Microdroplet Deposition Research Articles

Liquids microprinting through a novel film-free femtosecond laser based technique

Laser-induced forward transfer (LIFT) is a high resolution microprinting technique in which small amounts of material are transferred from a previously prepared donor thin film to a receptor substrate. The application of LIFT to liquid donor films allows depositing complex and fragile materials in solution or suspension without compromising the integrity of the deposited material. However, the main drawback of LIFT is the preparation of the donor material in thin film form, being difficult to obtain reproducible thin films with thickness uniformity and good stability. In this work we present a laser microprinting technique that is able to overcome the drawbacks associated with the preparation of the liquid film, allowing the deposition of well-defined uniform microdroplets with high reproducibility and resolution. The droplet transfer mechanism relies on the highly localized absorption of strongly focused femtosecond laser pulses underneath the free surface of the liquid contained in a reservoir. An analysis of the influence of laser pulse energy on the morphology of the printed droplets is carried out, revealing a clear correlation between the printed droplet dimensions and the laser pulse energy. Such correlation is interpreted in terms of the dynamics of the liquid displaced by a laser-generated cavitation bubble close to the free surface of the liquid. Finally, the feasibility of the technique for the production of miniaturized biosensors is tested.

Benzene monitoring by micro-machined sensors with SnO 2 layer obtained by using micro-droplet deposition technique

SnO 2 thin layers were deposited by the way of the micro-droplet technique. The sensor substrate consisted of a thin membrane developed on oxidised silicon wafer. The sensing layers were deposited by means of the micro-droplet technique into thin layers of about 100 nm. Such devices were tested for benzene detection. The obtained results showed a very high sensitivity for this chemical compound since 500 ppb were detected. The results presented in this paper were not focused on the reactional mechanism of benzene detection but rather on the development of a cheap and sensitive sensor using sol–gel and micro-droplet processes. Since these layers were elaborated using solely tin oxide, the as-obtained sensors are not selective but these one are intended to be used by coupling with additional devices such as chromatographic micro-column and micro-pre-concentrators.

Capillary-based liquid microdroplet deposition

Liquid droplet deposition through a capillary onto a substrate is studied. The application of pressure into the capillary causes a liquid meniscus to form at the outlet. Touching the substrate with the liquid meniscus and subsequent capillary retraction gives liquid deposition on the substrate. Theoretical and experimental studies of the steady liquid bridge structure between the capillary and substrate identified the range of parameters when deposition of small droplets (no blot) can be performed. Experiments revealed that in this range of parameters the size of deposited liquid droplets is less than 10%–15% of inner diameter of the capillary.

Research on accurate droplet generation for micro-droplet deposition manufacture

Micro-droplet deposition manufacture is a novel direct metal rapid prototyping for building the miniaturized parts. In this paper, the relationship between the droplet diameter and the operating parameters is investigated to obtain the optimal condition for the accurate droplet generation. The droplet generator is designed to adjust the parameters such as the infinitesimal disturbance and the jet velocity. Experiments with the different materials have been conducted to study the feasibility of the accurate droplet generation for the manufacturing. Spherical and uniform water micro-droplets within 2.4% variation are generated using an orifice diameter of 150 μm, and tin–lead alloy droplets are produced with an orifice diameter of 100 μm in the argon environment. The results show that the generator can produce accurate micro-droplets which meet the requirements of fabricating the miniaturized parts.

마이크로 Groove에서 액적충돌에 대한 수치적 연구

Microdroplet deposition in a micro-groove is studied numerically. The droplet shape is determined by a level-set method which is improved by incorporating a sharp-interface modeling technique for accurately enforcing the matching conditions at the liquid-gas interface and the no-slip and contact angle conditions at an immersed solid surface. The computations are carried out to investigate the droplet behavior derived by the interfacial characteristics between the liquid-gas-solid phases. The effects of contact angle, impact velocity and groove geometry on droplet deposition in a micro-groove are quantified.

Influences of disturbance frequency on the droplet generation for microdroplet deposition manufacture

The influences of the disturbance frequency on the metal droplet generation are investigated for microdroplet deposition manufacture. The polynomial distribution model is assumed for the jet velocity profile. The optimal disturbance frequency is predicted according to the linear stability analysis. The jet velocity converges and the uniform droplets are generated at the optimal disturbance frequency in the simulation. Experiments have been conducted with the orifice diameter of 150 μm. The formation and diameter of Sn60/Pb40 droplets are estimated under the actions of the different frequency. Results show that the accuracy of the droplet shape and diameter are improved. A narrow-sized log-normal distribution is obtained and the diameter deviation is about 2.59 per cent at the optimal disturbance frequency of 3.89 kHz. Good agreements between the theoretical predictions and experimental values indicate that the optimal disturbance frequency is well predicted with a frequency deviation of less than 1.03 per cent. These works are useful when selecting the operating parameters for manufacturing.

미세액적의 자기정렬 기법을 포함한 패터닝 공법에 대한 해석적인 연구

Numerical simulation is performed for microdroplet deposition on a pre-patterned micro-structure. The liquid-air interface is tracked by a level-set method, which is improved by incorporating a sharp-interface modeling technique for accurately enforcing the matching conditions at the liquid-gas interface and the no-slip condition at the fluid-solid interface. The method is further extended to treat the contact angle condition at an immersed solid surface. The present computation of a patterning process using microdroplet ejection demonstrates that the multiphase characteristics between the liquid-gas-solid phases can be used to improve the patterning accuracy.

Development of a multi-nozzle drop-on-demand system for multi-material dispensing

Besides its application in media printing, drop-on-demand (DoD) printing is becoming an attractive alternative to traditional micro-fabrication to build 3D structures with wide applications. In DoD printing, each type of nozzle (ejector) has its uniqueness and limitation in the dispensing material properties, driving parameters, and the ejected droplet dimension. This paper presents a multi-nozzle DoD printing machine to satisfy the increasing demand for multi-material dispensing in industry. On the hardware aspect, a microcontroller-based synchronizer is designed to synchronize the printing process from multi-nozzles with respect to a movable positioning stage. This real-time control contributes to accurate micro-droplet deposition. On the software aspect, a three-layer framework is proposed, including user interface, system manager, and hardware interface layer. This proposed structure greatly improves the system flexibility with which the user can dynamically configure the desired dispensers for 3D structure printing. Integrating these techniques into the printing system, the developed DoD system can achieve a flexible and friendly user interface, while dispensing a wide range of functional materials using various types of dispensing nozzles.

A numerical investigation on the influence of liquid properties and interfacial heat transfer during microdroplet deposition onto a glass substrate

This work investigates the impingement of a liquid microdroplet onto a glass substrate at different temperatures. A finite-element model is applied to simulate the transient fluid dynamics and heat transfer during the process. Results for impingement under both isothermal and non-isothermal conditions are presented for four liquids: isopropanol, water, dielectric fluid (FC-72) and eutectic tin–lead solder (63Sn–37Pb). The objective of the work is to select liquids for a combined numerical and experimental study involving a high resolution, laser-based interfacial temperature measurement to measure interfacial heat transfer during microdroplet deposition. Applications include spray cooling, micro-manufacturing and coating processes, and electronics packaging. The initial droplet diameter and impact velocity are 80 μm and 5 m/s, respectively. For isothermal impact, our simulations with water and isopropanol show very good agreement with experiments. The magnitude and rates of spreading for all four liquids are shown and compared. For non-isothermal impacts, the transient drop and substrate temperatures are expressed in a non-dimensional way. The influence of imperfect thermal contact at the interface between the drop and the substrate is assessed for a realistic range of interfacial Biot numbers. We discuss the coupled influence of interfacial Biot numbers and hydrodynamics on the initiation of phase change.

Microdroplet deposition of copper film by femtosecond laser-induced forward transfer

Copper microdroplets were deposited onto a quartz substrate by the laser-induced forward transfer (LIFT) using laser pulses of 148fs at a center wavelength of 775nm. The droplets with 2–3μm diameters were transferred at a laser pulse energy slightly above the threshold at which the copper film could be removed completely. The droplet formation was a result of the blow-off of a molten film from the quartz substrate by a compressive stress of plasma when the free surface was melted and was different from the microdroplet formation by the LIFT technique using nanosecond laser pulse.

High-Throughput Study of Poly(ethylene glycol)/Ibuprofen Formulations under Controlled Environment Using FTIR Imaging

Simultaneous analysis of many samples under identical conditions improves the effectiveness of research and accelerates product design. A novel spectroscopic imaging approach using a multichannel detector has been developed for parallel analysis of pharmaceutical formulations under controlled environments. Samples of formulations of ibuprofen in poly(ethylene glycol) have been prepared with ibuprofen concentrations ranging from 0 to 100% using a microdroplet deposition approach. The concentration of ibuprofen in PEG at which dimerization of ibuprofen molecules can be avoided has been determined via simultaneous measurement of all samples using in situ FTIR spectroscopic imaging. FTIR spectra from all samples have been analyzed to assess the molecular state of the drug and the degree of polymer swelling as a function of drug concentration. The effect of elevated temperature on the stability of all formulations was also studied. This high-throughput approach identified the concentration range for stable formulations and provided evidence that hydrogen bonding between ibuprofen and the polymer is responsible for enhanced stability at higher temperatures. This high-throughput imaging approach, based on a miniature sampling system, significantly reduces the experimental time by allowing many (potentially a few thousand) experiments to be run in parallel and increases the accuracy by minimizing variations between experiments.

Microdroplet deposition by laser-induced forward transfer

Laser-induced forward transfer was used to deposit aluminum and nickel microdroplets onto a substrate using a Q-switched neodymium:Yttrium-aluminum-garnet laser. The droplets have diameters of a few microns, much smaller than the laser spot diameter, and are transferred at fluences slightly above the melting threshold. Scanning electron microscopy shows that the original donor film is deformed after laser irradiation, such that the film protrudes outward from the center of the laser spot. The film expands during laser heating, but is constrained until the melt interface reaches the free surface. When this occurs, the film is no longer constrained, allowing the melt to rapidly expand, forming the protrusions from which droplets are ejected.

Preferential Crystallization of β-FeSi2 from Micro-droplets Generated by Laser Ablation.

ABSTRACTβ-FeSi2 was successfully fabricated at room temperature via the deposition of molten micro-droplets generated by the KrF excimer laser ablation. Only the molten droplets precipitated as the β-FeSi2 crystalline phase on a silicon substrate kept even at room temperature, whereas the rest of film was amorphous. The crystallization behavior of micro-droplets has been discussed in the light of non-equilibrium process due to rapid cooling on the substrate. After the deposition, pulsed laser annealing was also performed in order to improve the crystallinity of the β-FeSi2 microprecipitates-containing film.

Three-dimensional presolidification heat transfer and fluid dynamics in molten microdroplet deposition

The non-axisymmetric, coupled fluid mechanics and heat transfer of an impacting liquid solder droplet on a flat substrate is investigated numerically using the finite element method. The modelling of the fluid mechanics is based upon the full laminar Navier–Stokes equations employing a Lagrangian frame of reference. Due to the large droplet deformation, the surface (skin) as well as the volumetric mesh have to be regenerated during the calculations in order to maintain the high accuracy of the numerical scheme. The pressure and velocity fields are then interpolated on the newly created mesh. The energy equation is solved in both the droplet and the substrate domain. A time and space averaged thermal contact resistance is implemented between the two thermal domains (droplet and substrate). For the impact parameters used in this study (typical values We=2.38, Fr=16,300, Re=157), the droplet rolls along the substrate but its shape remains practically axisymmetric for all the impact angles within the range from 0° to 60°. The substrate/droplet contact area is not a monotonically decreasing function of time and the cooling of the droplet is markedly dependent on the impact angle.

An Investigation of Key Factors Affecting Solder Microdroplet Deposition

This paper combines a theoretical model with experimental measurements to elucidate the role of key operating parameters affecting solder microdroplet deposition in the electronics manufacturing industry. The experimental investigation is used to evaluate the final deposit (bump) shapes and trends predicted by the model. The effects of substrate temperature, material composition, layer thickness, and thermal contact resistance (including surface oxidation) are delineated. Solder-deposit shape comparisons between experiments and modeling suggest that the value of thermal contact resistance may change with process parameters, and is probably dependent on the solder phase. It is established that inferences regarding the overall shape or solidification times of solder bumps using limited modeling trends should be made only after careful consideration of the substrate composition, accurate representation of the thermal contact resistance, and adequate resolution of the fluid dynamical oscillatory motion and its effects on solidification rates. It is shown that modeling tools can be used in conjunction with experiments to promote our fundamental understanding of the transport processes in the complex solder jetting technology.

Metal ion implantation effects on surface properties of polymers

Polyethylene (PE), polycarbonate (PC) and polyetherimide (PEI) have been implanted with a variety of metallic ions from a metal vapor arc source at low energies yielding shallow depth implants. PC and PEI have been implanted with Cr, Ti, Si and Pt, while PE was implanted with Cr and Ti. Nanoindentation hardness and surface resistivity changes were measured for the implanted polymers. Results showed that the Ti and Si implantations caused similar hardness increases for PC and PEI while Cr implantation resulted in the largest hardness improvement for the two polymers. These effects were attributed to energy and dose effects. Pt implantation, on the contrary, had very little effect on hardness. This was attributed to a greater extent of chain scission caused by nuclear collisions by the heavy Pt ions than cross-linking caused by electronic excitation. In the case of PE, Ti and Cr yielded similar hardness increases although the measured hardness values were smaller than those for PC and PEI. The implantations also decreased electrical resistivities of the polymers. Ti, Si and Cr implantations exhibited trends for a decrease in resistivity similar to those for the increase in hardness suggesting similar underlying causes, namely cross-linking. It appears that irradiation-induced cross-links provide paths for increased mobility of carriers contributing towards decreased resistivity. Pt implantation yielded the largest increase in conductivity as a result of the high dose used as well as the deposition of microdroplets of Pt which form at the metal ion source causing Pt to accumulate at the polymer surface leading to metallic conduction. This study represents the first use of a mixed energy spectrum of ion implantation to modify surface properties of polymers.

Electronic Deposition of Sprayed Suspensions of Protein for Electron Microscopy

The deposition of microdroplets from a nebulized aerosol is enhanced by the application of an electric field. Electronic deposition facilitates the electron microscopic sampling of the aerosol by capturing those microdroplets that are electrically charged or whose dimensions are too small and therefore elude the more conventional methods of collection. The subsequent deposition of the microdroplets in the system described is a function of the strength of the electric field and the size of the individual microdroplets, and therefore negates the use of forced-air impaction of the microdroplets onto the collecting surface. Electronic deposition as applied to the collection of nebulized suspensions of particulate protein renders the resulting microdroplets (0.25 to 2.0 μ diameter size range) and their contents accessible to sampling under conditions suitable for high-resolution electron microscopy.