SciCombinator

Discover the most talked about and latest scientific content & concepts.

Concept: Electric field

51

The electrostatic confinement of massless charge carriers is hampered by Klein tunneling. Circumventing this problem in graphene mainly relies on carving out nanostructures or applying electric displacement fields to open a band gap in bilayer graphene. So far, these approaches suffer from edge disorder or insufficiently controlled localization of electrons. Here we realize an alternative strategy in monolayer graphene, by combining a homogeneous magnetic field and electrostatic confinement. Using the tip of a scanning tunneling microscope, we induce a confining potential in the Landau gaps of bulk graphene without the need for physical edges. Gating the localized states towards the Fermi energy leads to regular charging sequences with more than 40 Coulomb peaks exhibiting typical addition energies of 7-20meV. Orbital splittings of 4-10meV and a valley splitting of about 3meV for the first orbital state can be deduced. These experimental observations are quantitatively reproduced by tight binding calculations, which include the interactions of the graphene with the aligned hexagonal boron nitride substrate. The demonstrated confinement approach appears suitable to create quantum dots with well-defined wave function properties beyond the reach of traditional techniques.

Concepts: Electron, Electric charge, Energy, Quantum mechanics, Electric field, Condensed matter physics, Band gap, Boron nitride

43

The validity of the superposition principle and of Born’s rule are well-accepted tenants of quantum mechanics. Surprisingly, it has been predicted that the intensity pattern formed in a three-slit experiment is seemingly in contradiction with the most conventional form of the superposition principle when exotic looped trajectories are taken into account. However, the probability of observing such paths is typically very small, thus rendering them extremely difficult to measure. Here we confirm the validity of Born’s rule and present the first experimental observation of exotic trajectories as additional paths for the light by directly measuring their contribution to the formation of optical interference fringes. We accomplish this by enhancing the electromagnetic near-fields in the vicinity of the slits through the excitation of surface plasmons. This process increases the probability of occurrence of these exotic trajectories, demonstrating that they are related to the near-field component of the photon’s wavefunction.

Concepts: Electromagnetism, Photon, Quantum mechanics, Physics, Light, Electric field, Quantum field theory, Wave

38

Magnetically actuated ciliary microrobots were designed, fabricated, and manipulated to mimic cilia-based microorganisms such as paramecia. Full three-dimensional (3D) microrobot structures were fabricated using 3D laser lithography to form a polymer base structure. A nickel/titanium bilayer was sputtered onto the cilia part of the microrobot to ensure magnetic actuation and biocompatibility. The microrobots were manipulated by an electromagnetic coil system, which generated a stepping magnetic field to actuate the cilia with non-reciprocal motion. The cilia beating motion produced a net propulsive force, resulting in movement of the microrobot. The magnetic forces on individual cilia were calculated with various input parameters including magnetic field strength, cilium length, applied field angle, actual cilium angle, etc., and the translational velocity was measured experimentally. The position and orientation of the ciliary microrobots were precisely controlled, and targeted particle transportation was demonstrated experimentally.

Concepts: Electron, Electromagnetism, Magnetic field, Electric field, Magnetism, Ferromagnetism, Magnetic moment, Magnetic flux

32

Machine technology frequently puts magnetic or electrostatic repulsive forces to practical use, as in maglev trains, vehicle suspensions or non-contact bearings. In contrast, materials design overwhelmingly focuses on attractive interactions, such as in the many advanced polymer-based composites, where inorganic fillers interact with a polymer matrix to improve mechanical properties. However, articular cartilage strikingly illustrates how electrostatic repulsion can be harnessed to achieve unparalleled functional efficiency: it permits virtually frictionless mechanical motion within joints, even under high compression. Here we describe a composite hydrogel with anisotropic mechanical properties dominated by electrostatic repulsion between negatively charged unilamellar titanate nanosheets embedded within it. Crucial to the behaviour of this hydrogel is the serendipitous discovery of cofacial nanosheet alignment in aqueous colloidal dispersions subjected to a strong magnetic field, which maximizes electrostatic repulsion and thereby induces a quasi-crystalline structural ordering over macroscopic length scales and with uniformly large face-to-face nanosheet separation. We fix this transiently induced structural order by transforming the dispersion into a hydrogel using light-triggered in situ vinyl polymerization. The resultant hydrogel, containing charged inorganic structures that align cofacially in a magnetic flux, deforms easily under shear forces applied parallel to the embedded nanosheets yet resists compressive forces applied orthogonally. We anticipate that the concept of embedding anisotropic repulsive electrostatics within a composite material, inspired by articular cartilage, will open up new possibilities for developing soft materials with unusual functions.

Concepts: Electron, Electromagnetism, Electric charge, Magnetic field, Fundamental physics concepts, Electric field, Magnetic flux, Coulomb's law

32

Honeybees, like other insects, accumulate electric charge in flight, and when their body parts are moved or rubbed together. We report that bees emit constant and modulated electric fields when flying, landing, walking and during the waggle dance. The electric fields emitted by dancing bees consist of low- and high-frequency components. Both components induce passive antennal movements in stationary bees according to Coulomb’s law. Bees learn both the constant and the modulated electric field components in the context of appetitive proboscis extension response conditioning. Using this paradigm, we identify mechanoreceptors in both joints of the antennae as sensors. Other mechanoreceptors on the bee body are potentially involved but are less sensitive. Using laser vibrometry, we show that the electrically charged flagellum is moved by constant and modulated electric fields and more strongly so if sound and electric fields interact. Recordings from axons of the Johnston organ document its sensitivity to electric field stimuli. Our analyses identify electric fields emanating from the surface charge of bees as stimuli for mechanoreceptors, and as biologically relevant stimuli, which may play a role in social communication.

Concepts: Electron, Electromagnetism, Electric charge, Electromagnetic field, Fundamental physics concepts, Electrostatics, Electric field, Coulomb's law

32

Invisibility to electromagnetic fields has become an exciting theoretical possibility. However, the experimental realization of electromagnetic cloaks has only been achieved starting from simplified approaches (for instance, based on ray approximation, canceling only some terms of the scattering fields, or hiding a bulge in a plane instead of an object in free space). Here, we demonstrate, directly from Maxwell equations, that a specially designed cylindrical superconductor-ferromagnetic bilayer can exactly cloak uniform static magnetic fields, and we experimentally confirmed this effect in an actual setup.

Concepts: Electromagnetism, Magnetic field, Electric field, Maxwell's equations, Invisibility, James Clerk Maxwell, Equations, Electromagnetic wave equation

28

Self organization of large-scale structures in nature - either coherent structures like crystals, or incoherent dynamic structures like clouds - is governed by long-range interactions. In many problems, hydrodynamics and electrostatics are the source of such long-range interactions. The tuning of electrostatic interactions has helped to elucidate when coherent crystalline structures or incoherent amorphous structures form in colloidal systems. However, there is little understanding of self organization in situations where both electrostatic and hydrodynamic interactions are present. We present a minimal two-component oil-in-oil model system where we can control the strength and lengthscale of the electrohydrodynamic interactions by tuning the amplitude and frequency of the imposed electric field. As a function of the hydrodynamic lengthscale, we observe a rich phenomenology of exotic structure and dynamics, from incoherent cloud-like structures and chaotic droplet dynamics, to polyhedral droplet phases, to coherent droplet arrays.

Concepts: Electric charge, Fundamental physics concepts, Fluid dynamics, Magnetohydrodynamics, Electrohydrodynamics, Structure, Electrostatics, Electric field

25

The peptide drug enfuvirtide (T20) is the only HIV-1 fusion inhibitor in clinical use, but it easily induces drug-resistance, calling for new strategies for developing next-generation drugs. On the basis of the M-T hook structure, we recently developed highly potent short-peptide HIV-1 fusion inhibitors (MTSC22 and HP23), which mainly target the conserved gp41 pocket and possess high genetic barriers to resistance. Here, we focused on the selection and characterization of HIV-1 escape mutants to MTSC22, which revealed new resistance pathways and mechanisms. Two mutations, E49K and L57R, located at the inhibitor-binding site, and two mutations, N126K and E136G, located at the C-terminal heptad repeat region of gp41, were identified as conferring high resistance either singly or in combinations. While E49K reduced the C-terminal binding of inhibitors via an electrostatic repulsion, L57R dramatically disrupted the N-terminal binding of M-T hook structure and pocket-binding domain. Different from E49K and N126K that enhanced the stability of endogenous viral six-helical bundle core (6-HB), L57R and E136G conversely destabilized the 6-HB structure. We also demonstrated that both primary and secondary mutations caused the structural changes of 6-HB and severely impaired the ability of HIV-1 entry. Collectively, our data provide novel insights into the mechanisms of short-peptide fusion inhibitors targeting the gp41 pocket site and help our understanding for the structure and function of gp41 and HIV-1 evolution.

Concepts: HIV, Protein, Michael Faraday, Electrostatics, Electric field, CCR5, Inhibitor, Entry inhibitor

25

This study aimed to evaluate effects of pulsed electromagnetic fields (PEMF) in a double blind study on patients with knee joint osteoarthritis. The MAGCELL ARTHRO electrode-less therapy delivered a sinusoidal magnetic field, varying in frequency between 4 and 12 Hz. In 1 cm tissue depth, magnetic flux density was 105 mT. A total of n = 57 patients were randomly assigned to the verum, PEMF or placebo group (placebo device). Their average age was 61.6 ± 12.0 years. According to American College of Rheumatology criteria the osteoarthritis level was 2.8 ± 0.8. Treatment was performed twice a day for 5 min over a period of 18 days. Treatment with the MAGCELL device versus control (sham exposed) showed a highly significant reduction in pain (P < 0.001), a significant reduction in stiffness (P = 0.032) and a significant reduction in disability in daily activities (P = 0.005) according to the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scales-with a good overall treatment tolerance. In the placebo group there was no evidence of a significant change between the initial and final examination in any of the three above-mentioned WOMAC scales. Results of this partly randomized placebo-controlled double-blind study show clinically at any rate, that use of PEMF lead to highly significant better results in the treatment group compared to the placebo group with regard to the total WOMAC global score and especially for visual analogue scale. Patient assessment of the "effectiveness" was rated in 29.5% as very good and good in 27.3% compared to 0.0% and 15.4% in controls. This therapy is thus a useful complementary treatment option with no side effects. Bioelectromagnetics. 2015;9999:1-10. © 2015 Wiley Periodicals, Inc.

Concepts: Pharmacology, Electromagnetism, Electromagnetic field, Magnetic field, Electric field, Magnetism, Magnetic flux, Blind experiment

23

A series of three-dimensional (3-D) TiO2/graphene (TiO2/GR) hybrids with different TiO2 weight ratios were prepared using a self-assembly approach followed by the gaseous reduction in a hydrothermal system. The method was based on the electrostatic attraction between the positively charged titanium glycolate precursor and negatively charged graphene oxide in an aqueous medium without any surfactant or template. The structure, morphology, physical and optical properties of the as-synthesized hybrids were characterized, and the results showed that TiO2 spheres were homogeneously confined within the 3-D networks of graphene, and acted as pillars to effectively separate the graphene sheets from each other. By optimizing the ratio of TiO2 in the hybrids, the material was identified as an excellent photocatalyst to remove organic compound in water with high degradation efficiency. Additionally, TiO2/GR hybrids delivered high specific capacity, enhanced rate capability and excellent cyclic stability when used as a freestanding electrode for lithium ion batteries.

Concepts: Electric charge, Electrostatics, Electric field, Ratio, Titanium dioxide, Rechargeable battery, Lithium, Coulomb's law