BLOGS

Schematic of annulus-conical water jet-assisted laser machining. Fig 1

Topics: Applied Physics, Composite Materials, Fluid Mechanics, Lasers, Materials Science, Nanoengineering

Abstract

When processing SiC/SiC composites using nanosecond-pulsed lasers, thermal effects such as molten deposition and heat-affected zones (HAZs) will occur. In this study, an annulus-conical water jet (ACWJ) was introduced to assist nanosecond laser machining of SiC/SiC composites, aiming to suppress thermal damage. A comparative investigation between laser processing in air and under ACWJ assistance was conducted. The results demonstrated that ACWJ assistance effectively eliminated molten material deposition and HAZs, significantly improving surface quality. However, despite a short beam path in the water (approximately 2.5–3 mm), turbulence in the water stream during ACWJ processing caused beam divergence and focus drift, leading to a substantial reduction in laser energy density on the target surface, and thus a lower material removal efficiency compared to laser machining in air. Moreover, the beam focal position drifted within the turbulent water stream, resulting in broader and shallower machined features. The material removal rate during ACWJ-assisted laser processing was only approximately 3%–10% of that achieved in air. In groove ablation, achieving the same depth as that in air-based processing required a significantly larger number of scan passes under ACWJ conditions. In hole machining, the resulting hole diameters were approximately 240% greater than those achieved in air.

Processing of SiC/SiC composite using an annulus-conical water jet-assisted nanosecond-pulsed laser, Zhuang Liu, Weicheng Xu, Chenhao Li, Tianrui Liu, Changshui Gao, Journal of Applied Physics

Figure 1. The Vadret da Tschierva glacier in 1935 (top) and in 2022 (bottom).Photos courtesy of swisstopo, L. Hösli, G. Carcanade, M. Huss, VAW-ETHZ.

Topics: Civilization, Climate Change, Fluid Mechanics, Global Warming, Meteorology, Research

Glaciers—dynamic masses of ice descending from the mountain tops—have always been fascinating to humankind. They intrinsically belong to the high-alpine environment. Countless photographs immortalize their bright white beauty and the power they radiate. Glaciers have been depicted on oil and parchment for centuries, as if trying to capture their transience. They are constantly moving; under the influence of gravity, the ice generated at high elevation flows downwards and shapes tremendous glacier tongues that are speckled with deep fissures known as crevasses. Sometimes, the openings to these crevasses are hidden by a light dusting of snow; subsequently, mountaineers need a lot of experience to accurately judge their exposure to the ice.

Even though glaciers are not living things, they are not lifeless. For many mountain regions worldwide, glaciers function similarly to lungs: They absorb snow in wintertime and “breathe” out water during hot summer days. This glacier water is urgently needed, especially in dry periods.

Glaciers consequently have a relevance that goes far beyond the mountain peaks where they reside. A reduction in meltwater from glaciers would be painful for nature and the global economy: irrigation of fields would be restricted, the temperature and mineralization of rivers would change, and during periods of drought, serious bottlenecks could come into existence for the drinking water supply and for shipping on rivers. In addition, melting glacial ice contributes to sea-level rise and therefore directly or indirectly affects billions of people living near the coast.

Despite, or perhaps because of, their majestic appearance, glaciers can also pose an immediate threat. Glaciers can produce floods and ice avalanches that endanger villages in the valleys. Together with permafrost, glaciers also stabilize mountain flanks and therefore reduce the potential for rock falls and landslides—a role that is becoming increasingly lost.

The so-called “eternal” ice of glaciers tells a long and dynamic story. During the Ice Age, ice sheets covered a large part of the North American continent, as well as Europe. The last time this happened was around 20,000 years ago—a blink of the eye from a geological perspective. Since then, the climate has changed, due both to natural factors and to anthropogenic influences—human-caused factors—which have massively accelerated over the past 100 years (Marzeion et al., 2014). As a result, the glaciers are still present, but they are getting smaller every year.

The Alps’ iconic glaciers are melting, but there’s still time to save (most of) the biggest, Matthias Huss, Bulletin of the Atomic Scientists.

Left: the electron density isosurface from theoretical DFT calculations. S and W atoms are shown in yellow and blue respectively. Right: transmission electron microscopy image. Courtesy: R Boya

Topics: Fluid Mechanics, Materials Science, Nanofluidics, Nanotechnology

Gases flow through a porous membrane at ultrahigh speeds even when the pores’ diameter approaches the atomic scale. This finding by researchers at the University of Manchester in the UK and the University of Pennsylvania in the US shows that the century-old Knudsen description of gas flow remains valid down to the nanoscale – a discovery that could have applications in water purification, gas separation, and air-quality monitoring.

Gas permeation through nano-sized pores is both ubiquitous in nature and technologically important explains Manchester’s Radha Boya, who led the research effort along with Marija Drndić at Pennsylvania. Because the diameter of these narrow pores is much smaller than the mean free diffusion path of gas molecules, the molecules’ flow can be described using a model developed by the Danish physicist Martin Knudsen in the early decades of the 20^th century. During so-called Knudsen flow, the diffusing molecules randomly scatter from the pore walls rather than colliding with each other.

Until now, however, researchers didn’t know whether Knudsen flow might break down if the pores become small enough. Boya, Drndić, and colleagues have now shown that the model holds even at the ultimate atomic-scale limit.

Gas flows follow conventional theory even at the nanoscale, Isabelle Dumé, Physics World