Study of ripple formation on Si surface under Ga ion beam bombardment

The process of microrelief formation on Si (100) surface under 30 keV Ga+ ion beam bombardment with doses 2⋅1017 - 4⋅1018 ion/cm2 at incident angles θ = 0 - 50° was studied. It was found that wave-like structures form on the surface at θ = 25° - 35° and doses 6⋅1017 - 2⋅1018 ion/cm2. The nice ripple formed at θ = 30±2° incident angles and irradiation dose 1018 ion/cm2.

Experiments on the dynamic wetting of growing icicles

The distinctive shape of an icicle is the outcome of a highly non-equilibrium process involving heat and mass transport in the presence of fluid flowing over an evolving topography. It has previously been shown that the ripple patterns with a near universal wavelength that are observed on many icicles are correlated with small levels of impurities in the feed water. Models of icicle shape evolution, and of the origin of the ripple pattern, require a detailed understanding of how liquid water flows over a growing icicle. The impurity effect is not accounted for by any existing model of ripple formation. Here, we explore this flow dynamics using laboratory-grown icicles with a fluorescent dye as an impurity. Contrary to previous models, we find that the ice is incompletely wetted by the liquid phase, and that the whole process is much more stochastic than has been previously assumed. In addition, the presence of impurities modifies the wetting properties of the ice surface, while the emerging topography interacts with the liquid distribution. There is evidence for mixed-phase ice. These observations must inform any successful model of an impurity-driven rippling instability. Our results have general implications for the morphological evolution of many natural, gravity-driven, wet ice growth processes.

Sediment shell-content diminishes current-driven sand ripple development and migration

Shells and shell fragments are biogenic structures that are widespread throughout natural sandy shelf seas and whose presence can affect the bed roughness and erodibility of the seabed. An important and direct consequence is the effect on the formation and movement of small bedforms such as sand ripples. We experimentally measured ripple formation and the migration of a mixture of natural sand with increasing volumes of shell material in a racetrack flume. Our experiments reveal the impacts of shells on ripple development in sandy sediment, providing information that was previously lacking. Shells expedite the onset of sediment transport while simultaneously reducing ripple dimensions and slowing down their migration rates. Moreover, increasing shell content enhances near-bed flow velocity due to the reduction of bed friction that is partly caused by a decrease in average ripple size and occurrence. This, in essence, limits the rate and magnitude of bed load transport. Given the large influence of shell content on sediment dynamics as well as the high shell concentrations found naturally in the sediments of shallow seas, a significant control from shells on the morphodynamics of sandy marine habitats is expected.

An Evolving Understanding of Enigmatic Large Ripples on Mars

<p>Unlike terrestrial sandy deserts, Mars hosts two scales of ripples in fine sand. Larger, meter-scale ripples are morphologically distinct from small, decimeter-scale ripples, and their size, in particular, decreases with increasing atmospheric density. As a result, it was recently proposed that the equilibrium size of the larger ripples is set by an aerodynamic process, which makes them larger under thinner atmospheres. Under this hypothesis, large martian ripples would be distinct from smaller, decimeter-scale impact ripples in a mechanistic sense. Several workers have followed up on these initial observations to either corroborate, counter, or expand upon that hypothesis. Notably, a mechanistic model that not only corroborates the hypothesis that the size of large martian ripples is set by an aerodynamic process but also suggests that they arise from an aerodynamic instability, distinct from the grain-impact instability thought to be responsible for the formation of impact ripples, was developed. Conversely, other workers proposed that large ripples can develop from small impact ripples in a numerical model due to Mars’ low atmospheric pressure. In the latter model, the ripples’ growth-limiting mechanism is consistent with an aerodynamic process, but the large ripples would not be a separate class of ripples – they would simply be a larger version of the small impact ripples. Here, we explore this debate by synthesizing recent advances in large-ripple formation and offer potential avenues to address outstanding questions. Although significant knowledge gaps remain, it is clear that large martian ripples are larger where the atmosphere is less dense. The size of large martian ripples thus remain a powerful paleoclimate indicator.</p>

Sediment shell-content diminishes current-driven sand ripple development and migration

Shells and shell fragments are biogenic structures that are widespread throughout natural sandy shelf seas and whose presence can affect the bed roughness and erodibility of the seabed. An important and direct consequence is the effect on the formation and movement of small bedforms such as sand ripples. We experimentally measured ripple formation and migration of a mixture of natural sand with increasing volumes of shell material in a racetrack flume. Our experiments reveal the impacts of shells on ripple development in sandy sediment, providing information that was previously lacking. Shells expedite the onset of sediment transport while simultaneously reducing ripple dimensions and slowing down their migration rates. Moreover, increasing shell content enhances near-bed flow velocity due to the reduction of bed friction that is partly caused by a decrease in average ripple size and occurrence. This, in essence, limits the rate and magnitude of bedload transport. Given the large influence of shell content on sediment dynamics on the one hand, and the high shell concentrations found naturally in the sediments of shallow seas on the other hand, a significant control from shells on the morphodynamics of sandy marine habitats is expected.

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.