Optimization of femtosecond laser processing in liquids

In this paper we analyze femtosecond laser processing of metals in liquids searching for optimal conditions for predictable ablation. Incident laser pulses are stretched or compressed, self-focused and scattered on bubbles and on surface waves in the liquid environment. Influence of these effects on the laser intensity distribution on the target surface is discussed and optimal processing parameters are suggested

Scalable surface microstructuring by a fiber laser for controlled nucleate boiling performance of high- and low-surface-tension fluids

Nucleate boiling enables effective cooling and heat transfer at low temperature differences between a heated surface and the surrounding fluid. It is utilized in many applications, ranging from large power plants to small microelectronics. To enhance the boiling process by minimization of the surface temperature and increase the maximum attainable heat flux, several approaches for surface modifications were recently developed. However, each of them has at least one important drawback, including challenging and expensive production, mechanical and/or thermal instability or problematic scale-up. Herein, a straightforward, robust and flexible method using a nanosecond fiber laser for production of surfaces with multi-scale micro-cavities (with diameters ranging from 0.2 to 10 µm) is developed. Examination of these surfaces in two very contrasting fluids - water, which is polar, has high surface tension and high latent heat of vaporizationand non-polar, dielectric tetradecafluorohexane (FC-72) with low surface tension and much lower latent heat - confirms that such surfaces enable enhanced heat transfer and controlled boiling in combination with diverse fluids. This demonstration suggests that the developed method has the potential to overcome the current limitations for further miniaturization of microelectronic devices and to increase performance and safety in high heat flux systems

The impact of Participatory Budgeting on health and wellbeing:A scoping review of evaluations

Background: Participatory budgeting (PB), citizens deliberating among themselves and with officials to decide how to allocate funds for public goods, has been increasingly implemented across Europe and worldwide. While PB is recommended as good practice by the World Bank and the United Nations, with potential to improve health and wellbeing, it is unclear what evaluations have been conducted on the impact of PB on health and wellbeing. Methods: For this scoping review, we searched 21 databases with no restrictions on publication date or language. The search term ‘participatory budget’ was used as the relevant global label for the intervention of interest. Studies were included if they reported original analysis of health, social, political, or economic and budgetary outcomes of PB. We examined the study design, analysis, outcomes and location of included articles. Findings are reported narratively. Results: From 1458 identified references, 37 studies were included. The majority of evaluations (n = 24) were of PB in South America, seven were in Europe. Most evaluations were case studies (n = 23) conducting ethnography and surveys, focussing on political outcomes such as participation in PB or impacts on political activities. All of the quantitative observational studies analysing population level data, except one in Russia, were conducted in South America. Conclusion: Despite increasing interest in PB, evaluations applying robust methods to analyse health and wellbeing outcomes are scarce, particularly beyond Brazil. Therefore, implementation of PB schemes should be accompanied by rigorous qualitative and quantitative evaluation to identify impacts and the processes by which they are realised

Comment on "Bioinspired reversible switch between underwater superoleophobicity/superaerophobicity and oleophilicity/aerophilicity and improved antireflective property on the nanosecond laser-ablated superhydrophobic titanium surfaces"

Laser-textured surfaces enabling reversible wettability switching and improved optical properties are gaining importance in cutting-edge applications, including self-cleaning interfaces, tunable optical lenses, microfluidics and lab-on-chip systems. Fabrication of such surfaces by combining nanosecond-laser texturing and low-temperature annealing of titanium Ti-6Al-4V alloy was demonstrated by Lian et al. in ACS Appl. Mater. Inter.2020,12 (5), 6573-6580. However, it is difficult to agree with (i) their contradictory explanation of the wettability transition due to low-temperature annealing and (ii) their theoretical description of the optical behavior of the laser-textured titanium surface. This comment provides an alternative view - supported by both experimental results and theoretical investigation - on how the results by Lian et al. could be interpreted more correctly. The annealing experiments clarify that controlled contamination is crucial in obtaining consistent surface wettability alterations after low temperature annealing. Annealing of laser-textured titanium at 100 °C in contaminated and contaminant-free furnaces leads to completely different wettability transitions. Analysis of the surface chemistry by XPS and ToF-SIMS reveals that (usually overlooked) contamination with hydrophobic polydimethylsiloxane (PDMS) may arise from the silicone components of the furnace. In this case, a homogeneous thin PDMS film over the entire surface results in water repellency (contact angle of 161° and roll-off angle of 15°). Contrarily, the annealing under the same conditions, but in a contaminant-free furnace preserves the initial superhydrophilicity, while the annealing at 350 °C turns the hydrophobicity "off". The theoretical calculations of optical properties demonstrate that the laser-induced oxide layer formed during the laser texturing significantly influences the surface optical behavior. Consequently, the interference of light reflected by the air-oxide and the oxide-metal interfaces should not be neglected and enables several advanced approaches to exploit such optical propertie

Wettability and friction control of a stainless steel surface by combining nanosecond laser texturing and adsorption of superhydrophobic nanosilica particles

In this work, we present functionalization of AISI 316 L surfaces by nanosecond Nd:YAG laser texturing and adsorption of superhydrophobic fluoroalkylsilane functionalized 30-nm silica nanoparticles. Surface modification by varying the distance between laser-produced micro([micro])-channels leads to different surface roughnesses. After nanosilica coating, the superhydrophilic laser-textured surfaces change into superhydrophobic surfaces with the same [micro] -roughness. A higher [micro] -channel density leads to more hydrophobic surfaces after coating. This enables a study of the combined effect of surface wettability and morphology on the friction coefficient and wear resistance. Experiments were performed in dry and water environments. In the case of dry friction, increased [micro]-roughness leads to a higher friction coefficient, and the water-repellency modification by nanosilica particles has no influence on the tribological behaviour. In contrast, in the water environment, the wettability presents an important contribution to the properties of contact surfaces: hydrophobic surfaces exhibit a lower friction coefficient, especially at higher densities of [micro] -channels. Energy-dispersive X-ray spectroscopy analysis of surfaces before and after the tribological experiments is performed, revealing the difference in weight % of Si in the worn surface compared to the unworn surface, which varies according to the nature of the surface morphology due to laser texturing in both dry and water environments

Propulsion effects after laser ablation in water, confined by different geometries

A Nd:YAG laser with 7-ns pulses and pulse energies up to 10 mJ is used to induce an optical breakdown in the front surface of an aluminum rod, covered by a water layer. The rod is part of a ballistic pendulum. In this way, we study the propulsion effects by means of coupling coefficient and energy-conversion efficiency with respect to different confining geometries, volumes of water applied to the front surface of the rod, and the distance of this surface from the laser-beam focus. Holes with different dimensions are drilled on the target surface and filled with different volumes of water to examine the influence of the confinement by the liquid (a free boundary) and a solid-surface geometry on laser ablation effects. The rod movement and the water ejection after laser ablation are acquired by a high-speed camera with 10k frames per second. The results show that the confinement by cavity substantially increases the propulsion effects by shaping the ejected flow of the liquidwhile the cavitation bubble, induced inside the water layer, plays a significant role in propulsion efficiency. From the presented results, it follows that the laser-propelled rod carries below 0.5% of the total mechanical energy after propulsion, while the rest of this energy represents the kinetic energy of the ablated water. As expected, moving the target surface away from the focal position decreases the ablative-propulsion efficiency. When the focus is moved inside the solid target, the decrease occurs due to lower conversion of the pulse energy into the energy of the cavitation bubble. If the focus is moved from the surface outward, the bubble moves towards the liquid-gas interface and it is not able to efficiently eject all the liquid from the target

Observation of laser-induced elastic waves in agar skin phantoms using a high-speed camera and a laser-beam-deflection probe

We present an optical study of elastic wave propagation inside skin phantoms consisting of agar gel as induced by an Er:YAG (wavelength of 2.94 µm) laser pulse. A laser-beam-deflection probe is used to measure ultrasonic propagation and a high-speed camera is used to record displacements in ablation-induced elastic transients. These measurements are further analyzed with a custom developed image recognition algorithm utilizing the methods of particle image velocimetry and spline interpolation to determine point trajectories, material displacement and strain during the passing of the transients. The results indicate that the ablation-induced elastic waves propagate with a velocity of 1 m/s and amplitudes of 0.1 mm. Compared to them, the measured velocities of ultrasonic waves are much higher, within the range of 1.42-1.51 km/s, while their amplitudes are three orders of magnitude smaller. This proves that the agar gel may be used as a rudimental skin and soft tissue substitute in biomedical research, since its polymeric structure reproduces adequate soft-solid properties and its transparency for visible light makes it convenient to study with optical instruments. The results presented provide an insight into the distribution of laser-induced elastic transients in soft tissue phantoms, while the experimental approach serves as a foundation for further research of laser-induced mechanical effects deeper in the tissue