Anisotropy Of Diamagnetic Susceptibility Research Articles

Model of Magnetohydrodynamic Domains for a Nematic with Negative Anisotropy of Diamagnetic Susceptibility

Model of Magnetohydrodynamic Domains for a Nematic with Negative Anisotropy of Diamagnetic Susceptibility

Antiferromagnetic liquid-crystal suspensions of goethite nanorods: three mechanisms of magnetic field influence on orientational structure.

The study looks into magnetically induced orientational transitions in suspensions of goethite nanorods based on a nematic liquid crystal. The study considers magnetically compensated suspension, which is a liquid-crystal analogue of an antiferromagnet. Unlike conventional magnetic particles, goethite nanorods have a remanent magnetic moment directed along the long axis of the particle and also they have negative diamagnetic anisotropy. Thus, it can be claimed that liquid-crystal composites of goethite nanorods have three mechanisms of interaction with an external magnetic field. The first two mechanisms are originally quadrupolar and are related to diamagnetic susceptibility anisotropies of liquid-crystal matrix and impurity goethite nanorods. The third mechanism is a dipolar one and is due to a remanent longitudinal magnetic moment of each dispersed particle. The magnetic-field-induced birefringence is used to show that the presence of three competing orientational mechanisms of interaction with an external magnetic field can both increase and decrease the Fréedericksz transition threshold compared to a pure liquid crystal. Diagrams of orientational phases of the suspension were constructed, and cases of various orientational mechanism predominance were analysed. Besides, a representation of the free energy of the suspension near the Fréedericksz transition in the form of the Landau expansion was obtained. This made it possible to establish that the Fréedericksz transition can occur as a phase transition of both the first and second order.

Unraveling the Molecular Weight Dependence of High Magnetic Field to Manipulate the Semiconducting Polymer Molecular Orientation.

The magnetic alignment of molecules, which exploits the anisotropy of diamagnetic susceptibility, provides a clean and versatile approach to the structural manipulation of semiconducting polymers. Here, the magnetic-alignment dynamics of two molecular-weight (MW) batches of a diketopyrrolopyrrole (DPP)-based copolymer (PDVT-8) were investigated. Microstructural characterizations revealed that the magnetically aligned, high-MW (Mn = 53.7 kDa) PDVT-8 film exhibited a higher degree of backbone chain alignment and film crystallinity compared with the low-MW (Mn = 17.6 kDa) PDVT-8 film grown via the same magnetic alignment method. We found that as the MW increases, the degree of preaggregation of the polymer molecules in solution significantly increases and the aggregation mode changes from H-aggregation to J-aggregation through a cooperative assembly mechanism. These events improved the responsiveness of high-MW polymer molecules to magnetic fields. Field-effect transistors based on the magnetic aligned high-MW PDVT-8 films exhibited a 6.8-fold increase in hole mobility compared to the spin-coated films, along with a mobility anisotropy ratio of 12.6. This work establishes a significant correlation among chain aggregation behavior in solution, polymer film microstructures, magnetic responsiveness, and carrier transport performance in donor-acceptor polymer systems.

Three-dimensional blueprinting of molecular patterns in liquid crystalline polymers.

Exploiting the interplay of anisotropic diamagnetic susceptibility of liquid crystalline monomers and site selective photopolymerization enables the fabrication of 3D freeforms with highly refined microstructures. Utilizing chain transfer agents in the mesogenic inks presents a pathway for broadly tuning the mechanical properties of liquid crystalline polymers and their response to stimuli. In particular, the combination of 1,4-benzenedimethanethiol and tetrabromomethane is shown to enable voxelated blueprinting of molecular order, while allowing for a modulation of the crosslink density and the mechanical properties. The formulation of these monomers allows for the resolution of the voxels to approach the limits set by the coherence lengths defined by the anchoring from surfaces. These compositions demonstrate the expected thermotropic responses while allowing for their functionalization with photochromic switches to elicit photomechanical responses. Actuation strains are shown to outstrip that accomplished with prior systems that did not access chain transfer agents to modulate the structure of the macromolecular network. Test cases of this system are shown to create freeform actuators that exploit the refined director patterns during high-resolution printing. These include topological defects, hierarchically-structured light responsive grippers, and biomimetic flyers whose flight dynamics can be actively modulated via irradiation with light.

ON THE QUESTION OF HYDRODYNAMIC INSTABILITY OF NEMATICS IN A MAGNETIC FIELD. III: ACCOUNTING FOR THE INERTIAL TERM IN THE EQUATION OF MOTION IN THE MODEL OF MAGNETOHYDRODYNAMIC DOMAIN FOR SPLAY DISTORTION

A magnetohydrodynamic domain model describing the motion of a nematic liquid with positive anisotropy of diamagnetic susceptibility in the splay distortion geometry is considered. The inertial term is taken into account in the equation of motion of a nematic model. This approach makes it possible to introduce dimensionless parameters into the model: ratios of Eriksen numbers to Reynolds numbers. The introduction of such parameters allows one to experimentally assess the relative role of elastic and viscous forces in nematics of different nature.

Highly effective electromagnetic interference shielding composites with solvent-dispersed uniformly aligned graphene nanosheets enabled by strong intrinsic diamagnetism under magnetic field

Graphene nanosheets (GNS) have been regarded as one of the best fillers in polymers for electromagnetic interference (EMI) shielding composites, but it remains a challenge to prepare well-dispersed and highly aligned GNS for optimal EMI shielding despite many recently developed methods. Here we demonstrate a new technology that not only enables uniform alignment of GNS in polymer composites in any direction but also allows us to systematically tune the composites for optimal EMI performance. The technique makes use of graphene's strong intrinsic and anisotropic diamagnetic susceptibility. Unlike paramagnetism and ferromagnetism, diamagnetism allows us to fully control the orientation of GNS by a rotating magnetic field without causing any aggregation or introducing unwanted magnetic particles due to the repulsive force between GNS. This process for GNS composites further enables us to systematically modify GNS concentration and polymer matrix besides GNS orientations. A record-high shielding effectiveness (SE) of 95 dB is achieved with industrially mass-produced multilayer GNS aligned parallel to the surface of a mixed polyvinylidene fluoride-epoxy composite, and a 50% higher SE is further achieved with few-layer GNS. This technique is scalable, thus opening the practical applications of graphene in EMI shielding.

Combined experimental and computational 1H NMR study of water adsorption onto graphenic materials

The effects caused by the interaction with graphene-like layers on the 1H NMR spectra of water molecules adsorbed onto porous carbon materials were investigated by a combination of shielding calculations using density functional theory (DFT) and 1H NMR experiments. Experimental 1H NMR spectra were recorded for different water-containing carbon materials (activated carbons and milled graphite samples); the 1H NMR signals due to adsorbed water in these materials showed a strong shielding effect caused by the electron currents present in the graphene-like layers. This effect was enhanced for activated carbons prepared at high heat treatment temperatures and for milled graphite samples with short milling times, evidencing that the structural organization of the graphene-like layers was the key feature defining the magnitude of the shielding on the 1H nuclei in the water molecules adsorbed by the analyzed materials. The DFT calculations of the shielding sensed by these 1H nuclei showed an increased interaction with the graphitic layers as the distance between these layers (representing the pore size) was reduced. A continuous decrease of the 1H NMR chemical shift was then predicted for pores of smaller sizes, in good agreement with the experimental findings. These calculations also showed a large dispersion of chemical shifts for the several 1H nuclei in the water clusters, attributed to intermolecular interactions and to shielding variations within the pores. This dispersion, combined with the effects due to the locally anisotropic diamagnetic susceptibility of graphite-like crystallites, are the main contributions to the broadening of the 1H NMR signals associated with water adsorbed onto porous carbon materials.

Inertial Term Accounting in Navier-Stokes Equation of Magnetohydrodynamic Domains Model for a Nematic with Negative Anisotropy of Diamagnetic Susceptibility

Inertial Term Accounting in Navier-Stokes Equation of Magnetohydrodynamic Domains Model for a Nematic with Negative Anisotropy of Diamagnetic Susceptibility

Account for the Inertial Term in the Navier–Stokes Equation in the Model of Magnetohydrodynamic Domains for Nematics with Negative Anisotropy of Diamagnetic Susceptibility

Account for the Inertial Term in the Navier–Stokes Equation in the Model of Magnetohydrodynamic Domains for Nematics with Negative Anisotropy of Diamagnetic Susceptibility

Diamagnetic anisotropy and rotational viscosity of 6CHBT nematic liquid crystal

Nematic liquid crystal with polar terminal NCS group was studied by using acoustic and electro-optical methods. Experimental results on rotational viscosity have been obtained by two methods. The anisotropy of diamagnetic susceptibility of the studied LC with temperature variation was determined based on the comparison of these methods. The present results may be useful for testing physical models of the orientation dynamics of nematic liquid crystals in alternating electric and magnetic fields.

Angle Distribution Measurement of Scattered Light Intensity From Needle-Shaped Crystals in a Magnetic Field for Gout Diagnosis

Monosodium urate (MSU), which causes gout, forms needle-shaped crystals with large diamagnetic susceptibility anisotropy that orient in a magnetic field of up to 500 mT. The light transmittance and reflectance of MSU suspensions change under an applied magnetic field. To use this phenomenon to diagnose gout, it would be necessary to collect light scattering information from all angles, including transmission and reflection. This is because the scattered light intensity from MSU crystals is different in forward, sideward, and backward directions under near-infrared (NIR) light (785 nm), which has high biopermeability and is used in NIR spectroscopy. Therefore, we measured the angular distributions of light scattering by MSU crystals before and after applying a magnetic field and evaluated their characteristics. In the case of MSU crystals of the size found in gout patients, the scattered light intensity decreased at all angles under an applied magnetic field compared with that without a magnetic field. The change ratio of the scattered light intensity was large in the forward and backward directions and relatively small in the sideward directions. This was ascribed to the combined effects of the decrease in the scattering cross section caused by the magnetic orientation and the accompanying change in diffraction behavior. Our findings indicated that for gout diagnosis, the incident light should be horizontal and that if transmission measurement is possible, a detection angle of about 30° is suitable. If transmission measurement is not practical, it is better to avoid a detection angle of around 90°.

Statistical theory of magnetic field induced phase transitions in negative diamagnetic anisotropy liquid crystals doped with carbon nanotubes

A statistical mean-field theory is proposed for describing the magneto-orientational ordering of a composite material, namely a liquid crystal suspension of carbon nanotubes. A nematic liquid crystal with negative anisotropy of diamagnetic susceptibility with the molecules, which have long axes tending to be orthogonal to the applied magnetic field, has been considered as a dispersion medium. Since carbon nanotubes possess positive anisotropy of the diamagnetic susceptibility, they tend to align parallelly to the field, causing competition with the ordering of the liquid crystal matrix. Phase transitions induced by temperature and magnetic field have been studied for different values of the coupling energy of carbon nanotubes with a liquid crystal matrix. It is shown that depending on the temperature, magnetic field induces the transition from a highly ordered nematic phase to a weakly ordered paranematic phase with both the positive and the negative value of the order parameter of the liquid crystal, which respectively corresponds to the orientational anisotropy of the “easy-axis” and “easy-plane” types.

Rainbows in a vial: controlled assembly of 2D colloids in two perpendicular external fields

The ability to control the dynamics of self-assembly of anisotropic nanoparticles over multiple length scales by application of external fields is an important technological and fundamental challenge in colloidal science and soft condensed matter physics. Here we demonstrate the controlled assembly of two-dimensional (2D) colloidal liquid crystals under the influence of gravity and an external magnetic field. Due to the positive anisotropy of diamagnetic susceptibility (▵χ > 0), the colloidal nanoplates of monolayer zirconium phosphate could exhibit intriguing alignment with their normals parallel to the magnetic field, thus transforming the multi-domain nematic phase into a mono-domain superstructure. Interestingly, the quasi-equilibrium states of such 2D soft materials under the prolonged gravity-induced sedimentation were found to generate a rich pattern of interference color bands (referred to as rainbows in a vial) upon under the effect of an aligning magnetic field. The birefringence measurement of the resulting color bands suggested quadratically decreasing volume fraction with sample height. The osmotic pressure (П) as a function of nanoplate volume fraction (ϕ) is found to agree with the theoretical prediction of the equation of state of the nematic phase of hard nanoplates, with a positive deviation at higher volume fractions. To the best of our knowledge, such field-driven dynamic formation of rich interference color bands, imitating almost the full spectrum of Michel-Levy interference bands, which was a pure mathematical representation, is experimentally achieved for the first time in a single sample.

Influence of Ferromagnetic Carbon Nanotubes on Magnetic Transitions in Liquid Crystals

Magnetic field-induced orientational transitions in suspensions of carbon nanotubes (CNTs) in nematic liquid crystals have been studied in the framework of a continuum theory. Ferromagnetic CNTs are considered that possess, in addition to a large anisotropy of diamagnetic susceptibility, an additional magnetic moment due to ferromagnetic particles either situated on the CNT surface or encapsulated inside. It is established that, depending on the energy of coupling between disperse CNTs and liquid-crystalline matrix, the external magnetic field induces various sequences of orientational transitions. A threshold value of the coupling energy is determined, above which a transition from the initial homogeneous planar phase to inhomogeneous state (angular phase) becomes possible. At coupling energies below the threshold, the increasing magnetic field induces the following sequence of orientational transitions: homogeneous planar phase–inhomogeneous (angular) phase–homogeneous homeotropic phase–inhomogeneous angular phase. It is established that, depending on the intensity of segregation phenomena, these transitions can be of the first or second order. Analytical expressions for the fields of second-order transitions between phases are found. On an example of the optical phase difference between the ordinary and extraordinary rays of light passing through the cell of CNT suspension, it is shown that the suspension can exhibit optical bistability during the first-order orientational transition.

Computing the Diamagnetic Susceptibility and Diamagnetic Anisotropy of Membrane Proteins from Structural Subunits.

The behavior of large, complex molecules in the presence of magnetic fields is experimentally challenging to measure and computationally intensive to predict. This work proposes a novel, mixed-methods approach for efficiently computing the principal magnetic susceptibilities and diamagnetic anisotropy of membrane proteins. The hierarchical primary (amino acid), secondary (α helical and β sheet), and tertiary (α helix and β barrel) structure of transmembrane proteins enables analysis of a complex molecule using discrete subunits of varying size and resolution. The proposed method converts the magnetic susceptibility tensor for all protein subunits to a unit coordinate system and sums them to build the magnetic susceptibility tensor for the membrane protein. Using this approach, we calculate the diamagnetic anisotropy for all transmembrane proteins of known structure and investigate the effect of different subunit resolutions on the resulting predictions of diamagnetic anisotropy. We demonstrate that amino acid residues with aromatic side groups exhibit higher diamagnetic anisotropies. On average, high percentages of aromatic amino acid subunits, a β barrel tertiary structure, and a small volume are correlated with high volumetric diamagnetic anisotropy. Finally, we demonstrate that accounting for the spatial position of the residues with respect to one another is critical to accurately computing the magnetic properties of the complex protein molecule.

Cu-wire-mediated dipyrimidine base pairs as the building blocks for conductive and magnetic Cu–DNA nanowires

2Cu-mediated DNA dipyrimidine base pairs and 3Cu-mediated ones are designed and determined at the DFT level. The multi-Cu decoration modulates the orbital hybridization, which exhibits increased structural stability, reduced ionization potentials and HOMO–LUMO gaps. The Cu d orbitals in Cu-mediated base pairs function as bridges for improving $$\uppi $$ electron delocalization over two bases, which are verified by spin density delocalization and orbital composition changes. The 3Cu-mediated dipyrimidines exhibit radical properties as molecular magnets and their singly occupied HOMO( $$\upalpha )$$ , unoccupied LUMO( $$\upalpha )$$ and LUMO( $$\upbeta )$$ distribute over entire base pairs and thus largely promote conductivity. The aromaticity and magnetic properties are evaluated via nuclear independent chemical shift and anisotropic diamagnetic susceptibility. The electron localization function isosurfaces demonstrate effective longitudinal conductivity on aromatic bicyclic domain and strong $$\uppi $$ – $$\uppi $$ stacking between layers. Theoretical evidences are presented for the innovative multi-Cu decoration and dipyrimidine assembly on the improvement of conductive and magnetic properties for Cu–DNA nanowires.

Orientational transitions in antiferromagnetic liquid crystals

The orientational phases in an antiferromagnetic liquid crystal (ferronematic) based on the nematic liquid crystal with the negative anisotropy of diamagnetic susceptibility are studied in the framework of the continuum theory. The ferronematic was assumed to be compensated; i.e., in zero field, impurity ferroparticles with the magnetic moments directed parallel and antiparallel to the director are equiprobably distributed in it. It is established that under the action of a magnetic field the ferronematic undergoes orientational transitions compensated (antiferromagnetic) phase–non-uniform phase–saturation (ferrimagnetic) phase. The analytical expressions for threshold fields of the transitions as functions of material parameters are obtained. It is shown that with increasing magnetic impurity segregation parameter, the threshold fields of the transitions significantly decrease. The bifurcation diagram of the ferronematic orientational phases is built in terms of the energy of anchoring of magnetic particles with the liquid-crystal matrix and magnetic field. It is established that the Freedericksz transition is the second-order phase transition, while the transition to the saturation state can be second- or first-order. In the latter case, the suspension exhibits orientational bistability. The orientational and magnetooptical properties of the ferronematic in different applied magnetic fields are studied.

Correlation between manufacturing processes and anisotropic magnetic and electromagnetic properties of carbon nanotube/polystyrene composites

Abstract We present an original, easy to implement, reliable method of non-destructive testing of the orientation of carbon nanotubes by magnetic moment measurements performed in three perpendicular directions of magnetic field. Multi-wall carbon nanotubes/polystyrene composites were prepared by stretching and forge-rolling methods with the same nanotube loading. Unusually strong diamagnetic anisotropy in the composites prepared by the stretching method was observed and attributed to an additional diamagnetic response from the polystyrene aromatic rings wrapping the nanotubes. Strong anisotropy of diamagnetic susceptibility of the composites with highly aligned nanotubes correlates with anisotropic electromagnetic response and with improved microwave absorption properties. Both magnetic anisotropy and microwave absorbance is considerably lower in the composites prepared by the forge-rolled method. The magnetic results correlate well with polarized Raman spectroscopy. The research findings contribute to a better understanding of nanotube-polymer interface, alignment mechanisms, and ultimately the optimal design and performance of functional nanotube - aromatic polymer nanocomposites.

Orientational bistability in ferronematic liquid crystals with negative diamagnetic anisotropy

In the framework of continuum theory we study magnetic field induced orientational transitions in a ferronematic, i.e. suspension of single-domain magnetic particles in a nematic liquid crystal. We consider the case of negative diamagnetic susceptibility anisotropy of a liquid crystal and soft planar coupling of impurity particles with a liquid crystal matrix. We found tricritical behavior of the threshold transition in a magnetic field from perturbed state into uniform planar state. This transition can be the first or second order, depending on the parameter of the magnetic phase segregation. We analytically derive the expression for the tricritical segregation parameter that determines the character of a transition. We show that ferronematic has a large magneto-optical non-linearity which is the result of the director reorientation under external field. Comparison of results of numerical calculations with experimental data has been carried out.

Effects of magnetic fields on dissolution of arthritis causing crystals

The number of gout patients has rapidly increased because of excess alcohol and salt intake. The agent responsible for gout is the monosodium urate (MSU) crystal. MSU crystals are found in blood and consist of uric acid and sodium. As a substitute for drug dosing or excessive water intake, physical stimulation by magnetic fields represents a new medical treatment for gout. In this study, we investigated the effects of a magnetic field on the dissolution of a MSU crystal suspension. The white MSU crystal suspension was dissolved in an alkaline solution. We measured the light transmission of the MSU crystal suspension by a transmitted light measuring system. The magnetic field was generated by a horizontal electromagnet (maximum field strength was 500 mT). The MSU crystal suspension that dissolved during the application of a magnetic field of 500 mT clearly had a higher dissolution rate when compared with the control sample. We postulate that the alkali solution promoted penetration upon diamagnetic rotation and this magnetic field orienting is because of the pronounced diamagnetic susceptibility anisotropy of the MSU crystal. The results indicate that magnetic fields represent an effective gout treatment approach.