Spatial Period Research Articles

Laser-assisted surface alloying of titanium with silver to enhance antibacterial and bone-cell mineralization properties of orthopedic implants.

Orthopedic device-related infection (ODRI) poses a significant threat to patients with titanium-based implants. The challenge lies in developing antibacterial surfaces that preserve the bulk mechanical properties of titanium implants while exhibiting characteristics similar to bone tissue. In response, we present a two-step approach: silver nanoparticle (AgNP) coating followed by selective laser-assisted surface alloying on commonly used titanium alumina vanadium (TiAl6V4) implant surfaces. This process imparts antibacterial properties without compromising the bulk mechanical characteristics of the titanium alloy. Systematic optimization of laser beam power (8-40 W) resulted in an optimized surface (32 W) with uniform TiAg alloy formation. This surface displayed a distinctive hierarchical mesoporous textured surface, featuring cauliflower-like nanostructures measuring between 5-10 nm uniformly covering spatial line periods of 25 μm while demonstrating homogenous elemental distribution of silver throughout the laser processed surface. The optimized laser processed surface exhibited prolonged superhydrophilicity (40 days) and antibacterial efficacy (12 days) against Staphylococcus aureus and Escherichia coli. Additionally, there was a significant twofold increase in bone mineralization compared to the pristine Ti6Al4V surface (p < 0.05). Rockwell hardness tests confirmed minimal (<1%) change in bulk mechanical properties compared to the pristine surface. This innovative laser-assisted approach, with its precisely tailored surface morphology, holds promise for providing enduring antibacterial and osteointegration properties, rendering it an optimal choice for modifying load-bearing implant devices without altering material bulk characteristics.

Equivalent diffusion ratio distribution in the UGT15000 low pressure compressor

With spatial and temporal periodicity approach the results of steady state CFD simulation of compressible air flow in each blade-to-blade row of low pressure compressor (LPC) of the UGT15000 gas turbine engine (Ukrainian SSR constr. and prod.) are presented. Near the nominal rotation speed compressor map and adiabatic efficiency of each stage and the flow angles at leading and trailing blade edges are predicted and calculated. At the average radius stage flow and stage load coefficients are determined. It has been established that the reason of the low efficiency of the last stage is associated with the low stage reaction (≈0.5) leading to high airflow swirling at outlet. For LPC optimization a calculation of the radial distributions of the equivalent diffuser coefficient was performed. It is shown that the greatest total pressure losses are in the stator rows of stages 9, 8, 5 and in the rotor rows of stage 9. High losses are also shown on the upper half of the 4th, 3rd, 2nd stator blades and the lower half of the 1st rotor blades. The identified potential for increasing the efficiency of the LPC is up to 2% without significant construction changes.

Skin-Friction and Forced Convection from Rough and Smooth Plates

Since the 1930s, theories of skin-friction drag from plates with rough surfaces have been based by analogy to turbulent flow in pipes with rough interiors. Failure of this analogy at small fluid velocities has frustrated attempts to create a comprehensive theory. Utilizing the concept of a self-similar roughness that disrupts the boundary layer at all scales, this investigation derives formulas for a rough or smooth plate’s skin-friction coefficient and forced convection heat transfer given its characteristic length, root-mean-squared (RMS) height-of-roughness, isotropic spatial period, Reynolds number, and the fluid’s Prandtl number. This novel theory was tested with 456 heat transfer and friction measurements in 32 data-sets from one book, six peer-reviewed studies, and the present apparatus. Compared with the present theory, the RMS relative error (RMSRE) values of the 32 data-sets span 0.75% through 8.2%, with only four data-sets exceeding 6%. Prior work formulas have smaller RMSRE on only four of the data-sets.

Chemical and Topographical Changes upon Sub‐100‐nm Laser‐Induced Periodic Surface Structure Formation on Titanium Alloy: The Influence of Laser Pulse Repetition Rate and Number of Over‐Scans

Titanium and its alloys are known to allow the straightforward laser‐based manufacturing of ordered surface nanostructures, so‐called high spatial frequency laser‐induced periodic surface structures (HSFL). These structures exhibit sub‐100 nm spatial periods – far below the optical diffraction limit. The resulting surface functionalities are usually enabled by both, topographic and chemical alterations of the nanostructured surfaces. For exploring these effects, multi‐method characterizations were performed here for HSFL processed on Ti–6Al–4V alloy upon irradiation with near‐infrared ps‐laser pulses (1030 nm, ≈1 ps pulse duration, 1–400 kHz) under different laser scan processing conditions, i.e., by systematically varying the pulse repetition frequency and the number of laser irradiation passes. The sample characterization involved morphological and topographical investigations by scanning electron microscopy (SEM), atomic force microscopy (AFM), tactile stylus profilometry, as well as near‐surface chemical analyses hard X‐ray photoelectron spectroscopy (HAXPES) and depth‐profiling time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). This provides a quantification of the laser ablation depth, the geometrical HSFL characteristics and enables new insights into the depth extent and the nature of the non‐ablative laser‐induced near‐surface oxidation accompanying these nanostructures. This allows to answer the questions how the processing of HSFL can be industrially scaled up, and whether the latter is limited by heat‐accumulation effects.

Dynamical localization and slow thermalization in a class of disorder-free periodically driven one-dimensional interacting systems

We study if the interplay between dynamical localization and interactions in periodically driven quantum systems can give rise to anomalous thermalization behavior. Specifically, we consider one-dimensional models with interacting spinless fermions with nearest-neighbor hopping and density-density interactions, and a periodically driven on-site potential with spatial periodicity m=2 and m=4. At a dynamical localization point, these models evade thermalization either due to the presence of an extensive number of conserved quantities (for weak interactions) or due to the kinetic constraints caused by drive-induced resonances (for strong interactions). Our models therefore illustrate interesting mechanisms for generating constrained dynamics in Floquet systems which are difficult to realize in an undriven system.

Insights into ENSO Diversity from an Intermediate Coupled Model. Part I: Uniqueness and Sensitivity of the ENSO Mode

Abstract The basic dynamics of the spatiotemporal diversity for El Niño–Southern Oscillation (ENSO) has been the subject of extensive research and, while several hypotheses have been proposed, remains elusive. One promising line of studies suggests that the observed eastern Pacific (EP) and central Pacific (CP) ENSO may originate from two coexisting leading ENSO modes. We show that the coexistence of unstable EP-like and CP-like modes in these studies arises from contaminated linear stability analysis due to unnoticed numerical scheme caveats. In this two-part study, we further investigate the dynamics of ENSO diversity within a Cane–Zebiak-type model. We first revisit the linear stability issue to demonstrate that only one ENSO-like linear leading mode exists under realistic climate conditions. This single leading ENSO mode can be linked to either a coupled recharge-oscillator (RO) mode favored by the thermocline feedback or a wave-oscillator (WO) mode favored by the zonal advective feedback at the weak air–sea coupling end. Strong competition between the RO and WO modes for their prominence in shaping this ENSO mode into a generalized RO mode makes it sensitive to moderate changes in these two key feedbacks. Modulations of climate conditions yield corresponding modulations in spatial pattern, amplitude, and period associated with this ENSO mode. However, the ENSO behavior undergoing this linear climate condition modulations alone does not seem consistent with the observed ENSO diversity, suggesting the inadequacy of linear dynamics in explaining ENSO diversity. A nonlinear mechanism for ENSO diversity will be proposed and discussed in Part II.

A Method for Estimating the Spatial Period of Energy Release in Solar Flares

A Method for Estimating the Spatial Period of Energy Release in Solar Flares

Modulated distribution of vacancies within the post-irradiated bcc metals

The criterion of formation of a modulated structure by means of the spinodal mechanism in a spatial distribution of interacting vacancies (as strong concentrators of mechanical stresses) in metals as a target after irradiation is analyzed. For the irradiated body-centered cubic vanadium and tungsten, temperature dependences of the spatial periods of such a structure of the substitutional-vacancies’ subsystem are predicted taking into account the total mixing energies consisting of the energies of the (electro)chemical and strain-induced interactions between vacancies, which are cohesive by the nature in short distances and elastic by character at long distances, respectively.

A Method for Estimating the Spatial Period of Energy Release in Solar Flares

A method for estimating the spatial period of energy release in solar flares is proposed toanalyze the present-day satellite observations of arcades of flare coronal loops. The method is based on theapplication of a Fourier analysis to the difference images of flare arcades in the far ultraviolet. The operationof the method is demonstrated with the observations of the Bastille Day flare from the TRACE spacecraftin the 171 ˚A channel. The mean spatial period of energy release in the Bastille Day flare determined bythe proposed method is 5–8 Mm, in good agreement with the scenario for the development of thermalinstability of the preflare current layer in solar flares.

Minute-scale periodicity of neuronal firing in the human entorhinal cortex

Grid cells in the entorhinal cortex demonstrate spatially periodic firing, thought to provide a spatial map on behaviorally relevant length scales. Whether such periodicity exists for behaviorally relevant time scales in the human brain remains unclear. We investigate neuronal firing during a temporally continuous experience by presenting 14 neurosurgical patients with a video while recording neuronal activity from multiple brain regions. We report on neurons that modulate their activity in a periodic manner across different time scales-from seconds to many minutes, most prevalently in the entorhinal cortex. These neurons remap their dominant periodicity to shorter time scales during a subsequent recognition memory task. When the video is presented at two different speeds, a significant percentage of these temporally periodic cells (TPCs) maintain their time scales, suggesting a degree of invariance. The TPCs' temporal periodicity might complement the spatial periodicity of grid cells and together provide scalable spatiotemporal metrics for human experience.

The impact of freshening over the Antarctic Ocean on Pacific Decadal Oscillation

&lt;p&gt;The profound influence of the Antarctic Ocean freshening on the Pacific Decadal Oscillation (PDO) is investigated in this study by utilizing a series of fully coupled ocean-atmosphere 400-year-modeling experiments. The simulated results derived from the Fast Ocean-Atmosphere Model (FOAM) can reasonably identify the spatial pattern and time period (10–20 years and 20–50 years) of the observed PDO with slightly weak amplitudes. In the sensitivity experiment (Southern Ocean Water Hosing), 1.0 Sv (Sverdrup, 1Sv = 1.0 × 106 m&lt;sup&gt;3&lt;/sup&gt;/s) freshwater flux is uniformly imposed over the Antarctic Ocean for 400 years. As a response to this Antarctic Ocean freshening, the Tropical Pacific Ocean displays a normal “La Niña pattern”, while the low-frequency variability within the North Pacific Ocean is much weakened. Preserving the PDO’s spatial pattern, the multidecadal (20–50 years) magnitude becomes weak and shifts toward higher frequency. In contrast, the decadal magnitude of the PDO (10–20 years) is slightly reinforced and also shifts towards higher frequency. Dynamical analysis indicates that the shortening of the PDO multidecadal variability is mainly caused by the acceleration of the first-baroclinic-mode Rossby waves. The spreading of the fresh anomalies and associated increasing stratification in the North Pacific Ocean result in the shortening of the long Rossby wave propagation to cross the subtropical North Pacific basin. A heat budget analysis further shows that the upper-ocean thermodynamic variability in relationship to the stratification oscillation in the North Pacific Ocean is mainly associated with the anomalous behaviors of the meridional advection, heat flux and ocean mixing.&lt;/p&gt;

Trench Formation under the Tunable Nanogap: Its Depth Depends on Maximum Strain and Periodicity.

Metallic nanogaps have been studied for many years in the context of a significant amount of field enhancements. Nanogaps of macroscopic lengths for long-wave applications have attracted much interest, and recently one dimensional tunable nanogaps have been demonstrated using flexible PET substrates. For nanogaps on flexible substrates with applied tensile strain, large stress is expected in the vicinity of the gap, and it has been confirmed that several hundred nanometer-deep trenches form beneath the position of the nanogap because of this stress singularity. Here, we studied trench formation under nanogap structures using COMSOL Multiphysics 6.1. We constructed a 2D nanogap unit cell, consisting of gold film with a crack on a PDMS substrate containing a trench beneath the crack. Then, we calculated the von Mises stress at the bottom of the trench for various depths and spatial periods. Based on it, we derived the dependence of the trench depth on the strain and periodicity for various yield strengths. It was revealed that as the maximum tensile strain increases, the trench deepens and then diverges. Moreover, longer periods lead to larger depths for the given maximum strain and larger gap widths. These results could be applied to roughly estimate achievable gap widths and trench depths for stretchable zerogap devices.

Ghost imaging based on Fermat spiral laser array designed for remote sensing.

We propose a Fermat spiral laser array as illumination source in ghost imaging. Due to the aperiodic structure, the Fermat spiral laser array generates illuminating light field without spatial periodicity on the normalized second-order intensity correlation function. A single-pixel detector is used to receive the signal light from object for image reconstruction. The effects of laser array parameters on the quality of ghost imaging are analyzed comprehensively. Through experimental demonstration, the Fermat spiral laser array successfully achieves ghost imaging with high quality by combining with the compressive sensing reconstruction algorithm. This method is expected to be applied in remote sensing by combining with phased and collimated fiber laser array equipped with the high emitting power and high-speed modulation frequency.

Large eddy simulations of kagome and body centered cubic lattice cells

As advanced manufacturing methodologies such as additive manufacturing become accessible and cost effective, more complex and tailored internal cooling schemes can be achieved. One family of cooling geometries that has recently garnered interest within the heat transfer community are lattice structures. These structures offer enhanced mixing and large surface area-to-volume ratios, but their application in advanced heat exchangers is in relative infancy. This study explored the performance of two types of lattice unit cells, specifically Kagome and body centered cubic (BCC), by identifying the flow development and unsteady characteristics using large eddy simulations (LES) at a Reynolds number of 20,000. Simulation results agree well with experimental data, and differences between the two methods was found to be a function of the as-printed quality of the experimental geometry. Results indicate that both lattices feature spatial periodicity in the local Nusselt number and friction factor, due to the changes in flow area and flow interactions in the wake of each lattice strut. LES results show that the flow through the BCC lattice became fully developed, both hydrodynamically and thermally, with fewer unit cells than through the Kagome. In the entry region, the flow in the Kagome lattice became biased towards one side of the channel due to local wakes and interactions with the lattice struts, resulting in a bias to the local heat transfer. The simulations also revealed that the instantaneous vortices shed from the BCC lattice struts interacted with the duct endwalls, pulling the boundary layer from the wall and enhancing heat transfer.

Application of the Thermal Diffusivity Standard for the Heat Transfer Parameter Control in Absorbing Materials

Metrological support creation and use of heat transfer etalons are important stages in the development of modern materials science. This is especially concerned to the emergence of new materials in the world with previously unattainable thermophysical parameters. The purpose of this work was to develop and experimentally verify the idea of joint application of the transient gratings method which is well-known in nonlinear optics and the single thermal diffusivity etalon of conventional type for the heat transfer metrological control in materials of a wide values range. The method proposed is based on thermal diffusivity etalon application as a source of calibrated optical signals that are excited in it by short laser pulses. Their lifetime is formed by the etalon thermal diffusivity and on the transient grating spatial period. The etalon linear graph of gratings lifetimes as a function of the gratings periods squared and grating lifetime of the material under study are used for the thermal diffusivity calculation. Thermal diffusivity of thin sub-surface layers of the samples under study – duraluminium, monocrystalline silicon and thermoelectric lead telluride film was measured. The results obtained are in close agreement with the reference values.

MEG frequency tagging reveals a grid-like code during attentional movements

Grid-cells firing fields tile the environment with a 6-fold periodicity during both locomotion and visual exploration. Here, we tested, in humans, whether movements of covert attention elicit grid-like coding using frequency tagging. Participants observed visual trajectories presented sequentially at fixed rate, allowing different spatial periodicities (e.g., 4-, 6-, and 8-fold) to have corresponding temporal periodicities (e.g., 1, 1.5, and 2Hz), thus resulting in distinct spectral responses. We found a higher response for the (grid-like) 6-fold periodicity and localized this effect in medial-temporal sources. In a control experiment featuring the same temporal periodicity but lacking spatial structure, the 6-fold effect did not emerge, suggesting its dependency on spatial movements of attention. We report evidence that grid-like signals in the human medial-temporal lobe can be elicited by covert attentional movements and suggest that attentional coding may provide a suitable mechanism to support the activation of cognitive maps during conceptual navigation.

Sound transmission across locally resonant honeycomb sandwich meta-structures with large spatial periodicity.

Honeycomb sandwich structures have been widely used in the field of engineering owing to their outstanding mechanical properties. However, for a honeycomb sandwich structure with large spatial periodicity, there is a low-frequency sound insulation valley. Here, the sound transmission across locally resonant honeycomb sandwich meta-structures was investigated to overcome this sound-insulation valley. An analytical model was developed based on the space-harmonic approach and the low-frequency sound insulation valley was determined analytically and numerically. The results indicate that the resonator distributed at the center of the face panel has a significant impact on the sound transmission performance of the honeycomb sandwich structure, whereas the resonator distributed on the wall of the honeycomb core does not contribute to overcoming this sound-insulation valley. Based on the research results, a design strategy for overcoming this sound-insulation valley was determined by tuning the damping parameter and constructing graded resonators. Moreover, sound transmission under the excitation of oblique incidence sound waves was also investigated. Compared with the method of filling porous materials, the proposed design method is more effective, and more importantly, the mass of the resonator is only 1.23% of that of the porous materials.

Rogue waves as modulational instability result in one-dimensional nonlinear triatomic acoustic metamaterials

In this study, a triatomic acoustic nonlinear metamaterial was modelled. It is a network of identical periodic unit cells made up of three different masses. These masses interact with connections modelled by springs whose stiffnesses are linear and nonlinear. Handling the motion equations of nth unit cell, the semidiscrete approximation method is used to extract the three-component coupled nonlinear Schrödinger equations. The dispersion relationship was determined. The results showed that the dispersion relation of the one-dimensional triatomic chain contained an acoustic branch and two optical branches. The dispersion curves show that the proposed acoustic metamaterial exhibits two local resonance bandgaps independent of spatial periodicity. The spectral range and frequency bandgap are related to the mass of the three atoms in the original cell. Based on the theory of linear stability analysis, modulation instability is the condition for the existence of rogue waves. Analytically, it is shown that modulational instability and rogue wave phenomena are strongly influenced by variations in the nonlinearity, dispersion, and diffraction terms, which contain the values of the masses and stiffnesses.

Design of Linear Block Copolymers and ABC Star Terpolymers That Produce Two Length Scales at Phase Separation.

Quasicrystals (materials with long-range order but without the usual spatial periodicity of crystals) were discovered in several soft matter systems in the last 20 years. The stability of quasicrystals has been attributed to the presence of two prominent length scales in a specific ratio, which is 1.93 for the 12-fold quasicrystals most commonly found in soft matter. We propose design criteria for block copolymers such that quasicrystal-friendly length scales emerge at the point of phase separation from a melt, basing our calculations on the Random Phase Approximation. We consider two block copolymer families: linear chains containing two different monomer types in blocks of different lengths, and ABC star terpolymers. In all examples, we are able to identify parameter windows with the two length scales having a ratio of 1.93. The models that we consider that are simplest for polymer synthesis are, first, a monodisperse ALBASB melt and, second, a model based on random reactions from a mixture of AL, AS, and B chains: both feature the length scale ratio of 1.93 and should be relatively easy to synthesize.

Investigation of Laser-Induced Surface Structures (LIPSS) on quartz and evaluation of their influence on material wettability

We report here on a study of the generation of Laser-Induced Periodic Surface Structures (LIPSS) on quartz upon irradiation with linearly polarized femto-second laser pulses (pulse duration τ = 200 fs and central wavelength λ = 1030 nm). Two different regimes of LIPSS were observed, Low-Spatial-Frequency-LIPSS (LSFL) and High-Spatial-Frequency-LIPSS (HSFL), characterized by different spatial periods and orientations. The formation of these two types of structures was investigated by varying the laser fluence, the number of laser pulses and the laser repetition rate. Once established how these parameters affect the generation of LIPSS, extended HSFL patterns (25 × 25 mm2) were realized, and their wettability was compared to that of pristine quartz. Contact angle measurements showed that HSFL textured quartz exhibits a super-hydrophilic behaviour, with a measured contact angle equal to 7.6°, with respect to pristine quartz which results simply hydrophilic and shows a contact angle equal to 41.2°.