Ripple Morphology Research Articles

An Acoustic Sensor System to Measure Aeolian Ripple Morphology and Migration Rates.

Acoustic distance sensors have a long history of use to detect subaqueous bedforms. There have been few comparable applications for aeolian bedforms such as ripples. To address this, we developed a simple and reliable apparatus comprising a pair of distance sensors, a bracket upon which they are mounted, and a base upon which the bracket can slide. Our system relies on two Senix Corporation (Hinesburg, VT, USA), ToughSonic® model 14-TSPC-30S1-232 acoustic distance sensors: one to measure surface elevation changes (in this case, ripple morphology) and a second to measure horizontal location. The ToughSonic® vertical resolution was 0.22 mm and the horizontal scan distance was about 0.60 m with a locational accuracy of 0.22 mm. The measurement rate was 20 Hz, but we over-sampled at 1 KHz. Signal processing involves converting volts to meters, detrending the data, and removing noise. Analysis produces ripple morphologies and migration rates that conform with independent measurements. The advantages of this system relative to terrestrial laser scanning or structure from motion are described.

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.

Image Enhancement via Special Functions and Its Application for Near Infrared Imaging.

Image enhancement is important given that it can be used to highlight the area of interest in the images. This article designs four filters via special function for realizing image enhancement. Firstly, a filter based on the exponential function is designed. When the value of the progression is even, the edge feature can be extracted. When the value of the progression is odd, sharp contrast can be obtained. Secondly, a filter is built using hyperbolic cosine and its inverse function, where a printmaking feature can be extracted. Thirdly, a filter is made via a hyperbolic secant function and its inverse. It can lead to the extraction of image edge. When the progression value is increasing, marginal effect can be found and the brightness is decreasing. Ripple morphology can be found. Fourthly, a filter is constructed through a hyperbolic sine function and its inverse, where marginal features can be extracted. Furthermore, these filters are useful for extracting the marginal features even when a high noise density of 0.9 is added to the original images. They are useful for highlighting the images acquired from near infrared imaging.

Large-area regular periodic surface structures on 4H-SiC induced by defocused femtosecond laser

Femtosecond (fs) laser has been proved to induce periodic surface structure formation with various periods. Previous studies employed complex optical means and specific processing environments to induce nano-sized periodic surface structures. In this study, a defocused fs laser was utilized to realize a high-stability and straightforward method for generating periodic structures on silicon carbide (SiC) surfaces. Due to its more uniform energy distribution, the defocused laser effectively attenuates the unstable fluid flow caused by the Marangoni convection, thereby improving the consistency of laser- induced periodic surface structure morphology and size. The effects of laser processing parameters on the ripple period, density, and morphology were systematically investigated. Moreover, an appropriate physical process was adopted to explain the formation of the nanostructures. A high laser pulse number was considered the key to forming large-area dense nanoripples, which possessed a smooth edge profile and similar period. Under optimal parameters, large-area regular nanoripples with periods of ∼100 nm could be induced on the SiC surface by a defocused fs laser method. Meanwhile, a two-step laser-inducing method was proposed to fabricate an array of two-dimensional square-shaped nanocolumns with a size of 60 nm × 60 nm. The two-step laser-inducing method realized tuning in the surface nanostructure morphology and localized nanostructures rewriting by adjusting the laser processing parameters. This large-area periodic structure inducing method showed the possibility of making laser-writing technology to be flexible, straightforward and, hence, competitive for advanced industrial application based on surface nanostructuring.

Tunable non-Gilbert-type damping in Ni80Fe20 films sputtered on the rippled Al2O3 substrates

The ferromagnetic damping and relaxation mechanisms of Ni80Fe20 (NiFe) films sputtered on rippled Al2O3 substrates were investigated by in-plane angular dependence ferromagnetic resonance (FMR) measurements where these rippled Al2O3 substrates were prepared by thermal annealing. The linewidth of intrinsic contribution due to Gilbert damping mechanism and linewidth of extrinsic contribution related to two-magnon scattering (TMS) and inhomogeneous broadening of films were identified by filtering different magnetic relaxation mechanisms of films. The results show that the magnitude of ferromagnetic damping of films deposited on rippled Al2O3 substrates can be manipulated by modifying the ripple morphology of substrates and it is strongly dependent on the angle between the direction of the ripple and the external field. The non-Gilbert-type damping for these films is an adjustable, anisotropic contribution to the overall relaxation. The two-magnon scattering and inhomogeneous broadening act as non-Gilbert damping mechanism in magnetization relaxation and they dominate the variation of total linewidth. This work revealed the tunable magnetization relaxation in films deposited on rippled Al2O3 substrates with different surface morphology formed by annealing and it would be helpful for utilizing these rippled substrates to design magnetization relaxation characteristics of spintronic and magnonic devices.

Small-scale bedforms and associated sediment transport in a macro-tidal lower shoreface

Ripples and small-scale bedforms are ubiquitous in shallow water environments under the combined action of currents and waves. Small scale processes linked to their formation and migration are interconnected with sediment transport at larger scales (e.g. tens of metres to kilometres), both resulting in and being affected by large scale sediment transport and geomorphological evolution. The lower shoreface provides a key link between coasts and continental shelves, but the contribution of ripples and small-scale bedforms to sediment transport in this region has yet to be fully addressed. This work presents a study of sediment dynamic processes on the lower shoreface in the presence of small-scale bedforms. Observations were made during the winter of 2017 on the lower shoreface of Perranporth Beach, which is in the south west of the UK and exposed to Atlantic waves. The analysis of morphological expressions and the variability of ripples under waves, currents and wave-current conditions are assessed. Ripple morphology and associated dynamics are analysed for their potential contribution to the exchange of sediment between the lower and the upper shoreface. In the present study it was observed that even though ripples were evolving depending on the wave-current forcing, little ripple migration was observed due to low wave skewness. The implication is that ripple migration and bedload transport are only a small contribution to onshore sediment transport under low to moderate energy conditions. However, during more energetic conditions, ripples were washed out and the wave skewness increased, resulting in onshore sediment transport under a sheet flow regime. This suggests that ripple formation and migration can have little impact on the cross-shore supply of sediment from the lower shoreface to the upper shoreface and that more energetic wave conditions are required to significantly transport sediment towards the beach.

Transition of nano-ripple to nano-hillock pattern on ion bombarded Si with an enhanced hydrophobicity

Surface patterning in nano-scale dimension and the effect of surface heterogeneity to control surface properties like wettability of solid surfaces have been studied here. This work focuses primarily on the generation of Si nano-patterns at oblique incidence via ion beam assisted impurity co-deposition technique and its impact on the wetting property of liquid. When oblique ion irradiation is performed on a Si surface without impurity, ripple structures evolve that show an increase in its hydrophilic behavior with a rise in ion fluence. On the other hand, the inclusion of stainless steel as seeding material during bombardment brings about a noteworthy change in the surface topography with the emergence of hillocks structures over the ripple morphology. For a higher bombardment time, the Si rippled surface is seen to vanish and a transition from nano-ripple to nano-hillock pattern is witnessed. Such an unusual pattern evolution is aided by FeSi2 bond which further promotes an increase in surface hydrophobicity. Contact angle measurements show directional dependence behavior having a direct influence of the anisotropic patterns synthesized here.

Investigating the response of wave-generated ripples to changes in wave forcing

Wave-generated ripples are macroscopic roughness elements that influence fluid flow and sediment transport. For a major group of ripples (orbital ripples), morphology (height and wavelength) is set by the wave conditions. In natural conditions, where wave forcing is highly variable, ripple morphology is frequently changing. We investigate the rate of morphological change after changes in wave conditions (i.e., wave height and period), using laboratory experiments with a step-change in wave forcing. The adjustment time of ripple morphology to new conditions –– the hysteresis time scale (hereafter referred to simply as hysteresis) –– is proportional to changes in wave orbital diameter, and the coefficient of proportionality differs for decreasing and increasing orbital diameter. When the Shields parameter is lower than a threshold (0.043), there is no change in ripple morphology (or changes are extremely slow). In addition, we find the presence of defects (irregularities from straight parallel crestlines) reduces the hysteresis time scale. The dependence of hysteresis on defect density implies that a larger density of defects results in faster ripple adjustment, confirming previous theoretical and numerical model results.

Deep UV laser induced periodic surface structures on silicon formed by self-organization of nanoparticles

We have investigated the formation of laser-induced periodic surface structures (LIPSS or “ripples”) on silicon upon excitation with p-polarized excimer laser pulses in the deep ultraviolet region (λ = 193 nm, 20 ns). Well-pronounced ripples with a period close to the laser wavelength were observed for pulse numbers N ≥ 100, and the ripple period increased with the angle of incidence. While these results seem to be qualitatively consistent with the standard Sipe-theory, we observed a fundamentally different ripple formation mechanism and ripple morphology. At low pulse numbers, isolated nanoparticles with a size of a few tens of nanometers are observed at the silicon surface, which then start to agglomerate in 2D and self-organize to form ripples with a very shallow modulation depth as the pulse number increases. Employing a recently developed plasmonic model based on the propagation of a surface plasmon polariton on a rough surface, we demonstrate excellent quantitative agreement of the evolution of the ripple period with incidence angle. Finally, we show that surface regions exposed to lower laser fluence feature micro- and nanopores, which give rise to pronounced photoluminescence (PL) emission in the visible spectral region, as opposed to the nanoparticle-based ripples not showing PL.

Tailoring of uniaxial magnetic anisotropy in Permalloy thin films using nanorippled Si substrates

In this work the investigation of in-plane uniaxial magnetic anisotropy induced by the morphology due to ion beam erosion of Si(1 0 0) has been done. Ion beam erosion at an oblique angle of incidence generates a well-ordered nanoripple structure on the Si surface and ripple propagates in a direction normal to ion beam erosion. Permalloy thin films grown on such periodic nanopatterns show a strong uniaxial magnetic anisotropy with easy axis of magnetization in a direction normal to the ripple wave vector. The strength of uniaxial magnetic anisotropy is found to be high for the low value of ripple wavelength; it is decreasing with increasing value of ripple wavelength. Similarly, the strength of uniaxial magnetic anisotropy decreases with increasing Permalloy film thickness. Grazing incidence small angle x-ray scattering data reveals an anisotropic growth of Permalloy thin films with preferential orientation of grains in the direction normal to the ripple wave vector. Permalloy thin film growth is highly conformal with the film surface replicating the substrate ripple morphology up to a film thickness of 50 nm has been observed. Correlation between observed uniaxial magnetic anisotropy to surface modification has been addressed.

Video Transects Reveal That Tidal Sand Waves Affect the Spatial Distribution of Benthic Organisms and Sand Ripples

AbstractThe sandy seabed of shallow coastal shelf seas displays morphological patterns of various dimensions. The seabed also harbors a rich ecosystem. Increasing pressure from human‐induced disturbances necessitates further study on drivers of benthic community distributions over morphological patterns. Moreover, a greater understanding of the sand ripple distribution over tidal sand waves may improve morphological model predictions. Here we analyzed the biotic abundance and ripple morphology in sand wave troughs and crests using video transects. We found that both the epibenthos and endobenthos are significantly more abundant in sand wave troughs, where ripples are less abundant and more irregularly shaped. Finally, we show that camera systems are relatively quick and effective tools to study biotic spatial patterns in relation to seabed morphology.

Sputter yield of rippled surfaces: A simulation study

Abstract Atomistic (binary-collision) simulation is used to study the sputter yield from rippled surfaces in a wide range of ion incidence angles. Most simulations refer to amorphous carbon bombarded with 7 keV Ga ions, and sinusoidal ripple morphology is assumed. Results are compared with the analytical predictions and Monte Carlo simulations based on continuum models of sputtering from sine- and ridge-shaped surfaces. Both atomistic and Monte Carlo simulations do not confirm a sharp increase in the sputter yield with increasing the ripple amplitude predicted theoretically (Makeev and Barabasi, 2004). Significant differences between the results are also observed in the yield dependence on the angle of incidence.

Simulation of sputtering from ion-beam-roughened carbon surfaces

ABSTRACTEffects of ion-induced surface roughness on sputtering of amorphous carbon under ion bombardment are studied by means of binary-collision computer simulation in a wide range of incidence angles. Most simulations refer to 1–10 keV Ar ion bombardment, and sinusoidal ripple morphology is assumed. It is shown that surface roughness is a key factor to achieve quantitative agreement with experiment. The simulation results are compared with the analytical estimates of the yield from sine-shaped and ridged surfaces based on continuum models of ion sputtering. Some deviations between the results are discussed.

Nanoripple formation on GaAs (001) surface by reverse epitaxy during ion beam sputtering at elevated temperature

Self-organized pattern formation by the process of reverse epitaxial growth has been investigated on GaAs (001) surfaces during 1keV Ar+ bombardment at target temperature of 450°C for a wide range of incident angles. Highly ordered ripple formation driven by diffusion instability is evidenced at near normal incidence angles. Concurrent sample rotation shows that the ripple morphology and its orientation do not depend on the incident beam direction; rather they are determined by the symmetry of the crystal face.

Buckling behavior of carbon nanotubes under bending: From ripple to kink

This paper elucidates the buckling behavior of carbon nanotubes (CNTs) under bending. CNTs are modeled as continuous thin-wall circular tubes, and their buckling is governed by equations that take into account of the sectional Brazier effect and non-uniform structural deformation. The CNT governing equations (fourth-order ordinary differential nonlinear equations with integral conditions) are solved by introducing a continuation algorithm. In addition, the buckling behavior of CNTs under bending is simulated with objective molecular dynamics (OMD). The atomistic simulations are used to verify the continuum results. We show that there exist low- and high-strain phases during the bending process of CNTs, and the transition in between may divide the whole bending process into three stages: low-curvature stage, mixed-curvature stage and high-curvature stage. Ripples are generated on the CNT surfaces before the formation of kinks. Compared to single-walled CNTs (SWCNTs), hydrogen-filled CNTs have a longer mixed-strain stage owing to the presence of internal pressure, and are therefore more inclined to exhibit a ripple morphology. Our results offer better understanding of the buckling behavior of CNTs, and may open up new opportunities for the design and applications of novel CNT-based nanoelectronics.

Back‐flow ripples in troughs downstream of unit bars: Formation, preservation and value for interpreting flow conditions

AbstractBack‐flow ripples are bedforms created within the lee‐side eddy of a larger bedform with migration directions opposed or oblique to that of the host bedform. In the flume experiments described in this article, back‐flow ripples formed in the trough downstream of a unit bar and changed with mean flow velocity; varying from small incipient back‐flow ripples at low velocities, to well‐formed back‐flow ripples with greater velocity, to rapidly migrating transient back‐flow ripples formed at the greatest velocities tested. In these experiments back‐flow ripples formed at much lower mean back‐flow velocities than predicted from previously published descriptions. This lower threshold mean back‐flow velocity is attributed to the pattern of velocity variation within the lee‐side eddy of the host bedform. The back‐flow velocity variations are attributed to vortex shedding from the separation zone, wake flapping and increases in the size of, and turbulent intensity within, the flow separation eddy controlled by the passage of superimposed bedforms approaching the crest of the bar. Short duration high velocity packets, whatever their cause, may form back‐flow ripples if they exceed the minimum bed shear stress for ripple generation for long enough or, if much faster, may wash them out. Variation in back‐flow ripple cross‐lamination has been observed in the rock record and, by comparison with flume observations, the preserved back‐flow ripple morphology may be useful for interpreting formative flow and sediment transport dynamics.

Longevity of aeolian megaripples

Megaripples are distinguished from regular ripples by their larger dimensions and bimodal grain-size distributions. The interplay between wind, grain size and ripple morphology (height and wavelength) controls their development. Two main mechanisms limit megaripple height. The first, megaripple flattening due to winds that are above the fluid threshold of the coarse grains, destroys the armoring layer of the megaripple. The second is megaripple erosion by the impacts of fast-moving, fine saltating grains that propel the coarse grains constituting the armoring layer. For any given wind regime and grain size distribution, the potential megaripple dimensions are limited by these two mechanisms. Here we study the first mechanism and estimate the duration of strong winds (sustained above the fluid threshold) needed to flatten megaripples. Strong gusts of wind, in contrast, cannot destroy the megaripples but can cause ripple migration. Based on data from previous works on megaripples, we find a scaling law between the ripple morphology and the coarse mode of grains at the crest. Using this scaling relation allows us to calculate the wind velocity and duration needed for megaripple flattening. In general, the coarser the particles at the megaripple crest, the stronger the wind needed to flatten the megaripples. Moreover, the greater the strength of the wind required to flatten the megaripples, the lower the recurrence probability. Taken together, these findings increase the longevity of megaripples. We apply the results for a megaripple field in the southern Arava valley (Israel).

Discussion of the possible formation mechanism of near-wavelength ripples on silicon induced by femtosecond laser

In this paper, the effect of the irradiation pulse number on the morphology of the near-wavelength ripples perpendicular to the laser polarization was investigated. The results show that the ripple spacing first fast decreases with the increased irradiation pulse number and finally reaches a stable value. The change of ripple morphology with different incidence angles was also studied. The ripples are found keeping always perpendicular to the laser polarization and have respective ripple spacings of 650, 480 and 390 nm upon the laser irradiation with pulse number N = 16 and fluence of 1.33 J/cm2 at the incidence angles of 0°, 30° and 60°. The change of the ripple spacing with different excitation levels was also studied. The theory for interference of the incident wave with the surface plasmon polariton wave (SPP) and the Sipe–Drude theory are both employed to analyze the experimental results. Detailed calculations show that the Sipe–Drude theory works better for explaining the main features of the ripples on silicon wafer surface. We suggest that the interaction of the incident wave with some form of surface electromagnetic wave except SPP is most possibly responsible for the ripple formation on silicon surface.

Quantitative imaging of the magnetic configuration of modulated nanostructures by electron holography.

By means of off-axis electron holography the local distribution of the magnetic induction within and around a poly-crystalline Permalloy (Ni81Fe19) thin film is studied. In addition the stray field above the sample is measured by magnetic force microscopy on a larger area. The film is deposited on a periodically nanostructured (rippled) Si substrate, which was formed by Xe(+) ion beam erosion. This introduces the periodical ripple shape to the Permalloy film. The created ripple morphology is expected to modify the magnetization distribution within the Permalloy and to induce dipolar stray fields. These stray fields play an important role in spinwave dynamics of periodic nanostructures like magnonic crystals. Micromagnetic simulations estimate those stray fields in the order of only 10 mT. Consequently, their experimental determination at nanometer spatial resolution is highly demanding and requires advanced acquisition and reconstruction techniques such as electron holography. The reconstructed magnetic phase images show the magnetized thin film, in which the magnetization direction follows mainly the given morphology. Furthermore, a closer look to the Permalloy/carbon interface reveals stray fields at the detection limit of the method in the order of 10 mT, which is in qualitative agreement with the micromagnetic simulations.

Self-organizing microstructures orientation control in femtosecond laser patterning on silicon surface.

Self-organizing rippled microstructures are induced on silicon surface by linearly polarized femtosecond laser pulses. At a near threshold fluence, it is observed that ripple orientation is co-determined by the laser polarization direction and laser scanning parameters (scanning direction and scanning speed) in surface patterning process. Under fixed laser polarization, the ripple orientation can be controlled to rotate by about 40° through changing laser scanning parameters. In addition, it is also observed that the ripple morphology is sensitive to the laser scanning direction, and it is an optimal choice to obtain ordered ripple structures when the angle between laser scanning and laser polarization is less than 45°.