Ejection Regimes Research Articles

Numerical study on the influence of fluid properties in constant-voltage electrohydrodynamic pulsating jets

The pulsating jet is a common working mode in electrohydrodynamic printing (EHDP), and this process is highly influenced by operating parameters and material properties. In this paper, we investigated the behavior of pulsating jets in liquids with varying physical properties through numerical simulations. We established an electrohydrodynamic (EHD) solver and employed a charge flux restriction step to ensure a realistic distribution of free charges. Our simulations revealed three different ejection regimes: an oscillating cone (OC), a choked jet (CJ), and a stable cone–jet (SJ). We found that the ejection regime is primarily determined by three dimensionless numbers related to liquid properties: the Ohnesorge number, Q0εr/Q, and Q0/(QRe). Based on these dimensionless numbers, we analyzed the influence of liquid properties on pulsating jets in OC and CJ. In OC, the jet's breakage is mainly attributed to the significant oscillation of the Taylor cone, a phenomenon primarily influenced by viscosity and conductivity. In CJ, the emission of the jet is terminated due to the excessive resistant force in the cone–jet transition region. For liquids with low to medium viscosity, the dominant resistant force is either the polarization force or the viscous force depending on whether εrRe is larger or smaller than 1, respectively. In the cases of high viscosity liquids, the viscous force always plays a major role as the primary resistance. These findings provide deeper insight into the physical mechanisms of pulsating jets.

Numerical and experimental investigation of the melt removal mechanism and burr formation during laser cutting of metals

During laser fusion cutting, burr forms when the molten metal does not sufficiently exit the interaction zone. When it forms on the lower edge of the cut flank, burr becomes a factor limiting quality. Previous research has shown that a temporally regular and spatially localized melt flow can prevent the formation of burr. However, the high dynamics of the subprocesses involved can cause intrinsic instabilities that disrupt the flow and reduce the efficiency of the melt ejection. This paper presents a study on the correlation between process parameters, melt flow properties, and burr formation. It includes an experimental observation of the melt-flow dynamics using high-speed videography. In addition, a Computational Fluid Dynamics model was set up to examine fundamental flow properties, some of which are not observable experimentally. The dependency of the burr formation on the liquid Weber and Reynolds numbers is analyzed, and it is demonstrated how the magnitude and allocation of vapor pressure gradients in the kerf decisively affect melt ejection and burr formation. Additionally, a previously unknown melt ejection regime is identified in the thick section range, which occurs at feed rates close to the maximum cutting speed under specific high-power process conditions. This regime is characterized by a significantly increased process efficiency that could open up a new high-speed process window.

Numerical modeling to predict threshold fluence for material ejection in Laser-Induced forward transfer of metals

Laser-induced forward transfer (LIFT) uses a pulsed laser beam to propel material from a donor (containing a glass coated with a thin material film) onto a receiving substrate, resulting in pixellated material deposition. The deposition characteristics depend on the material ejection modes that vary with film thickness and laser parameters. This study develops a computational model based on the finite volume method for LIFT of gold films using nanosecond pulses, which captures two different ejection modes for droplet depositioncap ejection and jet ejection. The model computes the temperature distribution and predicts potential ejection regimes for different film thicknesses, providing an understanding of the material removal process. Cap ejection occurs at lower laser fluences when the entire film thickness in the irradiated zone is in the molten phase. In comparison, jet ejection occurs at higher laser fluences caused by vapor pocket formation at the glass-film interface. The model predicts threshold fluences with greater than 90% accuracy for film thickness less than 583 nm. However, for the film thickness of more than 583 nm, the simulations underpredict the threshold fluence, suggesting that laser absorption by the film decreases due to vapor formation at the glass-film interface. Besides, it is observed that higher fluences can cause melting and vaporization of the glass at the interface leading to possible contamination of the deposited material. The proposed model can be used to choose the operating window for laser parameters for droplet deposition in LIFT, which otherwise is challenging using experiments.

Axisymmetric gravity-capillary standing waves on the surface of a fluid

We conduct an experimental study of axisymmetric gravity-capillary standing waves generated by a vertically vibrating ring on a fluid surface. Different regimes of standing waves are highlighted at the basin center: linear, nonlinear, and ejection regimes. For moderate forcing, the spatial profile of standing waves breaks the up-down symmetry as predicted by a third-order nonlinear theory, whereas for stronger forcing, the maximum height reached by the axisymmetric gravity-wave crest, at the basin center, is found to increase linearly with its wavelength, as a consequence of the saturation of its steepness.

The effect of weak boundary layers on adhesion properties of laser pretreated aluminum alloy EN-AW 6082 surfaces

This study investigates the influence of the laser ablation regimes and the material removal mechanisms on the morphologies, topographies, and chemistries of laser formed surface structures on the aluminum alloy EN-AW 6082. The characteristics of these surface structures are correlated to the adhesion and durability of joints bonded adhesively with PA6. X-ray photoelectron spectroscopy showed that the chosen laser ablation regimes have almost no influence on surface chemistries while microscopic analysis indicated that the surface structures are affected significantly. The adhesion properties and durability of the adhesively bonded joints studied by single lap shear and aging tests are strongly affected by the ablation regimes. Optimal initial shear strengths and aging resistances were obtained for a single step laser treatment with parameters corresponding to the melt displacement mode, while multi-step treatments provided no further benefits. A failure analysis revealed that the micro- and nanostructures developed in melt ejection regime of laser ablation can act as weak boundary layers (WBLs) on metal substrates and affect adversely the adhesion and durability of the joints.

Elucidating ejection regimes of metal microdroplets in voxel-based laser-induced forward transfer

Voxel-based laser-induced forward transfer (VB-LIFT) is a promising micro additive manufacturing process to efficiently print microstructures by ejecting a metal microdroplet array. Elucidating the ejection regimes of microdroplets is crucial to manipulating the printing process. This study investigates the ejecting behaviors of microdroplets induced by irradiating a femtoliter metal voxel with an ultraviolet nanosecond laser. A time-resolved dark-field imaging system was developed to capture the ejecting microdroplets. The ejecting velocity increased linearly with the laser fluence but was independent of the voxel side length. The analysis of the laser energy conversion revealed that one-thousandth of the laser energy was converted to the kinetic energy of the ejecting microdroplets, and the majority of the remaining were converted into internal energy. Meanwhile, the relaxation of the thermal compressive stress induced by laser heating drove the metal voxel detaching from the donor glass to generate a microdroplet. Furtherly, the ejection map in the VB-LIFT is proposed, including the non-detaching, detaching without melting, detaching and melting, and broken regimes. This study provides insights to eject debris-free metal microdroplets with the desired temperature and velocity in laser-induced forward transfer. • We develop a dark-field imaging system to capture the ejecting metal microdroplets. • Ejecting velocity is proportional to laser fluence but independent of voxel length. • Metal voxel detaching is driven by thermal compressive stress due to laser heating. • An ejection map containing four regimes is proposed for the VB-LIFT process.

Quantum reflection of dark solitons scattered by reflectionless potential barrier and position-dependent dispersion.

We investigate theoretically and numerically quantum reflection of dark solitons propagating through an external reflectionless potential barrier or in the presence of a position-dependent dispersion. We confirm that quantum reflection occurs in both cases with a sharp transition between complete reflection and complete transmission at a critical initial soliton speed. The critical speed is calculated numerically and analytically in terms of the soliton and potential parameters. Analytical expressions for the critical speed were derived using the exact trapped mode, time-independent, and time-dependent variational calculations. It is then shown that resonant scattering occurs at a critical speed, where the energy of the incoming soliton is resonant with that of a trapped mode. Reasonable agreement between analytical and numerical values for the critical speed is obtained as long as a periodic multisoliton ejection regime is avoided.

Simulation of the Scalar Transport above and within the Amazon Forest Canopy

The parallelized large-eddy simulation model (PALM) was used to understand better the turbulent exchanges of a passive scalar above and within a forested region located in the central Amazon. Weak (2 ms−1) and strong (6 ms−1) wind conditions were simulated. A passive scalar source was introduced to the forest floor for both simulations. The simulations reproduced the main characteristics of the turbulent flow and of the passive scalar transport between the forest and the atmosphere. Noteworthily, strong and weak wind conditions presented different turbulence structures that drove different patterns of scalar exchange both within and above the forest. These results show how passive scalar concentration is influenced by the wind speed at the canopy top. Additionally, higher wind speeds are related to stronger sweep and ejection regimes, generating more intense plumes that are able to reduce the passive scalar concentration inside the forest canopy. This work was the first that used PALM to investigate scalar transport between the Amazon rainforest and the atmosphere.

Monitoring of high-speed laser welding process based on vapor plume

The increase of welding speed can improve the productivity of laser welding. The behavior of vapor plume is an important indicator reflecting the state of the laser welding process. However, the behavior of vapor plume in high-speed laser welding (whose welding speed is higher than 10 m/min) has not been studied adequately. In this research, images of the vapor plume in high-speed laser welding of SUS304 austenitic stainless steel are captured through high-speed imaging for the monitoring of the welding process. Characteristics of the vapor plume are extracted with image processing, and the influences of laser power and welding speed on these characteristics are analyzed. The relationships between the behavior of vapor plume and melt pool are discussed, and three ejection regimes of the vapor plume are proposed. In order to predict the occurrence of humping, which is a typical defect in high-speed laser welding, samples are built by applying a time sliding window and then a classification model is built with the random forest. In addition, the importance of features in the classification is analyzed. The test results show that the proposed method can predict the humping defect in high-speed laser welding accurately.

From macroscale to microscale characterization of droplet breakup regimes using a micromachined ultrasonic droplet generator

Controllable, uniform droplet generation is critical to many industrial processes and biomedical applications (e.g., materials synthesis, cell handling, and aerosol drug delivery). We have developed an ultrasonic droplet generator that can enable dynamic control over the droplet size distribution in the 2–75 μm range. The system uses a bulk piezoelectric transducer to generate a standing acoustic pressure field that drives fluid transport through an array of microscopic orifices. Our previous work suggests that certain resonances lead to variability in the pressure gradient at the orifices, affecting the droplet size distribution of the resultant spray. Here, we present a parametric study relating ejection uniformity to orifice size, operating frequency, and drive amplitude. We introduce a new system configuration that allows high-resolution stroboscopic imaging of the liquid-gas interface evolution at each orifice. Although the entire micronozzle array remains acoustically active, we use orifice size to control the flow resistance, restricting ejection to a single preselected orifice. Observed ejection regimes resemble classical descriptions of jet breakup (Rayleigh, wind-induced, and atomization), depending on drive voltage. A regime map is created using 5, 10, and 15 μm orifices for 0.5–2.0 MHz actuation from threshold ejection amplitude to the point of chaotic wind-induced breakup.

Printability regimes of pure metals using contactless magnetohydrodynamic drop-on-demand actuation

We demonstrate a computational study used to evaluate drop-on-demand printability of liquid metals via a contactless magnetohydrodynamic (MHD) pumping method. We show that the ejection regimes of pure liquid metal droplets can be categorized using two dimensionless quantities: We and a new dimensionless quantity S=Ha2Ca. By plotting We vs S, a linear relationship emerges which relates the velocity through the ejection orifice to the applied magnetic flux density. Additionally, satellite-free droplet generation is shown to be bounded by the ranges 1000≲S≲2000 and 10≲We≲20. These ranges, coupled with the linear We vs S relationship, allow one to predict the critical magnetic flux necessary to eject a satellite-free liquid metal droplet for any liquid metal with a very low viscosity to surface tension ratio (Oh<0.005). We discuss the physics underlying the MHD ejection process and relate the pump action to the dimensionless quantities. We use an MHD finite element model to parametrically sweep through applied magnetic fields and explore two-phase ejection of Al, Cu, Fe, Li, Sn, Ti, Zn, and Zr droplets from a 200 μm orifice. The model is validated using experimental high speed video ejection of Zn and Al, and the reported relationship between We and S can be used to connect the input flux density to the resulting ejection regime.

Investigation on the melt ejection and burr formation during laser fusion cutting of stainless steel

When the melt is ejected from the cut kerf by an assist gas during laser fusion cutting, a fraction of it may adhere to the lower cut edge and form a burr. Adjusting the process parameters to minimize burr formation is a challenging task that becomes more and more difficult as sheet thickness increases. The burr length has significant relevance when the quality of a cut is evaluated, but the underlying mechanisms that control its formation are still not fully understood. In this paper, new results of an experimental investigation focused on identifying characteristic melt ejection regimes are presented. The experiments were conducted on 6 mm thick stainless steel sheets using a disk laser at a power of up to 10 kW. The melt flow exiting the kerf channel and the temporal formation of burr were analyzed using two high-speed cameras. The dependency of melt ejection regimes from process parameters as well as their influence on burr formation is discussed. Furthermore, the authors demonstrate that melt ejection forming a compact and stable threefold outflow is a characteristic property of a burrfree cut.

Tubular catalytic micromotors in transition from unidirectional bubble sequences to more complex bidirectional motion

The generation of oxygen microbubbles in catalytic microtubes has attracted tremendous attention towards the exploration of unidirectional and overloaded bubble ejection regimes, leading to simple and more complex motions of micromotors. While it is widely believed that a bubble's frequency in a unidirectional regime (i.e., a bubble ejected from a single tubular opening) is random, this study shall demonstrate that periodic oxygen bubble frequencies and sequences can be experimentally controlled using various concentrations of hydrogen peroxide fuel and surfactants. When released from a substrate, unidirectional micromotors self-propel in straight, circular, and helical trajectories, leading to a class of well-predictable or simple micromachines. Under overloaded conditions, micromotors generate bubbles at both tubular openings, which influence the trajectories of micromotor motion strongly. A one-dimensional reaction-diffusion equation is formulated to explain the possible mechanisms of mass transport in microtubes and the transition from the unidirectional to the overloaded regime of micromotors.

Direct and opposite droplet ejections from metal films induced by nanosecond laser pulses: experimental observation and lattice Boltzmann modeling

Laser-induced forward transfer is a versatile method for the fabrication of surface microstructures. In this paper, the material ejection from copper films induced by nanosecond laser pulses was studied. Two reversely oriented ejections were induced simultaneously by a single laser shot. The opposite ejected droplets were significantly smaller than the direct ejected ones, and were separated from the remnant donor material clearly, which might be utilized to fabricate microstructures on the donor substrates. Three distinct ejection regimes: no ejection, stable ejection and sputtering were characterized with respect to the laser intensity. The thermal field and the phase transition within the donor film were analyzed with a finite element model. Correlations between the phase transitions and the ejection regimes were noticed, suggesting that the material ejection was mainly caused by the cavitation effect. The dynamics of the laser-induced cavitation bump and the formation of the jets were simulated with a vapor–liquid lattice Boltzmann model. The formation of the bump and the material ejections were clearly demonstrated. It was concluded that the jets were mainly induced by the collision of the cumulating flows, which were generated within the bump shell by the asymmetrical cavitation of the bump. The dynamics of the vapor also plays an important role in the development of the jets. The simulation results agree well with the present experimental observations.

Wind tunnel observations of weak and strong snow saltation dynamics

AbstractTheoretical considerations suggest that saltation dynamics is dominated by either aerodynamic entrainment or by a combination of ejection and rebound at a given time and location. Calling these two regimes “weak” and “strong” saltation, respectively, we have investigated high‐resolution snow mass flux measurements by shadowgraphy in a cold wind tunnel to determine whether these two regimes can be experimentally reproduced. In this contribution, we first suggest that aerodynamic entrainment should lead to a negative correlation in the growth rates of horizontal versus vertical mass fluxes, while for the ejection regime, a positive correlation is plausible. Based on this criterion, we find evidence of weak and strong saltation in our data. Weak saltation is characterized by smaller peaks of horizontal mass flux; however, these peaks have a higher growth rate and therefore are typically narrow. The larger peaks are associated with strong saltation, are wider, and their frequency of occurrence increases with increasing overall saltation mass flux.

Energy‐ and momentum‐conserving model of splash entrainment in sand and snow saltation

AbstractDespite being the main sediment entrainment mechanism in aeolian transport, granular splash is still poorly understood. We provide a deeper insight into the dynamics of sand and snow ejection with a stochastic model derived from the energy and momentum conservation laws. Our analysis highlights that the ejection regime of uniform sand is inherently different from that of heterogeneous sand. Moreover, we show that cohesive snow presents a mixed ejection regime, statistically controlled either by energy or momentum conservation depending on the impact velocity. The proposed formulation can provide a solid base for granular splash simulations in saltation models, leading to more reliable assessments of aeolian transport on Earth and Mars.

Time-resolved imaging of flyer dynamics for femtosecond laser-induced backward transfer of solid polymer thin films

We have studied the transfer regimes and dynamics of polymer flyers from laser-induced backward transfer (LIBT) via time-resolved shadowgraphy. Imaging of the flyer ejection phase of LIBT of 3.8μm and 6.4μm thick SU-8 polymer films on germanium and silicon carrier substrates was performed over a time delay range of 1.4–16.4μs after arrival of the laser pulse. The experiments were carried out with 150fs, 800nm pulses spatially shaped using a digital micromirror device, and laser fluences of up to 3.5J/cm2 while images were recorded via a CCD camera and a spark discharge lamp. Velocities of flyers found in the range of 6–20m/s, and the intact and fragmented ejection regimes, were a function of donor thickness, carrier and laser fluence. The crater profile of the donor after transfer and the resulting flyer profile indicated different flyer ejection modes for Si carriers and high fluences. The results contribute to better understanding of the LIBT process, and help to determine experimental parameters for successful LIBT of intact deposits.

Impact-driven ejection of micro metal droplets on-demand

On-demand metal droplet deposition will be a cornerstone technology in 3D metal printing. However, suitable small nozzles are hardly available, limiting the resolution and surface finish of final products. Here, the ejection of record-small metal droplets with a diameter of only 0.55±0.07 times the nozzle diameter was demonstrated. To this end, a novel metal drop-on-demand (DoD) generator for high-temperature metal processing was designed and manufactured. A metal rod was utilized to transfer a vibration pulse, which was required to eject a liquid droplet, from a low-temperature region to the high-temperature liquid metal close to the nozzle. The influence of the pulse characteristics on the droplet ejection regime was studied experimentally and numerically. A 2D axisymmetric numerical model revealed that the shorter pulses allow reducing the droplet size, with the pulse duration of 13μs resulting in the smallest feasible droplets. A novel method to create such short pulses, by impacting the metal-ring connected rod with a solid impactor was manufactured and tested, and the benefits of this method over more the spring-type pulse transfer was experimentally confirmed. This research provides a feasible way to achieve ejection of the small metal droplet on-demand.

Eggs and milk: Spinning spheres partially immersed in a liquid bath

When a hard-boiled egg spins through a pool of milk on the kitchen counter, the milk rises up the sides of the egg and droplets are ejected. This phenomenon occurs when any partially submerged object whose radius increases upward from the fluid surface (e.g., spheres, inverted cones, and rings) spins in a liquid bath. The fluid ejects from the surface near the maximum radius in one of three ejection modes: jets, sheets, or sheet breakup. Additionally, a surprisingly large flow rate is induced by the spinning object. In this study, we used spheres to determine the effects of experimental parameters on the induced flow rate. We characterized the modes of ejection and measured the sheet breakup distance using high-speed imaging. The basis of our closed form analytical model utilizes an integral momentum boundary layer analysis both beneath the free surface and in the thin film attached to the sphere. We present criteria defining the transitions between ejection modes and the radius where liquid sheets break up in the sheet ejection regime. Criteria defining the transitions between ejection modes and the radius where liquid sheets break up in the sheet ejection regime shows good agreement with experiments.

Ejection Regimes in Picosecond Laser-Induced Forward Transfer of Metals

Laser-induced forward transfer (LIFT) allows for high-resolution three-dimensional printing of e.g. small metal droplets, for which inkjet printing is not an option. Although the technique is almost 30 years old, a detailed understanding of its physics has been lacking. The authors observe and explain the high-speed mechanisms behind the ejection of pure metal microdrops during LIFT, which further allows parameter optimization for a particular job.