Ripple Wavelength Research Articles

Plasmon resonance tuning in Ag nanoparticles arrays grown on ripple patterned templates

A bottom-up approach to grow highly ordered self-assembled silver nanoparticles arrays on ripple patterns produced by low energy ion irradiation is reported. The advantage of this approach over other self-assembling or lithographic methods is the flexibility to tune the array periodicity down to 20nm with inter-particle gaps as low as 5nm along the ripples. The silver nanoparticle arrays grown on these rippled surfaces are optically anisotropic, i.e. they exhibit a direction dependent shift in their localised surface plasmon resonance. The reason for the observed anisotropy is a direction dependent plasmonic field coupling. In this way the capability to tune the plasmon resonance by varying the ripple wavelength is demonstrated.

Laboratory Investigation of Bedform Geometry under Regular and Irregular Surface Gravity Waves

Abstract Bedform (ripple) geometry is an important feature in the nearshore that can significantly enhance wave energy dissipation. Ripples regulate wave transformation and influence the sediment transport phenomenon. These issues are of particular importance when determining beach erosion and investigating other coastal processes. Although numerous studies have addressed the issue of bedform geometry under surface gravity waves, no single model is considered adequate. The relative roughness and subsequent wave energy dissipation are governed by attributes that include ripple height and wavelength. The comparatively large wave tank facility at Davidson Laboratory (Stevens Institute of Technology, Castle Point on the Hudson, Hoboken, New Jersey) provided an ideal opportunity for investigating bedform features for a flat bed, as well as the novel study of a sloped sediment bed. Our results provide new data that strengthen prior relationships and demonstrate that a sloped surface may be modeled in a similar ...

Growth process of GaAs ripples as a function of incident Ar-ion dose

We report periodic ripple formation on GaAs sputtered by 60keV Ar ions at an angle of 60° over a large range of ion doses from 1×1017 to 1×1019ions/cm2 under Atomic Force Microscopy (AFM) study. Initially in the dose range between 1×1017 and 4×1017ions/cm2, only very small roughness is formed on the surface and from the dose of 5×1017ions/cm2, the ripples start to form and attain a well-defined structure at a dose around 9×1017ions/cm2, remain stable and then from a dose of 4×1018ions/cm2, the ripple structures become very rough, periodicity breaks down and step-like features become prominent all over the surface. Parameters like rms roughness, ripple wavelength, amplitude etc. are measured from the AFM image analysis. The results are discussed with the help existing formalism with the understanding of preferential sputtering of one of the components of GaAs.

Genetic algorithm optimization of tunable wavelength selection photonic switch using a microring resonator

A generation algorithm is used to optimize a tunable optical wavelength selection switch (WSS) which is based on a cascaded silicon microring resonator. This method can help us find the optimal inter-ring coupling coefficients which can lead to smaller ripple and better wavelength selectivity over a predefined pass band range. In this paper, we validate this method by a real design and report the results of several, such GA-based WSS optimization. The results show that the GA method performs well even under the limit of physical properties of a silicon ring resonator.

Evolution of ripple morphology on Si(1 0 0) by 60-keV argon ions

In this paper, we report on evolution of ripple morphology on Si(100) surface due to 60keV Ar+-ion implantation to the fluence of 2×1018ionscm−2 and over a large incident angular window of 0–75°. Room temperature implantations were carried out by using a uniform current density of 20μAcm−2. Atomic force microscopic (AFM) studies indicate that ripple morphology starts to appear at an incident angle 45° and becomes more prominent at higher incident angles. AFM studies also reveal that while the ripple wavelength decreases with increasing angle of incidence, the amplitude increases with the same. We also observe a systematic variation in the surface roughness with incident angle. Micro-Raman studies show that the sub-surface silicon layer becomes amorphous whose depth keeps reducing at higher incident angles. The results are attributed to viscous flow mechanism.

Parameterization of bedform morphology and defect density with fingerprint analysis techniques

A novel method for parameterizing the morphology of seafloor ripples with fingerprint analysis numerical techniques is presented. This fully automated analysis tool identifies rippled areas in two-dimensional imagery of the seafloor, and returns ripple orientation and wavelength as well as a new morphological parameter, the spatial density of ripple defects. In contrast to widely used manual and spectral parameterization methods, this new technique yields a unique probability distribution for each derived parameter, which describes its spatial variability across the sampled domain. Here we apply this new analysis technique to synthetic and field collected side-scan sonar seafloor images in order to assess the methods capacity to define bed geometry across a wide range of simulated and observed morphological conditions. The resulting orientation and wavelength values compare favorably with those of the existing manual and spectral parameterization methods, and are superior under environmental conditions characterized by low signal to noise ratios as well as high planform ripple sinuosity. Furthermore, the resulting ripple defect density values demonstrate correlation with ripple orientation, wave direction, and the Shields parameter, which is consistent with recent investigations that have theoretically linked this parameter to hydrodynamic forcing conditions. The presented fingerprint analysis method surpasses the capacity of existing methods for ripple parameterization and promises to yield greater insight into theoretical and applied problems associated with the temporal and spatial variability of ripple morphology across a wide spectrum of marine environments.

Influence of substrate on edge rippling in graphene sheets

The morphology of graphene sheets overlying a substrate plays an important role in controlling the electronic behaviour of graphene-based electronic devices. Atomistic simulations and energetic analysis are performed to study the morphology of graphene edges on a model substrate. We find that the van der Waals interactions are able to significantly reduce the amplitude, penetration depth and wavelength of edge ripples for a single-layer graphene sheet, but suppression of the ripples is not observed in multilayer graphene sheets with unsaturated edges. We explain this difference using an analytical model that accounts for the energetics of rippling and van der Waals interactions.

Geometric bed roughness and the bed state storm cycle

[1] Temporal variation in the geometric roughness of the mobile sandy seabed during wave forcing events is investigated as a function of bed state. The bed states include irregular ripples, cross ripples, linear transition ripples, and flat bed, each appearing repeatedly within a different range of wave energies, as part of the bed state storm cycle. Ripple wavelengths determined from ensemble-averaged roughness spectra indicate that irregular and cross ripples are suborbital, whereas linear transition ripples are anorbital. The observed ripple steepnesses indicate that irregular and linear transition ripples fall slightly below Nielsen's (1981) field data relation, whereas cross ripple steepnesses are anomalously low in comparison. Time series of geometric roughness are coherent across spatial frequency. Abrupt changes in geometric roughness occurred on timescales of ∼3 h on average during both the onset and the waning stages of storm events: i.e., for both decreasing and increasing roughness, respectively. Predicted response times, based on ripple volume and bed load transport rate, are in agreement with the observations when the best fit form of the Meyer-Peter and Muller (1948)-type bed load relation obtained by Ribberink (1998) is used. In contrast, the more standard form of this relation and the associated parameters yield predicted response times much shorter than those observed.

Periodic ripples in suspended graphene

We study the mechanism of wrinkling of suspended graphene, by means of atomistic simulations. We argue that the structural instability under edge compression is the essential physical reason for the formation of periodic ripples in graphene. The ripple wavelength and out-of-plane amplitude are found to obey 1/4-power scaling laws with respect to edge compression. Our results also show that parallel displacement of the clamped boundaries can induce periodic ripples, with oscillation amplitude roughly proportional to the 1/4 power of edge displacement. The results are fundamental to graphene's applications in electronics.

Intrawave observations of sediment entrainment processes above sand ripples under irregular waves

[1] Measurements of intrawave sediment entrainment processes are reported above ripples under irregular waves generated in a large‐scale flume facility. The data consist of substantive observations of small‐scale processes collected at high spatial and temporal resolution under irregular wave forcing, typical of coastal environments. Acoustic measurements were made of water velocities, bed forms, and suspended sediment concentration over plane and rippled beds. During each rippled bed experiment, measurements were taken over the crests of steep sided ripples, above which flow separation was considered likely to occur. Above the ripple crests, a strong intrawave variation in the near‐bed suspended sediment, indicative of vortex formation and shedding, was observed to dominate wave half cycles where the orbital diameter was greater than 1.2 times the ripple wavelength l. The irregular waves were parameterized with a significant wave height Hs and a significant orbital diameter d0s. Thus, during irregular wave conditions and over steep ripples where d0s/l > 1.2, vortex shedding was deemed as the dominant sediment entrainment process. Furthermore, the percentage of wave half cycles where vortex shedding was observed, was found to scale as 26 d0s/l. These results demonstrate, for under irregular waves, the existence of a clear suspended sediment structure consistent with vortex shedding, previously observed under regular waves. Citation: O’Hara Murray, R. B., P. D. Thorne, and D. M. Hodgson (2011), Intrawave observations of sediment entrainment processes above sand ripples under irregular waves, J. Geophys. Res., 116, C01001, doi:10.1029/2010JC006216.

Control of surface ripple amplitude in ion beam sputtered polycrystalline cobalt films

We have grown both polycrystalline and partially textured cobalt films by magnetron sputter deposition in the range of thickness (50–200nm). Kinetic roughening of the growing film leads to a controlled rms surface roughness values (1–6nm) increasing with the as-grown film thickness. Ion erosion of a low energy 1keV Ar+ beam at glancing incidence (80°) on the cobalt film changes the surface morphology to a ripple pattern of nanometric wavelength. The wavelength evolution at relatively low fluency is strongly dependent on the initial surface topography (a wavelength selection mechanism hereby confirmed in polycrystalline rough surfaces and based on the shadowing instability). At sufficiently large fluency, the ripple wavelength steadily increases on a coarsening regime and does not recall the virgin surface morphology. Remarkably, the use of a rough virgin surface makes the ripple amplitude in the final pattern can be controllably increased without affecting the ripple wavelength.

Morphological instability of the solid-liquid interface in crystal growth under supercooled liquid film flow and natural convection airflow

Ringlike ripples on the surface of icicles are an example of morphological instability of the ice-water interface during ice growth under supercooled water film flow. The surface of icicles is typically covered with ripples of about 1 cm in wavelength, and the wavelength appears to be almost independent of external temperature, icicle radius, and volumetric water flow rate. One side of the water layer consists of the water-air surface and growing ice is the other. This is one of the more complicated moving phase boundary problems with two interfaces. A recent theoretical work [K. Ueno, Phys. Rev. E 68, 021603 (2003)] to address the underlying instability that produces ripples is based on the assumption of the absence of airflow around icicles. In this paper, we extend the previous theoretical framework to include a natural convection airflow ahead of the water-air surface and consider whether the effect of natural convection airflow on the wavelength of ripples produced on an ice surface is essential or not.

Simulation of the nanostructuring of surfaces under ion-beam bombardment

As top-down nanolithography is inexorably approaching its ultimate limits, the techniques based on bottom-up self-assembly become increasingly attractive. Among these, the ion-beam-induced self-assembly of thin films occupies a special place, as they combine both the top-down, focused micro-fabrication with the bottom-up self-assembly nano-processes. Because the ion-beam-induced self-assembly processes are not well understood, here we report on the design-orientated modelling and simulation of quasi-organised metallic nanostructures. It has been found that, because of the near-critical state of the ion-beam-induced self-assembly, the interplay between the angle of ion bombardment, ion flux and substrate temperature leads to nanotopographies that are either quasi-structured or random. Furthermore, minor variations in the angle of bombardment leads to a large variation of the parameters of the nano-organisation, i.e., the type of the ripples – lateral or transversal, and ripple wavelengths. The simulation results were benchmarked against comprehensive experimental results reported in the literature. This study demonstrates that simulation can be used in a feed-forward, design-orientated manner when attempting the fabrication of self-assembled nanostructures with micron-level ion-beam techniques.

Introducing artificial length scales to tailor magnetic properties

Magnetism is a collective phenomenon. Hence, a local variation on the nanoscale of material properties, which act on the magnetic properties, affects the overall magnetism in an intriguing way. Of particular importance are the length scales on which a material property changes. These might be related to the exchange length, the domain wall width, a typical roughness correlation length, or a length scale introduced by patterning of the material. Here we report on the influence of two artificially created length scales: (i) ion erosion templates that serve as a source of a predefined surface morphology (ripple structure) and hence allow for the investigation of roughness phenomena. It is demonstrated that the ripple wave length can be easily tuned over a wide range (25–175 nm) by varying the primary ion erosion energy. The effect of this ripple morphology on the induced uniaxial magnetic anisotropy in soft magnetic Permalloy films is studied. Only below a ripple wavelength threshold (≈60 nm) is a significant induced magnetic anisotropy found. Above this threshold the corrugated Permalloy film acts as a flat film. This cross-over is discussed in the frame of dipolar interactions giving rise to the induced anisotropies. (ii) Ion implantation through a lithographically defined mask, which is used for a magnetic property patterning on various length scales. The resulting magnetic properties are neither present in non-implanted nor in homogeneously implanted films. Here new insight is gained by the comparison of different stripe patterning widths ranging from 1 to 10 μm. In addition, the appearance of more complicated magnetic domain structures, i.e. spin-flop domain configurations and head-on domain walls, during hard axis magnetization reversal is demonstrated. In both cases the magnetic properties, the magnetization reversal process as well as the magnetic domain configurations depend sensitively on the artificially introduced length scale.

Ripple morphology under oscillatory flow: 1. Prediction

Recent laboratory and field experiments have shown the inability of existing oscillatory flow ripple predictors to accurately predict both ripple size and ripple planform geometry. However, at this time, only partial adaptations of these predictors have been proposed to account for the observed discrepancies with new experimental data. The present work starts by presenting a comprehensive data set compiled from 27 literature studies and our own experimental data, which are presented in a companion paper. Dimensional analysis is used both to clarify the importance of the different variables involved in the development of ripples and to generate the basic dimensionless framework for the study of ripple morphology under oscillatory flow. Several ripple size and planform geometry predictors are contrasted against the compiled data set. Afterward, a new ripple wavelength and height predictor and a new planform geometry predictor are proposed, which overcome several of the limitations found in the earlier formulations. The new predictors emphasize the existence of local and global sediment transport mechanisms over a ripple bed, each of them acting at different scales. In particular, sediment size is found to have an important effect on the size and geometry of bed forms that can be generated by oscillatory flows.

The molecular dynamics simulation of ion-induced ripple growth

The wavelength-dependence of ion-sputtering induced growth of repetitive nanostructures, such as ripples has been studied by molecular dynamics (MD) simulations in Si. The early stage of the ion erosion driven development of ripples has been simulated on prepatterned Si stripes with a wavy surface. The time evolution of the height function and amplitude of the sinusoidal surface profile has been followed by simulated ion-sputtering. According to Bradley-Harper (BH) theory, we expect correlation between the wavelength of ripples and the stability of them. However, we find that in the small ripple wavelength (lambda) regime BH theory fails to reproduce the results obtained by molecular dynamics. We find that at short wavelengths (lambda<35 nm) the adatom yield drops hence no surface diffusion takes place which is sufficient for ripple growth. The MD simulations predict that the growth of ripples with lambda>35 nm is stabilized in accordance with the available experimental results. According to the simulations, few hundreds of ion impacts in lambda long and few nanometers wide Si ripples are sufficient for reaching saturation in surface growth for for lambda>35 nm ripples. In another words, ripples in the long wavelength limit seems to be stable against ion-sputtering. A qualitative comparison of our simulation results with recent experimental data on nanopatterning under irradiation is attempted.

Utilizing High-Frequency Acoustic Backscatter to Estimate Bottom Sand Ripple Parameters

In some applications of underwater acoustics, it is important to know the ripple structure on shallow-water sediments. For example, the prediction of buried target detection via sound scattering by ripples depends critically on the ripple height and spatial wavelength. Another example is the study of sediment transport, where knowing the ripple structure and its evolution over time helps to understand the forcing on the bottom and the response of sediments. Here, backscatter data from a 300-kHz system are used to show that ripple wavelength and height can be estimated from backscatter images via a simple inversion formula. The inversion results are consistent with in situ measurements of the ripple field using an independent measurement system. Motivated by the backscatter data, we have developed a time-domain numerical model to simulate scattering of high-frequency sound by a ripple field. This model treats small-scale scatterers as Lambertian scatterers distributed randomly on the large-scale ripple field. Numerical simulations are conducted to investigate the conditions under which remote sensing of bottom ripple heights, wavelength, and its power spectrum is possible.

Estimation of the wave-related ripple characteristics and induced bed shear stress

A large data set on ripples was collected and examined. A set of new formulas for the prediction of the ripple characteristics is proposed with an emphasis on the disappearance of the ripples. The ripple wavelength was observed to be proportional to the bottom wave excursion but also to be a function of the grain-related Shields parameter and wave period parameter introduced by Mogridge et al. (1994). The ripple steepness was found to be nearly constant for orbital ripples, and with a sharp decrease for suborbital ripples. Two empirical functions are added including the effects of the critical Shields parameters (inception of transport and inception of sheet flow), i.e. giving the boundaries for the ripple existence's domain. The proposed formulas yield better prediction capabilities compared to the previously published formulas, especially when ripples are washed out. The effect of the ripple characteristics on the roughness height and the calculation of the bed shear stress is also discussed. It appeared that the bed shear stress calculation is more sensitive to the empirical coefficient a r introduced in the estimation of the ripple-induced roughness height or to the limits of existence of the ripples than the ripple characteristics themselves.

Controlled ripple texturing of suspended graphene and ultrathin graphite membranes

Graphene is nature's thinnest elastic material and displays exceptional mechanical and electronic properties. Ripples are an intrinsic feature of graphene sheets and are expected to strongly influence electronic properties by inducing effective magnetic fields and changing local potentials. The ability to control ripple structure in graphene could allow device design based on local strain and selective bandgap engineering. Here, we report the first direct observation and controlled creation of one- and two-dimensional periodic ripples in suspended graphene sheets, using both spontaneously and thermally generated strains. We are able to control ripple orientation, wavelength and amplitude by controlling boundary conditions and making use of graphene's negative thermal expansion coefficient (TEC), which we measure to be much larger than that of graphite. These results elucidate the ripple formation process, which can be understood in terms of classical thin-film elasticity theory. This should lead to an improved understanding of suspended graphene devices, a controlled engineering of thermal stress in large-scale graphene electronics, and a systematic investigation of the effect of ripples on the electronic properties of graphene.

Ion-induced self-organized ripple patterns on graphite and diamond surfaces

In order to investigate the allotropic effect on ripple pattern formation, highly oriented pyrolytic graphite (HOPG) and single crystalline diamond were irradiated with 10–200 keV Xe + at an incident angle of 60° with respective to the surface normal. The irradiation fluence was 2 × 10 17 cm −2 for all irradiations. Ripple patterns were observed on both HOPG and diamond surfaces. However, large differences in ripple wavelengths, amplitudes and surface roughnesses between HOPG and diamond were recognized. The reason for these differences is discussed.