A repulsive material: New hydrogel dominated by electrostatic repulsion

In a world-first achievement published in Nature, scientists from the RIKEN Center for Emergent Matter Science in Japan, along with colleagues from the National Institute of Material Science and the University of Tokyo, have developed a new hydrogel whose properties are dominated by electrostatic repulsion, rather than attractive interactions.

According to Yasuhiro Ishida, head of the Emergent Bioinspired Soft Matter Research Team, the work began from a surreptitious discovery, that when titanate nano-sheets are suspended in an aqueous colloidal dispersion, they align themselves face-to-face in a plane when subjected to a strong magnetic field. The field maximizes the electrostatic repulsion between them and entices them into a quasi-crystalline structure. They naturally orient themselves face to face, separated by the electrostatic forces between them.

To create the new material, the researchers used the newly discovered method to arrange layers of the sheets in a plane, and once the sheets were aligned in the plane, fixed the magnetically induced structural order by transforming the dispersion into a hydrogel using a procedure called light-triggered in-situ vinyl polymerization. Essentially, pulses of light are used to congeal the aqueous solution into a hydrogel, so that the sheets could no longer move.

By doing this, they created a material whose properties are dominated by electrostatic repulsion, the same force that makes our hair stand end when we touch a van generator.

Up to now, humanmade materials have not taken advantage of this phenomenon, but nature has. Cartilage owes its ability to allow virtually frictionless mechanical motion within joints, even under high compression, to the electrostatic forces inside it. Electrostatic repulsive forces are used in various places, such as maglev trains, vehicle suspensions and noncontact bearings, but up to now, materials design has focused overwhelmingly on attractive interactions.

The resultant new material, which contains the first example of charged inorganic structures that align co-facially in a magnetic flux, has interesting properties. It easily deforms when shear forces are applied parallel to the embedded nano-sheets, but strongly resists compressive forces applied orthogonally.

According to Ishida, "This was a surprising discovery, but one that nature has already made use of. We anticipate that the concept of embedding anisotropic repulsive electrostatics within a composite material, based on inspiration from articular cartilage, will open new possibilities for developing soft materials with unusual functions. Materials of this kind could be used in the future in various areas from regenerative medicine to precise machine engineering, by allowing the creation of artificial cartilage, anti-vibration materials and other materials that require resistance to deformation in one plane."

Selengkapnya »»  

How electrons split: New evidence of exotic behaviors

Electrons may be seen as small magnets that also carry a negative electrical charge. On a fundamental level, these two properties are indivisible. However, in certain materials where the electrons are constrained in a quasi one-dimensional world, they appear to split into a magnet and an electrical charge, which can move freely and independently of each other. A longstanding question has been whether or not similar phenomenon can happen in more than one dimension. A team lead by EPFL scientists now has uncovered new evidence showing that this can happen in quasi two-dimensional magnetic materials. Their work is published in Nature Physics.

A strange phenomenon occurs with electrons in materials that are so thin that they can be thought of as being one-dimensional, e.g. nanowires. Under certain conditions, the electrons in these materials can actually split into an electrical charge and a magnet, which are referred to as "fractional particles." An important but still unresolved question in fundamental particle physics is whether this phenomenon could arise and be observed in more dimensions, like two- or three-dimensional systems.

Henrik M. Rønnow and Bastien Dalla Piazza at EPFL and Martin Mourigal (recently appointed Assistant professor at Georgia Tech) have now led a study that provides both experimental and theoretical evidence showing that this exotic split of the electrons into fractional particles actually does take place in two dimensions. The scientists combined state-of-the-art polarized neutron scattering technology with a novel theoretical framework, and tested a material that normally acts as an electrical insulator. Their data showed that the electrons magnetic moment can split into two halves and move almost independently in the material.

The existence of fractional particles in more than one dimension was proposed by Nobel laureate PW Anderson in 1987 when trying to develop a theory that could explain high-temperature superconductivity: the ability of some materials to conduct electricity with zero resistance at very low, yet technologically feasible, temperatures. This phenomenon remains one of the greatest mysteries and has been extensively researched in the most promising high-temperature superconductors, the copper-containing cuprates.

Under temperatures close to absolute zero, electrons bind together to form an exotic liquid that can flow with exactly no friction. While this was previously observed at near-absolute zero temperatures in other materials, this electron liquid can form in cuprates at much higher temperatures that can be reached using liquid nitrogen alone. Consequently, there is currently an effort to find new materials displaying high-temperature superconductivity at room temperature. But understanding how it arises on a fundamental level has proven challenging, which limits the development of materials that can be used in applications. The advances brought by the EPFL scientists now bring support for the theory of superconductivity as postulated by Anderson.

"This work marks a new level of understanding in one of the most fundamental models in physics," says Henrik M. Rønnow. "It also lends new support for Anderson's theory of high-temperature superconductivity, which, despite twenty-five years of intense research, remains one of the greatest mysteries in the discovery of modern materials."

Selengkapnya »»  

Basic rules for construction with a type of origami

Hands-on kirigami: With a cut and a few folds, this structure could serve as a shelter or a microfluidic channel.

Origami is capable of turning a simple sheet of paper into a pretty paper crane, but the principles behind the paper-folding art can also be applied to making a microfluidic device for a blood test, or for storing a satellite's solar panel in a rocket's cargo bay.

A team of University of Pennsylvania researchers is turning kirigami, a related art form that allows the paper to be cut, into a technique that can be applied equally to structures on those vastly divergent length scales.

In a new study, the researchers lay out the rules for folding and cutting a hexagonal lattice into a wide variety of useful three-dimensional shapes. Because these rules ensure the proportions of the hexagons remain intact after the cuts and folds are made, the rules apply to starting materials of any size. This enables materials to be selected based on their relevance to the ultimate application, whether it is in nanotechnology, architecture or aerospace.

The study was conducted by Toen Castle, a postdoctoral researcher in the School of Arts & Science's Department of Physics and Astronomy; Shu Yang, a professor in the School of Engineering and Applied Science's Department of Materials Science and Engineering; and professor Randall Kamien, also of the Department of Physics and Astronomy. Also contributing to the study were undergraduate Xingting Gong and postdoctoral researcher Daniel Sussman, members of Kamien's research group; graduate student Euiyeon Jung, a member of Yang's group; and postdoctoral researcher Yigil Cho, who works in both groups.

It was published in the journal Physical Review Letters.

"If you see a fancy piece of origami," Kamien said, "it can have arbitrarily small folds. We want to make something much simpler. If there are standards for the size of folds and cuts, we can make the math apply to any length scale. We can make channels, gates, steps and other 3-D shapes without needing to know anything about the size of the sheet and then combine those building blocks into even more complex shapes."

A hexagonal lattice may seem like an odd choice for a starting point, but the pattern has advantages over a seemingly simpler tessellation, such as one made from squares.

"The connected centers of the hexagons make triangles," Castle said, "so, if you start with a hexagonal lattice, you get the triangles for free. It's like two lattices in one, whereas if you start with squares, you only get squares."

"Plus," Yang said, "it's easier to fill a space with a hexagonal lattice and move from 2-D to 3-D. That's why you see it in nature, in things like honeycombs."

Starting from a flat hexagonal grid on a sheet of paper, the researchers outlined the fundamental cuts and folds that allow the resulting shape to keep the same proportions of the initial lattice, even if some of the material is removed. This is a critical quality for making the transition from paper to materials that might be used in real-world applications.

"You can think of the sheet of paper as a template for a mesh of rods that you can lay on top of it," Castle said. "Alternatively, you can think of the paper as the membrane that attaches to a scaffolding. Both concepts are in the theory from the start; it's just a question of whether you want to build the rods or the material between them."

Having a set of rules that draws on fundamental mathematical principles means the kirigami approach can be applied equally across length scales, and with almost any material.

"The rules we lay out," Kamien said, "tell you how you make the cuts so you only have to fold on straight lines, and so that, when you fold them together, the rods remain the same length and the centers remain the same distance apart. You may have to bend [or put hinges on] some of the rods to make the folds, but you don't have to be able to stretch them. That also means the whole structure remains rigid when you're done folding."

"This means it's just a matter of picking the materials with the properties you want for your application," Yang said. "We can go from nanoscale materials like graphene to materials you would make clothing out of to materials you would see in a space station or satellite."

The rules also guarantee that "modules," basic shapes like channels that can direct the flow of fluids, can be combined into more complex ones. For example, iterating those folds and cuts can produce a ratcheting interface that can lock itself into place at different points. This structural feature could change the volume of a channel or even serve as an actuator for a robot.

Kirigami is particularly attractive for nanoscale applications, where the simplest, most space-efficient shapes are necessary, and self-folding materials would circumvent some of the fabrication challenges inherent in working at such small scales.

The research was supported by the National Science Foundation through its ODISSEI program, the American Philosophical Society and the Simons Foundation.

Selengkapnya »»