Ripple Formation Research Articles

Influence of Si Content on Fatigue Life for Heat Treated Cast Al-Si-mg Alloy Using Different Quenching Technique

Al-Si-Mg aluminum alloy samples were subjected to solution heat treatment at the temperature of 520 °C for 2 h to attain T42 condition. The heat-treated samples were quenched further in air and water mediums. After quenching, the fatigue life was analyzed with the total strain amplitude (TSA) of 0.4% at the cyclic frequency of 0.3 Hz using a low cycle fatigue testing machine. Air quenched sample has higher life with 1286 cycles than heat-treated and water quenched sample, which were having a fatigue life of 836 cycles and 708 cycles. Tensile mean stress was observed in all samples due to residual stress. Due to fatigue striation, ripple formation was observed in the air quenched sample, and the ripple having the size of 2-3 μm observed under SEM fractography. XRD peaks of Mg2Si and Al3Fe precipitates confirm the proper dispersion during the solution heat treatment.

Impact of Laser Fluence in Modifying the Surface Characteristics of Laser-Treated Monocrystalline Zinc

Zinc single crystal specimens were irradiated with pulsed Nd:YAG laser in vacuum. Keeping the number of laser shots constant (10 shots), laser energy was varied from 50 to 150 mJ, whereas the corresponding laser fluence was 97 to 292 J cm−2, respectively. Using optical microscope, the dimensions (area and perimeter) of laser ablated region on the surface of each specimen were measured. Both parameters increased, in general, on increasing laser pulse energy. Examination of surface morphology by scanning electron microscope revealed formation of pores, grooves, cracks, bubbles, micron-size rods, ripples, ridges, cavities, microcones, and solid flakes, etc. Surface roughness had no systematic dependence on the laser pulse energy. Structural parameters, i.e., texture coefficient, lattice strain, and crystallite size, were determined by means of Harris analysis as well as Williamson–Hall analysis of the XRD patterns of specimens. The variation of lattice strain and crystallite size with laser fluence or laser pulse energy was alike. Surface hardness decreased on increasing laser fluence, and followed classical Hall–Petch relation.

Super-hard coatings deposited under conditions of the dynamic shock wave and measurement features of control parameters during production

Abstract Super-hard coatings are of increasing scientific interest as they allow synthesizing materials with unique physical and chemical properties for further application in the industry at high-speed processing or tooling. This study is aimed to investigate tetrahedral amorphous carbon (ta-C) films, as well as the dependence of the pulsation parameters on the irradiation parameters and the potential of such pulsations to be further applied in practice. The article shows that ta-C films are completely amorphous and have up to 85% of sp3 bonds. Deposited films are characterized by high compression stresses ranging within 8-10 GPa. The possibilities of reducing these stresses by thermal and pulsed laser annealing have been examined. The formation of ripples in super-hard ta-C films were studied by applying femtosecond laser pulses with a wavelength of 775 nm, an average wavelength of 387 nm, and the pulse duration of 150 fs. Obtained results demonstrated that the orientation of the pulsations on smooth surfaces is perpendicular to the vector of the electric field of the linearly polarized laser beam. Besides, the period of pulsation reduces with decreasing the laser wavelength and/or increasing the angle of laser beam incidence on the substrate.

Splash function for the collision of sand-sized particles onto an inclined granular bed, based on discrete-element-simulations

This work is motivated by the lack of a model for the splash function associated with the collision of wind-blown sand grains onto an inclined granular surface. This splash function, which describes the velocity of rebound particles and the number and velocity of ejected particles, is an essential ingredient of theoretical models of sand transport and the formation of dunes and ripples. While previous models regarded the granular bed as a horizontal surface, there is experimental evidence that bed slopes can affect the splash process. However, since it is difficult to investigate particle trajectories near the bed from experiments, numerical simulations that resolve individual particle trajectories provide useful means to elucidate the effect of bed slope on the splash function. Here we employ numerical simulations by means of the Discrete-Element-Method (DEM) to investigate the splash function of sand-sized particles impacting onto inclined beds. To this end, we consider bed slopes consistent with those of the windward side of ripples and dunes. Specifically, the angle θ between the granular bed and the horizontal is varied from 0° to 15°, while a broad range of particle impact velocities (from 1 m/s to 7 m/s) is considered. Moreover, we consider values of incident particle diameter din within the range 120 ≤ din/μm ≤ 520, i.e., consistent with natural sand. We obtain expressions for all parameters of the splash function as a function of the incident particle diameter and velocity, and the bed slope. In particular, our simulations show that the effect of the bed slope on the splash function depends on the size of the impacting particle, which has implication for grain trajectories formative of ripples and dunes.

Nanostructures on Sapphire Surfaces Induced by Metal Impurity Assisted Ion Beam

The metal impurity assisted ion beam technology has shown its uniqueness and effectiveness in the formation and precise control of nanostructures on the surface of materials. Hence, the investigation in this area is vital. The morphology evolution of self-organized nanostructures induced by Fe co-deposition assisted Ar+ ion beam sputtering at a different distance from the impurity target was investigated on sapphire, using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). We also investigated the role of metal impurities on sapphire ripple formation. Experiments were carried out at an oblique angle of incidence 65° with constant ion beam current density 487 μA/cm2 and the erosion duration of 60 min at room temperature (20 °C). The introduction of Fe impurity increased the longitudinal height and roughness of the surface nanostructures. Moreover, the amounts of Fe deposited on the surface decreased with increasing distance, and the morphology of the smooth sapphire surface demonstrated a strong distance dependence. Differences in surface morphology were attributed to changes in metal impurity concentration. With an increase of impurity target distance, island-like structures gradually evolved into continuous ripples. At the same time, the orderliness of nanostructures was enhanced, the longitudinal height gradually decreased, while the spatial frequency was unchanged. In addition, there were very few metal impurities on the etched sample. During the ion beam sputtering process, island-like structures promoted the growth of ripples but destroyed their orderliness.

Formation and migration of ripple pattern due to pure wave

ABSTRACT This paper reports an experimental study carried out in a laboratory flume to study the ripple formation and migration for three different sediment sizes ranging from fine sand to coarse sand under the influence of surface waves of different frequencies. This study compares the ripple length, height, ripple patch formation and migration characteristics for different wave frequencies and sediment sizes. Three-dimensional velocity was measured by 3D micro-ADV (Acoustic Doppler Velocimeter) and the evolution of bed morphology was quantified by Ultrasonic Ranging System (URS), SeaTek Inc. It is found from experimental results that patches (bunch of ripples) do not appear all over the bed at a time, rather these patches appear at isolated locations and the separation between the patches scale with the wave length of surface waves. Further, the ripple migration rate is evaluated and its relationship with sediment grain size and frequency is presented.

Polarization-switchable nanoripples fabricated on a silicon surface by femtosecond-laser-assisted nanopatterning.

The polarization ripples on many materials have been intensively studied and have yielded significant applications such as modulating light fields, building hydrophobic or hydrophilic surfaces, and fabricating tunable thermal sources. It was shown that ripples are closely dependent on the ablation threshold of laser fluence and orientation of laser polarization. Here we demonstrate that laser polarization ellipticity also represents the switching effect on the formation of ripples. Two significantly different damage morphologies, ripples and hollows, are respectively observed below and above the switching value of incident laser polarization ellipticity. Furthermore, it is demonstrated that this ellipticity switching value varies with laser pulse energy and pulse number. These intriguing phenomena are qualitatively explained using a laser-surface plasmon polariton interference mechanism. Finally, we achieve the analogous laser-assisted nanopatterning by using a femtosecond laser beam with spatially inhomogeneous polarization state, demonstrating the application potential of these switchable nanoripples in laser-assisted nanopatterning.

Transverse Piezo- and Pyroelectric Effects in 2D Nanoallotropes of Boron Nitride Caused by the Ripple Formation

The ripple formation in the structure of 2D allotropes of boron nitride is shown to lead to the appearance in them of the transverse piezo- and pyroelectric effects with coefficients depending on temperature. These coefficients are calculated for various 2D allotropes of boron nitride. It is shown that due to a small ripple height the transverse piezoeffect in these nanoallotropes is three order of magnitude weaker than the longitudinal effect, whereas the pyroelectric effect is quite comparable with analogous effect in fluorographane and can find a practical application.

Numerical investigation of ripple in oscillating water tank by DEM-MPS coupled solid–liquid two-phase flow model

The height of ripples, representing roughness in the shallow-water area, is an important factor in the prediction of coastal processes. However, the characterization of ripples under violent flow conditions has proven challenging. For instance, in shallow water in the swash zone, significant water fluctuation occurs owing to interactions between the up-rush and down-rush of waves. Because such phenomena shows strong nonlinearity, sufficient knowledge of swash-ripple formation remains lacking. In this study, experiments generating ripples in an oscillating water tank are conducted. Furthermore, numerical simulation using the three-dimensional DEM-MPS coupled method is performed. By comparing numerical and experimental results, validation of the present numerical simulation is demonstrated, followed by an investigation of the mechanics of ripple formation. Numerical results show that not only the drag force but pressure gradient force resulting from the water surface gradient considerably affect sand-particle motion, revealing the need for accurate evaluation of the free surface flow. Moreover, pressure gradient force plays an important role in the ripple formation process, so it should be reflected in the numerical model for ripple simulations.

Theory of nanoscale ripple topographies produced by ion bombardment near the threshold for pattern formation.

Nanoscale pattern formation on the surface of a solid that is bombarded with a broad ion beam is studied for angles of ion incidence, θ, just above the threshold angle for ripple formation, θ_{c}. We carry out a systematic expansion in powers of the small parameter ε≡(θ-θ_{c})^{1/2} and retain all terms up to a given order in ε. In the case of two diametrically opposed, obliquely incident beams, the equation of motion close to threshold and at sufficiently long times is rigorously shown to be a particular version of the anisotropic Kuramoto-Sivashinsky equation. We also determine the long-time, near-threshold scaling behavior of the rippled surface's wavelength, amplitude, and transverse correlation length for this case. When the surface is bombarded with a single obliquely incident beam, linear dispersion plays a crucial role close to threshold and dramatically alters the behavior: highly ordered ripples can emerge at sufficiently long times and solitons can propagate over the solid surface. A generalized crater function formalism that rests on a firm mathematical footing is developed and is used in our derivations of the equations of motion for the single and dual beam cases.

Wrinkling poly(trimethylene 2,5-furanoate) free-standing films: Nanostructure formation and physical properties

Polymer nanostructures were developed on fully bio-based poly(trimethylene furanoate) (PTF) films, by using the technique of Laser Induced Periodic Surface Structures (LIPSS). We found that irradiation times between 1 and 8 min allowed the formation of periodic and nanometric ripples, when using an UV pulsed laser source at a fluence of 8 mJ/cm2. The wrinkled surfaces were studied by a combined macro- and nanoscale approach. We evaluated possible physicochemical changes taking place on the polymer surface after irradiation by infrared spectroscopy, contact angle measurements and atomic force microscopy. The macroscopic properties of PTF showed almost no changes after nanostructure formation, differently from the results previously found for the terephthalic counterparts, as poly(ethylene terephthalate), PET, and poly(trimethylene terephthalate), PTT. At the nanoscale, the surface mechanical properties of the nanostructured PTF were found to be improved, as evidenced by force spectroscopy measurements. In particular, stiffer surfaces characterized by an increased Young's modulus were measured for the nanostructured sample.

The Geometry of Sand Ripples in a Uniformly Rotating and Librating Horizontal Cylinder

Regular ripples at the interface between the granular medium and the fluid in a rapidly rotating cylinder are experimentally studied. We consider two types of ripple formation. First, the ripple formation in a uniformly rotating horizontal cylinder filled with granular medium, fluid and air. In this case, oscillations of the multiphase system in the rotating reference frame are excited by gravity. Second, the ripple formation under the fluid oscillations in a cylinder filled with fluid and granular medium and rotating with a modulated rotation rate (librating cylinder). For this type of ripple formation, the effect of gravity is negligible compared to that of centrifugal force of inertia. It is found that the ripple azimuthal length is determined by the angular amplitude of the fluid oscillations in the vicinity of the sand bed and is not affected by the frequency of the fluid oscillations. The experimental data on the steepness of both types of the ripples favorably agree with each other and predictions of (Nielsen in J. Geophys. Res. Oceans 86(C7): 6467-6472, 1981). Differences in experimental research between normal gravity and microgravity are highlighted.

Heat transfer and pressure drop of film condensation in a horizontal minitube for HFO1234yf refrigerant

This paper applies the computational fluid dynamics (CFD) simulation to study the condensate two-phase thermofluid characteristics of refrigerant HFO-1234yf flowing through horizontal straight and convergent passages for guiding the design of minitube heat exchangers. The theoretical analysis employs the volume-of-fluid (VOF) method to model the progression of film condensation process for determining the distributions of velocity, pressure, temperature, vapor volume fraction and film thickness for liquid-vapor interfacial flows. The Lee model is formulated as a user defined function to treat the phase change effect at the interface. The predictions of heat transfer coefficients and pressure drops agree reasonably well with the measured data from the referencing literature at the mass and heat fluxes of 200–800 kg/m2 s and 9.9–24.3 kW/m2 for validation of the computational model. The simulated results are presented to capture the complex two-phase flow behaviors during the film condensation progression, including the formation of interfacial ripples of annular films, wave-shaped films with elongated necking regions, shedding of liquid ligaments and breakup of these ligaments entrained into the vapor core. The VOF calculations also estimate the average heat transfer coefficient and pressure drop up to 11.3 kW/m2 K and 55.1 kPa at a mass flux of 800 kg/m2 s for the straight tube, showing similar thermal performance to refrigerant HFC-134a reported by open literature. The convergent passage essentially raises the average heat transfer coefficient and pressure drop because of more severe shear stresses over the film surface. The correlations of heat transfer coefficient and pressure drop are compared with CFD predictions for straight and convergent tubes. The performance assessment is thus conducted using the validated correlations to guide the redesign of minitube heat exchangers with HFO1234yf as an alternative refrigerant in various energy systems. Two design changes are presented using HFO1234yf refrigerant to enlarge the total surface area up to 13.49% in straight tubes or broaden the cooling surface area of only 6.4% in convergent tubes for matching the performance of R134a.

Ar+-sputtered Ge (001) surface nanostructuring at target temperature above the recrystallization threshold

Nanoscale pattern formation on Ge (001) surface by 500 eV Ar+ bombardment has been studied for a wide range of ion incidence angles at a temperature of 300° C. In the angular regime 0° to 65°, a fourfold symmetric topography forms which shows a remarkable transition into highly regular one-dimensional asymmetric pattern at grazing incidences, known as perpendicular mode ripples. The four-fold symmetric patterns are found to retain their symmetry under the concurrent substrate rotation during sputtering, while the ripples show degeneration into hole structure with a weak fourfold symmetric pattern. The dynamics of the observed patterns has also been investigated in a wide range of ion fluence from 1 × 1017 to 1 × 10 20 ions cm−2. The square topographies are found to be determined by Ehrlich–Schwoebel barrier induced diffusion bias driven growth process, whereas, the ripple formation implies the role of incidence ion beam direction.

An experimental study on the spatiotemporal evolution of sand waves/ripples in turbulent boundary layer airflow

An experimental study was conducted to investigate the spatiotemporal evolution of sand waves/ripples submerged in a turbulent boundary layer airflow. While a digital image projection technique was applied to achieve temporally resolved measurements of the dynamically evolving sand surface morphology, a combined particle tracking/imaging velocimetry technique was also used to reveal the two-phase (i.e., air–sediment) flow field during the dynamic sand wave/ripple evolution process. It was found that the sand bed surface would evolve from initial random three-dimensional (3D) sand wavelets to two-dimensional (2D) sand waves and further into well-organized sequences of 3D chevron-shaped sand ripples that are separated by longitudinal streaks, when exposed to the turbulent boundary layer airflow. A discrepancy of the local sand wave propagation at different transverse locations was revealed based on the wavelet analysis of the time-series of the sand bed height variation, which was suggested to contribute to the formation of the 3D chevron-shaped sand ripples. It was also found that the evolving sand waves/ripples could dramatically affect the near-bed two-phase (i.e., air–sediment) flow structures as indicated by the dramatically disturbed air–sediment flow structures. By correlating the sand surface profiles and the near-surface sand particle velocity patterns, a complete description of the dynamic sand bedform evolution was revealed with five dominant phases being defined: (I) initial strengthening phase, (II) transition phase, (III) ripple-modulated re-strengthening phase, (IV) stabilizing phase, and (V) longitudinal phase.

Ultrafast dynamics of subwavelength periodic ripples induced by single femtosecond pulse: from noble to common metals

The electron energy band structures and optical properties of silver and nickel are notably different, and they are identified as representative of noble and common metals, respectively. The ultrafast dynamics of the formation of periodic ripples on the surface of silver and nickel films induced by a single 800 nm, 50 fs laser pulse was investigated using the collinear pump-probe imaging method. The dependence of the transient ripple periods on the laser fluences was also studied.A surface plasmon polariton (SPP) model was developed to explain the formation of periodic ripples on noble and common metals induced by a femtosecond laser pulse. The evolution of the electron and the lattice temperature was investigated by numerically solving the equations of the two-temperature model. In addition, the effects of electron collision, localization of hot electrons, and inter-band transition on the dielectric constant in the excited states are discussed in detail as functions of laser fluence. The ripple periods obtained using the developed SPP model agree well with the experimental results, which supports the view that the SPP excitation plays an important role in the formation of periodic ripples. This model is expected to be a general approach for understanding the formation of subwavelength periodic ripples on metals induced by a femtosecond laser pulse.

Controllable formation of periodic wrinkles in Marangoni-driven self-assembled graphene film for sensitive strain detection

Controllable formation of microstructures in the assembled graphene film could tune the physical properties and broaden its applications in flexible electronics. Many efforts have been made to control the formation of wrinkles and ripples in graphene films. However, the formation of orderly wrinkles in graphene film remains a challenge. Here, we reported a simple strategy for the fabrication of graphene film with periodic and parallel wrinkles with a pre-stretched polydimethylsiloxane substrate. The width of the wrinkles in graphene can be controlled by changing the pre-stretched strain of the substrate. The average width of wrinkles in graphene film on the substrate with pre-stretched strain of 10%, 20%, and 50% was about 3.68, 2.99 and 2.01 µm, respectively. The morphological evolution of wrinkled double-layered graphene under mechanical deformation was observed and studied. Furthermore, a strain sensor was constructed based on the wrinkled graphene, showing high sensitivity, large working range and excellent cyclic stability. These strain sensors show great potential in real-time motion detection, health surveillance and electronic skins.

Mechanical properties of graphene oxide: The impact of functional groups

In the current study, mechanical characteristics of graphene oxide (GO) as a promising substitute of graphene are systematically studied through molecular dynamics simulation. For this purpose, several GO samples having different concentrations of epoxide and hydroxyl functional groups are considered. The results reveal that increasing the epoxide coverage causes a noticeable deterioration in the mechanical characteristics of GO systems. This change is correlated with the increase of the formation of ripples in the structure upon increasing the epoxide coverage. Moreover, investigating the bond lengths in the system, it is concluded that the higher epoxide percentage leads to an increase in the length of single and hybrid resonance bonds leading to an overall deterioration of the mechanical properties of GO samples. Additionally, our results demonstrate that high concentration of functional groups can lead to a negative Poisson’s ratio. Increasing the amount of hydroxyl groups shows the same declining effect on the Young’s modulus. In a graphene system containing both epoxide and hydroxyl groups, it is deduced that a higher percentage of the former can result in a higher residual strain because of the formation of more ripples within the system.

Surface ripples induced by gas cluster ion beam on copper surface at elevated temperatures

The temperature dependence of the ripples formation on a copper surface under off-normal gas cluster ion beam irradiation was investigated. At room temperature, well developed asymmetric ripples perpendicular to the beam were obtained with “droplets” of copper at the crests. At elevated temperatures, surface smoothing was found instead of rippling. On some grains the ripples perpendicular to the ion beam still remained. To explain this fact, electron backscattered diffraction analysis of the samples surfaces was performed. It showed that the ripple formation at elevated temperatures is governed both by the ion beam direction and by the crystalline orientation of a grain with respect to the beam.

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.