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Mechanism and stability investigation of a nozzle-free droplet-on-demand acoustic ejector
Abstract
This paper investigates the mechanism of a new acoustic micro-ejector using a Lamb wave transducer array, which can stably generate picoliter (pL) droplet jetting without nozzles. With eight transducers arranged as an octagon array, droplets are ejected based on the mechanism of combined acoustic pressure waves and acoustic streaming. The acoustic focusing area is designed as a line at the liquid center, which is the key factor for a large working range of liquid height. The experimental results show that the ejector can produce uniform water droplets of 22 μm diameter (5.6 pL in volume) continuously at a rate of 0.33 kHz with high ejection stability, owing to a large liquid height window and high acoustic wave frequency. By delivering precise ∼pL droplets without clogging issues, the acoustic ejector has great potential for demanding biochemical applications.
... This array has a structure similar to that of C-IDTs, and it focuses the Lamb wave acoustic focusing area on a single vertical line at the center. By combining the mechanisms of acoustic pressure waves and acoustic flow, jetting in a large working range of liquid height can be achieved [122]. ...
... However, Lamb waves have weaker fluid driving capabilities compared to Rayleigh AW, so reasonable optimization of the IDT is necessary to enhance the driving capability. Ning et al. [122] focused the Lamb waves using eight transducers arranged in an octagonal array and achieved a continuous injection rate of 0.33 kHz of uniform water droplets with a diameter of 22 µm and a volume of 5.6 pL. Fu et al. [118] proposed an optimized double-sided SPUDT to enhance the anti-symmetric A0 mode of Lamb waves and fabricated a Lamb wave device on a double-sided polished 128 • YX LiNbO 3 silicon wafer with a frequency of 7.9 MHz, verifying the feasibility of injection and atomization. ...
... La the use of various types of SAW to drive sessile droplet ejection has emerged. Recen Lamb waves [122] and high-order waves [116] have successfully enabled jetting of ses droplets. ...
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.
... The sample solution passes through a solenoid valve and requires periodic cleaning, rendering it suitable for scenarios demanding continuous liquid transport. Peristaltic pump pipetting [16] entails theh precise control of the peristaltic pump's rotation for microliter pipetting [17], albeit with lower precision and a heightened risk of sample solution cross-contamination. Ultrasonic pipetting [18] employs a transducer positioned beneath the liquid reservoir to emit precisely focused ultrasonic waves. These waves, leveraging their focused acoustic radiation force, continuously propel tiny liquid droplets upward from the surface of the source container into the target receptacle positioned above the liquid, facilitating the high-precision, non-contact transfer of minute liquid volumes. ...
In this study, to improve the efficiency of the pipetting workstation and reduce the impact of the pipetting device on the stability performance of the workstation, a novel fully automatic pipetting method is proposed. Based on this method, a lightweight, multifunctional, and quantitative twelve-channel pipetting device was designed. This device can achieve simultaneous quantitative liquid absorption for twelve channels and sequential interval liquid discharge for each channel. Initially, the overall functional requirements were determined, and with the aim of a lightweight design, the total weight of the device was controlled to be within 580 g through a reasonable structural design, material selection, and choice of driving source. The device’s overall dimensions are 170 mm × 70 mm × 180 mm (length × width × height), with a micropipetting volume ranging between 1.3 μL and 1.4 μL. Subsequently, factors affecting liquid suction stability were experimentally analyzed, and appropriate pipetting parameters were selected. The stability performance of this pipetting method during prolonged operation was investigated. Finally, the twelve-channel pipetting device was validated through experiments, demonstrating results that meet the national standards for the stability of a pipetting device. In summary, the device designed in this study exhibits novel design features, low cost, and modularity, thus demonstrating promising potential for applications in high-speed micro-volume pipetting.
... Gentle acoustic fields with the ability to maintain cells in their native state in their original culture without harm shows the potential to generate and print the droplet containing single cells [45,46,[100][101][102][103][104][105]. In 2007, Demirci et al. applied a gentle acoustic field to generate picolitre droplets containing cells from a microfluidic chip and increased the reliability of cell encapsulation efficiency by as high as 98.4% (Figure 2a) [45]. ...
Single-cell analysis is becoming an indispensable tool in modern biological and medical research. Single-cell isolation is the key step for single-cell analysis. Single-cell printing shows several distinct advantages among the single-cell isolation techniques, such as precise deposition, high encapsulation efficiency, and easy recovery. Therefore, recent developments in single-cell printing have attracted extensive attention. We review herein the recently developed bioprinting strategies with single-cell resolution, with a special focus on inkjet-like single-cell printing. First, we discuss the common cell printing strategies and introduce several typical and advanced printing strategies. Then, we introduce several typical applications based on single-cell printing, from single-cell array screening and mass spectrometry-based single-cell analysis to three-dimensional tissue formation. In the last part, we discuss the pros and cons of the single-cell strategies and provide a brief outlook for single-cell printing.
Inkjet droplet-based vitrification is an emerging cryopreservation technique that leverages precise droplet formation and ultrafast cooling to preserve cells with high viability and functionality. Unlike conventional methods, such as slow freezing and bulk vitrification, this approach minimizes ice crystal formation by generating microdroplets, achieving rapid cooling rates while reducing cytotoxicity through lower concentrations of cryoprotective agents. This review summarizes the fundamentals of vitrification, including the theoretical background of the cooling rates and cryoprotective effects, and examines the principles, applications, and challenges of inkjet-based techniques. Comparisons with other droplet generation methods highlight its advantages in terms of precision and scalability for small sample volumes, particularly in regenerative medicine and biobanking. The importance of thawing processes is also discussed, emphasizing the need for optimized and automated warming techniques to prevent recrystallization. Future developments are expected to focus on improving automation and reproducibility, making inkjet-based cryopreservation a versatile tool for advanced biological storage.
Microfluidics, with its remarkable capacity to manipulate fluids and droplets at the microscale, has emerged as a powerful platform in numerous fields. In contrast to conventional closed microchannel microfluidic systems, free‐boundary microfluidic manufacturing (FBMM) processes continuous precursor fluids into jets or droplets in a relatively spacious environment. FBMM is highly regarded for its superior flexibility, stability, economy, usability, and versatility in the manufacturing of advanced materials and architectures. In this review, a comprehensive overview of recent advancements in FBMM is provided, encompassing technical principles, advanced material manufacturing, and their applications. FBMM is categorized based on the foundational mechanisms, primarily comprising hydrodynamics, interface effects, acoustics, and electrohydrodynamic. The processes and mechanisms of fluid manipulation are thoroughly discussed. Additionally, the manufacturing of advanced materials in various dimensions ranging from zero‐dimensional to three‐dimensional, as well as their diverse applications in material science, biomedical engineering, and engineering are presented. Finally, current progress is summarized and future challenges are prospected. Overall, this review highlights the significant potential of FBMM as a powerful tool for advanced materials manufacturing and its wide‐ranging applications.
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Cryopreservation is currently a key step in translational medicine that could provide new ideas for clinical applications in reproductive medicine, regenerative medicine, and cell therapy. With the advantages of a low concentration of cryoprotectant, fast cooling rate, and easy operation, droplet-based printing for vitrification has received wide attention in the field of cryopreservation. This review summarizes the droplet generation, vitrification, and warming method. Droplet generation techniques such as inkjet printing, microvalve printing, and acoustic printing have been applied in the field of cryopreservation. Droplet vitrification includes direct contact with liquid nitrogen vitrification and droplet solid surface vitrification. The limitations of droplet vitrification (liquid nitrogen contamination, droplet evaporation, gas film inhibition of heat transfer, frosting) and solutions are discussed. Furthermore, a comparison of the external physical field warming method with the conventional water bath method revealed that better applications can be achieved in automated rapid warming of microdroplets. The combination of droplet vitrification technology and external physical field warming technology is expected to enable high-throughput and automated cryopreservation, which has a promising future in biomedicine and regenerative medicine.
Over the past few decades, acoustofluidics, one of the branches of microfluidics, has rapidly developed as a multidisciplinary cutting edge research topic, covering many biomedical and bioanalytical applications. Acoustofluidics usually utilizes acoustic pressure and acoustic streaming effects to manipulate liquids and bioparticles. Acoustic manipulation using acoustic radiation force has been widely studied; however, with the recent development of new piezoelectric devices that enable faster acoustic streaming, particle manipulations using drag force induced by acoustic streaming have attracted more attention. Despite many review articles on acoustic radiation force-based acoustophoresis, acoustic streaming is less frequently covered. Here, we review the recent development of microscale acoustic streaming, especially high-frequency transducer-induced high-speed streaming, confinement and programed streaming, and acoustic streaming tweezers, which combine the acoustic radiation force and drag force to tackle the size limitations of conventional acoustic manipulations. A brief review of acoustic streaming theory and its generation is summarized. Recent progress in applying acoustic streaming for fluidic handling and bioparticle manipulations is reviewed. Representative applications of micro acoustic streaming are provided, and the key issues in these applications are analyzed. Finally, the future prospects of micro acoustic streaming in bioanalytical and biomedical applications are discussed.
Acoustics-based tweezers provide a unique toolset for contactless, label-free, and precise manipulation of bioparticles and bioanalytes. Most acoustic tweezers rely on acoustic radiation forces; however, the accompanying acoustic streaming often generates unpredictable effects due to its nonlinear nature and high sensitivity to the three-dimensional boundary conditions. Here, we demonstrate acoustohydrodynamic tweezers, which generate stable, symmetric pairs of vortices to create hydrodynamic traps for object manipulation. These stable vortices enable predictable control of a flow field, which translates into controlled motion of droplets or particles on the operating surface. We built a programmable droplet-handling platform to demonstrate the basic functions of planar-omnidirectional droplet transport, merging droplets, and in situ mixing via a sequential cascade of biochemical reactions. Our acoustohydrodynamic tweezers enables improved control of acoustic streaming and demonstrates a previously unidentified method for contact-free manipulation of bioanalytes and digitalized liquid handling based on a compact and scalable functional unit.
Advances in lab-on-a-chip technologies are driven by the pursuit of programmable microscale bioreactors or fluidic processors that mimic electronic functionality, scalability, and convenience. However, few fluidic mechanisms allow for basic logic operations on rewritable fluidic paths due to cross-contamination, which leads to random interference between “fluidic bits” or droplets. Here, we introduce a mechanism that allows for contact-free gating of individual droplets based on the scalable features of acoustic streaming vortices (ASVs). By shifting the hydrodynamic equilibrium positions inside interconnected ASVs with multitonal electrical signals, different functions such as controlling the routing and gating of droplets on rewritable fluidic paths are demonstrated with minimal biochemical cross-contamination. Electrical control of this ASV-based mechanism allows for unidirectional routing and active gating behaviors, which can potentially be scaled to functional fluidic processors that can regulate the flow of droplets in a manner similar to the current in transistor arrays.
On-demand droplet dispensing systems are indispensable tools in bioanalytical fields, such as microarray
fabrication. Biomaterial solutions can be very limited and expensive, so minimizing the use of solution per
spot produced is highly desirable. Here, we proposed a novel droplet dispensing method which utilizes a
gigahertz (GHz) acoustic resonator to deposit well-defined droplets on-demand. This ultra-high frequency
acoustic resonator induces a highly localized and strong body force at the solid–liquid interface, which
pushes the liquid to generate a stable and sharp “liquid needle” and further delivers droplets to the target
substrate surface by transient contact. This approach is between contact and non-contact methods, thus
avoiding some issues of traditional methods (such as nozzle clogging or satellite spots). We demonstrated
the feasibility of this approach by fabricating high quality DNA and protein microarrays on glass and flexible
substrates. Notably, the spot size can be delicately controlled down to a few microns (femtoliter in volume). Because of the CMOS compatibility, we expect this technique to be readily applied to advanced biofabrication processes.
This paper presents a transverse mode suppression theory and its experimental verification through aluminum nitride Lamb wave resonators (LWRs) operating at 142 MHz. An effective 2-D approximation model of the LWR is proposed, based on which the origin of transverse modes in LWR is investigated. The displacement distribution, resonant frequencies, and electromechanical coupling coefficients (k 2 t) of the main mode and its auxiliary transverse modes are obtained. A spurious mode suppression theory in terms of the expression of k 2 t in the 2-D model is proposed. Three kinds of electrodes are designed to suppress the transverse mode adjacent to the main mode, including a novel interdigital transducer gap technique that is reported for the first time. With the applicable geometries, these methods reduce the spurious response from 11.8 to <0.5 dB, without significantly affecting the figure of merit of the resonator. Index Terms— Aluminum nitride (AlN), Lamb wave resonator (LWR), micro electromechanical system (MEMS), spurious mode suppression.
A design guideline for one-port aluminum nitride (AlN) Lamb wave resonators (LWRs) working at S0 mode with high performance is reported. A fabricated 252 MHz LWR, with an aperture of 200 μm, 12 fingers, and 1.5 μm thick AlN, is found to have a remarkably high figure of merit (FOM), which exhibits a high ratio of the resistance at parallel resonance (Rp) to the resistance at series resonance (Rs) of 1317 and a corresponding product of the effective coupling coefficient (k²eff) and quality factor (Q) exceeding 52. Consisting of such resonators, a 6-stage ladder filter with a low pass-band insertion loss (IL) of 4.5 dB and steep filter skirts is achieved, offering significant advantage of size savings.
The volume and velocity of droplets ejected from a piezoelectric droplet generator, as used in ink-jet printing, have been studied for a range of concentrated suspensions of submicron alumina particles as a function of driving signal voltage, frequency, and peak shape. Drop velocity and volume are found to show a linear relation with driving voltage, but show a more complicated and periodic behavior with changing frequency and peak width. This periodic dependence is shown to be a function of the acoustic properties of the fluid-filled chamber in the droplet generator. However, a simple model considering propagation of pressure waves along the tubular actuator is not consistent with experimental data if the ends are modeled as step changes in acoustic impedance. By comparing the data with that in the literature, it is proposed that the acoustic boundary condition (impedance change) at the orifice where drops are ejected is a function of orifice size and extrapolates to a closed boundary condition, as the orifice diameter tends to zero. At equivalent driving signal frequencies, the drop volume ejected, normalized by actuator volume displacement, is shown to be a function of the Ohnesorge number of the orifice through which the drops are ejected.
This work for the first time describes a centrifugal technique for the production and manipulation of highly monodisperse water droplets (CV of droplet diameter below 2%) immersed in a continuous flow of immiscible oil. Within a given working range, droplet volumes (5–22 nL) and their mutual spacing is governed by the channel geometry and the frequency of rotation. Different regimes of liquid–liquid flows are presented. We also demonstrate capabilities like droplet splitting and sedimentation as well as the production of two colored droplets, thus setting the stage for a novel centrifugal platform for multiphase flows.
We exploit large accelerations associated with surface acoustic waves to drive an extraordinary fluid jetting phenomena. Laterally focusing the acoustic energy to a small region beneath a drop placed on the surface causes rapid interfacial destabilization. Above a critical Weber number We, an elongated jet forms for drops with dimensions greater than the fluid sound wavelength. Further increases in We lead to single droplet pinch-off and subsequent axisymmetric breakup to form multiple droplets. A simple equation based on a momentum balance is derived to predict the jet velocity.
A 2-D array of 10 × 10 diffractive lenslets was fabricated and tested. Each lenslet has a rectangular aperture and a size of 1.5 mm × 1.5 mm. The focal length of each lenslet is 47 mm. The array was produced by depositing thin films of silicon monoxide on a quartz glass substrate and by using photolithographic techniques. The performance of the lenslets is based on the diffraction of light at a Fresnel zone plate (FZP). The FZP pattern was implemented as a phase structure with eight discrete levels. The diffraction efficiency was measured to be 91%.
Acoustic tweezers are a versatile set of tools that use sound waves to manipulate bioparticles ranging from nanometer-sized extracellular vesicles to millimeter-sized multicellular organisms. Over the past several decades, the capabilities of acoustic tweezers have expanded from simplistic particle trapping to precise rotation and translation of cells and organisms in three dimensions. Recent advances have led to reconfigured acoustic tweezers that are capable of separating, enriching, and patterning bioparticles in complex solutions. Here, we review the history and fundamentals of acoustic-tweezer technology and summarize recent breakthroughs.
We report the nonlinear acoustic streaming effect and the fast manipulation of microparticles by microelectromechanical Lamb-wave resonators in a microliter droplet. The device, consisting of four Lamb-wave resonators on a silicon die, generates cylindrical traveling waves in a liquid and efficiently drives nine horizontal vortices within a 1−μl droplet; the performance of the device coincides with the numerical model prediction. Experimentally, the particles are enriched at the stagnation center of the main vortex on the free surface of the droplet in open space without microfluidic channels. In addition, the trajectories of the particles in the droplet can be controlled by the excitation power.
We report a miniaturised platform for continuous production of single or multiple liquid droplets with diameters between 60 and 500 μm by interfacing a capillary-driven self-replenishing liquid feed with pulsed excitation of focussed surface acoustic waves (SAWs). The orifice-free operation circumvents the disadvantages of conventional jetting systems, which are often prone to clogging that eventuates in rapid degradation of the operational performance. Additionally, we show the possibility for flexibly tuning the ejected droplet size through the pulse width duration, thus avoiding the need for a separate device for every different droplet size required, as is the case for systems in which the droplet size is set by nozzles and orifices, as well as preceding ultrasonic jetting platforms where the droplet size is controlled by the operating frequency. Further, we demonstrate that cells can be jetted and hence printed onto substrates with control over the cell density within the droplets down to single cells. Given that the jetting does not lead to significant loss to the cell's viability or ability to proliferate, we envisage that this versatile jetting method can potentially be exploited with further development for cell encapsulation, dispensing and 3D bioprinting applications.
Droplet-based bioprinting (DBB) offers greater advantages due to its simplicity and agility with precise control on deposition of biologics including cells, growth factors, genes, drugs and biomaterials, and has been a prominent technology in the bioprinting community. Due to its immense versatility, DBB technology has been adopted by various application areas, including but not limited to, tissue engineering and regenerative medicine, transplantation and clinics, pharmaceutics and high-throughput screening, and cancer research. Despite the great benefits, the technology currently faces several challenges such as a narrow range of available bioink materials, bioprinting-induced cell damage at substantial levels, limited mechanical and structural integrity of bioprinted constructs, and restrictions on the size of constructs due to lack of vascularization and porosity. This paper presents a first-time review of DBB and comprehensively covers the existing DBB modalities including inkjet, electrohydrodynamic, acoustic, and micro-valve bioprinting. The recent notable studies are highlighted, the relevant bioink biomaterials and bioprinters are expounded, the application areas are presented, and the future prospects are provided to the reader.
We investigated the unusual droplet jetting formation as a response to the high intensity of a focused acoustic wave on superhydrophobic surface. When focused surface acoustic waves come into contact with a free surface droplet, an elongated pinched liquid column is formed due to the translation of the acoustic radiation force into the inertial body force on the bulk of the droplet. This phenomenon, however, was found to differ as the surface wettability changed. We examined this phenomenon by conducting an experimental observation of the droplet deformation, and a further analysis was carried out using a numerical study, providing a quasi-quantitative analysis of the acoustic radiation pressure distribution.
Rapid, precise, and reproducible deposition of a broad variety of functional materials, including analytical assay reagents and biomolecules, has made inkjet printing an effective tool for the fabrication of microanalytical devices. A ubiquitous office device as simple as a standard desktop printer with its multiple ink cartridges can be used for this purpose. This Review discusses the combination of inkjet printing technology with paper as a printing substrate for the fabrication of microfluidic paper-based analytical devices (μPADs), which have developed into a fast-growing new field in analytical chemistry. After introducing the fundamentals of μPADs and inkjet printing, it touches on topics such as the microfluidic patterning of paper, tailored arrangement of materials, and functionalities achievable exclusively by the inkjet deposition of analytical assay components, before concluding with an outlook on future perspectives.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
The complex chemistry occurring at the interface between liquid and vapor phases contributes significantly to the dynamics and evolution of numerous chemical systems of interest, ranging from damage to the human lung surfactant layer to the aging of atmospheric aerosols. This work presents two methodologies to eject droplets from a liquid water surface and analyze them via mass spectrometry. In bursting bubble ionization (BBI), droplet ejection is achieved via the formation of a jet following bubble rupture at the surface of a liquid to yield 250 µm diameter droplets (10 nL volume). In interfacial sampling by an acoustic transducer (ISAT), droplets are produced by focusing pulsed piezoelectric transducer-generated acoustic waves at the surface of a liquid, resulting in the ejection of droplets of 100 μm in diameter (500 pL volume). In both experimental methodologies, ejected droplets are aspirated into the inlet of the mass spectrometer, resulting in the facile formation of gas-phase ions. We demonstrate the ability of this technique to readily generate spectra of surface-active analytes, and we compare the spectra to those obtained by electrospray ionization. Charge measurements indicate that the ejected droplets are near-neutral (<0.1% of the Rayleigh limit), suggesting that gas-phase ion generation occurs in the heated transfer capillary of the instrument in a mechanism similar to thermospray or sonic spray ionization. Finally, we present the oxidation of oleic acid by ozone as an initial demonstration of the ability of ISAT-MS to monitor heterogeneous chemistry occurring at a planar water/air interface.
This is a review on recent developments in the field of conductive nanomaterials and their application in printed electronics, with particular emphasis on inkjet printing of ink formulations based on metal nanoparticles, carbon nanotubes, and graphene sheets. The review describes the basic properties of conductive nanomaterials suitable for printed electronics (metal nanoparticles, carbon nanotubes, and graphene), their stabilization in dispersions, formulations of conductive inks, and obtaining conductive patterns by using various sintering methods. Applications of conductive nanomaterials for electronic devices (transparent electrodes, metallization of solar cells, RFID antennas, TFTs, and light emitting devices) are also briefly reviewed.
This paper describes the synthesis of a 9-mers-long peptide ladder structure of glycine on a modified glass surface using a nano-liter droplet ejector. To synthesize peptide on a glass substrate, SPOTTM peptide synthesis protocol was followed with a nozzleless acoustic droplet ejector being used to eject about 300 droplets of pre-activated amino acid solution to dispense 60 nL of the solution per mer. The coupling efficiency of each mer was measured with FITC fluorescent tag to be 96 %, resulting in net 70 % efficiency for the whole 9-mer-long peptide of glycine. Usage of a nano-liter droplet ejector for SPOTTM peptide synthesis increases the density of protein array on a chip.
Theories for calculating steady streaming associated with sound fields are reviewed, comparing the methods and approximations of various authors. Two illustrative problems are worked out, both for rectilinear flow due to irrotational sound fields. The first deals with a single attenuated plane wave traveling down a tube, as in Cady's quartz wind experiments. In the second, a pair of crossed plane waves is treated, giving rise to a quite different kind of streaming. In obtaining solutions, attention is given to boundary conditions; here, gradients of She excess static pressure, another second‐order quantity, come into consideration. Significantly, streaming speeds depend critically upon α, the attenuation constant, where α may be due to any common cause, such as heat conduction, scattering, thermal relaxation, etc. From these results it appears that streaming measurements cannot be used to distinguish between absorption mechanisms. Numerical values are given for a few cases; high flow speeds may be expected in a bubbly medium.
Recent interest in MEMS devices in general, and in micro-fluidic devices specifically, has spurred a great deal of research into the behavior of fluids in very small-scale devices. Many novel techniques have been proposed for the propulsion of fluids in small scales devices including peristaltic and electrokinetic. More recently, investigations have considered the use of acoustic streaming: that is, the imposition of steady fluid momentum via nonlinear acoustic effects. The purpose of this manuscript is to overview the physics of acoustic streaming, discus the various physical phenomena which generate the effect, and to highlight the favorable scaling issues of acoustic streaming that make it a viable alternative in micro-fluidic devices.
In this article, we present an acoustically actuated two-dimensional (2D) micromachined ejector array for a zero waste and spinless droplet-by-droplet photoresist deposition method. The theory of operation, the experimental results obtained with acoustic focused 2D micromachined microdroplet ejector array is demonstrated. The ejector operation at 34.7 MHz and generation of 21 mum diam photoresist solvent droplets in drop-on-demand and continuous modes of operation are demonstrated. Photoresist droplets are ejected onto a wafer surface by this acoustic ejector array. Photoresist droplets are ejected on drop on demand and interact with each other by surface tension forces to generate photoresist coverage on the wafer surface during photoresist deposition by droplet generation. By overlapping ejected photoresist droplets, formation of a uniform thickness, line coverage is achieved. Multiple photoresist lines are printed simultaneously by a 3×3 ejector array. By overlapping photoresist lines, coverage of a 4 in. silicon wafer with photoresist is achieved.
The electromechanical coupling coefficients of fundamental and higher‐order ultrasonicLamb wave modes propagating in a piezoelectric plate have been calculated using two different methods. The first approach calculates the coupling coefficient K from the well‐known equation K 2=2(Δv/v)=2(v 0−v m )/v 0, where v 0 and v m are the wavevelocities on a free and a metallized surface, respectively. The second approach, which involves considerably more calculations, is based on the Green’s function method of analysis. In the case of surface acoustic waves, it has been shown that the two approaches give nearly identical results. It is found that, in most cases, this is also true for Lamb waves. The only exception occurs for the lowest‐order symmetric (S 0) and antisymmetric (A 0) modes, if the ratio h/λ (h=plate thickness, λ=acoustic wavelength) is greater than 1. In this case the Δv/v method gives erroneous results. This occurs because, in this case, the surfacemetallization causes strong perturbation of the mechanical displacements associated with the waves. In all other cases, the surfacemetallization produces a weak perturbation of the acoustic wave, and the simple Δv/v method can be used to calculate the coupling coefficient.
The basic principles and technology for the development of lateral-field-excited Lamb acoustic wave resonators on sputter-deposited c-oriented thin aluminum nitride films are presented. The experimental results demonstrate that Lamb waves can be successfully used as an alternative to high-velocity surface acoustic waves.
This letter reports picoliter liquid droplet generation using an orifice-free acoustic ejector operating at its harmonic frequencies. For an acoustic ejector working at the thickness-mode resonance, the droplet size is primarily determined by the acoustic wavelength, which is proportional to the piezoelectric substrate thickness. In our design, we do not need to lap the bulk piezoelectric lead zirconate titanate (PZT) substrate or deposit high temperature processing PZT thin film, but we use harmonic frequencies of the bulk form to reduce the wavelength. The fabricated acoustic ejector with a size of 1200×1200 μm2 has been shown to be very effective up to the ninth harmonic (180 MHz), continuously ejecting ∼ 10 μm diameter droplets, corresponding to droplet volumes as small as 0.5 pl.
In this work, pressure sensitivities of aluminium nitride (AlN) thin film plate acoustic resonators (FPAR) operating at the lowest-order symmetric (S0), the first-order asymmetric (A1) as well as the first-order symmetric (S1) Lamb modes are theoretically and experimentally studied in a comparative manner. The finite element method analysis has also been performed to get a further insight into the FPAR pressure sensitivity. The theoretical predictions are found to be in good agreement with the experiment. The S0 Lamb mode is identified as the most pressure-sensitive FPAR mode, while the A1 and S1 modes are found to be much less sensitive. Further, the S0 and the A1 modes exhibit almost equal temperature sensitivities, which can be exploited to eliminate the temperature drift by comparing the resonance frequencies of the latter two modes.
We report the use of focused acoustic beams to eject discrete droplets of controlled diameter and velocity from a free‐liquid surface. No nozzles are involved. Droplet formation has been experimentally demonstrated over the frequency range of 5–300 MHz, with corresponding droplet diameters from 300 to 5 μm. The physics of droplet formation is essentially unchanged over this frequency range. For acoustic focusing elements having similar geometries, droplet diameter has been found to scale inversely with the acoustic frequency. A simple model is used to obtain analytical expressions for the key parameters of droplet formation and their scaling with acoustic frequency. Also reported is a more detailed theory which includes the linear propagation of the focused acoustic wave, the coupling of the acoustic fields to the initial surface velocity potential, and the subsequent dynamics of droplet formation. This latter phase is modeled numerically as an incompressible, irrotational process using a boundary integral vortex method. For simulations at 5 MHz, this numerical model is very successful in predicting the key features of droplet formation.
This article reviews acoustic microfluidics: the use of acoustic fields, principally ultrasonics, for application in microfluidics. Although acoustics is a classical field, its promising, and indeed perplexing, capabilities in powerfully manipulating both fluids and particles within those fluids on the microscale to nanoscale has revived interest in it. The bewildering state of the literature and ample jargon from decades of research is reorganized and presented in the context of models derived from first principles. This hopefully will make the area accessible for researchers with experience in materials science, fluid mechanics, or dynamics. The abundance of interesting phenomena arising from nonlinear interactions in ultrasound that easily appear at these small scales is considered, especially in surface acoustic wave devices that are simple to fabricate with planar lithography techniques common in microfluidics, along with the many applications in microfluidics and nanofluidics that appear through the literature.
This paper reports on the design, fabrication and testing of novel one and two port piezoelectric higher order contour-mode MEMS resonators that can be employed in RF wireless communications as frequency reference elements or arranged in arrays to form banks of multi-frequency filters. The paper offers a comparison of one and two port resonant devices exhibiting frequencies approximately ranging from 200 to 800 MHz, quality factor of few thousands (1000–2500) and motional resistances ranging from 25 to 1000 Ω. Fundamental advantages and limitations of each solution are discussed. The reported experimental results focus on the response of a higher order one port resonator under different environmental conditions and a new class of two port contour resonators for narrow band filtering applications. Furthermore, an overview of novel frequency synthesis schemes that can be enabled by these contour-mode resonators is briefly presented.
The capability to encapsulate single to few cells with micrometre precision, high viability, and controlled directionality via a nozzleless ejection technology using a gentle acoustic field would have great impact on tissue engineering, high throughput screening, and clinical diagnostics. We demonstrate encapsulation of single cells (or a few cells) ejected from an open pool in acoustic picolitre droplets. We have developed this technology for the specific purpose of printing cells in various biological fluids, including PBS and agarose hydrogels used in tissue engineering. We ejected various cell types, including mouse embryonic stem cells, fibroblasts, AML-12 hepatocytes, human Raji cells, and HL-1 cardiomyocytes encapsulated in acoustic picolitre droplets of around 37 microm in diameter at rates varying from 1 to 10,000 droplets per second. At such high throughput levels, we demonstrated cell viabilities of over 89.8% across various cell types. Moreover, this ejection method is readily adaptable to other biological applications, such as extracting data from single cells and generating large cell populations from single cells. The technique described in the current study may also be applied to investigate stem cell differentiation at the single cell level, to direct tissue printing, and to isolating pure RNA or DNA from a single cell at the picolitre level. Overall, the techniques described have the potential for widespread impact on many high-throughput testing applications in the biological and health sciences.
This paper presents a synthesis technique for any random deoxyribonucleic acid (DNA) sequences on different substrates such as glass, plastic or silicon by an array of directional droplet ejectors. Any DNA sequence can be synthesized by ejecting droplets of DNA bases by an ultrasonic transducer having lens with air-reflectors (LWARs) that requires no nozzle. The LWAR is capable of ejecting liquid droplets around 80 mum in diameter, and reduces the amount of reagents needed for the synthesis from most of conventional microarray techniques. One major advantage of the nozzleless ejector is that it can eject droplets in any direction, so that a spot can be inked by four ejectors (carrying four DNA bases) without moving the ejector. The directional ejection of the droplets removes the need for aligning the substrate with the ejector, and minimizes the automation and control circuitry. To demonstrate the DNA synthesis capability of the directional droplet ejectors, four LWAR ejectors were used to synthesize a 15-mer 5'-CGCCAAGCAGTTCGT-3' on a substrate surface. This paper describes the concept and scheme of the on-demand DNA synthesis (with MEMS ejector integrated with microfluidic components) along with experimental results of an actual DNA synthesis by four directional droplet ejectors.
This paper reports theoretical analysis and experimental results on a new class of rectangular plate and ring-shaped contour-mode piezoelectric aluminum nitride radio-frequency microelectromechanical systems resonators that span a frequency range from 19 to 656 MHz showing high-quality factors in air (Qmax=4300 at 229.9 MHz), low motional resistance (ranging from 50 to 700 Omega), and center frequencies that are lithographically defined. These resonators achieve the lowest value of motional resistance ever reported for contour-mode resonators and combine it with high Q factors, therefore enabling the fabrication of arrays of high-performance microresonators with different frequencies on a single chip. Uncompensated temperature coefficients of frequency of approximately -25 ppm/degC were also recorded for these resonators. Initial discussions on mass loading mechanisms induced by metal electrodes and energy loss phenomenon are provided
This paper presents the theory of operation, fabrication, and experimental results obtained with a new acoustically actuated two-dimensional (2-D) micromachined microdroplet ejector array. Direct droplet based deposition of chemicals used in IC manufacturing such as photoresist and other spin-on materials, low-k and high-k dielectrics by ejector arrays is demonstrated to reduce waste contributing to environmentally benign fabrication and lower production cost. These ejectors are chemically compatible with the materials used in IC manufacturing and do not harm fluids that are heat or pressure sensitive. A focused acoustic beam overcomes the surface tension and releases droplets in air in every actuation cycle. The ejectors were operated most efficiently at 34.7 MHz and generated 28mum diameter droplets in drop-on-demand and continuous modes of operation as predicted by the finite element analysis (FEA). Photoresist, water, isopropanol, ethyl alcohol, and acetone were ejected from a 4times4 2-D micromachined ejector array. Single photoresist droplets were printed onto a silicon wafer by drop-on-demand and continuous modes of operation. Parallel photoresist lines were drawn and a 4-in wafer was coated by Shipley 3612 photoresist by using acoustically actuated 2-D micromachined microdroplet ejector arrays
This paper describes the fabrication and characterization of a thermal ink jet (TIJ) printhead suitable for high speed and high-quality printing. The printhead has been fabricated by dicing the bonded wafer, which consists of a bubble generating heater plate and a Si channel plate. The Si channel plate consists of an ink chamber and an ink inlet formed by KOH etching, and a nozzle formed by inductively couple plasma reactive ion etching (ICP RIE). The nozzle formed by RIE has squeezed structures, which contribute to high-energy efficiency of drop ejector and, therefore, successful ejection of small ink drop. The nozzle also has a dome-like structure called channel pit, which contributes to high jetting frequency and high-energy efficiency. These two wafers are directly bonded using electrostatic bonding of full-cured polyimide to Si. The adhesive-less bonding provided an ideal shaped small nozzle orifice. Use of the same material (Si substrate) in heater plate and channel plate enables the fabrication of high precision long printhead because no displacement and delamination occur, which are caused by the difference in thermal expansion coefficient between the plates. With these technologies, we have fabricated a 1" long printhead with 832 nozzles having 800 dots per inch (dpi) resolution and a 4 pl. ink drop volume.
This paper describes the design and performance of micromachined,
self-focusing acoustic-wave liquid ejector (AWLE) that requires no heat,
nozzle, nor acoustic lens. The AWLE has a very simple device structure
and is easy to fabricate. Three versions of AWLE have been designed,
fabricated, and tested for an ink-jet printing application. Also
developed are computer simulation and design aids that take into account
the acoustic loss in water and the two-time wave reflections at the
water-air and water-transducer interfaces. The AWLE has been observed to
eject water droplets of about 5 μm in diameter with radio frequency
(RF) pulses of 5 μs pulsewidth. Overall, the AWLE has been shown to
be capable of improving the printing resolution and speed of ink-jet
printing significantly
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