We report a systematic experimental study on the impingement of two eccentric microjets at low to moderate Weber numbers. By conducting high-speed imaging and parametric scans, we identify several distinct film instability modes and construct a Weber number (We)-eccentricity (B) phase diagram to delineate transitions from closed to open films. In contrast to centric collisions, where jets are aligned, we find that eccentric impingement, with the jet axes offset, produces thinner films under comparable We. A momentum-based tilting model that incorporates a parabolic jet velocity profile, which reproduces measured film deflection angles and suggests the jets remain partially unrelaxed before collision. By adopting an equivalent merged-jet radius in Rayleigh-Plateau instability (RPI) scaling, we accurately predict the observed monodisperse droplet generation frequencies. These findings shed light on fluid dynamic mechanisms in eccentric microjet impingement and inform strategies for producing self-refreshing thin liquid films for liquid target or mirror applications.

Nd:YAG solid-state lasers are considered a promising candidate to replace CO2 lasers as the driver laser for extreme ultraviolet (EUV) light sources. Laser-produced plasma (LPP) EUV driver lasers must simultaneously achieve higher than 10 kHz high repetition rate, hundred-millijoule pulse energy, ten-nanosecond pulse width, good beam quality, and long-term operational stability. Achieving these specifications imposes significant technical challenges, particularly in managing waste heat and suppressing thermally induced distortion at elevated temperatures. In response to the requirements of LPP EUV driver lasers, a master oscillator power amplifier (MOPA) architecture is adopted, employing an end-pumped Nd:YAG slab gain module with a zig-zag propagation path as the power amplifier. Short-pulse extraction is achieved through triple-pass amplification in a single slab combined with temporal pulse sequencing. Thermal distortion is mitigated using microchannel coolers laterally bonded to the slab surfaces. A 4f imaging relay system is implemented to image the beam profile, minimizing diffraction-induced intensity modulation during propagation. Based on a laser kinetic model, numerical simulations are carried out to compare the amplification behavior of ∼10 ns short pulses with that of long-pulse or continuous-wave (CW) signals. Output power at various repetition rates is also calculated. A laser system is subsequently constructed, producing laser pulses with a duration of 7.8 ns and an average output power exceeding 1500 W across repetition rates from 10 to 50 kHz. At 10 kHz, an average output power of 1507 W is achieved, corresponding to a peak power of 19.28 MW, and the beam quality factors in the slab thickness and width directions are 1.98 and 3.23, respectively. The single-slab configuration results in a compact overall laser structure, making it suitable as the main-pulse source for benchtop EUV systems. Using this laser as the driver to irradiate liquid metal droplets, the spreading and propulsion velocities of the droplets under various single-pulse energies are experimentally compared.

Noncontact-actuated drop-on-demand (DOD) jetting isolates the actuator from harsh liquid environments, improving reliability and material compatibility. Here, we develop a lead zirconate titanate (PZT)-connector-tube-nozzle indirect axial-vibration architecture and clarify how the architecture transmits and reshapes PZT-side vibration into nozzle motion from low-frequency pulsed actuation to high-frequency periodic excitation. Under low-frequency single-pulse drive, experiments and simulations show that coupled residual nozzle vibration and meniscus oscillation can trigger multiple ejection events; introducing a precisely delayed sub-pulse suppresses the residual nozzle motion and enables single-ejection operation under a single-pulse command. Under high-frequency periodic excitation, the system approaches a steady-state forced-vibration regime. Time- and frequency-domain analyses reveal that, in the PZT-connector-tube-nozzle architecture, the nozzle response differs from the PZT-side vibration because of connector-assisted dynamic transmission. The assembled architecture exhibits a dominant response near 14 kHz, where the nozzle displacement is similar to 2.8 times that of the PZT, accompanied by attenuation of non-dominant harmonic components. This resonant amplification reduces the required drive to reach the jetting threshold and improves droplet generation for challenging fluids with high surface tension and/or high viscosity. With coordinated control of voltage, frequency, and liquid supply pressure, the device achieves reproducible droplet generation for a glycerol/ethanol (1:2, v/v) mixture and enables Galinstan droplet formation under a nitrogen atmosphere. These results provide transferable design and waveform-guidance for noncontact DOD jetting in functional material printing and extreme-environment droplet generation.

Meniscus guided printing (MGP) is a promising technique for fabricating a large area active layer of organic photovoltaic cells (OPVs). The flexible microcomb printing (FMCP) is a particular type of MGP that offers high shear rates and relaxed requirements on the gap control. However, printing large film with good thickness uniformity remains a challenge for FMCP. This work systematically studies the hydrodynamics of the film formation and deposition process by FMCP, and provides two optimization ideas: the flow rate automatically regulated by the back pressure, and the internal chamfer that stabilizes the contact line. Combined with the evaporation convection mechanism, the effects of different parameters on the morphology of the film are studied, and the hydrodynamic causes of the formation of stripes in the process of evaporation deposition are clarified. The power conversion efficiency (PCE) of the bulk heterostructure of the donor/acceptor device made by FMCP is 16.72%, and the PCE of the layer-by-layer printed device by FMCP is further improved to 16.93%. This work shows that the improved FMCP is an efficient, robust, and scalable film printing technology.

We experimentally and numerically investigated the hydrodynamics, fragmentation mechanisms, and debris distribution arising from the interaction of nanosecond laser pulses with a gallium-indium-tin (Ga-In-Sn) liquid film of micron-scale thickness. High-speed stroboscopic shadow photography was employed to visualize the splash crown and ejection of debris. The velocities of this debris, ranging from 329 to 4211 m s−1, were found to scale with laser pulse energy (Ep = 0.9–36 mJ) and film thickness (h) according to U ∝ E 5/9 /h. This velocity was accurately described by a modified ablation and propulsion model. The numerical simulations provided insights into the underlying physics, including the expansion of high-pressure plasma zone, shock wave propagation, and the formation of significant negative pressure regions conducive to cavitation. Furthermore, the direction of minimal debris deposition is found to align with peak plasma luminous intensity, which is normal to the liquid film.

We experimentally study the blast of a millimeter-sized drop of water dyed red subjected to a tightly focused nanosecond 532 nm laser pulse. The red water drop is opaque to the green laser wavelength, but is transparent to the red illumination, which allows the detailed visualization of the phenomena inside the drop. The laser induced plasma or localized boiling at the drop front surface leads to a splash crown originating from the blast center. The various stages of this physical process are analyzed based on four characteristic time scales with different focal point positions and power densities. The initial blast on the drop front surface sends spherical shock waves into the drop. The spherical drop surface focuses the reflected rarefaction waves to the rear side of the drop, inducing cavitation bubbles as well as the ejections of liquid jets on the rear end. Various laser energy densities lead to different splash-crown-modes, which in turn affect the response of the opaque drop at the capillary time scale. The temporal evolution of the splash crown waist diameter follows a power law with respect to time, resembling characteristics of craters from mechanical impacts. Through the scaling law of the splash crown growth, we highlight the similarities among different splash processes that are triggered by various methods of point energy deposition.

The rapid evolution of conformal electronics necessitates advancements in high-resolution printing on three-dimensional curved insulating surfaces directly. A recently developed technique, known as plasma-induced electrohydrodynamic (PiE) printing, is a promising technique that enables micron and submicron scale printing on arbitrary dielectric substrates. However, factors that govern the deposited charges and the effects of the constructed electric field on induced jetting behavior have not been fully comprehended. Here, we investigate the role of plasma-surface interaction in PiE printing and its impact on the printing process. The local electric field, constructed by site-selective deposition of positive charges on the dielectric surfaces through plasma jet impingement, forms a spatial region of effective influence. The geometrical and electrical parameters of the PiE printing system significantly influence the spreading range and intensity of plasma on the surface and hence the effective region, and therefore determines the induced ink jetting behavior and the jets/droplets traction and deflection. Our study reveals the relationship between the plasma jet and inkjet dynamics in PiE printing, providing insights into the interplay between the plasma spreading behavior and process parameters. The optimization of these parameters can enable better performance of high-quality PiE printing.

We report an experimental investigation on the explosion of a liquid microjet induced by nanosecond laser. The jet is periodically perturbed by a piezoelectric actuator to generate highly controllable jet pinch-off. The laser is introduced co-axially and propagates through the liquid jet by total internal reflections. The pinch-off region serves as a light funnel to confine and concentrate the laser beam, and the optical power flux may exceed the threshold to induce plasmas and explosions. The explosive phenomenon evolves over four different time scales spanning from 10 ns to 100 mu s. The growth of the explosion gap follows power laws behavior, while the growth of the mist cloud diameter remains linear with respect to time, both of which can be described by models relating plasma volume to deposited energy.

Printing of electrodes to replace thermal evaporation of metals for back contacts in perovskite solar cells (PSCs) is essential for scalable manufacturing. However, PSCs incorporating printed electrodes typically exhibit lower power conversion efficiencies (PCEs) than those with evaporated metals. Low-melting-point alloys (LMPAs) are promising candidates for PSC electrodes due to their matched work functions, high electrical conductivities, and chemical stability. This study proposes a convenient strategy of blade printing to pattern In-Sn-Bi LMPAs as back electrodes in inverted PSCs. These LMPAs, with moderate melting points (62 degrees C, 80 degrees C, and 120 degrees C), are printed above their melting points and solidify at room temperature without additional post-treatment. PSCs with LMPA electrodes show high built-in potential and fast charge extraction, achieving PCEs of 22.48%, comparable to their evaporated-metal counterparts. Charge transport and recombination dynamics reveal that PSCs with LMPA electrodes are more stable than those with evaporated copper electrodes after aging in air without encapsulation. Morphological analysis of LMPAs and perovskite layers after aging shows no noticeable corrosion. PSCs with blade-printed LMPA electrodes retain similar to 80% of their peak PCE after 1500 hours of aging, demonstrating significantly higher stability than PSCs with evaporated copper or silver electrodes.

We found that a hemispherical water drop of radius of the capillary length lc suspended on a substrate can generate a pair of bursting jets after experiencing one cycle of oscillation. The drop of initial base radius R0 first undergoes the downward acceleration stage and reaches a peak velocity Vp with a characteristic Weber number We equivalent to rho Vp2R0/gamma. At the end of the downward acceleration, the drop is compressed into a pancake shape with the maximum spreading diameter that scales with We1/10. Subsequently, the acceleration is reversed to upward, pulling up a cylindrical cavity. The depth of the cavity increases with larger We. The axisymmetric cavity pinch-off is logarithmically slow and the variation of neck radius r with time tau exhibits the scaling law: root r proportional to tau 1/2e- -log tau/2(Eggers et al., 2007). The collapse of a small cavity does not trap any bubble, forming a single Worthington-type jet. The intermediate sized cavity collapses to form two opposite bursting jets, and the inner jet is absorbed by the enclosed bubble. The collapse of the sufficiently deep cavity produces two bursting jets that are self-similar in terms of jet radius and velocity. A phase diagram of We vs R = R0/lc is presented for categorizing the three different cavity collapse behaviors.

Top electrodes of organic photovoltaics (OPVs) are usually thermally evaporated in the vacuum, which is non-continuous and time-consuming and has been the bottleneck for the OPV fabrication process. Printable top electrodes that are free of vacuum, high temperature, and solvents will make OPVs more attractive. Low-melting-point alloys (LMPAs) are promising candidates for printable OPV electrodes thanks to the merits of matching work functions, high electron conductivity, high environment stability, and no need for post-treatment. Here, LMPA electrodes are directly deposited on OPVs by simply falling a single LMPA droplet onto the substrate. The LMPA droplet spreads to form a thin film with a smooth interface intimately contacting the substrate. The electrode area can be tailored by adjusting the droplet diameter or the Weber number, which is the ratio of inertia to surface tension. The interface morphology is mainly affected by the contact temperature. The degree of oxidation and charges on the droplet can also influence the electrode area and interface morphology. OPVs with droplet-impacted LMPA electrodes exhibit power conversion efficiencies of up to 16.17%. This work demonstrates the potential of single-droplet impact deposition as a simple method for printing OPV electrodes for scalable manufacturing.

The performance of organic solar cells is sensitive to the film thickness and the uniformity of the donor and acceptor distribution in the photoactive layer. Especially, in the quasi-planar heterojunction fabricated by layer-by-layer process, the donor and acceptor materials may be non-homogeneously distributed across the entire photoactive layer. Visualization of the spatial distributions of donor and acceptor components sheds light on the relationship between nanostructures and device performances. Based on the distinct infrared absorption characteristics of the organic donor and acceptor, a quasi-monochromatic light transmittance (QMLT) technique is proposed to quantify the donor and/or acceptor thickness in the photoactive layer blend across the entire device area with nanometer precision. The variation in thickness of donor and acceptor can be clearly and separately revealed. Accordingly, the impacts of spin coating techniques (such as dynamic or static dispensing, and on-center or off-center spin coating), as well as solvent choices, on film morphology and device performance can be correlated by the noncontact, nondestructive, and convenient QMLT method. A quasi-monochromatic light transmittance method is proposed to visualize the spatial thickness distribution of each component in a semitransparent blend film in a large area. This method provides an effective way to understand the relationship between active layer nanostructures and the performances of organic photovoltaics and sheds light on choices of processing methods and solvents for high-performance layer-by-layer processed devices.image

Electrohydrodynamic (EHD) printing enables large-area, ultra-high-resolution manufacturing across a broad range of ink viscosities, but inevitably encounters difficulties when printing on electrically insulating three-dimensional substrates due to unpredictable electric field and surface residual charges. To overcome these obstacles, a novel approach called plasma-induced electrohydrodynamic (PiE) printing has been proposed. PiE printing employs plasma to directly create a controllable local charge region directly on substrate surfaces, which triggers EHD ink ejection and mitigates the effect of residual charges. However, the underlying mechanisms of the jetting behavior with respect to printing parameters, such as the charge-induced electric field, remain unexplored. Here, we conduct a numerical investigation, based on the Taylor-Melcher leaky dielectric model and the level set method, on the jetting behavior of substrate surface charge-induced EHD printing. We first introduce the dynamics behavior throughout the entire printing process. Then, we carry out a comprehensive investigation on surface charge-induced EHD printing under four crucial parameters: the amount of preset surface charge, the radius of preset surface charge, the duration of preset surface charge, and liquid electrical conductivity. By analyzing the induced electric field, induced charge density, fluid velocity, jet diameters, and deposited droplet sizes obtained from the numerical results, we elucidate the influence of these parameters on the dynamic behavior, durations of jetting process, and printing quality. These findings offer valuable insights into surface charge-induced EHD jetting, advancing the understanding and optimization methods for this useful micro-/nano-manufacturing technology.

The details of the interaction of human thermal plume and breathing activities are simulated in the current study of an unsteady turbulent flow field in an elevator cabin. Air velocity and temperature distributions of the circulation flow pattern (i.e., the macroenvironment), the breathing-scale microenvironment's characteristics, and the thermal plume's fate are analyzed. The current study is aimed at showing how respiratory activities such as breathing and human thermal plumes affect the flow field and respiratory contaminants dispersion pattern in a nonventilated enclosed environment (the elevator cabin). The results from three cases, i.e., breathing thermal manikins, nonbreathing thermal manikins, and isothermal breathing manikins, are contrasted to unveil better the effects of human thermal plume and breathing on the flow field, including the velocity distribution, dispersion pattern of the exhaled contaminant, the human body's heat transfer coefficient, and the large-scale flow pattern. Results reveal that breathing inhalation increases the upward velocity of the thermal plume on the one hand, which directly affects the microenvironment and indirectly impacts the macroenvironment due to the more vigorous reflected thermal plume. On the other hand, the upward thermal plume reduces the penetration length of the exhaled jet. Breathing activities create ring vortices that connect the microenvironment and the macroenvironment. The circulation flow features a downward flow in the cabin's center, affecting the vortex strength and contaminant dispersion pattern. Overall, the human thermal plume and human breathing make comparable contributions to the resulting elevator-cabin flow characteristics.

Electronic thin films play a ubiquitous role in microelectronic devices and especially hold great promise for flexible electronics, energy conversion and storage, and biomedical applications. Their characterizations, including ultra-thin, large-scale dimensions, stretchability, and conformal ability to curved or 3D structures, present new challenges for thin film fabrication based on the solution method. Electrospray deposition emerges as a feasible method for fabricating large-area, flexible, and curved films. It offers many advantages such as material adaptability, controlled atomization, tunable film morphology, and shape retention on complex substrates. These advantages make it a key method for fabricating high-performance films on large-area, 3D surfaces. This work presents a comprehensive review of the mechanisms, processes, applications, and equipment of electrospray deposition. First, the fundamental principles of electrospray deposition are introduced, focusing on the mechanisms and scaling laws of liquid atomization. Moreover, the control methods for electrospray modes, structures, and film morphology are discussed. These advanced control methods pave the way for the fabrication of smart skins, wearable devices, and energy conversion and storage components. Finally, this work introduces three types of electrospray deposition manufacturing equipment to illustrate the advantages of electrospray deposition for large-area, and 3D surface manufacturing. Electronic thin films are pivotal in microelectronics, particularly in flexible electronics, energy storage, and biomedical fields. Challenges arise in their fabrication due to characteristics like ultra-thinness, scalability, and adaptability to curved surfaces. Electrospray deposition emerges as a viable solution, offering controlled atomization and tailored film morphology. This review delves into its mechanisms, applications, and equipment, showcasing its potential for large-area and 3D surface manufacturing.image

Precisely controlling the optical characteristics of the microlens array (MLA) is a key issue for the MLA's fabrication and its applications. A cost-effective method for fabricating MLA with different curvatures and diameters by surface functionalization and electrohydrodynamic inkjet (E-jet) printing is proposed. The surface with self-assembled monolayers has low surface energy, which greatly improves the contact angle of the microlens. The UV-ozone treatment and neutral density filters with different optical densities are used to control the wettability of the substrates. Based on this method, the curvatures of the microlens can be controlled easily and the numerical aperture (NA) of the microlens can be adjusted from 0.18 to 0.49. Compared with traditional fabrication techniques, the method we demonstrated is flexible, low-cost, rapid, and capable of fabricating MLA with large area, high packing density, desirable NA, and diameters.

In contrast to the existing research on the liquid jet colliding with a physical target to produce a single type of liquid sheet or bell, we used an opposing gas jet target to generate a series of diverse shapes of axisymmetric liquid films. Specifically, we experimentally investigated the outcome of a 0.4-mm diameter water jet impinging on an opposing gas jet of 0.3 mm initial diameter. Five different axisymmetric film shapes are identified for the experimental parameters explored in this work, and the momentum flux ratio (X) of the gas jet to the water jet is the dictating factor for the exact film shape: (i) X = 0: smooth bells that share the same shape and profile model with classic water bells of a liquid jet impacting a solid target. (ii) 0 < X < 0.4: crumpled bells being lifted off from the gas nozzle by a thin layer of squeezing gas flow that is quantifiable by the lubrication model. (iii) 0.4 < X < 0.9: transient bells with mildly wrinkled surfaces that undergo periodic cycles from closure to bursting at a gas Reynolds number of ∼2400. (iv) 0.9 < X < 1.4: turbulent parachutes with curved and rough shapes. (v) X > 1.4: turbulent sheets that are flattened and atomized by the relatively strong gas stream. This work demonstrated that an opposing gas jet is a simple yet versatile approach to achieving a rich collection of phenomena by diverting the liquid jet momentum in multiple ways. This investigation aims to explore and rationale the rich phenomenology of water bells resulting from the impingement of a water jet on a gaseous jet target.

The dynamic response of a micrometer-sized gallium–indium (Ga-In) jet in nitrogen subjected to intense Neodymium-doped Yttrium Aluminum Garnet (Nd: YAG) laser pulses with energy ranging from 0.35 to 5.0 mJ per pulse is investigated experimentally. The rapid deformation of the jet was visualized using timed-delayed stroboscopic shadowgraphy. The laser pulse disrupts the jet to form a gap, and the length of the gap grows according to a logarithm relationship with respect to the dimensionless time normalized a characteristic timescale τ, which is determined by the pulse energy Ep. The ablation impulse bends and flattens the jet into a thin curved film that resembles a wind-blown sail. The area of the sail increases with t6/5Ep13/15, where t is time. The sail eventually breaks up into fine mist. Additionally, we found that the laser-blast-induced initial bending velocity of the jet could be predicted using the semi-empirical laser-ablated propulsion model for an In-Sn droplet of tens of micrometers.