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The supersonic core length of the air microjets flowing from rectangular micro-nozzles with different aspect ratios has been experimentally studied. The height and width of the nozzles varied from 119 to 135 μm and from 927 to 2185 μm, respectively.

Liquid-assisted laser ablation has the advantage of relieving thermal effects of common laser ablation processes, whereas the light scattering and shielding effects by laser-induced cavitation bubbles, suspended debris, and turbulent liquid flow generally deteriorate laser beam transmission stability, leading to low energy efficiency and poor surface quality. Here, we report that a continuous and directional high-speed microjet will form in the laser ablation zone if laser-induced primary cavitation bubbles asymmetrically collapse sequentially near the air-liquid interface under a critical thin liquid layer. The laser-induced microjet can instantaneously and directionally remove secondary bubbles and ablation debris around the laser ablation region, and thus a very stable material removal process can be obtained. The shadowgraphs of high-speed camera reveal that the average speed of laser-induced continuous microjet can be as high as 1.1 m s −1 in its initial 500 μ m displacement. The coupling effect of laser ablation, mechanical impact along with the collapse of cavitation bubbles and flushing of high-speed microjet helps achieve a high material removal rate and significantly improved surface quality. We name this uncovered liquid-assisted laser ablation process as laser-induced microjet-assisted ablation (LIMJAA) based on its unique characteristics. High-quality microgrooves with a large depth-to-width ratio of 5.2 are obtained by LIMJAA with a single-pass laser scanning process in our experiments. LIMJAA is capable of machining various types of difficult-to-process materials with high-quality arrays of micro-channels, square and circle microscale through-holes. The results and disclosed mechanisms in our work provide a deep understanding of the role of laser-induced microjet in improving the processing quality of liquid-assisted laser micromachining. A LIMJAA approach is developed for achieving high-quality micromachining by instantaneously removing bubbles and debris from ablation zone. A continuous and directional high-speed microjet forms from the asymmetrical collapse of sequentially laser-induced cavitation bubbles in a critical thin liquid layer. Through parameters optimizing, high-quality microgrooves with a large depth-to-width ratio of 5.2 are manufactured by a single-pass laser scanning process. The proposed approach is capable of machining high-quality arrays of micro-channels and micro-through-holes in various types of difficult-to-process materials.

Ultra-high-pressure micro-jet (UHP) sterilization technology has received significant attention as a non-thermal food processing method. However, during highpressure jet operation, cavitation at the nozzle outlet rapidly damages the nozzle structure, resulting in extremely poor durability and consequently limiting the application of UHP micro-jet technology in food processing. To address this issue, this study first built a CFD model of the ultra-high-pressure jet system to simulate the spatial distribution of cavitation regions in the flow field near the nozzle. The model’s accuracy was then validated through accelerated cavitation-erosion experiments on jet nozzles. Finally, based on this model, we calculated the effects of different jet outlet pressures on cavitation distribution near the nozzle. The results show that increasing the jet outlet pressure can effectively suppress cavitation in the flow field near the nozzle, providing a simple and effective approach to improving the service life of ultra-high-pressure jet nozzles.

Experimental studies of the electronic structure of excess electrons in liquids—archetypal quantum solutes—have been largely restricted to very dilute electron concentrations. We overcame this limitation by applying soft x-ray photoelectron spectroscopy to characterize excess electrons originating from steadily increasing amounts of alkali metals dissolved in refrigerated liquid ammonia microjets. As concentration rises, a narrow peak at ~2 electron volts, corresponding to vertical photodetachment of localized solvated electrons and dielectrons, transforms continuously into a band with a sharp Fermi edge accompanied by a plasmon peak, characteristic of delocalized metallic electrons. Through our experimental approach combined with ab initio calculations of localized electrons and dielectrons, we obtain a clear picture of the energetics and density of states of the ammoniated electrons over the gradual transition from dilute blue electrolytes to concentrated bronze metallic solutions.

The structure of the electrical double layer has been debated for well over a century, since it mediates colloidal interactions, regulates surface structure, controls reactivity, sets capacitance, and represents the central element of electrochemical supercapacitors. The surface potential of such surfaces generally exceeds the electrokinetic potential, often substantially. Traditionally, a Stern layer of nonspecifically adsorbed ions has been invoked to rationalize the difference between these two potentials; however, the inability to directly measure the surface potential of dispersed systems has rendered quantitative measurements of the Stern layer potential, and other quantities associated with the outer Helmholtz plane, impossible. Here, we use x-ray photoelectron spectroscopy from a liquid microjet to measure the absolute surface potentials of silica nanoparticles dispersed in aqueous electrolytes. We quantitatively determine the impact of specific cations (Li+ , Na+ , K+ , and Cs+ ) in chloride electrolytes on the surface potential, the location of the shear plane, and the capacitance of the Stern layer. We find that the magnitude of the surface potential increases linearly with the hydrated-cation radius. Interpreting our data using the simplest assumptions and most straightforward understanding of Gouy-Chapman-Stern theory reveals a Stern layer whose thickness corresponds to a single layer of water molecules hydrating the silica surface, plus the radius of the hydrated cation. These results subject electrical double-layer theories to direct and falsifiable tests to reveal a physically intuitive and quantitatively verified picture of the Stern layer that is consistent across multiple electrolytes and solution conditions.

A cavitation bubble expanding and collapsing near a rigid boundary develops a directed jet flow towards the boundary. In the case of a perforated plate, some of the jet flow passes through the plate and thus the bubble acts as a pump transporting liquid from one side of the plate to the opposite side. The transport is rather complex, is time dependent and varies with the geometric parameters of the bubble and the connecting channel. Therefore, we first model the transport of liquid through a perforated rigid plate for a large range of parameters and then compare some regimes with experiments using single laser-induced bubbles. The simulations are based on a Volume-of-Fluid solver in OpenFOAM and account for surface tension, compressibility and viscosity. The resulting flux and generated velocity in the channel obtained in the simulations are discussed with regards to the dependence of the channel geometry, liquid viscosity and stand-off distance of the bubble to the plate. In general, high flow rates are achieved for long cylindrical channels that have a similar width as the jet produced by the collapsing bubble. At low stand-off distances combined with thick plates, an annular inflow creates a fast and thin jet, also called needle jet, which is approximately a magnitude faster and significantly thinner than the usually encountered microjet. In contrast, for thin plates and small stand-off distances, liquid is pumped in the opposite direction via a reverse jet.

This work seeks to investigate jet array impingement liquid cooling of CPU processors that utilise Integrated Heat Spreaders. In this study, single and centralised heat sources of various sizes (10 mm to 20 mm) were investigated, and a series of single and multi-objective optimisation studies were carried out to determine the optimum arrangement of microjets for different heat source sizes based on variable geometric inputs of the jet orifice plate. The results indicated that the design of the optimum jet orifice strongly depends on the size of the heat source as well as whether the target optimisation strategy is to achieve minimum thermal resistance, or both minimum thermal resistance and minimum pumping power simultaneously. A clustering of quite different orifice designs in optimal zones is observed, where approximately the same hydraulic and/or thermal performance is observed.

Microbubbles, as key triggering factors of cavitation phenomena, are of significant importance for research on cavitation effects. This paper systematically investigates the collision and coalescence dynamics of microbubbles through a combined approach of theoretical analysis, experimental observation, and numerical simulation. First, a compact microbubble generation device was designed. By integrating a high-speed impeller-accelerated flow field with gas-liquid mixing pressurization technology, efficient and accurate controlled generation of microbubbles was achieved. Second, an observation system based on a high-magnification microscopic lens and optimized reflective optical path was developed, overcoming the technical challenge of capturing transient behaviours at the micrometer scale. Furthermore, a mesh-free numerical model based on Smoothed Particle Hydrodynamics (SPH) was established. By incorporating surface tension corrections and interfacial force coupling algorithms, the simulation accuracy of microscale microbubble motion was significantly enhanced. Experimental and numerical results demonstrate that: Microbubble coalescence requires multiple collisions, with the number of collisions positively correlated with the amplitude of surface oscillations. Microjets and local pressure differentials are the key physical mechanisms triggering coalescence. The coalescence process follows a transient evolutionary pattern described as a 'contact-compression-rupture-reconstruction' cycle. This study provides novel observation techniques and theoretical models for microscale microbubble dynamics, offering valuable reference for cavitation effect prediction, drag and noise reduction in underwater equipment, and the optimal design of hydraulic machinery.

Fast microjets can emerge out of liquid pools from the rebounding of drop-impact craters, or when a bubble bursts at its surface. The fastest jets are the narrowest and are a source of aerosols both from the ocean and from a glass of champagne, of importance to climate and the olfactory senses. The most singular jets, which we observe experimentally at a maximum velocity of $137\\pm 4\\ {\\rm m}\\ {\\rm s}^{-1}$ and a diameter of $12\\ \\mathrm {\\mu }{\\rm m}$, under reduced ambient pressure, are produced when a small dimple forms at the crater bottom and rebounds without pinching off a small bubble. The radial collapse and rebounding of this dimple is purely inertial, but highly sensitive to initial conditions. High-resolution numerical simulations reveal a new focusing mechanism, which drives the fastest jet within a converging conical channel, where an entrained air sheet provides effective slip at the outer boundary of the conically converging flow into the jet. This configuration bypasses any viscous cutoff of the jetting speed and explains the extreme sensitivity to initial conditions observed in detailed experiments of the phenomenon.

Extreme ultraviolet high-harmonic generation (HHG) from bulk liquids has only recently been demonstrated (T.T. Luu, Z. Yin et al, Nat. Comm. 9, 3724, (2018)). This has opened new prospects for the development of bright high-harmonic sources and the development of liquid-phase high-harmonic spectroscopy (HHS). Here, we report on the first observation of HHG in liquids driven by few-cycle (∼7 fs) pulses. We observe the emission of a broad quasi-continuum in the extreme ultraviolet, which is strongly modulated on a photon-energy scale much finer than the central photon energy of the driver. We show that these modulations arise from an etalon effect inside the flat microjet, which we use to perform an in-operando measurement of the flat-jet's thickness. The possibility to directly characterize flat microjets during HHG will greatly support their optimization for light-source applications and facilitate the development of liquid-phase HHS.