a particular slip length will entail under a certain flow condition, at least for turbulence-free (laminar) flows. Micro and nanofabrication technologies using microelectromechanical systems (MEMS) have played a key role in advancing the field. By enabling researchers to construct SHPo surfaces with exact and deterministic micro and nano structures (figure 2a), rather than random roughness (figure 2b), these technologies have made it possible to confirm theoretical predictions about SHPo behaviour. In a nutshell, we have learned that a SHPo surface will be more slippery (that is, its slip length will be large) if its microstructures are slender (more void spaces and fewer solid portions) and dispersed (greater distances between the structure peaks).
These theoretical predictions have been confirmed for laminar flows. If we extrapolate this to turbulent flows, it seems that a highly slippery surface, consisting of slender microstructures dispersed far apart, is required before a boat will enjoy an appreciable reduction in drag. By fabricating SHPo surfaces full of parallel trenches tens of microns apart with a large void space between them (figure 2c), our lab has reported drag reductions as large as 75% in turbulent flows. This level of reduction was obtained under well-controlled flow experiments using a small (2 × 2 cm) SHPo surface made precisely by MEMS technology, and it may not be reproduced with large surfaces in field conditions. Nevertheless, it demonstrates the potential of SHPo drag reduction. The theory also suggests that more typical SHPo surfaces (figure 2b) with microscopic random roughness would have a slip length that is too small to induce any appreciable drag reduction for macroscale applications such as a boat in open water.
2 Exact construction vs random roughness These periodic microstructures were made precisely by microelectromechanical systems and helped to establish a quantitative relationship between a superhydrophobic surface’s geometry and how slippery it is. The percentages are the proportion of void spaces on the superhydrophobic surface; surfaces with higher void percentages proved more slippery in tests. (Courtesy: CJ Kim)
Maintaining an air layer
Recall that in the early days, SHPo surfaces were considered promising for drag reduction because of their presumed ability to retain an air layer (plastron) between the surface and water even when fully submerged. The assumption was that this plastron would persist for as long as it was needed to keep the surface lubricated. The reality is not so simple, and plastron behaviour is currently the most critical issue in the field of SHPo drag reduction.
Unlike in air, where microscopic voids between microstructures or roughness in a SHPo surface stay dry even after temporary wetting by water droplets, a SHPo surface fully submerged in water will become wetted once it loses the plastron. And unfortunately, the plastron is easily lost underwater because hydrostatic pressure forces the surrounding water into the spaces between microstructures. The smaller the spaces, the more persistent the plastron – but as we have already noted, narrowly packed SHPo structures are not very slippery. We cannot have it both ways: careful quantitative studies have shown that SHPo surfaces capable of providing an appreciable (>10%) drag reduction for a boat simply cannot retain the plastron if the SHPo surface is submerged to a depth of more than a few centimetres. For most applications the surface would need to be much deeper than that.
If the plastron will be lost for most applications, the principle of SHPo drag reduction won’t apply to them either. Faced with this fundamental limitation, the only reasonable approach that is valid for all flow applications is to replenish the lost gas – ideally, using a method that is simple to implement and consumes a minimal amount of energy. Our lab is pioneering such an approach, but much still needs to be learned before it becomes practical for real-world applications.
Most drag-reduction research has been performed using SHPo surfaces with microstructures that are randomly rough. This is mainly because such surfaces are easy to fabricate: all you need to do is apply a commercially available SHPo spray coating. In contrast, SHPo surfaces with well-defined periodic microstructures, and thus a superior slip, would be simply too expensive to manufacture and implement on a ship hull – or so the discussion goes. The inconvenient fact, however, is that a surface with microscopic random roughness is simply not capable of providing enough slip to achieve meaningful drag reduction in most practical applications. A better approach would be to address this economic challenge by developing techniques to mass-manufacture SHPo surfaces that do have a chance of providing appreciable drag reduction.
Another reason why random SHPo surfaces continue to be studied, despite the well-established theory that indicates their severe limitations, is that one can also find many successful results reported in the literature. The reason for this apparent contradiction lies in the way drag reductions are tested. Most drag-reduction experiments have been performed in a water tunnel, in keeping with traditional flow experiments. But as my group confirmed recently, in flow tests using a water tunnel, the water quickly becomes supersaturated with air. In this supersaturated condition a very thick plastron forms on random SHPo surfaces, assisted by the few tallest rough protrusions. Naturally, one obtains a large drag reduction in these conditions: basically, you get the plastron you would expect from a SHPo surface with slender microstructures spaced far apart.
The problem, unfortunately, is that this thick plastron would disappear in open-water conditions, where the water is mostly undersaturated and tends to take the gas away. This dissimilarity between the water tunnel and open-water conditions is most likely why apparently successful lab studies have so far never been repeated in the real marine environment. In fact, the rare studies carried out in tow tanks actually reported an increase in drag, rather than a reduction, with random SHPo surfaces. The increase is understandable because high peaks of the random roughness will penetrate into the water once the plastron becomes thin, impeding the flow – rather like a coating of tiny barnacles.
On the other hand, a SHPo surface that is slippery enough to produce appreciable drag reduction in macroscale applications (such as a boat) is difficult to test in open-water conditions. These types of SHPo surface have large spaces between microstructures so they lose the plastron easily, and since they must be fabricated using MEMS technologies, it is difficult to manufacture the relatively large samples (more than 1 m2) used for open-water tests. Actual boats are, of course, even bigger, and their hulls are curved. But these challenges are practical rather than fundamental. By developing ways to get around them, rather than confronting them head on (which will take much longer), my lab is currently performing experiments using a motor boat that replicates field conditions as closely as possible.
Clear water ahead?
After nearly two decades of research we now understand the potential as well as the limitations of SHPo drag reduction much better than we once did. We can predict how slippery a SHPo surface is and how much drag reduction is possible at a given flow condition. Although most of our understanding deals with laminar flows, we can extrapolate this to a certain extent for turbulent flows, including open-water conditions. My group has very recently obtained a 30% reduction with a boat in ocean water. This result is preliminary, but its unprecedented success is grounded in the extensive body of scientific knowledge summarized briefly in this article. It is, perhaps, a peek into the future of marine transport.
A new technique for detecting the faint light from exoplanets that orbit two or more stars has been proposed by Artur Aleksanyan, Nina Kravets, and Etienne Brasselet at the University of Bordeaux in France. Their method is an improvement on vortex coronagraphy, a telescope-based technique that was developed in 2005. It involves sending light from a star through a mask that puts the light on an outwardly spiralling trajectory. When the light strikes the telescope camera, it is shifted some distance from the actual position of the star. This makes it possible to see faint objects such as exoplanets that are nearby to the star. While the technique works well for exoplanets orbiting a single star, it cannot resolve exoplanets that orbit two or more stars. Brasselet and colleagues have now used reconfigurable defects in a liquid crystal to create a mask that they say will send light from several different stars along spiral paths. This involves using laser light to configure the defects so that the mask will work for a specific arrangement of stars. The trio tested their scheme by simulating an exoplanet in a three-star system using four beams of light. Writing in Physical Review Letters, they describe how the view of the simulated planet was enhanced significantly (see image above).
Flat lens for immersion microscopy is a first
Nanofins: electron microscope image of the flat lens for immersion microscopy. The horizontal distance covered by the image is about 4 μm. (Courtesy: Capasso Lab)
The first flat lens for use with an immersion microscope has been made at Harvard University in the US – according to researchers there. Liquid-immersion microscopy involves placing the front lens of a microscope and the specimen in a liquid – usually water or oil. Liquids have higher indices of refraction than air and this improves the resolving power of the microscope. Front lenses used in high-performance microscopes are usually hand-polished to very high specifications, making the lenses very expensive. Furthermore, each lens will only work with fluids with specific indices of refraction. Now, Federico Capasso, Alexander Zhu, Wei Ting Chen and colleagues have created a flat lens comprising an array of titanium-dioxide “nanofins” that can be tailored for use with different immersion liquids (see image above). The nanofins are just a few hundred nanometres tall and “can be mass-produced with existing foundry technology or nanoimprinting for cost-effective high-end immersion optics”, according to Chen. The lenses can also be tailored to work in samples such as skin that have multiple layers, each having a different index of refraction. “Our immersion meta-lens can take into account the refractive indices of epidermis and dermis to focus light on the tissue under human skin without any additional design or fabrication complexity,” says Zhu. The new lens is described in Nano Letters.
Caltech students protest return of suspended astrophysicist
Students at the California Institute of Technology (Caltech) in the US staged a sit-down protest yesterday against a temporary return to campus by the astrophysics professor Christian Ott. Suspended from his job in 2015 for violating the university’s sexual-harassment policy, Ott was at Caltech at the request one of his graduate students to observe the student’s thesis presentation. According to BuzzFeed News, Ott was then escorted off campus by two faculty members. Ott is expected to return to his job in August 2017, which has raised concerns from some astrophysics students. Maya Fuller told BuzzFeed that she is “really uncomfortable” with the possibility that she may have to take a course taught by Ott. Graduate student Io Kleiser, who a Caltech investigation says was subject to gender-based harassment by Ott, said: “I personally would not like him to be on campus at all, ever.” Fiona Harrison, division chair for physics, math and astronomy at Caltech, suggested that Ott’s “behaviour and progress during his suspension” will be assessed before his return.
