Magnetic assembly of nonmagnetic particles into photonic crystal structures

We report the rapid formation of photonic crystal structures by assembly of uniform nonmagnetic colloidal particles in ferrofluids using external magnetic fields. Magnetic manipulation of nonmagnetic particles with size down to a few hundred nanometers, suitable building blocks for producing photonic crystals with band gaps located in the visible regime, has been difficult due to their weak magnetic dipole moment. Increasing the dipole moment of magnetic holes has been limited by the instability of ferrofluids toward aggregation at high concentration or under strong magnetic field. By taking advantage of the superior stability of highly surface-charged magnetite nanocrystal-based ferrofluids, in this paper we have been able to successfully assemble 185 nm nonmagnetic polymer beads into photonic crystal structures, from 1D chains to 3D assemblies as determined by the interplay of magnetic dipole force and packing force. In a strong magnetic field with large field gradient, 3D photonic crystals with high reflectance (83%) in the visible range can be rapidly produced within several minutes, making this general strategy promising for fast creation of large-area photonic crystals using nonmagnetic particles as building blocks.     

Template‐Assisted Electroforming of Fully Semi‐Hard‐Magnetic Helical Microactuators

The authors report on the fabrication of semi‐hard‐magnetic microhelices using template‐assisted electroforming. The method consists of electrodepositing a material on a sacrificial mandrel on which a pattern has been previously written. To electroform the helical microswimmers, a helical template on a polymer‐coated metallic mandrel is created using a laser, which precisely ablates the polymer coating and exposes the mandrel surface. Subsequently, the semi‐hard‐magnetic material is electrodeposited in the trenches produced by the laser. In this investigation, the helical structures are obtained from an electrolyte, which enables the production of hard‐magnetic CoPt alloys. The authors also show that electroformed semi‐hard‐magnetic helical microswimmers can propel in viscous environments such as silicon oil in three dimensions and against gravity. Their manufacturing approach can be used for the fabrication of more complex architectures for a wide range of applications and can be potentially extended to any electroplatable material.

     

Rotating‐Electric‐Field‐Induced Carbon‐Nanotube‐Based Nanomotor in Water: A Molecular Dynamics Study

Using molecular dynamics simulations, it is shown that a carbon nanotube (CNT) suspended in water and subjected to a rotating electric field of proper magnitude and angular speed can be rotated with the aid of water dipole orientations. Based on this principle, a rotational nanomotor structure is designed and the system is simulated in water. Use of the fast responsiveness of electric‐field‐induced CNT orientation in water is employed and its operation at ultrahigh‐speed (over 10¹¹ r.p.m.) is shown. To explain the basic mechanism, the behavior of the rotational actuation, originated from the water dipole orientation, is also analyzed . The proposed nanomotor is capable of rotating an attached load (such as CNT) at a precise angle as well as nanogear‐based complex structures. The findings suggest a potential way of using the electric‐field‐induced CNT rotation in polarizable fluids as a novel tool to operate nanodevices and systems.     

Forced‐ and Self‐Rotation of Magnetic Nanorods Assembly at the Cell Membrane: A Biomagnetic Torsion Pendulum

In order to provide insight into how anisotropic nano‐objects interact with living cell membranes, and possibly self‐assemble, magnetic nanorods with an average size of around 100 nm × 1 µm are designed by assembling iron oxide nanocubes within a polymeric matrix under a magnetic field. The nano–bio interface at the cell membrane under the influence of a rotating magnetic field is then explored. A complex structuration of the nanorods intertwined with the membranes is observed. Unexpectedly, after a magnetic rotating stimulation, the resulting macrorods are able to rotate freely for multiple rotations, revealing the creation of a biomagnetic torsion pendulum.     

A Modular System for the Design of Stimuli‐Responsive Multifunctional Nanoparticle Aggregates by Use of Host–Guest Chemistry

A self‐assembly approach for the design of multifunctional nanomaterials consisting of different nanoparticles (gold, iron oxide, and lanthanide‐doped LiYF₄) is developed. This modular system takes advantage of the light‐responsive supramolecular host–guest chemistry of β‐cyclodextrin and arylazopyrazole, which enables the dynamic and reversible self‐assembly of particles to spherical nanoparticle aggregates in aqueous solution. Due to the magnetic iron oxide nanoparticles, the aggregates can be manipulated by an external magnetic field leading to the formation of linear structures. As a result of the integration of upconversion nanoparticles, the aggregates are additionally responsive to near‐infrared light and can be redispersed by use of the upconversion effect. By varying the nanoparticle and linker concentrations the composition, size, shape, and properties of the multifunctional nanoparticle aggregates can be fine‐tuned.     

Magnetostatic interactions in planar ring-like nanoparticle structures

Numerical calculations of equilibrium state energies and local magnetic fields in planar ring-like nanoparticle structures were performed. The dipole–dipole, Zeeman and magnetic anisotropy interactions were included into the model. The result of their competition depends on the value of the external magnetic field, magnetic parameters of an individual nanoparticle, size and shape of the structures. Flux-closed vortexes, single domain, two-domain “onion”-like, “hedgehog”-like and more complex spin structures can be realized. The critical field, providing a sharp transition from the flux-closed vortex to the “onion”-like state, can be regulated by a variation of the particle magnetization and anisotropy constant, their easy directions, and particle space arrangement.     

Hydrophobic,Electrostatic, and Dynamic Polymer Forces at Silicone Surfaces Modified with Long‐Chain Bolaform Surfactants

Surfactant self‐assembly on surfaces is an effective way to tailor the complex forces at and between hydrophobic‐water interfaces. Here, the range of structures and forces that are possible at surfactant‐adsorbed hydrophobic surfaces are demonstrated: certain long‐chain bolaform surfactants—containing a polydimethylsiloxane (PDMS) mid‐block domain and two cationic α, ω‐quarternary ammonium end‐groups—readily adsorb onto thin PDMS films and form dynamically fluctuating nanostructures. Through measurements with the surface forces apparatus (SFA), it is found that these soft protruding nanostructures display polymer‐like exploration behavior at the PDMS surface and give rise to a long‐ranged, temperature‐ and rate‐dependent attractive bridging force (not due to viscous forces) on approach to a hydrophilic bare mica surface. Coulombic interactions between the cationic surfactant end‐groups and negatively‐charged mica result in a rate‐dependent polymer bridging force during separation as the hydrophobic surfactant mid‐blocks are pulled out from the PDMS interface, yielding strong adhesion energies. Thus, (i) the versatile array of surfactant structures that may form at hydrophobic surfaces is highlighted, (ii) the need to consider the interaction dynamics of such self‐assembled polymer layers is emphasized, and (iii) it is shown that long‐chain surfactants can promote robust adhesion in aqueous solutions.     

Directed Self‐Assembly of Templatable Block Copolymers by Easily Accessible Magnetic Control

Magnetic control has been a prosperous and powerful contactless approach in arraying materials into high‐order nanostructures. However, it is tremendously difficult to control organic polymers in this way on account of the weak magnetic response. The preparation of block copolymers (BCPs) with high magnetostatic energy is reported here, relying on an effective electrostatic coupling between paramagnetic ions and polymer side chains. As a result, the BCPs undergo a magnetically directed self‐assembly to form microphase‐segregated nanostructures with long‐range order. It is emphasized that such a precisely controlled alignment of the BCPs is performed upon a single commercial magnet with low‐intensity field (0.35 Tesla). This strategy is profoundly easy‐to‐handle in contrast to routine electromagnetic methods with high‐intensity field (5–10 Tesla). More significantly, the paramagnetic metal component in the BCP samples can be smartly removed, providing a template effect with a preservation of the directed self‐assembled nanofeatures for patterning follow‐up functionalized species through the original binding site.     

N‐Type Superconductivity in an Organic Mott Insulator Induced by Light‐Driven Electron‐Doping

The presence of interface dipoles in self‐assembled monolayers (SAMs) gives rise to electric‐field effects at the device interfaces. SAMs of spiropyran derivatives can be used as photoactive interface dipole layer in field‐effect transistors because the photochromism of spiropyrans involves a large dipole moment switching. Recently, light‐induced p‐type superconductivity in an organic Mott insulator, κ‐(BEDT‐TTF)₂Cu[N(CN)₂]Br (κ‐Br: BEDT‐TTF = bis(ethylenedithio)tetrathiafulvalene) has been realized, thanks to the hole carriers induced by significant interface dipole variation in the spiropyran‐SAM. This report explores the converse situation by designing a new type of spiropyran monolayer in which light‐induced electron‐doping into κ‐Br and accompanying n‐type superconducting transition have been observed. These results open new possibilities for novel electronics utilizing a photoactive SAMs, which can design not only the magnitude but also the direction of photoinduced electric‐fields at the device interfaces.     

The propagation of magnetic bubbles on permalloy disks

A magnetic bubble generator consisting of a Permalloy disk and a current conductor loop has been used recently in a mass memory design utilizing magnetic bubble technology. The bias field range in which the disk can hold the seed bubble is measured in this report as a function of of the rotating field frequency. Above a critical frequency fc, the bias field margins begin to decrease. The dependence of fcon disk size is obtained for disks with diameters from 16 μm up to 43 μm at rotating fields of 20 and 30 Oe. The separation between Permalloy disks and the garnet film is kept at 0.8 μm or 1.6 μm. Results show that at a fixed rotating field, a smaller disk is preferable at higher frequency for a magnetic bubble material with a given mobility. The critical frequency fcobtained is in good agreement with a theoretical calculation using the viscous damping model by Rossol et al. For frequencies below fc, the bias field margin on the disk is equal to that of the propagating channel and circuit failure due to the loss of the generator seed bubble can be eliminated.     

Chiromagnetic Plasmonic Nanoassemblies with Magnetic Field Modulated Chiral Activity

Chiral plasmonic nanoassemblies, which exhibit outstanding chiroptical activity in the visible or near‐infrared region, are popular candidates in molecular sensing, polarized nanophotonics, and biomedical applications. Their optical chirality can be modulated by manipulating chemical molecule stimuli or replacing the building blocks. However, instead of irreversible chemical or material changes, real‐time control of optical activity is desired for reversible and noninvasive physical regulating methods, which is a challenging research field. Here, the directionally and reversibly switching optical chirality of magneto‐plasmonic nanoassemblies is demonstrated by the application of an external magnetic field. The gold‐magnetic nanoparticles core–satellite (Au@Fe₃O₄) nanostructures exhibit chiral activity in the UV–visible range, and the circular dichroism signal is 12 times greater under the magnetic field. Significantly, the chiral signal can be reversed by regulating the direction of the applied magnetic field. The attained magnetic field‐regulated chirality is attributed to the large contributions of the magnetic dipole moments to polarization rotation. This magnetic field‐modulated optical activity may be pivotal for photonic devices, information communication, as well as chiral metamaterials.     

Orientations of overdamped magnetic nanorod-gyroscopes

Overdamped magnetic nanorod-gyroscopes driven by a rotating magnetic field undergo a series of reorientations when sedimenting on top of a surface in a viscous liquid. By changing the amplitude and the rotation frequency of the driving magnetic field, the nanorod-gyroscope either synchronizes or desynchronizes with the field and rotates either around its long or short axis. The different regimes of motion are explained theoretically by coupling the nanorod-gyroscopes motion to the creeping flow equations of the surrounding fluid. It is shown that friction anisotropy plays an important role for the orientation of the nanorod-gyroscopes.     

3D Fabrication of Fully Iron Magnetic Microrobots

Biocompatibility and high responsiveness to magnetic fields are fundamental requisites to translate magnetic small‐scale robots into clinical applications. The magnetic element iron exhibits the highest saturation magnetization and magnetic susceptibility while exhibiting excellent biocompatibility characteristics. Here, a process to reliably fabricate iron microrobots by means of template‐assisted electrodeposition in 3D‐printed micromolds is presented. The 3D molds are fabricated using a modified two‐photon absorption configuration, which overcomes previous limitations such as the use of transparent substrates, low writing speeds, and limited depth of field. By optimizing the geometrical parameters of the 3D molds, metallic structures with complex features can be fabricated. Fe microrollers and microswimmers are realized that demonstrate motion at ≈20 body lengths per second, perform 3D motion in viscous environments, and overcome higher flow velocities than those of “conventional 3D printed helical microswimmers.” The cytotoxicity of these microrobots is assessed by culturing them with human colorectal cancer (HCT116) cells for four days, demonstrating their good biocompatibility characteristics. Finally, preliminary results regarding the degradation of iron structures in simulated gastric acid liquid are provided.     

Magnetic rotating disk viscometer

A rotating disk viscometer has been developed aiming at measuring the viscosity at the critical consolute point of binary mixtures in the true hydrodynamic limit. The viscometer consists of a small magnetized disk set into slow rotation by a uniformly rotating magnetic field. The liquid is continuously sheared and no frequency effects should manifest even at the critical point. The phase delay between field and disk depends on the torques acting on the disk, including that of viscous nature. The viscosity can be calculated from the knowledge of the viscous torque obtained after calibration of the viscometer using literature data for ethanol. Preliminary testing results obtained with this novel apparatus are reported.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Magnetic forces on monocomponent toner

The magnetic field strengths and magnetizations of an assembly of magnetic (monocomponent) particles in a constant external field has been calculated. Due to the equilibrium configuration of the particles, forming chains or treelike structures, a one-dimensional approach has been employed. Using the dipole approximation, the problem can be cast as an eigenvalue problem, whose solution yields the magnetizations and field strengths, where the nonlinear interaction between particles has been explicitly accounted for in the formalism.     

胶囊机器人在黏液中旋进运动的研究

研究表面螺旋胶囊机器人在外部旋转磁场作用下在黏液中的运动特性。首先介绍外磁推进的胶囊机器人的系统构成,给出表面螺旋胶囊在黏液中旋进的磁主动矩的表达式,然后制作了镶有条形强磁片的胶囊机器人样品,搭建了旋转磁场试验台,配置了不同动力黏度的二甲基硅油以模拟人体体液。试验模拟旋转磁场对胶囊机器人在不同黏性液体中的旋转,证明了设计的合理性和可行性。当外部磁场强度在6.281 A/m左右时,胶囊机器人受到的驱动力矩为1.0 mN·m左右。在胶囊机器人的设计中,决定需要的外部磁场强度以及所受到的阻力矩大小的因素依次为胶囊直径、设计的工作转速和黏液的黏度。     

Effects of viscous dissipation and Joule heat on heat transfer near a rotating disk in the presence of intensive suction

Heat transfer in the bounday layer of an electrically conducting incompressible liquid near a disk rotating in an axial magnetic field is investigated for the case of intensive, uniform suction. The thermal flux intensity near the disk surface is determined in relation to the magnetic field strength and the rotation speed of the disk with an allowance for the viscous and the Joule dissipation.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 55, No. 5, pp. 740–743, November, 1988.     

Polymer Microstructures through Two‐Photon Crosslinking

Two‐photon crosslinking of polymers (2PC) is proposed as a novel method for the fabrication of freestanding microstructures via two‐photon lithography. During this process in the confocal volume, two‐photon absorption leads to (formal) C,H‐insertion reactions, and consequently to a strictly localized crosslinking of the polymer. To achieve this, the polymer is coated as a solvent‐free (glassy) film onto an appropriate substrate, and the desired microstructure is written by 2PC into this glass. In all regions outside of the focal volume where no two‐photon process occurs, the polymer remains uncrosslinked and can be washed away during a developing process. Using a self‐assembled monolayer containing the same photoreactive group allows covalent attachment of the forming freestanding structures to the substrate, and thus guarantees an improved stability of these structures against shear‐induced detachment. As the two photon process is carried out in the glassy state, in a simple way, multilayer structures can be used to write structures having a varying chemical composition perpendicular to the surface. As an example, the 2PC process is used to build a structure from both protein‐repellent and protein‐adsorbing polymers so that the resulting 3D structure exhibits spatially controlled protein adsorption.     

Controllable Self‐Assembled Microstructures of La0.7Ca0.3MnO3:NiO Nanocomposite Thin Films and Their Tunable Functional Properties

Two‐phase self‐assembled nanocomposite films have attracted increasing interest in recent years because of their potential applications in novel technological devices. However, tuning the physical properties by modulating the microstructure of self‐assembled nanocomposite films is still a challenge. In this study, epitaxial La_0.7Ca_0.3MnO₃:NiO nanocomposite films are synthesized by pulsed laser deposition. In the composite films with a NiO ratio of 50%, microstructures with nanomultilayer, nanogranular, and nanocolumnar characteristics are successfully obtained by using different growth modes. The metal–insulator transition and magnetic transition can be separately modulated by tuning the microstructures. By precisely modulating the microstructure, a significantly enhanced low‐field magnetoresistance (>80% at a magnetic field of 1 T) with an unusual plateau in the temperature interval from 10 to 110 K is realized in these films, which is expected to be applicable in field‐sensor devices that can be operated in a wide temperature range.     

Highly Stretchable,Adaptable, and Durable Strain Sensing Based on a Bioinspired Dynamically Cross‐Linked Graphene/Polymer Composite

Flexible strain sensors can detect physical signals (e.g., temperature, humidity, and flow) by sensing electrical deviation under dynamic deformation, and they have been used in diverse fields such as human motion detection, medical care, speech recognition, and robotics. Existing sensing materials have relatively low adaptability and durability and are not stretchable and flexible enough for complex tasks in motion detection. In this work, a highly flexible self‐healing conductive polymer composite consisting of graphene, poly(acrylic acid) and amorphous calcium carbonate is prepared via a biomineralization‐inspired process. The polymer composite shows good editability and processability and can be fabricated into stretchable strain sensors of various structures (sandwich structures, fibrous structures, self‐supporting structures, etc.). The developed sensors can be attached on different types of surfaces (e.g., flat, cambered) and work well both in air and under water in detecting various biosignals, including crawling, undulatory locomotion, and human body motion.