Ripple Formation Research Articles

Ultrafast thermal modulation dynamics during ripples formation induced by femtosecond laser multi-pulse vortex beam

Abstract This work theoretically investigated the ultrafast thermal modulation dynamics during early formation of ripples on Au film induced by femtosecond laser vortex beam irradiation. An extended two-temperature dynamics model that comprehensively considers the optical interference modulation for the formation of seed ripples, transient reflectivity, and non-nonequilibrium thermal transfer was self-consistently built to predict high-contrast ripples formation. The 2D evolutions of the electron and phonon temperature modulations during ripples formation in high non-nonequilibrium state of Au film were obtained via femtosecond laser vortex beam irradiation. It was revealed that the ripples contrast in can be significantly amplified by shortening the laser wavelength, increasing the pulse number, or enlarging the laser fluence of vortex beam. Moreover, the electron-phonon coupling time during ripples formation was fully explored in detail. The studies provide valuable insights into optimizing laser parameters for controlled high-contrast ripple formation on Au films.

Predicting the Probability of Abrupt Changes to Wave‐Generated Seafloor Sand Ripples

AbstractA new, non‐dimensional ripple reset parameter and a stochastic point process model is used to estimate the likelihood of propagating ocean waves to form ripples on sandy seabeds. The ripple reset parameter is a function only of water depth, significant wave height, and mean grain size. Ripple formation is estimated by the magnitude of an intensity function based on a time series of the ripple reset parameter. The point process model is trained with a time series of observed waves and ripple change, and is then applied to predict the probability that a ripple field with a different geometry will form within a given time interval from another time series of wave data. The model is trained and tested with four field deployments at three field sites to determine its skill in predicting the ripple formation (a) at one field site over one time period after being trained with observations from the same site over a different time period, and (b) at one field site after being trained with observations from another field site. Results show that while the model is sufficient at predicting ripple formation in both scenarios, it is sensitive to the quality and quantity of the training data. Increasing the amount of training data greatly improves model performance. Employing a stochastic model based on a simple ripple reset parameter reduces tunable model parameters and provides a prediction of the probability for ripple formation given only a water depth, grain size, and time series of wave heights.

On the threshold value of the vertical vibrations amplitude causing Faraday ripples on the charged surface of a viscous liquid

The influence of the surface electric charge on the regularities of the formation of Faraday ripples on the horizontal surface of a viscous liquid is studied on the base of the approximation of small-amplitude perturbations. The typical horizontal dimensions of the Faraday ripples are established which is most significantly affected by the surface change density and viscosity.

Impact of laser interaction on morphological and mechanical properties of palladium thin film

This research work investigates surface morphology, plasma plume intensity and the hardness of palladium thin film deposited on silicon substrates under the action of laser light. For this purpose, a pulsed Nd:YAG laser (1064 nm, 10 mJ, 12 ns) was used to irradiate the specimen with repetitive laser pulses. The experiment was repeated under the illumination with UV light. The surface morphology of the specimens was examined through an Optical and Field-Emission Scanning Electron Microscopy showing thermal conduction, exfoliation, crater formation, re-deposition of the material, ripples formation, debris, heat-affected zone, holes formation and the formation of nanoparticles. The diameter and the depth of the crater formed on non-UV illuminated Pd thin film were greater than the crater formed on UV illuminated Pd thin film, because of its higher absorption. The photographs of the plasma plume were captured with a computer-controlled image capture system and examined by image processing software. The intensity of UV-illuminated Pd thin film was higher than non-UV illuminated Pd thin film because UV illumination induces defects in the specimen that have additional electronic states. These defects cause higher absorption and more ionization of the target material and emerge a more luminous plasma plume. A nanoindentation test was applied on the material surface to investigate the mechanical properties such as hardness and elastic modulus of the target specimen. The hardness and elastic modulus of the material were increased by cumulative laser shots.

Ripple formation on TA2 pure titanium surface induced by annular laser beam at low pressure environment

Formation of the micron-scale circular ripples on metal surfaces produced by continuous wave laser irradiation in low-pressure environment was investigated experimentally and numerically in this manuscript. It is found that splashing of the molten material and oxidation of the metal surface, which usually happened in the atmospheric environment, were avoided in low-pressure environment. Therefore, circular ripples were formed during the solidification process of the molten pool. Numerical simulations based on the finite element method (FEM) and the moving mesh method were applied to investigate the evolution of the flow field behavior of the molten pool. It is found that the Marangoni effect and capillary force during solidification process dominate the formation of the micron-scale circular ripples. In addition, the ripple spacing can be controlled by the temperature gradient. At the same power density, a higher temperature gradient on the molten surface can be produced by an annular laser beam, compared to a Gaussian beam, thus a denser circular ripple structure were produced, which were confirmed by the experimental results. The results of this paper may enrich the investigations on the metal surface texturing methods, laser vacuum processing and laser material engineering.

Spatially Variable Ripple and Groove Formation on Gallium Arsenide Using Linear, Radial, and Azimuthal Polarizations of Laser Beam

We demonstrated the linear, radial, and annular ripple formation on the surface of GaAs. The formation of linear ripples was optimized by the number of shots and the fluence of 30 ps, 532 nm pulses. The radial and annular nanoripples were produced under the ablation using doughnut-like beams possessing azimuthal and radial polarizations, respectively. We compare the ripples and grooves formed by a linearly polarized Gaussian beam relative to an annular vector beam. The joint overlap of sub-wavelength grooves with ripples formed by azimuthally and radially polarized beams was reported. The conditions under which the shape of radial and ring-like nano- or micro-relief on the GaAs surface can be modified by modulating the polarization of laser pulse were determined. The resultant surface processing of GaAs using a laser beam with different polarization modes is useful for exploring valuable insights and benefits in different applications.

Nanoscale pattern formation on surfaces by cluster ion beam irradiation

Ion beam sputtering is a solid-surface nanostructuring procedure applicable to any type of material. In this study, we are interested in the bombardment of a metallic surface, specifically, of a Co(110) surface bombarded with Ar clusters at oblique incidence. The bombardment with clusters accentuates the effect of surface ripple formation. Sputter erosion and surface atomic redistribution are the processes that determine the morphological evolution of the surface from the collisional point of view. The importance of these processes was analyzed for different angles of incidence in the bombardment with very small clusters and atoms while always maintaining the same fluence. The sputter yield increases with the number of atoms in the cluster for angles greater than 50°; while for the rest, it barely experiences any increase. Unlike sputtering, displacements increase as the cluster size increases above the critical angle. Moreover, horizontal displacement only shows some sign of saturation for angles close to the normal. An analysis of redistributed volumes and previous data suggest that sputtering drives the ripple effect for grazing angles and ones close to these. It is also responsible for the enhancement effect in the cluster bombardment. For the rest of the angles, the weight of the redistribution is decisive. There is a proportional relationship between the emptied volume and the horizontal displacement.

Field observations on the characteristics of sand ripples on tidal flats

Ripples on tidal flats significantly influence bedform roughness and near-bed turbulence, yet their dynamics in sandy and muddy environments remain incompletely understood. While laboratory studies have elucidated the effects of mud content and extracellular polymeric substances (EPSs) on ripple formation and stability, the interactions between ripple characteristics, EPS, and mud content in natural settings are more complex and not thoroughly explored. To address this gap, we conducted field studies on sand ripples at the central Jiangsu coast, China, using high-resolution drone photogrammetry to accurately measure ripple dimensions. We performed bedload sampling at both ripple crests and troughs to analyze median grain size, mud content, and EPS concentrations. Our results show that the investigated ripple wavelengths range from 34 to 46 mm, and heights vary between 2.7 and 5.3 mm. Ripples developed near tidal creeks show pronounced asymmetry. Significantly, EPS concentrations are markedly higher at the ripple crests than at the troughs, and it follows a power-law relationship with the median grain size. These results highlight the complex intricate relationship between the ripple morphology, environmental forcing conditions, and EPS content. Our findings enrich the understanding of ripple development and the factors influencing current-dominated ripples in natural environments. This research also adds to the better prediction of the bedform roughness and quantification of the near bed sediment transport.

Simulations of pulsed overpressure jets: formation of bellows and ripples in galactic environments

ABSTRACT Jets from active nuclei may supply the heating which moderates cooling and accretion from the circum-galactic medium. While steady overpressured jets can drive a circulatory flow, lateral energy transfer rarely exceeds 3 per cent of jet power, after the initial bow shock has advanced. Here, we explore if pulses in high-pressure jets are capable of sufficient lateral energy transfer into the surrounding environment. We answer this by performing a systematic survey of numerical simulations in an axisymmetric hydrodynamic mode. Velocity pulses along low Mach jets are studied at various overpressures. We consider combinations of jet velocity pulse amplitude and frequency. We find three flow types corresponding to slow, intermediate, and fast pulsations. Rapid pulsations in light jets generate a series of travelling shocks in the jet. They also create ripples which propagate into the ambient medium while a slow convection flow brings in ambient gas which is expelled along the jet direction. Long period pulses produce slowly evolving patterns which have little external effect, while screeching persists as in non-pulsed jets. In addition, rapid pulses in jets denser than the ambient medium generate a novel breathing cavity analogous to a lung. Intermediate period pulses generate a series of bows via a bellows action which transfer energy into the ambient gas, reaching power efficiencies of over 30 per cent when the jet overpressure is sufficiently large. This may adequately inhibit galaxy gas accretion. In addition, such pulses enhance the axial out-flow of jet material, potentially polluting the circum-galactic gas with metal-enriched interstellar gas.

Physical parametric study of bacterial biofilm disruption and removal by jet impingement: A CFD investigation

Bacterial biofilm formation is one of the most critical challenges in industrial and health systems. It is the leading cause of nearly 80 % of clinical infections. Biofilms are usually evaluated from the physiological point of view as microbial cells enclosed in an extracellular polymer and represent a complex, dynamic bacterial community. Cohesive mechanical forces hold the bacteria within the biofilm together. These forces contribute to the biofilm's mechanical and structural stability, resulting in its viscoelastic properties. Studying the biofilm's mechanical properties as a viscoelastic matter can provide a better understanding of the biofilm's characteristics and the survival strategies of bacteria in the biofilm state of life. In the present study, a 2D-axisymmetric RANS-CFD simulation model using the Volume Of Fluid (VOF) method was developed to track changes in biofilm surface and ripple formation across six biofilm thicknesses ranging from 15 μm to 115 μm, two substrate geometries including flat and curved, and two substrate roughnesses of 0.4 μm and 0.9 μm height. A modified Herschel-Bulkley model was employed to capture the biofilm's non-Newtonian shear-thinning behavior under high-velocity gas jet impingement. The results revealed the importance of biofilm thickness, substrate geometries, and properties like roughness in biofilm's mechanical behavior. Higher thickness of biofilm resulted in smaller jet-impingement region and more significant ripples formation. Biofilm on curved substrates decreased in thickness more rapidly than those on flat substrates. The substrate's geometry impacted biofilm folding patterns, removal, and mechanical behavior. Selected substrate roughnesses did not create any resistance against the mechanical removal of biofilms. Here, we successfully captured biofilms' non-Newtonian and shear-thinning behavior with numerical simulations consistent with previously reported experimental results. This could benefit industrial and health systems by tackling biofilm-related challenges and developing more accurate and detailed models of biofilms to optimize tools for drug testing or screening and enhance the models for in-vitro and in-vivo analysis.

Investigation of Ripple Formation on Surface of Silicon by Low-Energy Gallium Ion Bombardment.

Regular wave patterns were created by a 2 kV gallium ion on Si(111) monocrystals at incidence angles between 60° and 80° with respect to the surface normal. The characteristic wavelength and surface roughness of the structured surfaces were determined to be between 35-75 nm and 0.5-2.5 nm. The local slope distribution of the created periodic structures was also studied. These topography results were compared with the predictions of the Bradley-Harper model. The amorphised surface layers were investigated by a spectroscopic ellipsometer. According to the results, the amorphised thicknesses were changed in the range of 8 nm to 4 nm as a function of ion incidence angles. The reflectance of the structured surfaces was simulated using ellipsometric results and measured with a reflectometer. Based on the spectra, a controlled modification of reflectance within 45% and 50% can be achieved on Si(111) at 460 nm wavelength. According to the measured results, the characteristic sizes (periodicity and amplitude) and optical property of silicon can be fine-tuned by low-energy focused ion irradiation at the given interval of incidence angles.

Surface Ripple Formation by Bombardment with Clusters: Influence of Mass

Nanostructure formation on Co(110) surfaces was studied by using irradiation with cluster ion beams with oblique incidence and an energy of 250 eV/atom. In this work, the effect of the mass of the cluster projectiles on the process was analyzed. The launched clusters were formed by different types of charged atoms: He, Ne, Ar, Kr, and Xe. Due to the different collision processes, the formed surface patterns stand out more if the mass of the projectile atoms is greater, regardless of the angle of incidence of the clusters. Two processes control the morphological evolution of the surface during the bombardment phase: sputtering erosion and surface atomic redistribution. At grazing angles, the contribution of sputtering is greater during the process. In fact, heavier species give greater sputtering, and the redistribution factor becomes lower. The weight of redistribution is greater for intermediate angles above the critical angle (50° and 60°), since the displacement is greater for heavier species, and the redistribution factor takes substantially higher values. The experimental results point to a shift in the critical angle with the mass of the projectile atom. In the case of He, a very light ion, the results are marked by channeling and vertical displacements.

On icicle ripples

Natural icicles have an overall conical shape modulated by surface ripples. It has been noted from many observations of icicles formed in nature and in the laboratory that the wavelength of the ripples has a very narrow spectrum between about 8 and 12 mm and that, as time evolves, the phase of the ripples migrates upwards. In this pedagogical review, I explore some of the physical mechanisms that can cause and mediate the formation and migration of ripples on icicles using simple mathematical models. To keep the mathematics more straightforward and transparent, I confine attention to two dimensions. A key physical parameter is the surface tension between the film of water that coats an icicle and the air that surrounds it, which causes a phase shift between the film–air interface and the ice–film interface. I show that the wavelength of ripples is dominantly proportional to the cube root of the square of the gravity-capillary length times the thickness of the water film. At high film-flow rates, advection-dominated heat transfer coupled with the interfacial phase shift becomes the dominant driver of instability. Gibbs–Thomson undercooling provides an unexpectedly large stabilisation of small wavelengths at these large flow rates, sufficient to maintain wavelength selection at millimetre scales.

Field monitoring of alluvium accumulation in the riverine floodplain of the Oka River, European Russia

Sedimentation traps were used to assess the annual surface renewal dynamics of the low floodplain of the Oka River (a major tributary of the Volga River in central European Russia). Trap-mats and platforms made of crushed bricks were installed in positions near the meandering and relatively straight riverbed, in different sedimentation environments. Stationary research in 2014–2020 covered a section of the bottom of the Oka River valley with a length of more than 400 km along the main channel. The graphical-analytical processing of field data using Ferret's Triangles and sedimentation diagrams showed that transport and deposition of suspended sediment dominated in the accumulation of 87% of alluvium samples. The formation of ripples was not recorded, which was lithologically reflected in the horizontal layering of the new sediment. The determination of the granulometric composition of the removed sediment and their thickness on the traps showed the absence of statistically significant differences according to the Kruskal–Wallis test between the data samples from the trap-mats and trap-platforms. The reference to the daily calendar of synoptic mechanisms according to the classification of Dzerdzeevsky contributed to the identification of meteorological prerequisites for the variation of the hydrograph curve of the Oka River in its middle reaches. Prolonged floods caused by the premature arrival of spring lead to massive deposition of silt and clay particles even on sandbanks. On the other hand, short (15–35 d) March floods and abnormal high water in June, caused by Atlantic cyclone intrusions, can stabilize sand accumulation on the riverine floodplain. The thickest sediment layers on the traps were obtained in 2018 after a very cold March and a powerful April flood, and the overall distribution of alluvium thickness and its particle size distribution also depends on the morphology of the riverine relief. The siltation is caused by the accumulation of silts; the most finely dispersed sediment was deposited in those facies environments for which siltation was also characteristic in historical times.

Multiple Evolution Modes of Megaripples in the Qaidam Basin and Implications for Ripple‐Like Aeolian Landforms on Mars

AbstractAeolian landforms provide valuable insights into the planetary surface environment and its evolutionary history. In this study, the formation and evolution of megaripples in the Qaidam Basin and their relationship with the development environment are analyzed. By quantifying the wind environment, morphology, grain‐size distribution, sedimentary structure, and luminescence age of megaripples, we propose for the first time that there are multiple megaripple evolution modes. Investigation revealed that three evolution modes were responsible for forming megaripples in different equilibrium states: transient, stable, and metastable. Well‐sorted coarse sand grains accumulate on ridges and overlay poorly sorted fine sand grains to form transient megaripples. Stable megaripples have alternating sedimentary bedding of coarse and fine sand grains. Metastable megaripples have a secondary ripple formation on the surface. Throughout their formation, coarse and fine sand grains undergo regrouping. The response of coarse grains to the change in wind speed lags behind that of fine grains. This process controls the erosion and accumulation of megaripples and affects their size and sedimentary structures. The evolution mode, scale, and sedimentary structure of megaripples are influenced by the grain‐size range under the same wind conditions. The luminescence ages of the coarse‐grained megaripple sediments are less than 700 years. This study provides a fresh perspective on the coexistence of various sand ripples and transverse aeolian ridges found on Mars.

The influence of laser fluences on surface properties, plasma formation, and microhardness of Mg-alloy

The impact of variation in laser fluence on the surface modifications and plasma parameters of biocompatible, Magnesium (Mg) alloy has been studied. A 1064 nm Nd: YAG laser with, 10 ns pulse duration and a repetition rate of 1 to 10 Hz has been used for this purpose. Mg-alloy targets were subjected to fluences ranging from 1.3 J·cm−2 to 10.47 J·cm−2 in Argon (Ar) ambient. The surface morphology of irradiated targets was analyzed using a Scanning Electron Microscope (SEM). Formation of ripples, ridges, conical structures, pores, and cavities are observed on the surface of the irradiated Mg targets and are shown in the SEM topographical images. A higher initial fluence causes a rise in the size and density of these structures, which subsequently falls off as the fluence increases more to its highest value. Plasma parameters, such as electron temperature (Te) and number density (ne), are measured using Laser-Induced-Breakdown-Spectroscopy (LIBS). It was also found that both Te and ne increase with increasing fluence. XRD analysis is performed to study properties such as phase purity and structural studies were carried out through X-ray diffraction (XRD) technique, aiding in the investigation of material behavior and guiding the development of new materials with specific properties. Using a Vickers Micro hardness tester, we haveinvestigated the hardness of irradiated Mg targets. From the present investigation it is observed that the hardness value of irradiated targets is higher than the pristine Mg target. It is also found that Te and ne play a significant role for the enhancement of hardness as well as for the growth of surface structures on Mg-alloy.

Exploring wetting & scaling behavior of rough silicon surfaces

Here we study the roughening behavior, scaling properties and the wetting nature of the Silicon surfaces that have been irradiated with 3 keV Ar ions. These surfaces illustrate the formation of nanostructures and ripples, which evolve at higher fluences. The scaling studies have been applied to understand the evolution of the surfaces. Height- Height correlation method has been utilized to extract important scaling parameters, and the derived Hurst exponents (H) indicate these surfaces to be of self-affine type. The derived correlation lengths (ξ) reflect long ranged correlations on the surface and evolution of the nanostructures. The nano-patterned surfaces show near hydrophobic behavior, which gets further enhanced with increasing irradiation.

Superhydrophobic Coatings for Corrosion Protection of Stainless Steel

Corrosion is a persistent challenge in the aviation industry, affecting the safety, performance, and maintenance costs of aircraft. While composite materials have gained widespread use due to their lightweight properties and corrosion resistance, certain critical parts, such as the wing and empennage leading edges and the engine inlet, demand alternative solutions. Aluminum, titanium, and stainless steel emerge as mandatory materials for such components, given their exceptional strength and durability. However, protecting these metallic components from corrosion remains crucial. In this paper, we present a study aimed at evaluating the corrosion resistance of stainless steel, employed as an erosion shielding panel for a composite vehicle’s wing, layered with a superhydrophobic coating. The samples with and without coating have been characterized by contact angle measurements, microscopy (optical and electronic), and visual inspection after immersion in two solutions, NaCl and NaOH, respectively. The application of the superhydrophobic coating demonstrated a significant reduction in corrosion extent, especially in the demanding NaCl environment. This was evidenced by diminished formation of ripples and surface roughness, decreased iron oxide formation from oxidative processes, and a lower Surface Free Energy value in both liquid environments. Notably, the surface maintained its superhydrophobic properties even following an 8-day immersion in NaCl and NaOH solutions, demonstrating the reliability of the superhydrophobic coating offering as a potential solution to enhance the longevity and reliability of aircraft structures.

First-principles modeling of molecular adsorption on an InSe monolayer

In this paper, it is demonstrated that the calculated physical adsorption energies, substrate–adsorbent distances and substrate distortions strongly depend on the size of the employed supercell and particularly on the type of optimization in the case of very flexible two-dimensional monolayers, such as indium (II) selenide (InSe). It has been established that calculations with optimization of only atomic positions and calculations with optimization of atomic positions and lattice parameters can give energies of different signs and values. In-plane (stretching and compression) and out-of-plane (ripple formation) distortions also lead to significant changes in the calculated adsorption energies. The influence of substrate flexibility and adsorption on the electronic structure and optical properties is also discussed.

Ambient Pressure Influence on the Electrical Resistance of Tracks Fabricated by Picosecond Laser Pulses on the Surface of AlN Ceramic

Herein, results are presented on the air pressure influence on the resistance of surface structures formed by picosecond processing of aluminum nitride ceramic. It is found that scanning the ceramic surface by laser radiation at the wavelengths of 1064 and 355 nm emitted by a neodymium‐doped yttrium aluminum garnet laser system can produce conductive lines. The resistance value is measured of structures fabricated under different processing conditions related to varying the laser fluence, pulse overlapping, and wavelength at different ambient air pressures from 10−4 Torr to atmospheric pressure. Based on the obtained dependences, the processing parameter windows are defined, allowing formation of a conductive material. It is demonstrated that changing the experimental conditions may change the resistance value by several orders of magnitude. The ambient pressure also affects the morphology of the ablated zones, with different structures being observed, including formation of ripples. The analyses performed of the processed surface indicate that conductivity arises from the presence of aluminum following a thermally induced ceramic decomposition. The influence of the gas flow during the laser processing on the resistance and the possibility of laser scribing of the ceramic are also experimentally estimated.