Drop Ejection Research Articles

Pneumatic conveying inkjet bioprinting for the processing of living cells.

Inkjet printing techniques are often used for bioprinting purposes because of their excellent printing characteristics, such as high cell viability and low apoptotic rate, contactless modus operandi, commercial availability, and low cost. However, they face some disadvantages, such as the use of bioinks of low viscosity, cell damage due to shear stress caused by drop ejection and jetting velocity, as well as a narrow range of available bioinks that still challenge the inkjet printing technology. New technological solutions are required to overcome these obstacles. Pneumatic conveying printing, a new type of inkjet-based printing technique, was applied for the bioprinting of both acellular and cellular fibrin-hydrogel droplets. Drops of a bioink containing 6 × 106 HEK293H cells/ml were supplied from a sterile nozzle connected to a syringe pump and deposited on a gas stream on a fibrinogen-coated glass slide, here referred to as biopaper. Fibrinogen film is the substrate of the polymerization reaction with thrombin and Ca2+ present in the bioink. 
The pneumatic conveying printing technique operates on a mechanism by which drop ejection and deposition in a stream of gas occurs. The percentage of unprinted and printed dead HEK293H cells was 5 ± 2 % and 7 ± 4 %, respectively. Thus, compared to normal handling, pneumatic conveying printing causes only little damage to the cells. The velocity of the drop approaching the biopaper surface is below 0.2 m/s and does not cause any damage to the cells. The cell viability of printed cells was 93%, being an excellent value for inkjet printing technology. The HEK293H cells exhibited approximately a 24 h lag time of proliferation that was preceded by intense migration and aggregation. Control experiments proved that the cell migration and lag time were associated with the chemical nature of the fibrin hydrogel and not with cell stress. &#xD.

Drop-on-demand bioprinting: A redesigned laser-induced side transfer approach with continuous capillary perfusion

We present a drop-on-demand (DOD) bioprinting method based on a novel implementation of laser-induced side transfer (LIST). Our approach involves continuous bioink perfusion through a glass capillary featuring a laser-machined hole in the capillary wall, serving as a nozzle. Focused low-energy nanosecond laser pulses are employed for precise droplet ejection. This innovative design separates the control of the bioink flow rate inside the capillary from the printing rate (drop ejection), leading to an enhanced printing workflow. We assessed the impact of key printing parameters, such as laser energy and flow conditions, on printing quality. Furthermore, we utilized the redesigned LIST to bioprint human umbilical vein endothelial cells (HUVECs). Our findings indicate that the printed HUVECs exhibit no viability loss and demonstrate the ability to recruit perivascular cells, including pericytes and fibroblasts. The redesigned LIST can be utilized in tissue engineering applications requiring DOD cell printing.

Synchrotron X-ray phase-contrast imaging of ultrasonic drop atomization

Ultrasonic atomization is employed to generate size-controllable droplets for a variety of applications. Here, we minimize the number of parameters dictating the process by studying the atomization of a single drop pending from an ultrasonic horn. Spatiotemporally resolved X-ray phase-contrast imaging measurements show that the number-median sizes of the ejected droplets can be predicted by the linear Navier–Stokes equations, signifying that the size distribution is controlled by the fluid properties and the driving frequency. Experiments with larger pendant water drops indicate that the fluid–structure interaction plays a pivotal role in determining the ejection onset of the pendant drop. The atomization of viscoelastic drops is dictated by extended ligament formation, entrainment of air, and ejection of drop-encapsulated bubbles. Existing scaling laws are used to explain the required higher input amplitudes for the complete atomization of viscoelastic drops as compared to inviscid drops. Finally, we elucidate the differences between capillary wave-based and cavitation-based atomization and show that inducing cavitation and strong bubble oscillations quickens the onset of daughter drop ejection but impedes their size control.

Effect of surface viscoelasticity on top jet drops produced by bursting bubbles.

Jet drops resulting from bubble bursting at a liquid surface play a key role in various mass transfer processes across the interface, including sea spray aerosol generation and pathogen transmission. However, the impact of structurally compound interfaces, characterized by complex surface rheology introduced by surface-active contaminants, on the jet drop ejection still remains unclear. Here, we experimentally investigate the influence of surface viscoelasticity on the size and velocity of the top jet drops from surface bubble bursting, examining both pure protein and mixed protein-surfactant solutions. We document that for bubble bursting at a pure-protein-laden surface where surface elasticity dominates, the increase in Ec, i.e. the interfacial elastocapillary number as the ratio between the effects of interfacial elasticity and capillarity, efficiently increases the radius and decreases the velocity of the top jet drop, ultimately inhibiting the jet drop ejection. On the other hand, considering the mixed protein-surfactant solution, we show that the top jet drop radius and velocity exhibit a different variation trend with Ec, which is attributed to the additional dissipation on the capillary waves as well as the retardation and resistance on the converging flow for jet formation from surface viscoelasticity. Our work may advance the understanding of bubble bursting dynamics at contaminated liquid surfaces and shed light on the potential influence of surface viscoelasticity on the generation of bubble bursting aerosols.

Secondary Bubble Entrainment via Primary Bubble Bursting at a Viscoelastic Surface.

Bubble bursting at liquid surfaces is ubiquitous and plays a key role for the mass transfer across interfaces, impacting global climate and human health. Here, we document an unexpected phenomenon that when a bubble bursts at a viscoelastic surface of a bovine serum albumin solution, a secondary (daughter) bubble is entrapped with no subsequent jet drop ejection, contrary to the counterpart experimentally observed at a Newtonian surface. We show that the strong surface dilatational elastic stress from the viscoelastic surface retards the cavity collapse and efficiently damps out the precursor waves, thus facilitating the dominant wave focusing above the cavity nadir. The onset of daughter bubble entrainment is well predicted by an interfacial elastocapillary number comparing the effects of surface dilatational elasticity and surface tension. Our Letter highlights the important role of surface rheology on free surface flows and may find important implications in bubble dynamics with a contaminated interface exhibiting complex surface rheology.

A Comprehensive Review of Surface Acoustic Wave-Enabled Acoustic Droplet Ejection Technology and Its Applications.

This review focuses on the development of surface acoustic wave-enabled acoustic drop ejection (SAW-ADE) technology, which utilizes surface acoustic waves to eject droplets from liquids without touching the sample. The technology offers advantages such as high throughput, high precision, non-contact, and integration with automated systems while saving samples and reagents. The article first provides an overview of the SAW-ADE technology, including its basic theory, simulation verification, and comparison with other types of acoustic drop ejection technology. The influencing factors of SAW-ADE technology are classified into four categories: fluid properties, device configuration, presence of channels or chambers, and driving signals. The influencing factors discussed in detail from various aspects, such as the volume, viscosity, and surface tension of the liquid; the type of substrate material, interdigital transducers, and the driving waveform; sessile droplets and fluid in channels/chambers; and the power, frequency, and modulation of the input signal. The ejection performance of droplets is influenced by various factors, and their optimization can be achieved by taking into account all of the above factors and designing appropriate configurations. Additionally, the article briefly introduces the application scenarios of SAW-ADE technology in bioprinters and chemical analyses and provides prospects for future development. The article contributes to the field of microfluidics and lab-on-a-chip technology and may help researchers to design and optimize SAW-ADE systems for specific applications.

Drop impact dynamics on the hydrophobic leaf surface of an aquatic plant: a case study of Pistia stratiotes.

Pistia stratiotes is an aquatic plant with a complex structure that allows it to stay afloat. It grows quickly, and in large numbers becomes an undesirable plant as an invasive species. Describing the dynamics of a water drop splash on P. stratiotes leaves can contribute to increasing knowledge of its behavior and finding alternative methods for eradicating it or using it for the benefit of the environment. The non-wettable surface of P. stratiotes presents a complex structure-simple uniseriate trichomes and also ridges and veins. We analyzed the drop impact on a leaf placed on the water surface and recorded it by high-speed cameras. Based on the recordings, quantitative and qualitative analyses were performed. After impacting the leaf, the water drop spread until it reached its maximum surface area accompanied by the ejection of early droplets in the initial stage. Thereafter, three scenarios of water behavior were observed: (i) drop receding and stabilization; (ii) drop receding and ejection of late droplets formed in the later stage as an effect of elastic deformation of the leaf; and (iii) drop breaking apart and ejection of late droplets. The results indicated that the increasing kinetic energy of the impacting drops expressed by the Weber number and the complex leaf surface have an effect on the course of the splash. The simple uniseriate trichomes of the P. stratiotes leaf and the high energy of the falling drops were responsible for the formation and characteristics of the early droplets. The presence of ridges and veins and the leaf's mechanical response had an impact on the occurrence of late droplets.

FRAGMENTATION OF WATER DROPS IN COLLISION WITH A SMALL OBSTACLE

The study is dedicated to the general features of the processes of deformation and fragmentation of liquid drops have been studied when they collide with obstacles. Masks and filters, protecting against airborne infections, are among the possible obstacles. Coughing, sneezing, and talking cause the ejection of drops of saliva and bronchial mucus. A local drop-mask or drop-filter collision is modeled by the impact of a drop on a small obstacle as the simplest hydrodynamic case with a minimum number of influencing factors. For water-based oral and bronchial drops with a typical diameter <i>d<sub>i</sub></i> = 100 μm and impact velocity of the order of <i>v<sub>i</sub></i> = 10 m/s, the impact Weber number is about We<sub><i>i</i></sub> = ρ<i>v<sub>i</sub></i><sup>2</sup><i>d<sub>i</sub></i>/γ = 139. As a starting point in the problem of the drop breakup in a collision with a solid obstacle, we consider the coaxial impact of an inviscid liquid drop with a diameter of 2.8 mm on a disk with a diameter of 4.0 mm. In laboratory experiments, the similarity was provided by impact velocities of 1.88-3.57 m/s, which gives impact Weber numbers We<sub><i>i</i></sub> = 137-496. Such collisions are controlled only by inertia and capillarity, while the influence of all other factors is negligible. A round liquid lamella with a torus-shaped rim is formed upon the collision. The rim first expands and then retracts, forming radially directed liquid fingers in the rim. At low impact velocities, the fingers retained continuity, while at sufficiently high velocities, the fingers spattered into secondary droplets. Experiments have shown that the critical Weber number corresponding to the transition to spattering lies between 137 and 206. Approximately the same values of the Weber number occur when infected drops hit masks or filters.

Experimental study of the stable droplet formation process during micro-valve-based three-dimensional bioprinting

Three-dimensional (3D) bioprinting offers great potential for the fabrication of complex 3D cell-laden constructs for clinical and research applications. The droplet formation process is the important first step in droplet-based 3D bioprinting, affecting the positional accuracy and printing fidelity. In this paper, the drop ejection behavior, thresholds for stable droplet generation, and formation of satellite drops are studied, under various ink properties, printing conditions, and input cell concentrations using a micro-valve-based 3D bioprinter. Three droplet ejection behaviors are identified under different conditions: an isolated stable droplet, satellites coalescing into a single droplet, and the presence of one/multiple satellites. The droplet state is represented by a phase diagram bounded by a dimensionless Z number (the inverse of the Ohnesorge number) and a jet Weber number, Wej, to define the printability of the utilized bioprinter. The printability range is defined as 2 < Z < 15 and 10 < Wej < 25 by considering characteristics, such as stable single droplet formability and sufficient drop falling velocity. There is no fatal damage on cells within this printability range. The results show there is no strong influence of an actuation system on droplet-based bioprinting printability. As the input cell concentration increases, the bioink's density and viscosity increases, and surface tension decreases, which, in turn, causes the Z number to slightly decrease. The change in the cell concentration (from 0 to 1×107 cells/ml), within a Newtonian bioink, has negligible impact on the droplet volume, falling velocity, drop ejection behavior, breakup time, and ligament length in microvalve-based bioprinting.

Novel genetic algorithms for femtoliter jetting using multi-nozzle MEMS printheads (femtoliter jetting using MEMS printheads)

Piezoelectric inkjets have enabled highly precise, customized material deposition for the micro and nanofabrication industries. Inkjet performance relies on a delicate balance between hardware manufacturing precision and tolerancing, material rheology, and the associated actuation-waveform for drop ejection. For a multi-nozzle inkjet system, the important performance metrics include drop volume resolution, inter-nozzle drop volume variation, overall drop-placement accuracy, jetting frequency (related to throughput), and long-term reliability. Cutting-edge applications typically require minimizing drop volume to 1 pL or less; drop placement accuracy of <10 mm, 3-σ; maintaining high jetting frequency of >10 kHz; and achieving reliable long-term operation, e.g., >1 month of continuous operation without any faults. Here, we strive towards a global optimum in drop resolution while adhering to constraints related to placement accuracy and reliability, at a fixed jetting frequency of 14.2 kHz. Our approach is to explore novel genetic algorithms that generate highly complicated waveforms (>100 parameter waveforms) to achieve our global optimum goals. Our automated genetic algorithm (a) starts from rudimentary waveforms that are easily constructed based on empirical knowledge and manual tuning, and (b) systematically increases complexity to generate highly sophisticated waveforms with increasingly greater number of parameters, an approach that is unattainable through manual tuning processes. This approach was developed and implemented using a commercial MEMS-based Fujifilm Samba G3L piezo-inkjet system with 2048 sub-20μm nozzles, with an aqueous diethylene glycol-based ink supplied by Fujifilm. The genetic algorithm operated based on a variable-length chromosome which started with 10 parameter waveforms and evolved into waveforms defined by > 100 parameters, resulting in drop-volume resolution of <600 fL, and maintaining drop placement accuracy to within 10 μm, 3-σ with reliable jetting over at least 30 print fields with the same waveform, each field nominally composed of 340 drops. The best result presented yielded drops with an average volume of 568 fL, which exceeded the manufacturer's specification for drop volume of 2.4 pL by 4.23 times. In summary, we have developed a novel framework for machine-in-the-loop waveform optimization in multi-nozzle inkjet systems to achieve previously unattained drop resolution with high reliability and accuracy.

Evolution of jets during drop impact on a deep liquid pool

The impact of a liquid drop on a liquid pool has been widely investigated. The transition regimes between coalescence and splashing of drops include jet formation with single or multiple secondary drops. One of the main features in this regime is the formation of a central liquid jet followed by breakup of the jet in the form of drops. Earlier studies have shown that the diameter of the secondary drop lies between 0.58 and 0.94 times the diameter of the impacting drop. We perform investigations based on a coupled level-set and volume-of-fluid method to elucidate the earlier observations. The investigations reveal the creation of a variety of secondary drops depending on the impact conditions. The present study also reveals that secondary drops larger than the initial drop can be obtained at higher impact velocities. We identify the importance of cavity shapes on the formation of jets and the pertaining parameters that are responsible for drop ejection.

Contactless electrostatic shaping of a capillary jet for drop-on-demand purposes

The contactless electrostatic shaping of a capillary jet can be adjusted so as to generate a monodisperse spray in the jetting regime. To demonstrate this, an actuator based on stacked electrodes is developed to generate a spatially modulated electric stress. The latter promotes the parametric excitation of a wavelength along a deformed capillary jet. The objective is to better control the breakup length of the capillary jet and to phase lock drop ejection. Experiments are carried out based on light absorption and fast imaging. The breakup length and the drop size distribution are measured. The comparison with a stability model allows us to identify resonant frequencies which monitor the size or the number density of drops. The concept of synchronization frequencies is introduced here with possible use in future developments of electrodynamic actuators.

The ejection of large non-oscillating droplets from a hydrophobic wedge in microgravity

When confined within containers or conduits, drops and bubbles migrate to regions of minimum energy by the combined effects of surface tension, surface wetting, system geometry, and initial conditions. Such capillary phenomena are exploited for passive phase separation operations in micro-fluidic devices on earth and macro-fluidic devices aboard spacecraft. Our study focuses on the migration and ejection of large inertial-capillary drops confined between tilted planar hydrophobic substrates (a.k.a., wedges). In our experiments, the brief nearly weightless environment of a 2.1 s drop tower allows for the study of such capillary dominated behavior for up to 10 mL water drops with migration velocities up to 12 cm/s. We control ejection velocities as a function of drop volume, substrate tilt angle, initial confinement, and fluid properties. We then demonstrate how such geometries may be employed as passive no-moving-parts droplet generators for very large drop dynamics investigations. The method is ideal for hand-held non-oscillatory ‘droplet’ generation in low-gravity environments.

Inkjet Printability Assessment of Weakly Viscoelastic Fluid: A Semidilute Polyvinylpyrrolidone Solution Ink Case Study.

Here, we present an integrated approach to the weakly viscoelastic fluid printability assessment by using global dimensionless criteria (DC). The problem was studied on a model semidiluted polyvinylpyrrolidone water-based ink. For the study purpose, the ink composition was kept as simple as possible. First, the solution density, viscosity, and surface tension were determined. Obtained data were used for testing limitations of DC printability diagrams already available for Newtonian fluids. A replotted version of the original Kim and Baek's map was developed emphasizing the importance of surface tension in the drop formation process. Another set of DC (e.g., Ec and De) was also used for a real evaluation of the viscoelasticity effect on both jetting conditions and drop formation. The polymer relaxation time as a crucial parameter for viscoelasticity was shown to be calculated using the Kuhn segment length rather than from Zimm and Rouse theories for diluted polymer systems. Then, a two-dimensional diagram using four DC (Oh and De with Ec and El as parameters) is proposed based on the famous McKinley's work. The diagram describes the interplay of possible forces responsible for filament thinning and breakup processes. Obtained results were supported by further experiments involving drop ejection and formation, determination of critical polymer concentration, and others. The proposed diagram promises a useful initial step in further investigations of viscoelasticity of polymer compounds by inkjet printing.

Inkjet Printing of a Benzocyclobutene-Based Polymer as a Low-k Material for Electronic Applications.

Polymeric materials with a low dielectric constant are widely used in the electronic industry due to their properties. In particular, polymer adhesives can be used in many applications such as wafer bonding and three-dimensional integration. Benzocyclobutene (BCB) is a very interesting material thanks to its excellent bonding behavior and dielectric properties. Usually, BCB is applied by spin-coating, although this technology does not allow the fabrication of complex patterns. To obtain complex patterns, it is necessary to use a printing technology, such as inkjet printing. However, inkjet printing of BCB-based inks has not yet been investigated. Here, we show the feasibility of printing complex patterns with a BCB-based ink, reaching a resolution of 130 μm. We demonstrate that with a proper dilution, BCB-based inks enter the printability window and drop ejection is achieved without the formation of satellite drops. In addition, we present the conditions in which there is an appearance of the coffee ring effect. Inks that feature a too high interaction with the substrate are more likely to show the coffee ring effect, deteriorating the printing quality. We also observe that it is possible to achieve a better film uniformity by increasing the number of printed layers, due to redissolution of the BCB-based polymer that helps to level possible inhomogeneities. Our work represents the starting point for an in-depth study of BCB-based polymer fabrication using jet printing technologies, as a comparison of the bonding quality obtained with different materials and different technologies could give more information and broaden the perspective regarding this field.

Controlling droplet behaviour and quality of DoD inkjet printer by designing actuation waveform for multi-drop method

There are two techniques that can be used to improve the print quality of an inkjet printer, namely binary and greyscale techniques. In the greyscale technique, the method used is the multi drop ejection method. In the multidrop ejection method, it is more difficult to control droplet behaviour because the possibility of residual vibration and crosstalk will be higher, which can cause satellites and ligaments that can make poor print quality. In this study, we reveal how to control droplet behaviour in a multidrop ejection method so that it can produce good quality droplets. The comparison of droplet image that generated from basic waveform with the waveform using preliminary and suppressing vibration, without and with adjustments of actuation voltage was also studied.

Stability of Lines with Zero Receding Contact Angle Produced by Inkjet Printing at Small Drop Volume.

We present an experimental study of the maximum and minimum bounding drop spacing for a parallel-sided liquid line produced by inkjet printing with drop volumes of 1.5 and 8.5 pL, on substrates with advancing contact angles of 46 and 54°, and zero receding contact angle. The results are used to validate models of the limiting bounds for the formation of stable parallel-sided lines as a function of drop spacing and transverse printing speed. The model for the maximum drop spacing bound (minimum line width) shows a good agreement with our results, but, when used to predict the stable line width, there is an influence of printing speed not captured by the model. This is probably because of a coupling between printed drop volume and ejection velocity outside the scope of the model. The minimum drop spacing bound (maximum stable line width) is limited by a bulging instability, and our results agree with the existing model, except for printing with the largest drop volumes at low temperature. It is shown that under these conditions, there is a different mechanism for bulging that occurs after printing over a period of minutes, if the liquid bead is present on the surface for a significant period of time before drying. Our results suggest that this mechanism is possibly triggered by imperfections on the substrate.

Predictive modelling of drop ejection from damped, dampened wings by machine learning.

The high frequency, low amplitude wing motion that mosquitoes employ to dry their wings inspires the study of drop release from millimetric, forced cantilevers. Our mimicking system, a 10-mm polytetrafluoroethylene cantilever driven through ±1 mm base amplitude at 85 Hz, displaces drops via three principal ejection modes: normal-to-cantilever ejection, sliding and pinch-off. The selection of system variables such as cantilever stiffness, drop location, drop size and wetting properties modulates the appearance of a particular ejection mode. However, the large number of system features complicate the prediction of modal occurrence, and the transition between complete and partial liquid removal. In this study, we build two predictive models based on ensemble learning that predict the ejection mode, a classification problem, and minimum inertial force required to eject a drop from the cantilever, a regression problem. For ejection mode prediction, we achieve an accuracy of 85% using a bagging classifier. For inertial force prediction, the lowest root mean squared error achieved is 0.037 using an ensemble learning regression model. Results also show that ejection time and cantilever wetting properties are the dominant features for predicting both ejection mode and the minimum inertial force required to eject a drop.

Optimization of drop ejection frequency in EHD inkjet printing system using an improved Firefly Algorithm

Swarm intelligence approaches have been used to solve various optimization real-world applications in recent years. Firefly Algorithm (FA) is one of the popular stochastic swarm intelligence paradigms developed in the recent past. In order to solve the slow convergence speed of the standard FA, a new improved Firefly Algorithm (iFA) is proposed in this research. Instead of keeping a constant initial brightness coefficient, a new rule has been proposed for updating the brightness of the fireflies based on a selection probability during generations which leads to a better balance between exploration and exploitation. The efficiency of the iFA has been tested by solving benchmark mathematical functions as well as in a real-world engineering problems. In order to comprehensively compare the performance of the iFA, several other metaheuristic algorithms were used for solving the same benchmark functions and the real-world problems. The iFA shows its superiority in almost all the cases for finding better optima in terms of the objective function value. The droplet ejection speed of an electrohydrodynamic inkjet printing system has been significantly improved by the iFA, which hence fully demonstrates its potential to solve real-world problems. Additionally, the iFA proves its efficiency in solving some challenging, classic engineering design problems with unknown search space. The source codes of the iFA are publiclyavailable at https://www.amitball.com/projects/iFA.

Numerical Study of a Liquid Metal Oscillating inside a Pore in the Presence of Lorentz and Capillary Forces

In order to ensure stable power exhaust and to protect the walls of fusion reactors, liquid metals that are fed to the wall surface through a capillary porous system (CPS) are considered as alternative plasma-facing components (PFCs). However, operational issues like drop ejection and plasma contamination may arise. In this study, the unsteady flow of a liquid metal inside a single pore of the CPS in the presence of Lorentz forces is investigated. A numerical solution is performed via the finite element methodology coupled with elliptic mesh generation. A critical magnetic number is found (Bondm = 4.5) below which the flow after a few oscillations reaches a steady state with mild rotational patterns. Above this threshold, the interface exhibits saturated oscillations. As the Lorentz force is further increased, Bondm &gt; 5.8, a Rayleigh–Taylor instability develops as the interface is accelerated under the influence of the increased magnetic pressure and a finite time singularity is captured. It is conjectured that eventually, drop ejection will take place that will disrupt cohesion of the interface and contaminate the surrounding medium. Finally, the dynamic response of different operating fluids is investigated, e.g., gallium, and the stabilizing effect of increased electrical conductivity and surface tension is demonstrated.