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Graphene oxide weaved into ‘paper’

First isolated in 2004, graphene is a one-atom-thick sheet of graphite that, aside from having unique electronic properties, is very strong. But as of yet there is no way of producing it in large quantities, which has limited its potential as a building block for new types of specialist materials.

Now, however, a group from Northwestern University in the US including Rodney Ruoff have discovered that large quantities of oxidized graphene can be weaved together to create a new type of “paper” that is stiffer and stronger than other thin materials.

“My dream has been to disassemble graphite into individual sheets, and then reassemble those sheets in different ways,” Ruoff told physicsworld.com. To do this his group begins by oxidizing graphite to make graphite oxide, which leaves roughly half the carbon atoms with an attached oxygen atom. When graphite oxide is mixed into water, these oxygen atoms repel water molecules, forcing the individual layers – graphene oxide – to disperse or “exfoliate”. The researchers filter this exfoliated mixture through a membrane, which collects the layers in such an arrangement to produce graphene oxide paper.

Normal graphite has a delicate structure, needing only a small lateral force to break apart its regularly-stacked layers. Conversely, the layers in graphene oxide paper interweave with one another and wrinkle on larger scales. This allows load to be distributed across the structure, making it stronger than graphite foil and “bucky paper”, which is made from carbon nanotubes. In fact, Ruoff claims, the only material stronger could be diamond.

Many layers

The interwoven structure also lets individual layers shift over each other, so that the collective layers become pliable. But most importantly the paper can be chemically tuned by altering the amount of oxygen on the layers. Reducing the oxygen content, for example, would take it from being an electrical insulator to a good conductor. Moreover, the paper could be infused with polymers, ceramics or metals, to make composite materials that outperform their pure counterparts.

This wide array of properties could mean applications as diverse as membranes with controlled permeability to supercapacitors for energy storage.

Artificial swimmers have no moving parts

Making micrometre-sized objects swim is no easy task because over very short distances, water behaves like a very viscous fluid such as honey. Some bacteria manage to swim by using highly specialized undulating whips called flagella – and while some progress has been made in creating artificial flagella, they have proved very difficult to mimic in a tiny machine.

In 2005 Ramin Golestanian, a theoretical physicist at the University of Sheffield in the UK and colleagues proposed a much simpler way of propelling tiny objects that uses no moving parts. Now a team led by fellow Sheffield physicist Richard Jones has created such a propulsion system for making particles swim in a solution of water and hydrogen peroxide.

The team used polystyrene balls that were about 1.6µm in diameter and had one side coated with platinum — a catalyst that boosts the rate at which hydrogen peroxide is converted into oxygen and water. This reaction decreases the concentration of hydrogen peroxide in the region near the platinum-coated side of the sphere, causing water to flow away from the region in order to maintain equilibrium. This flowing water pushes the object in a specific direction relative to the coating– if the platinum is on the right hand side of the sphere, for example, the sphere would move to the left.

By looking at the system with an optical microscope, the researchers saw that the balls could reach speeds of 5µm/s — which is not far off the 10µm/s observed in similarly-sized bacteria. According to Golestanian, the propulsion technique could be adapted to work in other liquids including blood, which could someday allow micromachines to swim within the body to deliver drugs to specific locations.

However like bacteria, the swimmers also have to contend with another consequence of being very small –- being knocked off course by random collisions with water molecules in a process called Brownian motion. Indeed, after a few seconds of motion in a specific direction, the Sheffield swimmers followed completely random paths.

Golestanian told physicsworld.com that thermodynamics makes it impossible to design a tiny object that would be able to avoid Brownian motion on its own and travel in a straight line. Instead, he believes that the objects could be guided externally – for example, if a magnetic dipole could be placed in the object, it could be steered using a magnetic field.

Tiny magnets help drugs reach the spot

Many pulmonary diseases, such as asthma, cystic fibrosis and lung cancer, need drugs to be inhaled so that they can reach the affected area. To do this, patients have to gasp on an inhaler that emits the particulate drugs into the windpipe.

But the effectiveness of these inhalers is not great: typically only 4% of the drug makes it through the windpipe, forcing doctors to administer higher doses, which can exacerbate unwanted side effects.

A better way, according to Carsten Rudolph at Ludwig-Maximilians University in Munich and co-workers from elsewhere in Germany, is to mix the drugs with iron-oxide magnetic nanoparticles and microdroplets of water, or so-called “nanomagnetosols”. These nanomagnetosols can then be guided directly to problem areas using a magnetic field. The idea is not new, but Rudolph’s group show for the first time that it can be performed in a real organism – in this case, a mouse.

The researchers began by creating a computer simulation of a mouse’s airways where the windpipe forks into two bronchi, taking into account air flow rates measured in previous physiological studies. Assuming they were to use nanomagnetosol droplets with an average diameter of 3.5 µm, they predicted that they could use a magnetic probe placed close to a bronchus to get up to 16% coverage of the microdroplets.

Rudolph’s group tested their prediction by opening up the chest of a mouse, and placed a specially designed magnetic tip probe with a high flux gradient of 100 Tm^-1 next to one of the lungs. When they squirted the nanomagnetosol droplets into the mouse’s airways, they found that the lung next to the probe received eight times more drug coverage than the one without. Upon placing the probe on another mouse with its chest intact, the benefit was reduced, with just two and a half times more coverage.

Performing the same feat in humans will not be so straightforward, however. Human lungs are much larger and more intricate, so it will be difficult to guide the nanomagnetosol droplets with the same accuracy. Moreover, a much more powerful magnetic probe will be required to overcome the additional distance between the probe and inner lung.

Supersolid saga continues

The first convincing evidence for supersolidity came in 2004, when US physicists Moses Chan and Eun-Song Kim were monitoring the rotation of a sample of helium-4 supported inside a torsion oscillator. As they reduced the temperature below 230 mK, they noticed that the oscillations sped up slightly, and concluded that 1% of the sample had become a supersolid and was therefore staying still in the lab frame.

At first, this was thought to be proof of a phase transition predicted in the late 1960s, which suggested that close to absolute zero any lattice vacancies present in a sample would collapse into the same quantum state, becoming a so-called Bose-Einstein condensate. In this supersolid phase, the vacancies would be able to move throughout the rest of the solid effortlessly like a superfluid.

More recent calculations, however, have shown that there would not be enough vacancy condensation at low temperature to give supersolid signal as large as 1%.

One alternative explanation is that the signal actually comes from supersolidity involving tiny “grain boundaries”, but this was ruled-out experimentally last month by Chan, who told Physics Web that he suspected the signal could be originating in dislocations within the crystal lattice.

Now, Massimo Boninsegni from the University of Alberta and collaborators from the US and Switzerland have backed-up this suspicion by simulating a “screw” dislocation in a microscopic helium-4 crystal. Screw dislocations form when a break in the crystal lattice causes atoms to stack-up in a structure akin to a spiral staircase, and for complex helium-4 crystals have been too difficult to simulate accurately in the past.

Using a new computer algorithm Boninsegni’s group found that, as they reduced the temperature parameter towards absolute zero, the core of the screw dislocation acted like a tube through which some of the atoms could flow freely – essentially in one dimension – as a superfluid. “This is an important aspect experimentally, as a one-dimensional system has distinct physical signatures that can be probed by measurement,” Boninsegni said.

The researchers say that a network of these superfluid cores could produce a supersolid signal in a real sample, although it would not produce one as high as the 1% originally recorded by Kim and Chan. This signal strength, they suggest, might be due to auxiliary sources, such as “pockets” of superfluid.

Partnership to boost fuel-cell research

Fuel cells are typically made up of an anode and a cathode, separated by a membrane that conducts protons but not electrons. When a fuel, such as hydrogen, is supplied to the anode, it is broken into its constituent protons and electrons. The protons are attracted to the cathode, while the electrons – unable to travel through the membrane – are harnessed in an adjacent circuit to produce electrical energy.

Fuel-cell technologies could bring important environmental benefits because they are much cleaner than traditional sources of power such as internal combustion engines. At worst, fuel cells release small amounts of carbon dioxide, but often the only by-product is water.

In this agreement, physicists at Oak Ridge National Laboratory (ORNL), which is owned by the US Department of Energy, will use their skills in the imaging of solids and surfaces to develop metal and carbon bipolar plates for the anodes and cathodes. Jülich researchers will share their materials and fabrication techniques.

The researchers will concentrate on two types of fuel cells: proton-exchange membrane fuel cells (PEMFCs) and direct-methanol fuel cells (DMFCs), both of which are already used to some extent commercially. Because of their relatively high output, PEMFCs are primarily being developed for the automotive industry. DMFCs can currently only produce small amounts of power, albeit over a long period of time, which makes them better suited to portable devices such as mobile phones, digital cameras or laptops.

“This agreement underscores the Department of Energy’s commitment to the [US] president’s hydrogen fuel initiative,” said Tim Armstrong, the manager of ORNL’s fuel cells program.

Liquid jets bounce along

The photographs were taken by Matthew Thrasher and colleagues at the University of Texas at Austin, who built a rotating oil bath into which a stream of oil was dropped under the watchful eye of a video camera. The photographs reveal that when the stream strikes the surface of the bath, it slides along the surface on a thin layer of air. Furthermore, the force of impact creates a bowl-shaped indentation in the surface of the liquid that acts as a ramp, launching the stream back into the air (see figure “Bouncing jet”). The stream then arcs over the surface and plunges back into the liquid, sometimes emerging in a second arc.

Skimming

Bouncing was observed in a number of different silicone oils with viscosities ranging from 56 to 560 times that of water. The arc became smaller when the bath was rotated more quickly until the jet no longer lifted off the surface, but rather skimmed its surface (see figure “Skimming”)

Thrasher told Physics Web that a crucial requirement for bouncing is that the layer of air supporting the jet must not break into air bubbles, which would disrupt the stream. He adds that an understanding of why some liquids bounce while others bubble could help improve the metal casting process — in which molten material is poured into a mould and bubbles must be carefully avoided because they weaken the solid metal. Conversely, plunging jets are often used to introduce air bubbles into liquids – a familiar example being bath bubbles that form under a running tap. Therefore Thrasher believes that those designing liquid aeration systems should understand how to avoid the bouncing of fluids.

Thrasher came up with the idea for the experiment when he was pouring oil from one container to another and noticed that the stream of poured oil would sometimes bounce across the surface of the container. In their paper, the researchers suggest several simple experiments for observing bouncing jets in the classroom or even at home.

Nanostructures help mosquitoes walk on water

Scientists know pond skaters stay afloat because of a special “hairy” nanostructure on their legs that holds pockets of air. These prevent the legs from ever getting wet, thus allowing the insects to be propped up on water’s surface tension. Flies, on the other hand, are adept at clinging vertically or upside down to smooth surfaces, using either bristles that produce attractive forces or claws on their feet.

To see if mosquitoes exploit a similar nanostructure to pond skaters for landing on water, a group led by Chengwei Wu from the Dalian University of Technology in China used a scanning electron microscope (SEM) to examine the six legs, between 5 and 7 mm in length, of mosquitoes caught in their local area.

In addition to claws for clinging to surfaces, they found that the legs were covered in a profusion of scales, of which each was adorned with six to twelve randomly-spaced ridges with a thickness of about 200 nm. These ridges were linked by many smaller “ribs” with a thickness of 100 nm.

Wu’s team thought that, like pond skaters, air would be contained in the gaps created between these ridges and ribs to make the legs stay afloat in water. To see if this was right, they connected the severed leg from a mosquito to a moveable steel needle, and positioned it over a container of water sitting on a force balance. They then inclined the leg to a 30° angle – roughly the angle that would be used for landing – and pushed it against the water’s surface.

Surprisingly, the researchers found that the leg resisted with a force of up to 600 µN – roughly 23 times the mosquito’s body weight – before breaking the surface. In contrast, a pond skater can only receive a force equal to 15 times its body weight through its legs. A common fly, the researchers later found, could barely support its own body weight at all. This more refined water-repelling nanostructure could provide greater stability when mosquitoes land on the surface of water.

To check whether the ability was not simply a result of a low-density – that is, more buoyant – material, Wu’s team repeated the experiment with the leg pitched vertically, which would prevent any air from being trapped. As expected, the force from the leg was heavily reduced, almost by a factor of 50.

Mosquitoes do not have the best reputation. Although in developed nations their damage tends to be limited to itchy bites, in the third world they cause millions of deaths each year by spreading malaria. “Most of the previous studies on mosquitoes were focused on the harmful sides that they bring to human beings,” said the researchers. They added that among the special skills of the mosquitoes, the water-repelling ability “might be the most surprising.”

Dark-energy teams win cosmology prize

Each team presented its findings in two key papers, published early in 1998. Perlmutter, who is at the Lawrence Berkeley National Laboratory and the University of California at Berkeley, was leader of the Supernova Cosmology Project. Brian Schmidt, who headed the High-Z Supernova Search, is based at the Australian National University in Canberra. Perlmutter and Schmidt take home $125,000 in prize money each, while the other team members will share the remaining $250,000.

Brian Schmidt

Both teams discovered the accelerating expansion of the universe by studying distant type Ia supernovae. These exploding stars are believed to all have the same luminosity, which means that their brightness can be used to determine how far away they are. However, Perlmutter and Schmidt’s teams were surprised to find that the light from these supernovae was fainter than expected for a given expansion velocity, which indicated that the supernovae were further away than expected.

Since the 1998 breakthrough, the accelerating expansion of the universe has been confirmed by independent studies including those looking at the cosmic microwave background and the distribution of thousands of galaxies. These and other studies have strengthened the argument for the existence of dark energy, which some physicists believe accounts for nearly three-quarters of the mass of the universe. However, dark energy has yet to be observed directly, either the laboratory or the cosmos.

The prize citation describes the discovery as “a crazy result that was hard to accept”. It also commends both teams for developing “new techniques that use supernovae exploding within distant galaxies to measure precise distances across a large fraction of the observable universe”. The Gruber prize has been awarded annually since 2000 by the Peter and Patricia Gruber Foundation, which is based in the US Virgin Islands.

First “heat transistor” unveiled

The transistor is based on a design for a single-electron refrigerator that was unveiled earlier this year by Jukka Pekola and colleagues at the Helsinki University of Technology and NEST in Pisa, Italy (see Refrigerator cools one electron at a time).

Heat is transported by electrons as they tunnel though junctions between a metal and a superconductor (see figure). Only one electron at a time can squeeze through a tiny junction because mutual repulsion between electrons prevents multiple tunnelling. This orderly process ensures that only the hottest electrons can leave the metal, thereby causing heat to flow from the metal to the superconductor.

The researchers found that the heat flux though the transistor was a function of the temperature of the metal electrode, leading them to suggest that the device could be used as an accurate thermometer for measuring temperatures in the range of hundreds of millikelvin. Because the device allows for the precise study of the heat transported by single electrons, it could also help physicists to design better conventional electronic refrigerators, which also use electrons to transfer heat.

Pekola told Physics Web that while the heat transistor could someday be used to control the temperature of electronic devices, its very small cooling power rules out most practical applications.

Steel balls make a splash in sand

Two years ago, Detlef Lohse and a team of physicists from the University of Twente devised a mechanism for crater formation by dropping metal balls in beds of sand. They found that the impact first throws sand outwards in a crown-shaped “splash”, leaving the ball to penetrate further into the surface and create a void. The pressure of the sand then forces the grains to fill the void, causing a vigorous jet of sand to shoot up from the centre (see related story: “Extraterrestrial impact created in the lab”).

The striking conclusion for the researchers was that this mechanism is very similar to what would happen in a liquid. This meant that, in certain circumstances, the well-established equations for fluid dynamics could potentially be used to describe poorly-understood granular systems such as sand. Now, however, Lohse and co-workers have gone one step further and found that the ambient pressure of the air above the sand is related to the height of the jet and the depth of penetration of the ball.

In a new set of experiments, they dropped 1.4 cm steel balls into a bed of sand 40 cm deep. But this time they put the entire apparatus inside a container that could be partially evacuated with an air pump, and attached a taut thread with markers to the balls so that the penetration depth could be read off easily.

With more air evacuated from the container, and hence less pressure, they discovered that the jet was less vigorous, and that the ball did not penetrate as deep. The researchers claim that this ties in with their existing mechanism: a lower pressure means there is less air surrounding the ball and the sand grains, thus increasing drag and making the sand behave less like a fluid. The net result is a shallower penetration, and a shorter jet. “It reveals the importance of air in fine granular matter,” Lohse told Physics Web.

Gabriel Caballero, a co-author of the study, said that the research could eventually have applications ranging from the design of space probes that have to land on other planets, to how powdered chemicals are mixed in pharmaceuticals. The team are now looking to investigate how the splash of the sand creates the ensuing crater.

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