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The thermal management issue of high electronic chips with heat flux is one of the most challenge during the development of electronic devices. In this work, the design of micro pin fin arrays (MPFA) of micro-jet heat sink was proposed to achieve effective cooling of electronic chips. The MPFA with different cross-sectional shapes were used to improve the thermal performance of the heat sink. The results show that heat sink design with MPFA exhibit excellent cooling performance while consuming approximately the same amount of power as smooth surface. In addition, the hexagonal cross-section has better thermal performance, the temperature uniformity of the heating surface can be improved by up to 70 % and the average temperature can be reduced by 7–20K. Moreover, the manufacturing cost and cooling performance per unit cost were investigated and the results show that the square cross-section is the most commercially viable. Based on this, the square cross-section was parametrically studied, including the length to height ratio and fins number. It was founded that the heat sink exhibits better thermal performance for a length-to-height ratio of 1.4 and a number of fins of 169. Finally, the ultimate heat flux of the electronic chip can be withstood was also tested, and the optimal heat sink design can handle heat flux of 400W/cm2. Therefore, the heat sink with MPFA design can significantly improve the thermal performance without consuming the pump power, which can be used to cooling the high electronic chips with high heat flux in the future. • Micro pin fin arrays greatly improve the thermal performance of the heat sink. • Enhanced heat transfer without additional power consumption of heat sink. • Manufacturing costs and cooling benefits of heat sinks were analyzed. • Optimal heat sink design can handle up to 400W/cm2 on the chip surface. [ABSTRACT FROM AUTHOR]

• Collapsing dynamics of a cavitation bubble between a particle and wall is experimentally investigated with high-speed camera. • Depending on the directions of micro-jets, bubble collapsing behavior could be categorized into three cases. • Four different kinds of transitions among three cases are demonstrated. In the present paper, the collapsing dynamics of a laser-induced cavitation bubble between a spherical particle and a solid wall is fully revealed using a high-speed camera based cavitation experimental system. For the purpose of comparisons, experiments of cases without the presence of the particle are also conducted and demonstrated. Influences of several paramount parameters (including the bubble radius, the particle radius, and the relative positions among the bubble, the particle and wall) on the bubble collapsing dynamics, the motions of bubble boundaries and the positions of the bubble centroid are all investigated. Based on the directions of the micro-jets during the bubble collapse, the bubble collapsing behavior could be categorized into three cases with their primary characteristics revealed. Furthermore, the transitions among three different cases are clearly demonstrated using several examples in terms of high-speed photography. Based on a large amount of experimental data, quantitative discussions on the parameter zone of the identified three typical cases are given in terms of the three non-dimensional parameters. Generally speaking, the particle scale does not show significant influences on the bubble dynamics behavior except some special positions of the bubble. [ABSTRACT FROM AUTHOR]

In the present paper, the interactions between a cavitation bubble and a spherical particle are experimentally investigated with the aid of high-speed camera. The cavitation bubble is generated by a focused laser beam with the bubble size adjusted through energy dissipater. A series of experimental work is conducted with variations of the bubble sizes, the particle sizes and the distance between the bubble and the particle. Based on the collapsing process of cavitation bubble near the particle, three cases (mushroom-shaped, pear-shaped and spherical-shaped collapses) are quantitatively identified and the criterions for separating three cases are also given in terms of non-dimensional parameters. Furthermore, transitions between the aforementioned cases during parameter variations are also shown together with several examples. Finally, some special cases (e.g. collapse of two bubbles near the particle) are also discussed with typical collapsing process shown. [ABSTRACT FROM AUTHOR]

In this paper we present a numerical study investigating the effects of nozzle geometry on stability, shape and flow characteristics of micron-sized liquid jets, produced by injection molded gas dynamic virtual nozzles (GDVNs) operating in vacuum. The jet characteristics are described as a function of (i) capillary-to-orifice distance, (ii) nozzle outlet orifice diameter, and (iii) liquid feeding capillary angle. An experimentally verified numerical model of GDVN with laminar two-phase Newtonian compressible flow, based on finite volume method and volume of fluid interface tracking, is used to assess the changes. The study is performed for two sets of liquid flow rates while keeping the gas flow rate constant. It is observed that for each value of capillary-to-orifice distance and nozzle outlet diameter there is a minimum liquid flow rate below which the jet is unstable. We find that the nozzle outlet diameter has the biggest influence on the jet diameter, length and velocity, while liquid capillary angle has no observable effect on jet characteristic. Varying capillary-to-orifice distance does not affect the flow field around micro-jet. It is found that the liquid and the gas interaction near the meniscus primarily affect the jet stability and shape. [ABSTRACT FROM AUTHOR]

We measure the minimum liquid flow rate Q min to produce steady jetting in gaseous flow focusing with orifices of diameters down to 30 μ m. We compare the results obtained for different diameters and liquids. Using dimensional analysis, we determine the influence of the gas viscosity on this stability limit. For low and moderately low-viscosity liquids, there is an optimum value of the gas viscosity for which Q min reaches its lowest value. This optimum value leads to the minimum jet diameter in front of the discharge orifice. • The stability limit of steady jetting in gaseous flow focusing was measured. • We use the orifice-plate configuration with diameters down to 30 micras. • We use dimensional analysis to study the gas viscosity effect without changing it. • There is an optimum gas viscosity for moderately low viscosity liquids. [ABSTRACT FROM AUTHOR]

In ultrasonic-assisted machining, the synergistic effect of the cavitation effect and micro-abrasive particles plays a crucial role. Studies have focused on the investigation of the micro-abrasive particles, cavitation micro-jets, and cavitation shock waves either individually or in pairs. To investigate the synergy of shock waves and micro-jets generated by cavitation with micro-abrasive particles in ultrasonic-assisted machining, the continuous control equations of a cavitation bubble, shock wave, micro-jet, and micro-abrasive particle influenced by the dimensionless amount (R/R 0), a particle size-velocity–pressure model of the micro-abrasive particle was established. The effects of ultrasonic frequency, sound pressure amplitude, and changes in particle size on micro-abrasive particle velocity and pressure were numerically simulated. At an ultrasonic frequency of 20 kHz and ultrasonic sound pressure of 0.1125 MPa, a smooth spherical SiO 2 micro-abrasive particle (size = 5 µm) was obtained, with a maximum velocity of 190.3–209.4 m/s and pressure of 79.69–89.41 MPa. The results show that in the range of 5–50 μm, smaller particle sizes of the micro-abrasive particles led to greater velocity and pressure. The shock waves, micro-jets, and micro-abrasive particles were all positively affected by the dimensionless amount (R/R 0) of cavitation bubble collapse, the larger the dimensionless quantity, the faster their velocity and the higher their pressure. [ABSTRACT FROM AUTHOR]

• A comprehensive geometrical study is performed on the passive planar mixers with bent baffles. • A novel planar micromixer with Z-shaped baffles and octagonal mixing units achieves a high mixing efficiency at the short distance of 2800 µm. • The mixing mechanisms inside the proposed mixer includes the reduction of molecular lateral diffusion distance, the split and recombination of fluids, vortex formation and asymmetric flow. • The micromixer has great potential in biochemical engineering applications. As an important pre-treatment part of the micro-total analysis system, the micromixer that ensures the uniform mixing of samples and reagents in a short distance is essential for the improvement of the detection accuracy. Here, a type of passive planar micromixer with narrow gaps and obstacles arranged in the mixing units is proposed. The mixer utilizes the micro-jets and bent baffles to split and recombine streams and promote vortex formation, which breaks the laminar flow when the Reynolds number (Re) varies from 0.1 to 50. The performance of the unoptimized mixer decreases first and then increases with the increase of Re. However, a poor mixing quality is found in the range of 0.5 ≤ Re ≤ 15. To surmount the above shortcomings, a comprehensive geometrical study is performed on this micromixer and the effects of various parameters such as the width and length of the gap, the shape of the baffle, and mixing unit on the mixing quality and pressure drop are analyzed. The results show that the reduction of gap width and the increase of the gap length result in the improvement of mixing efficiency. The staggered Z-shaped baffles accelerate the mixing process compared to traditional V-shaped baffles owing to asymmetric flow and vortex, which leads to excellent mixing. The results also show that the octagonal mixing unit increases mixing efficiency without basically changing pressure drop because the progressive and tapered structure enhances the convection. Lastly, a novel micromixer whose mixing performance is close to 0.8 at Re = 0.5 at the short distance of 2800 µm is proposed, fabricated and tested. This work provides a reference for the development of high-efficiency mixers in the pre-processing part of the micro-total analysis system. [ABSTRACT FROM AUTHOR]

• Non-lethal blasts can produce cerebrospinal fluid (CSF) cavitation. Blast induced traumatic brain injury (bTBI) has been a prevalent injury in recent conflicts. Post-mortem studies have shown damage in the brain tissue close to the cerebrospinal fluid (CSF) in bTBI cases compared to non-blast TBI cases. CSF cavitation is a potential mechanism for this brain/CSF interface injury. In this study, our aim was to explore the possibility and mechanism of blast induced CSF cavitation. We first developed a one-dimensional simplified human head surrogate and exposed it to nonlethal blast waves using a shock tube. High-speed videography and pressure sensors data showed the formation and collapse of cavitation in the CSF simulant. Then, we explored the mechanism of the cavitation using a finite element model of the head surrogate. We found that the pressure waves transmitting through the skull (outer wave) and tissue simulants (inner wave) are responsible for the generation and collapse of the cavitation bubbles, respectively. Next, we used this insight to explore the possibility of CSF cavitation in the human head using a detailed finite element model. The simulations verified the role of the inner and outer waves in the generation and collapse of cavitation. Our results suggested that CSF cavitation is likely to happen in the human head under blast loading. Finally, we studied the CSF cavitation in head surrogate models with different lengths. The results showed that the head length significantly affected the CSF cavitation, indicating the potential drawback of using small animals to study bTBI in human head. Our findings can improve our understanding of the brain/CSF interface injury after blast exposure and inform the design of protection systems and animal tests. [ABSTRACT FROM AUTHOR]

Abstract: This work investigates the relation between shock wave risetime and the amount of micro-scale fragments ejected from a grooved aluminium surface under shock loading condition. Using smoothed particle hydrodynamics, we calculate the formation of micro-jet from the groove of metal surface, and analyze the dependence on the width of loading wave front. The simulation results compare well with the experiment ones, and reveal the dependence of the micro-jet on the wave front: both the mass and the maximal velocity of the ejection will decrease with the increasing of the width of wave front. It is also found that the micro-jet originates from the folium near the groove, which can acquire axial velocity and impact at axis when the shock wave releases at the metal/vacuum interface. The folium becomes smaller as the wave front widens. This is because some matter will satisfy the lock condition of jet strength and can eject no more. [Copyright &y& Elsevier]

Abstract: Nowadays, molecular dynamics simulation has been a significant method in detecting the atomistic-scale kinetics of materials. The objective of this work is to detect the dynamic properties of micro-jet from a nano-grooved aluminium surface by molecular dynamics simulations. For a large range of shock pressure, we investigate the variation of micro-jet mass and morphology. The stacking fault, amorphous state, and release melting during the jetting are all observed, and the effect of release melting on the micro-jet is analyzed. It is found that, the micro-jet mass keeps a linear increase with the piston velocity prior to release melting, the occurrence of release melting can intensify the jetting evidently, and when the velocity of release melting is above a threshold, the jetting mass shows a linear increase with the piston velocity again. [Copyright &y& Elsevier]

Cavitation damage is a micro, high-speed, multi-phase complex phenomenon caused by the near-wall bubble group collapse. The current numerical simulation method of cavitation mainly focuses on the collapse impact of a single cavitation bubble. The large-scale simulation of the cavitation bubble group collapse is difficult to perform and has not been studied, to the best of our knowledge. In this study, the equivalent model of impact loading of acoustic bubble collapse micro-jets is proposed to study the cavitation erosion damage of materials. Based on the theory of the micro-jet and the water hammer effect of the liquid–solid impact, an equivalent model of impact loading of a single acoustic bubble collapse micro-jet is established under the principle of deformation equivalence. Since the acoustic bubbles can be considered uniformly distributed in a small enough area, an equivalent model of impact loading of multiple acoustic bubble collapse micro-jets in a micro-segment can be derived based on the equivalent results of impact loading of a single acoustic bubble collapse micro-jet. In fact, the equivalent methods of cavitation damage loading for single and multiple near-wall acoustic bubble collapse micro-jets are formed. The verification results show the law of cavitation deformation of concrete using equivalent loading is consistent with that of a micro-jet simulation, and the average relative errors and the mean square errors are insignificant. The equivalent method of impact loading proposed in this paper has high accuracy and can greatly improve the calculation efficiency, which provides technical support for numerical simulation of concrete cavitation. [ABSTRACT FROM AUTHOR]

• Effect of surface tension on acoustic cavitation is studied by adding surfactant. • Micro bubble in certain size is generated in liquid with different surface tension σ. • Acoustic-driven bubble evolution near a rigid wall in liquid with various σ are recorded. • Effect mechanisms of σ on bubble dynamics in ultrasonic field are discussed. • Mechanisms of lower σ on efficiency increase of ultrasonic applications are analyzed. Acoustic cavitation is a very important hydrodynamic phenomenon, and is often implicated in a myriad of industrial, medical, and daily living applications. In these applications, the effect mechanism of liquid surface tension on improving the efficiency of acoustic cavitation is a crucial concern for researchers. In this study, the effects of liquid surface tension on the dynamics of an ultrasonic driven bubble near a rigid wall, which could be the main mechanism of efficiency improvement in the applications of acoustic cavitation, were investigated at the microscale level. A synchronous high-speed microscopic imaging method was used to clearly record the temporary evolution of single acoustic cavitation bubble in the liquids with different surface tension. Meanwhile, the bubble dynamic characteristics, such as the position and time of bubble collapse, the size and stability of the bubbles, the speed of bubble boundaries and the micro-jets, were analyzed and compared. In the case of the single bubbles near a rigid wall, it was found that low surface tension reduces the stability of the bubbles in the liquid medium. Meanwhile, the bubbles collapse earlier and farther from the rigid wall in the liquids with lower surface tension. In addition, the surface tension has no significant influence on the speed of the first micro-jet, but it can substantially increase the speed of second and the third micro-jets after the first collapse of the bubble. These effects of liquid surface tension on the bubble dynamics can explain the mechanism of surfactants in numerous fields of acoustic cavitation for facilitating its optimization and application. [ABSTRACT FROM AUTHOR]

The removal of the adsorbed oil droplet is critical to deoiling treatment of oil-bearing solid waste. Ultrasonic cavitation is regarded as an extremely useful method to assist the oil droplets desorption in the deoiling treatment. In this paper, the effects of cavitation micro-jets on the oil droplets desorption were studied. The adsorbed states of oil droplets in the oil-contaminated sand were investigated using a microscope. Three representative absorbed states of the oil droplets can be summarized as: (1) the individual oil droplet adsorbed on the particle surface (2) the clustered oil droplets adsorbed on the particle surface; (3) the oil droplet adsorbed in a gap between particles. The micro-jet generation during the bubble collapse near a rigid wall under different acoustic pressure amplitudes at an ultrasonic frequency of 20 kHz was investigated numerically. The desorption processes of the oil droplets at the three representative absorbed states under micro-jets were also simulated subsequently. The results showed that the acoustic pressure has a great influence on the velocity of micro-jet, and the initial diameter of cavitation bubbles is significant for the cross-sectional area of micro-jets. The wall jet caused by a micro-jet impacting on the solid wall is the most important factor for the removal of the absorbed oil droplets. The oil droplet is broken by the jet impinging, and then it breaks away from the solid wall due to the shear force generated by the wall jet. In addition to a higher sound pressure, the cavitation bubble at a larger initial diameter is more important for the desorption of the clustered oil droplets. Conversely, the micro-jet generated by the cavitation bubble at a smaller initial diameter (0.1 mm) is more appropriate for the desorption of the oil droplet in a narrow or sharp-angled gap. [ABSTRACT FROM AUTHOR]

• Experimental characterization of droplet velocity emerging from gas-focused micro-jet breakup. • Validation of the numerical model of micro-jet in terms of its velocity and shape. • Improved capability in virtual design of micro-nozzles for sample delivery in femtosecond crystallography. Compressible multiphase numerical simulations of gas-focused micro-jets are compared with the experimental data obtained with the dual pulse imaging laser-induced fluorescence drop velocimetry. Such jets, originating from a 3D printed gas dynamic virtual nozzle into a low-vacuum (150 Pa) environment, are increasingly being used for sample delivery in serial femtosecond crystallography. The distance traveled by a detaching drop from the jet is measured between the two consecutive illumination pulses with a known time delay at the positions 200 µm and 450 µm from the nozzle. Additionally, the high-speed camera images are used to analyze the shape of the jet. An axisymmetric, compressible, Newtonian two-phase helium-water mixture model is numerically solved within the framework of the volume of fluid and the finite volume method. The experimental and the computational studies are performed with a constant volumetric liquid flow rate of 14 μl/min and the gas mass flow rate in the range from 4.6 mg/min to 20 mg/min. The related jet Reynolds number ranges from 120 to 220 and Weber number from 30 to 150. The maximum difference between the measurements and the results of the numerical model in terms of the droplet velocity and jet diameter is within 10 %. The study provides new information on the jet velocities for micron-sized gas-focused nozzles. The validated numerical model can be used as a design tool for the nozzles dedicated to the specific needs of the femtosecond crystallography experiments. [ABSTRACT FROM AUTHOR]

• Micro-jets perpendicularly hammering the material cause the greatest damage to the material surface and generate the eccentric bowl-shaped erosion pit are given. • The magnitude of material surface stress is positively related to the velocity of water hammer, the surface stress in the hammering area is the largest, and the stress nephogram is conical. • Neglecting the damage at the end of hammering, the surface of the material produced the maximum stress at the moment when the micro-jets contact the metal plate, and the magnitude of stress decreases with the impact process. In order to obtain the fatigue damage characteristics of materials from the cavitation bubble collapse zone, a hammering strategy of micro-jet water hammer is conceived, and a hammering of metal plate by micro-jet water hammer with cavitation bubbles is studied by numerical simulation method. The stress response characteristics of the metal plate(HT-200) in the instantaneous and whole hammering process are explored. It is also revealed that the maximum stress will be generated when water hammer with the same velocity hammering the metal plate at 90 degrees angle, and a bowl-shaped impact will be formed. However, eccentric bowl-shaped pit will be produced when the metal plate is hammered at a non-90 degree angle. Meanwhile, the simulation results are compared with the experimental cavitation pits, and the validity of the numerical simulation is verified. [ABSTRACT FROM AUTHOR]

• The size effect of materials should be considered in the formation of cavitation pits. • The prediction models of cavitation impact load, impact pressure and velocity of micro-jet with size effect were established. • A size effect coefficient 1 + 54 h p α 2 μ 2 b d p 2 σ JC 2 was defined. • The predicted values of parameters increased by about 11%–88% for the ten typical cavitation pits when considering the size effect. High-speed micro-jet produced by cavitation collapse near the wall is the main mechanism of material damage, and cavitation pit is the most typical damage feature. The reason why high-pressure and high-speed micro-jet can only cause nano- and microscale cavitation pit is that the micro-jet is a short-term impact load of nano- and microscale, and the material shows size effect during the formation of pits. To further explore the cavitation damage characteristics and deformation mechanism of materials, the theoretical framework of indentation test and J-C constitutive model were adopted, and the size effect of materials during the process of cavitation pit formation was mainly considered, and the prediction models of cavitation impact load, impact pressure and velocity of micro-jet were established. The results showed that the equivalent stress and strain of cavitation pit and the impact pressure and velocity of micro-jet are only related to the diameter-to-depth ratio of pit without size effect, and also to the diameter of pit with size effect. Larger diameter and deeper depth of the pit infers greater cavitation impact load, and the influence of the pit diameter is more obvious. When considering the size effect, there is an additional size effect coefficient: 1 + 54 h p α 2 μ 2 b d p 2 σ JC 2 . In the selected size range of pit, the cavitation impact load, impact pressure and velocity of micro-jet predicted with size effect increase by 0.9408%–322.5% compared with those without size effect. The maximum increase ratio appears at the minimum of diameter-to-depth ratio of pit (d p = 2 μm and d h = 2 μm), that is, the smaller the pit diameter is and the greater the depth is, the greater the increase ratio is. Ten typical cavitation pits were selected for inversion analysis. The impact pressure and velocity of micro-jet with and without size effect are 473–1131 MPa and 355–848 m/s, and 427–604 MPa and 320–453 m/s, respectively. The predicted values increase by about 11%-88% when considering the size effect, and the micro-jet velocity predicted is closer to that observed by high-speed cameras, which confirms the necessity and rationality of size effect in the inversion analysis of cavitation pits. [ABSTRACT FROM AUTHOR]