Ripple Orientation Research Articles

Surface slope effects for ripple orientation on sand dunes in López crater, Terra Tyrrhena region of Mars

Abstract Ripple orientations on small sand dunes (dunes lacking substantial slip faces) at widely distributed sites across Mars have been documented using High Resolution Imaging Science Experiment (HiRISE) images, in an effort to determine the last formative aeolian sediment transport direction experienced at these locations. Howard (1977) used field measurements and first principles to derive an expression for determining how much the surface slope on a sand dune deflects the orientation of sand ripples with respect to the formative wind direction. A Digital Terrain Model derived from stereo HiRISE images was used to assess the potential deflection of ripples on sand dunes on the floor of Lopez crater on Mars. Three-quarters of the area covered by sand dunes within the DTM has a surface slope

Sand Ripple Characterization Using an Extended Synthetic Aperture Sonar Model and Parallel Sampling Method

The aim of this work is to characterize the seafloor by estimating invariant sand ripple parameters from synthetic aperture sonar (SAS) imagery. Using a hierarchical Bayesian framework and a known sensing geometry, a method for estimating sand ripple frequency, amplitude, and orientation values from a single SAS image, as well as from sets of SAS imagery over an area, is presented. This is accomplished through the development of an extended model for sand ripple characterization and a Metropolis-within-Gibbs sampler to estimate sand ripple frequency, amplitude, and orientation characteristics for multiaspect high-frequency side-look sonar data. Results are presented on synthetic and measured SAS imagery that indicate the ability of the proposed method to estimate desired sand ripple characteristics.

Recent near-surface wind directions inferred from mapping sand ripples on Martian dunes

The High Resolution Imaging Science Experiment (HiRISE) provides the capability to obtain orbital images of Mars that are of sufficient resolution to record wind ripple patterns on the surfaces of sand dunes. Ripple patterns provide valuable insights into aeolian erosion and deposition on Earth and Mars. In this study, we develop a systematic mapping procedure to examine sand ripple orientations and create surface process maps to evaluate the recent wind flow over the dunes, as well as the interplay of wind and dune shape. By carefully examining the morphology of the dunes and the location of grainflow and grainfall on dune slipfaces, the recent near-surface wind direction (short-term wind) can be identified. Results from the analysis of three dune fields on the floors of craters west of Hellas Basin show regional N, NW, SE, and ESE wind directions. In the three adjacent dune fields, surface process and flow maps suggest a complex wind pattern. The comparison of short-term wind with dune-constructing wind (long-term wind) shows NE and ESE winds may be persistent at least for the past thousands of years. The results also show that the orientation of inferred wind direction on linear dunes is correlated with the crestlines, which suggest that form–flow interaction may take place. The results of local wind flow documentation should improve Martian surface wind modeling and advance our understanding of sand transport, as well as the rates of sand mobility on both Mars and Earth.

A spectral model for estimating temporal and spatial evolution of rippled seabeds

Existing time variable ripple prediction models have focused on ripple wavelength and height yet have not accounted for the effect of wave directionality on ripple evolution. These models use an equilibrium ripple sub-module that provides the target geometry of the seabed. Temporal variability of the hydrodynamic conditions cause ripple geometry to lag behind and not always agree with that predicted by equilibrium models. In this contribution, previous work is extended through the development of a new 2-D, time variable, spectral ripple model that allows for the estimation of ripple wavelength, height, and orientation. Since ripple irregularity is associated with directionality, the new model also has the ability to predict the irregularity of the rippled seabed. A comparison of the 2-D and existing time-dependent models is carried out using both synthetic and field ripple data that consist of time series representing a variety of wave-forcing conditions. Under strong wave forcing with no orientation changes, the 2-D model predicts ripple wavelength and orientations that agree with those of existing models. During periods of change in wave directions, the 2-D model predicts a significantly smaller ripple height during ripple reorientation. Under weaker energy flows, the 2-D model yields improved estimates especially for relict ripple geometry. In addition, the 2-D model’s spectrum yields a prediction of the seabed irregularity.

Study on morphology of high-aspect-ratio grooves fabricated by using femtosecond laser irradiation and wet etching

Morphologies of high-aspect-ratio silicon grooves fabricated by using femtosecond laser irradiation and selective chemical etching of hydrofluoric acid (HF) were studied. Oxygen was deeply doped into silicon under femtosecond laser irradiation in air, and then the oxygen-doped regions were removed by HF etching to form high-aspect-ratio grooves. After HF etching, periodic nano-ripples which were induced in silicon by femtosecond laser were observed on the groove sidewalls. The ripple orientation was perpendicular or parallel to the laser propagation direction (z direction), which depended on the relative direction between the laser polarization direction and the scanning direction. The formation of nano-ripples with orientations perpendicular to z direction could be attributed to the standing wave generated by the interference of the incident light and the reflected light in z direction. The formation of nano-ripples with orientations parallel to z direction could be attributed to the formation of self-organized periodic nanoplanes (bulk nanogratings) induced by femtosecond laser inside silicon. Materials in the tail portion of laser-induced oxygen doping (LIOD) regions were difficult to be etched by HF solution due to low oxygen concentration. The specimen was etched further in KOH solution to remove remaining materials in LIOD regions and all-silicon grooves were fabricated.

Realization of diverse displays for multiple color patterns on metal surfaces

Enhanced colors can be formed when white light is irradiated on the surface ripples induced by femtosecond laser. In this paper, we have demonstrated the ability to display the diverse colors by simultaneously adjusting the incident white light angle and the ripples orientation. Furthermore, our investigation revealed that multi-patterns constituted by ripples with different orientations could be designed on metal surfaces. The diverse display for the desired ones can be realized by exquisitely varying the incident light angle and rotating sample angle. More interestingly, it is found that, although the same patterns could be displayed under different conditions, the colors might be different. These findings can provide a novel method to carry and identify high quantity of information, which may find potential applications in the fields of information storage, identifying codes and anti-counterfeiting patterns.

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°.

Three-dimensional vegetation structure of Tillandsia latifolia on a coppice dune

Coppice dunes covered by epiarenic Tillandsia fog vegetation occur in the Peruvian coastal desert. To analyse the three-dimensional vegetation structure on such a coppice dune, a photogrammetric structure-from-motion approach is applied. A photogrammetric plant height map was derived from a digital surface model and a digital terrain model created by this approach. This plant height map is analysed in conjunction with stand-scale vegetation patterns, regional wind direction derived from migrating dunes, near-surface wind direction derived from the orientation of sand ripples and ground observations. Tillandsia vegetation patterns are non-random. Banded and reticulate patterns occur at stand scale. At metre to sub-metre scale, groups of tall plants occur in clusters as well as in rows aligned with air movement. While self-organisation processes related to maximising fog moisture capture by the plants and to coppice dune growth appear to control the development of the banded to reticulate pattern, retaining an upright growing position (and hence continued exposure to fog moisture on decadal time scales) by minimising aeolian erosion at the base of these rootless plants appears to control the development of rows and clusters at metre to sub-metre scale.

Temporal and spatial evolution of wave‐induced ripple geometry: Regular versus irregular ripples

AbstractConcurrent observations of inner shelf near bed hydrodynamics and acoustic imagery of the seabed are used to relate wave‐induced ripple geometry (wavelength and orientation) to near bed directional wave velocities. The observations were collected on the continental shelf of the South Atlantic Bight at water depths of 9.5 and 30 m off the coasts of South Carolina (median size 177 µm) and Georgia (388 µm), respectively. 2‐D spectral analysis techniques are performed on the imagery to automate detection of ripple wavelength, orientation, and irregularity. Our analysis shows that ripple irregularity is a time‐dependent process dependent on magnitude, direction, and duration of wave forcing. During energetic events, ripple geometry changes rapidly and the ripples align with the main wave direction. During periods of low energy, close to the critical conditions for initiation of sediment motion, ripple evolution occurs at a much slower rate often leading to irregularities such as terminations and bifurcations along the ripple crest. Under constantly changing wave direction, the rippled bed becomes highly disorganized. Six types of ripples are defined based on newly developed irregularity parameters: linear, bifurcating‐linear, linear‐bifurcating, bifurcating and cross, irregular, and disorganized beds. Ripple irregularity depends on the time history of the bed; ripples remain irregular until their wavelength and orientation attain a nearly equilibrium geometry. The observations collected provide significant information on the response of the seabed to wave forcing and identify processes that should be reproduced by any time‐dependent ripple prediction model. Ripple irregularity can only be predicted using such time‐dependent models.

Laser ablation of polished and nanostructured titanium surfaces by nanosecond laser pulses

A comparison of the IR nanosecond laser ablation parameters for polished and nanostructured titanium samples has been performed. The titanium foil was mechanically polished and pres-structured by multiple 744-nm femtosecond laser pulses producing large surface spots covered by ripples with periods in range of 400–500nm. In order to evaluate the influence of such nanoripples, the nanosecond laser ablation and laser plasma properties were compared for polished surface, surface with nanoripples parallel and orthogonal to the laser beam polarization. A substantial decrease of the nanosecond ablation threshold was observed for the nanostructured in contrast to polished surface was detected while no influence of the ripple orientation vs. beam polarization was revealed. The comparison of plasma spectra for the ablation cases demonstrated that intensity of basic atomic lines and plasma emission duration were 2–5 times larger for the polished sample while spectra evolution was faster for the nanostructured sample. Plasma temperature and electron density were slightly lower for nanostructured sample while laser beam polarization has no effect on plasma properties.

Ion beam induced surface patterns due to mass redistribution and curvature-dependent sputtering

Recently it was reported that ion-induced mass redistribution would solely determine nano pattern formation on ion-irradiated surfaces. We investigate the pattern formation on amorphous carbon thin films irradiated with Xe ions of energies between 200 eV and 10 keV. Sputter yield as well as number of displacements within the collision cascade vary strongly as function of ion energy and allow us to investigate the contributions of curvature-dependent erosion according to the Bradley-Harper model as well as mass redistribution according to the Carter-Vishnyakov model. We find parallel ripple orientations for an ion incidence angle of 60\ifmmode^\circ\else\textdegree\fi{} and for all energies. A transition to perpendicular pattern orientation or a rather flat surface occurs around 80\ifmmode^\circ\else\textdegree\fi{} for energies between 1 keV and 10 keV. Our results are compared with calculations based on both models. For the calculations we extract the shape and size of Sigmund's energy ellipsoid (parameters $a$, \ensuremath{\sigma}, \ensuremath{\mu}), the angle-dependent sputter yield, and the mean mass redistribution distance from the Monte Carlo simulations with program SDTrimSP. The calculated curvature coefficients ${S}_{x}$ and ${S}_{y}$ describing the height evolution of the surface show that mass redistribution is dominant for parallel pattern formation in the whole energy regime. Furthermore, the angle where the parallel pattern orientation starts to disappear is related to curvature-dependent sputtering. In addition, we investigate the case of Pt erosion with 200 eV Ne ions, where mass redistribution vanishes. In this case, we observe perpendicular ripple orientation in accordance with curvature-dependent sputtering and the predictions of the Bradley-Harper model.

Control of periodic ripples growth on metals by femtosecond laser ellipticity

Formation of the periodic ripples on metallic surfaces is investigated comprehensively using variable ellipticities of femtosecond lasers. Compared with the linearly polarized incidence, the well defined grating-like ripple structures rather than the uniform arrays of nanoparticle can always be obtained for the elliptical polarization lasers. The ripple orientation is slanted clockwise or anticlockwise depending on the laser helicity but always display a maximum angle of 45°. Theoretical analyses indicate that no circular polarization is achieved for femtosecond lasers passing through quarter waveplate, and the induced ripple orientation is determined by the major axis of the polarization ellipse. The simulation results agree well with the experimental observations.

Ordered rippling of polymer surfaces by nanolithography: influence of scan pattern and boundary effects

We demonstrate how AFM nanolithography, with a proper choice of scan pattern, can induce an exceptionally ordered alignment of ripples on the surface of polymer films on the first scan. By analogy with the manipulation of nanoparticles, the orientation of the ripples is determined by the material flow, which is ultimately fixed by the direction of motion of the probing tip. This makes a raster scan pattern the best choice for orienting the ripples, as opposed to the zigzag scan pattern commonly adopted by most AFM setups. Our hypothesis is substantiated by a series of measurements on a solvent-enriched ultrathin film of PET, which allowed ripple formation on the first scan. We also show how the ripple orientation is significantly modified by the boundary conditions appearing when nanolithography is performed on circular, triangular and L-shaped areas on the polymer surface.

Ripple formation in various metals and super-hard tetrahedral amorphous carbon films in consequence of femtosecond laser irradiation

Ripple formation in consequence of ultrashort laser pulse irradiation of materials is a well-known phenomenon. We have investigated the formation of ripples in various metals, i.e. steel, tungsten carbide hard metal, as well as in superhard ta-C films, where we used femtosecond laser pulses of 775 nm and 387 nm mean wavelength and 150 fs pulse duration. The aim was to investigate how the ripple parameters depend on irradiation parameters, and if such ripples have a potentiality for applications. In the paper, we will show that on smooth surfaces the ripple orientation is perpendicular to the electric field vector of the linearly polarized laser beam, as is well-known. Moreover, it will be shown that the ripple period decreases with decreasing laser wavelength and/or increasing angle of incidence of the laser beam on the substrate. By using optimum parameters large areas of the materials and films can be rippled swiftly, which would be important for applications. For instance, the improvement of frictional and wear behavior of tribologically stressed surfaces by ripples was investigated on ta-C coated steel surfaces.

Selective appearance of several laser-induced periodic surface structure patterns on a metal surface using structural colors produced by femtosecond laser pulses

Ripples with a subwavelength period were induced on the surface of a stainless steel (301 L) foil by femtosecond laser pulses. By optimizing the irradiation fluence of the laser pulses and the scanning speed of the laser beam, ripples with large amplitude (∼150nm) and uniform period could be obtained, rendering vivid structural colors when illuminating the surface with white light. It indicates that these ripples act as a surface grating that diffracts light efficiently. The strong dependence of the ripple orientation on the polarization of laser light offers us the opportunity of decorating different regions of the surface with different types of ripples. As a result, different patterns can be selectively displayed with structural color when white light is irradiated on the surface from different directions. More interestingly, we demonstrated the possibility of decorating the same region with two or more types of ripples with different orientations. In this way, different patterns with spatial overlapping can be selectively displayed with structural color. This technique may find applications in the fields of anti-counterfeiting, color display, decoration, encryption and optical data storage.

Parametric decay of laser into two electromagnetic waves in a rippled density plasma

The presence of a density ripple in an unmagnetized plasma is shown to allow parametric decay of an electromagnetic wave into two electromagnetic waves, which is otherwise not allowed due to wave number mismatch between the decay waves. The static density ripple accounts for the mismatch. The decay occurs at plasma densities below the quarter critical density and the decay electromagnetic waves propagate at angles to the pump laser. The requisite ripple wave number q increases with the increase in pump wave frequency. However, as the ripple orientation with respect to the pump θR increases, q decreases. The growth rate for the parametric instability initially decreases with the frequency of the lower frequency electromagnetic wave, attains a minimum and then increases. The growth rate is higher for lower values of θR.

Prediction of time-evolving sand ripples in shelf seas

The ability to predict the presence and physical properties of small-scale sand ripples on the sea bed is important for determining the bed roughness felt by currents and waves, for sediment transport applications, and for acoustic applications. A fully time-evolving model is presented for predicting the height, length and orientation of sand ripples generated by currents, waves or both, which includes processes dealing with threshold of motion, ripple wash-out and biological degradation. We believe that this is currently the only predictor to feature all these properties, and is thus more widely applicable to shelf seas than other more sophisticated, but less comprehensive, methods. Tests against a large data-set of observations using an Acoustic Ripple Profiler at a site on the east coast of England, where ripples are generated by both tidal currents and waves, showed that the predictor captures most of the main features observed. A modified criterion for the wave/current dominance in the prediction equations improved some aspects of the agreement. The properties of current-generated ripples are predicted accurately, but the wavelength, and to a lesser extent the height, of wave-generated ripples show some under-prediction. Ripple orientations are modelled in an approximate way, but the main features of the observed orientations are captured. The bio-degradation feature of the predictor could not be tested with this data-set.

The linear and nonlinear optical response of native‐oxide covered rippled Si templates with nanoscale periodicity

AbstractFabrication of dense arrays of nanowires by growth on nanostructured substrates appears to be a promising approach for producing functional nanoscale devices. Ion beam irradiation under carefully controlled conditions produces self‐organized ripple patterns on silicon substrates, on which Ag nanowire arrays with nanoscale periodicity have been grown successfully. Here, the linear and nonlinear optical response from native‐oxide‐covered Si(001) templates, with ripple periodicity between ∼20 and ∼50 nm, is reported. Reflection anisotropy spectroscopy (RAS) shows a small, broad peak at ∼2.5 eV of ∼1 × 10−3 amplitude. The RAS response decreases as the periodicity of the ripple structure increases. The nonlinear second‐harmonic (SH) response from the samples was also measured, using unamplified femtosecond excitation at 800 nm, by rotating the linear polarized input and detecting either the s‐ or p‐polarized SH output. A decrease in response with increased ripple structure periodicity was observed for the p‐in/p‐out configuration, when the plane of incidence was orthogonal to the average ripple orientation. Possible origins of the response and future experiments are discussed.

Onset of thermal ripples at the interface of an evaporating liquid under a flow of inert gas

The dynamics of thermal ripples at the interface of a volatile pure liquid (C2H5OH) is studied experimentally and numerically. Liquid evaporates under a flow of inert gas (N2) circulating along the interface. The evaporation rate is varied by regulating both the gas flow rate and the gas pressure. Experiments in microgravity environment allowed to identify a transition to “interfacial turbulence,” along which some particular events such as nearly periodic and possible intermittent behaviors. Direct numerical simulations have been performed, and compare qualitatively well with experimental results, offering new insights into the physical mechanisms involved. Small-scale ripples appear to arise from a secondary instability of large-scale convection cells and their motion seems to follow the corresponding large-scale surface flow. The relative role of surface tension and buoyancy in triggering these flows is assessed by comparing experiments and simulations in both microgravity and ground conditions. Qualitative features compare satisfactorily well such as typical speed and orientation of the thermal ripples, as well as spiral flow in the bulk.

AUV-enabled adaptive underwater surveying for optimal data collection

A new adaptive strategy for performing data collection with a sonar-equipped autonomous underwater vehicle (AUV) is proposed. The approach is general in the sense that it is applicable to a wide range of underwater tasks that rely on subsequent processing of side-looking sonar imagery. By intelligently allocating resources and immediately reacting to the data collected in-mission, the proposed approach simultaneously maximizes the information content in the data and decreases overall survey time. These improvements are achieved by adapting the AUV route to prevent portions of the mission area from being either characterized by poor image quality or obscured by shadows caused by sand ripples. The peak correlation of consecutive sonar returns is used as a measure for image quality. To detect the presence of and estimate the orientation of sand ripples, a new innovative algorithm is developed. The components of the overall data-driven path-planning algorithm are purposely constructed to permit fast real-time execution with only minimal AUV onboard processing capabilities. Experimental results based on real sonar data collected at sea are used to demonstrate the promise of the proposed approach.