Low Wave Vector Research Articles

Turbulent spot formation in stably stratified three-dimensional Yukawa liquids

A plane-Couette flow (PCF) is considered in a stably stratified three-dimensional (3D) Yukawa liquid, perturbed initially with a finite amplitude 3D perturbation. Stable stratification in density is achieved by subjecting the medium to external gravity. We demonstrate turbulent spot formation in a stably stratified PCF. The dynamics of the system is shown to depend upon the value of κ, which is associated with the range of interaction. We have performed “first principles” classical 3D molecular dynamics (MD) simulations for “hypergravity” (1.3g0, g0 is Earth's gravitational force for unit mass in our normalized unit) and “milligravity” (9×10−3g0) cases for κ, 1.0 and 4.0, respectively. We extract relevant fluid quantities from MD data. For the hypergravity case, when the system is evolved in time under stable stratification, the kinetic energy is observed to deposit in the lower wave vectors (Kx, Kz), leading to “inverse cascade” in the plane perpendicular to the direction of stratification. As a result, we observe large-scale structure formation in velocity and streamwise vorticity fields. Nucleation of velocity streak is observed for the first time in the stably stratified case in our simulation. The coherent structures in the velocity and streamwise vorticity fields are found to sustain for longer period of time for the stably stratified cases as compared to the unstratified case. For the milligravity case, the large-scale dynamics is observed to enhance. Unlike unstratified PCF, a background flow in Y-averaged streamwise fluid velocity field is observed for the stably stratified case. We believe that our results using “first principles” classical MD simulations on subcritical turbulence in stably stratified Yukawa liquid, may have ramifications to wider class turbulence problems. Published by the American Physical Society 2024

On the effect of the height distribution function in adhesion of non Gaussian random rough surfaces, and a simple analytical model (BAM)

Two apparently very different theories of adhesion of rough surfaces called BAM (bearing-area-model), and that of Persson–Tosatti lead to similar predictions concerning the stickiness of Gaussian surfaces: the role of roughness is mainly governed by the low wavevector content of the PSD. Here, we study the adhesion of non-Gaussian surfaces by means of a DMT-based simplified BEM (Boundary-Element-Method), and report a clear difference in the adhesion and stickiness of surfaces with different height distribution function. We then obtain analytical equations for a generalized BAM model. Results of the extended BAM theory compare reasonably well with numerical BEM-DMT predictions. We also notice that any theory based on the surface energy, would show instead insensitivity to height distribution.

Transition between distinct hybrid skyrmion textures through their hexagonal-to-square crystal transformation in a polar magnet

Magnetic skyrmions, topological vortex-like spin textures, garner significant interest due to their unique properties and potential applications in nanotechnology. While they typically form a hexagonal crystal with distinct internal magnetisation textures known as Bloch- or Néel-type, recent theories suggest the possibility for direct transitions between skyrmion crystals of different lattice structures and internal textures. To date however, experimental evidence for these potentially useful phenomena have remained scarce. Here, we discover the polar tetragonal magnet EuNiGe3 to host two hybrid skyrmion phases, each with distinct internal textures characterised by anisotropic combinations of Bloch- and Néel-type windings. Variation of the magnetic field drives a direct transition between the two phases, with the modification of the hybrid texture concomitant with a hexagonal-to-square skyrmion crystal transformation. We explain these observations with a theory that includes the key ingredients of momentum-resolved Ruderman–Kittel–Kasuya–Yosida and Dzyaloshinskii-Moriya interactions that compete at the observed low symmetry magnetic skyrmion crystal wavevectors. Our findings underscore the potential of polar magnets with rich interaction schemes as promising for discovering new topological magnetic phases.

Bead-Spring Simulation of Ionomer Melts-Studying the Effects of Chain-Length and Associating Group Fraction on Equilibrium Structure and Extensional Flow Behavior.

Ionomers are associative polymers with diverse applications ranging from selective membranes and high-performance adhesives to abrasion- and chemical-resistant coatings, insulation layers, vacuum packaging, and foamed sheets. Within equilibrium melt, the ionic or associating groups are known to form thermally reversible, associative clusters whose presence can significantly affect the system's mechanical, viscoelastic, and transport properties. It is, thus, of great interest to understand how to control such clusters' size distribution, shape, and stability through the designed choice of polymer architecture and the ionic groups' fraction, arrangement, and interaction strength. In this work, we represent linear associating polymers using a Kremer-Grest type bead-spring model and perform large-scale MD simulations to explore the effect of polymer chain-length (l) and fraction (fs) of randomly placed associating groups on the size distribution and stability of formed clusters. We consider different chain-lengths (below and above entanglement), varying fractions of associating groups (represented by 'sticky' beads) between 5 and 20%, and a fixed sticky-sticky nonbond interaction strength of four times that between regular non-associating beads. For all melts containing associating groups the equilibrium structure factor S(q) displays a signature ionomer peak at low wave vector q whose intensity increases with increasing fs and l. The average cluster size Nc increases with fs. However, the effect of chain-length on Nc appears to be pronounced only at higher values of fs. Under extensional flows, the computed stress (and viscosity) is higher at higher fs and l regardless of strain rate. Beyond a critical strain rate, we observe fragmentation of the associative clusters, which has interesting effects on the stress/viscous response.

Microphase versus macrophase separation in the square-well-linear fluid: A theoretical and computational study.

Due to the presence of competing interactions, the square-well-linear fluid can exhibit either liquid-vapor equilibrium (macrophase separation) or clustering (microphase separation). Here we address the issue of determining the boundary between these two regimes, i.e., the Lifshitz point, expressed in terms of a relationship between the parameters of the model. To this aim, we carry out Monte Carlo simulations to compute the structure factor of the fluid, whose behavior at low wave vectors accurately captures the tendency of the fluid to form aggregates or, alternatively, to phase separate. Specifically, for a number of different combinations of attraction and repulsion ranges, we make the system go across the Lifshitz point by increasing the strength of the repulsion. We use simulation results to benchmark the performance of two theories of fluids, namely, the hypernetted chain (HNC) equationand the analytically solvable random phase approximation (RPA); in particular, the RPA theory is applied with two different prescriptions as for the direct correlation function inside the core. Overall, the HNC theory proves to be an appropriate tool to characterize the fluid structure and the low-wave-vector behavior of the structure factor is consistent with the threshold between microphase and macrophase separation established through simulation. The structural predictions of the RPA theory turn out to be less accurate, but this theory offers the advantage of providing an analytical expression of the Lifshitz point. Compared to simulation, both RPA schemes predict a Lifshitz point that falls within the macrophase-separation region of parameters: in the best case, barriers roughly twice higher than predicted are required to attain clustering conditions.

Spin waves in two-dimensional placoid-like ferromagnetic lattices

The topology of placoid scales is explored as a model of a two-dimensional ferromagnetic lattice. We start from Heisenberg’s Hamiltonian, adopting in low temperature regime and performing the study of free modes of the magnetic excitations. The Holstein–Primakoff approximation was employed, restricting our attention to the bilinear terms in the quantized second operators. The dispersion relation was obtained and the properties of the magnetic modes were studied in terms of the physical fit parameter J2, which reveals the energy dynamics for different regions of the Brillouin Zone. A comparison was made with the energy maps from the square and triangular lattices in order to verify the equivalence of the effects from the placoid geometry. We concluded that this topology imprints a significant energy growing in the regime of low wave vectors with more effective rates than those observed in the triangular lattice, a fact that can be explained by the presence of 4 sites in the unit cell.

Low-temperature and high-pressure phase transitions in 1-decyl-3-methylimidazolium nitrate ionic liquid

The phase behavior of 1-decyl-3-methylimidazolium nitrate ([C10mim][NO3]) was investigated at a low temperature (LT) under ambient pressure and a high pressure (HP) under ambient temperature. [C10mim][NO3] is an ionic liquid (IL) possessing a long alkyl chain length. Detailed crystal structures of the IL at LT and HP were determined by synchrotron radiation experiments. The weak Bragg reflections in the low wavevector (Q) region were clearly observed. At LT, the Bragg reflection at the lowest Q was positioned differently than the prepeak in the liquid state. The 00ℓ Bragg reflections at Q00ℓ indicate a hybrid-layered structure with a lattice constant of 4.3 nm. In addition to the 00ℓ Bragg reflections, the double peak occurred in the region of Q002 at HP; thus new HP crystal structures were observed in [C10mim][NO3].

Crucial role of S8-rings in structural, relaxation, vibrational, and electronic properties of liquid sulfur close to the λ transition.

Liquid sulfur has been studied by density-functional based molecular dynamics simulations at different temperatures ranging from 400 up to 700K across the well-documented λ transition. Structure models containing either a majority of Sn chains or S8 rings are considered and compared to experimental data from x-ray scattering. The comparison suggests a liquid structure of a majority of twofold sulfur at low temperature, dominated by S8 rings that open progressively upon temperature increase. Typical features associated with such rings are analyzed and indicate that they contribute to a specific third correlating distance in the pair correlation function and to a contribution at low wavevector k in the reciprocal space. The vibrational properties of liquid sulfur are also considered and indicate a contribution at 60meV that is associated with both chains and rings, albeit the latter lead to a more intense peak at this wavenumber. The underlying network structure also impacts the dynamic properties of the melts which display enhanced dynamic heterogeneities when S8 rings are present. The analysis of the electronic Kohn-Sham energies shows insulating character with a gap of about ≃2.0eV, albeit the presence of localized mid-gap states is acknowledged that can be associated, in part, with the presence of S6 rings.

Intermolecular interactions in tetrabutylammonium chloride based deep eutectic solvents: Classical molecular dynamics studies

Deep eutectic solvents (DESs) based on tetrabutylammonium chloride have been studied using atomistic molecular dynamics simulations. Two different hydrogen bond donors (HBDs) glycerol and ethylene glycol were used to explore the effect of HBDs on the structure of DESs. Interaction between the anion and the HBD was found to be the most dominant interaction from the structural analysis using radial and spatial distribution functions. Anions and HBDs interact via hydrogen bonds formed between them. The mean hydrogen bond lifetime for these bonds was found to be relatively shorter in case of ethylene glycol compared to that of glycerol, which agrees with the binding energy of the complexes in the gas phase determined using quantum chemical calculations. Diffusivity of components in DES having ethylene glycol as HBD is higher than that of the DES having glycerol due to the greater extent of hydrogen bonding in the latter. The presence of peaks and anti-peaks in the lower wave vector region in both the DESs signifies the existence of long-range structural ordering. The vapor – liquid interface of both the systems showed enrichment of ions at the interface compared to the bulk region.

Effect of metal nanoparticles on polariton dispersion in low dimensional systems

The polariton dispersion of a metal nanocomposite-based system is studied for various low dimensional systems. The polariton dispersion of a novel LiNbO3 piezo-electric superlattice is studied for different periods. Al metal nanoparticles are doped in the novel superlattice system and the polariton dispersion is studied for various filling factors. It is found that when percentage of Al metal nanoparticle increases, the frequency of the upper mode increases and also the gap shifts towards longer wave vector region. The polariton dispersions at the brillouin Zone center and at the edge of LiNbO3/LiTaO3 Quantum Well, Quantum Wire and Quantum Dot superlattice systems are studied and compared. Additional modes of propagation are obtained in the dispersion of Al nanoparticle doped low dimensional systems. Some of which reflect the metal behavior of Al particles and some other modes of propagation are found to be degenerate at transverse optical phonon frequencies of LiNbO3/LiTaO3 and at interfacial frequency modes. It is found that the polaritonic gap shift towards lower wave vector region with filling factor. This type of tuning of polaritonic gap can be exploited in the fabrication of novel optical devices especially in the field of communication.

Antenna design for ferromagnetic resonance and spin wave spectroscopy

Abstract This paper presents a comprehensive study of antennas based on co-planar waveguides (CPW) for ferromagnetic resonance (FMR) detection and spin wave spectroscopy in ferromagnetic insulators. We studied the FMR detection waveguide for both flip-chip method and on-chip waveguide. An antenna can work with low-wave vectors on commercial substrates such as RO4003/Al2O3 and ferromagnetic insulators such as yttrium iron garnet. A higher value of wave vector k is required by antennas to excite spin wave with a shorter wavelength, which strongly depends on the width of the antenna. A co-plane waveguide antenna can excite k1 between 0 and 6 rad/μm with the spin wave group velocity and excitation-spectrum width Δk = 3.58 rad/μm and inner signal line 0.5 μm. The value of k1 for antenna excited spin wave using a 0.5 μm short-circuit line of CPW varied between 0 and 7 rad/μm with Δk = 1.98 rad/μm. Antennas with different width (0.5–10 μm) were also studied. Meander-shaped short-circuit lines were designed to modify the k vector. The maximum amplitude of the k vector of the designed antenna appeared at k3 ≈ 3 rad/μm for the line width of 0.5 μm.

Effect of Network Architecture and Linker Polarity on Ion Aggregation and Conductivity in Precise Polymerized Ionic Liquids.

Four polymerized ionic liquids (PILs) were systematically designed to study the effect of polymer architecture and linker polarity on ion aggregation and transport. Specifically, linear and network PILs with the same ammonium cations (Am) and bis(trifluoromethane)sulfonimide (TFSI) anions were prepared by step-growth polymerization, and polarity was tuned by incorporating two precise linkers, either polar tetra(ethylene oxide) (4EO) linker or nonpolar undecyl (C11) linker. The glass transition temperature (Tg) substantially increased with the nonpolar C11 linker or upon cross-linking to form a network. The low wave-vector (q) ion aggregation peak from wide-angle X-ray scattering (WAXS) was not observable in the linear 4EO PIL, while it was most pronounced in the network C11 PIL. The network C11 PIL exhibited the strongest decoupling, where the ionic conductivity at Tg is greater than 1 order of magnitude higher than the other PILs. This systematic comparison suggests that network structure and nonpolar linkers can promote both ion aggregation and ionic conductivity close to Tg.

Structural behavior of aqueous t-butanol solutions from large-scale molecular dynamics simulations.

Large-scale molecular dynamics simulations are reported for aqueous t-butanol (TBA) solutions. The CHARMM generalized force field (CGenFF) for TBA is combined with the TIP4P/2005 model for water. Unlike many other common TBA models, the CGenFF model is miscible with water in all proportions at 300 K. The main purpose of this work is to investigate the existence and nature of a microheterogeneous structure in aqueous TBA solutions. Our simulations of large systems (128 000 and 256 000 particles) at TBA mole fractions of 0.06 and 0.1 clearly reveal the existence of long-range correlations (>10 nm) that show significant variations on long time scales (∼50 ns). We associate these long-range slowly varying correlations with the existence of supramolecular domainlike structures that consist of TBA-rich and water-rich regions. This structure is always present but continually changing in time, giving rise to long-range slowly varying pair correlation functions. We find that this behavior appears to have little influence on the single particle dynamics; the diffusion coefficients of both TBA and water molecules lie in the usual liquid state regime, and mean square displacements provide no indication of anomalous diffusion. Using our large system simulations, we are able to reliably calculate small angle x-ray scattering and small angle neutron scattering spectra, except at a very low wave vector, and the results agree well with recent experiments. However, this paper shows that simulation of the relatively simple TBA/water system remains challenging. This is particularly true if one wishes to obtain properties such as Kirkwood-Buff factors, or scattering functions at a low wave vector, which strongly depend on the long-range behavior of the pair correlations.

Differences in self‐assembly features of thermoresponsive anionic triblock copolymers synthesized via one‐pot or two‐pot by atom transfer radical polymerization

ABSTRACTThe self‐assembly process in aqueous solutions of the methoxyl‐poly(ethylene glycol)‐block‐poly(2‐acrylamido‐2‐methyl‐1‐propanesulfonic sodium)‐block‐poly(N‐isopropyl acrylamide) (PNIPAAM) triblock copolymer, synthesized via two different atomic transfer radical polymerization methods, namely “one‐pot” (P3‐sample) and “two‐pot” (P2‐sample), was studied by various experimental techniques. The “one‐pot” procedure leads to a copolymer (P3) where the PNIPAAM block is contaminated with a minor quantity of 2‐acrylamido‐2‐methyl‐1‐propane sulfonate (AMPS) residuals and this sample does not form micelles over the considered temperature region, but unimers and temperature‐induced aggregates coexist in the presence of a small amount of salt. The P2 polymer forms micelles and intermicellar structures, but the former moieties disappear at high temperatures, whereas the latter species contract with increasing temperature. Small‐angle neutron scattering results revealed correlation peaks, both for P3 and P2, and no micelle formation for P3, but a pronounced upturn of the scattered intensity at low wavevector values at elevated temperatures for the P2 copolymer. The findings from this study clearly show that the spurious AMPS residuals have a drastic influence on the self‐assembly and micelle formation of the triblock copolymer. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 524–534

Anisotropic Diffusion and Phase Behavior of Cellulose Nanocrystal Suspensions.

In this paper, we use dynamic light scattering in polarized and depolarized modes to determine the translational and rotational diffusion coefficients of concentrated rodlike cellulose nanocrystals in aqueous suspension. Within the range of studied concentrations (1-5 wt %), the suspension starts a phase transition from an isotropic to an anisotropic state as shown by polarized light microscopy and viscosity measurements. Small-angle neutron scattering measurements also confirmed the start of cellulose nanocrystal alignment and a decreasing distance between the cellulose nanocrystals with increasing concentration. As expected, rotational and translational diffusion coefficients generally decreased with increasing concentration. However, the translational parallel diffusion coefficient was found to show a local maximum at the onset of the isotropic-to-nematic phase transition. This is attributed to the increased available space for rods to move along their longitudinal axis upon alignment. This increased parallel diffusion coefficient thus confirms the general idea that rodlike particles gain translational entropy upon alignment while paying the price for losing rotational degrees of freedom. Once the concentration increases further, diffusion becomes more hindered even in the aligned regions due to a reduction in the rod separation distance. This leads once again to a decrease in translational diffusion coefficients. Furthermore, the relaxation rate for fast mode translational diffusion (parallel to the long particle axis) exhibited two regimes of relaxation behavior at concentrations where significant alignment of the rods is measured. We attribute this unusual dispersive behavior to two length scales: one linked to the particle length (at large wavevector q) and the other to a twist fluctuation correlation length (at low wavevector q) along the cellulose nanocrystal rods that is of a larger length when compared to the actual length of rods and could be linked to the size of aligned domains.

A semianalytical "reverse" approach to link structure and microscopic interactions in two-Yukawa competing fluids.

We study theoretically a prototype hard-sphere two-Yukawa model with competing interactions, under thermodynamic conditions associated with the formation of clusters. We adopt the analytically solvable random phase approximation and show that this theory predicts reasonably well the structure of the fluid-in comparison with exact Monte Carlo results-within a unique parameterization of the direct correlation function inside the hard core of particles. In particular, the theory follows correctly the development, in the structure factor, of a local peak at low wavevectors, as peculiarly associated with the onset of aggregation. We then model the direct correlation function in the same wavevector regime by a Gaussian function, so as to systematically investigate, in a "reverse" scheme, how varying the properties of the local peak modifies the original underlying competing interaction. We show that large variations in the height of the peak are generally associated with comparatively smaller variations in the height of the microscopic repulsive barrier; moreover, the shrinking and shifting towards lower wavevectors of the peak may be interpreted in terms of the displacement of the barrier, producing a substantial enlargement of the range of both the attractive and repulsive contributions to the interaction potential. Finally, we document the way the repulsive barrier tends to vanish as the two-Yukawa fluid approaches a "simple fluid" behavior, heralding the onset of a liquid-vapor phase separation.

Modulation Instability of a Gravity Wave and Generation of a Direct Cascade of Vortex Energy on the Surface of Water

The development of instability of gravity-capillary waves on the surface of water excited by two perpendicular plungers has been experimentally observed. As a result of a four-wave process, waves with a frequency of 8 Hz scatter in pairs into waves with frequencies of 3.92 and 4.08 Hz, as well as 11.98 and 12.02 Hz. The amplitude of low-frequency waves increases exponentially with a characteristic time of about 90 s which exceeds the time of viscous wave damping almost by an order of magnitude. Along with the main pumping mode, the appeared low-frequency harmonics, propagating on the surface of water at an angle of 15° to each other, form large-scale vortex flows on the surface of water. The wave energy is transferred from the pumping region directly to vortices with a size comparable to the length of a bath wall. In a vortex system, a direct energy cascade with the energy distribution close to E(k) ~ k–5/3 is formed from the region of low wave vectors.

The effects of thermal and correlated noise on magnons in a quantum ferromagnet

The dynamics and thermal equilibrium of spin waves (magnons) in a quantum ferromagnet as well as the macroscopic magnetisation are investigated. Thermal noise due to an interaction with lattice phonons and the effects of spatial correlations in the noise are considered. We first present a Markovian master equation approach with analytical solutions for any homogeneous spatial correlation function of the noise. We find that spatially correlated noise increases the decay rate of magnons with low wave vectors to their thermal equilibrium, which also leads to a faster decay of the ferromagnet’s magnetisation to its steady-state value. For long correlation lengths and higher temperature we find that additionally there is a component of the magnetisation which decays very slowly, due to a reduced decay rate of fast magnons. This effect could be useful for fast and noise-protected quantum or classical information transfer and magnonics. We further compare ferromagnetic and antiferromagnetic behaviour in noisy environments and find qualitatively similar behaviour in Ohmic but fundamentally different behaviour in super-Ohmic environments.

Multiple Interacting Collective Modes and Phonon Gap in Phospholipid Membranes.

We combine Brillouin neutron scattering measurements with recent inelastic X-ray scattering [ Zhernenkov et al. Nat. Commun. 2016 , 7 , 11575 ] to propose a model for the collective dynamics of phospholipid bilayers. Neutron and X-ray spectra were fitted by the model response function associated with the Hamiltonian of an interacting-phonon system. This approach allows for a comprehensive and unprecedented picture of the vibrational collective features of phospholipids. At low wavevectors Q, the dispersion relations can be interpreted in terms of two acoustic-like modes, one longitudinal and one transverse, plus a dispersionless optic-like mode. The transverse mode of the liquid phase shows a phonon gap that can be linked to a passive transport mechanism through membranes, an interpretation that was proposed in Zhernenkov et al. At higher Q values, the interaction of the longitudinal acoustic excitation with the dispersionless mode gives rise to a pattern that is consistent with avoided-crossing behavior. Evidence is found for a slow- to fast-sound transition, similar to bulk water and other biomolecules.

Compressibility of ferrofluids: Towards a better understanding of structural properties

.This paper addresses a computational method aimed at obtaining the isothermal compressibility of ferrofluids by means of molecular dynamics (MD) simulations. We model ferrofluids as a system of dipolar soft spheres and carry out MD simulations in the NPT ensemble. The obtained isothermal compressibility computed via volume fluctuations provides us with a strong evidence that dipolar interactions lead to a higher compressibility of dipolar soft sphere systems: the stronger the dipolar interactions, the bigger is the deviation of the compressibility from the one of a system with no dipoles. Furthermore, we use the isothermal compressibility to calculate the structure factor of ferrofluids at low values of wave vectors, i.e. in the range where it is difficult to predict its behaviour because of a problem with accounting for long-range particle correlations that give the main contribution to the structure factor in this range. Our approach based on the interpolation of the structure factor and the computed isothermal compressibility allows us to obtain the smooth structure factor in the range of low wave vectors and the reliable fractal dimension of the clusters formed in the system.Graphical abstract