This work proposes a physically consistent numerical model to simulate ultrashort laser absorption by a metallic workpiece at the water–metal interface when optical breakdown of the dielectric occurs. The simulation couples the framework of the finite difference time-domain method used in computational electromagnetics with the constitutive relation derived from both the model of direct ablation of metals and the first-order model of water breakdown. The simulation is used to describe interface ablation processes such as laser-induced plasma micromachining (LIPMM). Applied to the water–aluminum interface, the model is able to describe the metal absorption and the dielectric breakdown threshold in three-dimensional (3D) geometry. It is an extensible monolithic approach in which the absorption by different materials can be described by simply changing the constitutive relations.

Thermal drawing from a preform recently emerges as a scalable manufacturing method for the high volume production of continuous metal microwires for numerous applications. However, no model can yet satisfactorily provide effective understanding of core diameter and continuity from process parameters and material properties during thermal drawing. In this paper, a long wavelength model is derived to describe the dynamics of a molten metal micro-jet entrained within an immiscible, viscous, nonlinear free surface extensional flow. The model requires numerical data (e.g., drawing force and cladding profile) be measured in real time. Examination of the boundary conditions reveals that the diameter control mechanism is essentially volume conservation. The flow rate of molten metal is controlled upstream while the flow velocity is controlled downstream realized by solidification of the molten metal. The dynamics of the molten metal jet are found to be dominated by interfacial tension, stress in the cladding, and pressure in the molten metal. Taylor's conical fluid interface solution (Taylor, 1966, “Conical Free Surfaces and Fluid Interfaces,” Applied Mechanics, Springer, Berlin, pp. 790–796.) is found to be a special case of this model. A dimensionless capillary number Ca = 2 F a / γ A ( 0 ) is suggested to be used as the indicator for the transition from continuous mode (i.e., viscous stress dominating) to dripping mode (i.e., interfacial tension dominating). Experimental results showed the existence of a critical capillary number Ca cr , above which continuous metal microwires can be produced, providing the first ever quantitative predictor of the core continuity during preform drawing of metal microwires.

Molecular dynamics (MD) simulations are used to gain insights into the process conditions that cause separation of graphene layers from a highly ordered pyrolytic graphite (HOPG) source in a polydimethylsiloxane (PDMS) stamp-assisted mechanical exfoliation process. Specifically, the effects of selected exfoliation process parameters and pre-existing defects, such as layer discontinuities in the graphite source, on the exfoliation process are investigated. The results show that exfoliation of individual and few layer graphene requires delicate control of the normal force applied to the HOPG by the PDMS stamp. The study also shows that defects (e.g., discontinuities) in the HOPG have a significant impact on the thickness of separated layers and the layer separation force. The insights derived from this study are expected to be very useful in the development of a low-cost, scalable, large area graphene production process.

A high electrical and thermal conductivity coupled with low costs make copper (Cu) an enticing alternative to aluminum for the fabrication of interconnects in packaging applications. To tap into the benefits of the ever-reducing size of transistors, it is required to increase the input/output pin count on electronic chips, and thus, minimize the size of chip to board interconnects. Laser sintering of Cu nanoparticle (NP) inks can serve as a promising process for developing these micron sized, 3D interconnect structures. However, the exact processing windows for Cu NP sintering are not well known. Therefore, this paper presents an extensive experimental investigation of the sintering processing window with different lasers including femtosecond (fs), nanosecond (ns), and continuous-wave (CW) lasers. The dependence of the processing window on Cu layer thicknesses and laser exposure durations has also been investigated. A simplified model to estimate optimum laser sintering windows for Cu NPs using pulsed lasers is presented and the predicted estimates are compared against the experimental results. Given the simplicity of the model, it is shown to provide good estimates for fluence required for the onset of sintering and the processing window for good sintering of Cu NPs.

To adapt with today's rapidly changing world, fabrication of intricate microparts is becoming an urgent need. Manufacturing of these microparts with stringent requirements necessitates the early adoption of different microfabrication techniques. Wire electrochemical machining (WECM) is such a process which removes excess metal by dissolving it electrochemically. This process can easily generate features downscaled to micron ranges and offers several advantages like the requirement of very simple setup, fabrication of accurate complex microfeatures without undergoing any thermal stress, burr formation, and tool wear, which make it superior from other existing micromachining processes. However, this process is new, and little is known about its applicability and feasibility. Hence, the present work is directed towards developing suitable WECM setup to fabricate microfeatures by introducing proper means for enhancing the mass transport phenomenon. The tungsten tool wire for machining has been in situ etched to a diameter of 23.43 μ m by a novel approach for retaining its regular cylindrical form and has been implemented during machining. Moreover, the influences of high duty ratio and applied frequency have been investigated on the corresponding width of the fabricated microslits and the experimental results have been represented graphically where the minimum width of the microslit is obtained as 44.85 μ m. Furthermore, mathematical modeling has been developed to correlate duty ratio and applied frequency with generated slit width. Additionally, the mathematical modeling has been validated with practical results and complex stepped type microfeatures have been generated to establish process suitability.

Molecular dynamics (MD) simulations are performed to investigate the wettability of gold substrate interacting with nanosized droplets of water. The effects of droplet size, temperature variation, and impingement velocity are evaluated using molecular trajectories, dynamic contact angle, spread ratios, radial distribution function (RDF), and molecular diffusion graphs. Droplets of 4 nm and 10 nm were simulated at 293 K and 373 K, respectively. Stationary droplets were compared to droplets impinging the substrate at 100 m/s. The simulations were executed on high-end workstations equipped with NVIDIA ® Tesla graphical processing units (GPUs). Results show that smaller droplets have a faster stabilization time and lower contact angles than larger droplets. With an increase in temperature, stabilization time gets faster, and the molecular diffusion from the water droplet increases. Higher temperatures also increase the wettability of the gold substrate, wherein droplets present a lower contact angle and a higher spread ratio. Droplets that impact the substrate at a higher impingement velocity converge to the same contact angle as stationary droplets. At higher temperatures, the impingement velocities accelerate the diffusion of water molecules into vapor. It was revealed that impingement velocities do not influence stabilization times. This research establishes relationships among different process parameters to control the wettability of water on gold substrates which can be explored to study several nanomanufacturing processes.

The manufacture of micro–nano structures in transparent dielectrics is becoming increasingly important due to the applications in medical and biological sciences. The femtosecond pulsed laser, with its selectivity, high precision, and three-dimensional direct writing nature, is an ideal tool for this processing technology. In this paper, an improved model for the prediction of ablation crater shape and fluence threshold in femtosecond laser processing of fused silica is presented, in which self-trapping excitons and electrons' relaxation are involved to depict ionization process, Thornber's and Keldysh's models are employed to estimate ionization rate precisely, and a novel ablation criterion is proposed to judge ablation. Moreover, the relationship between the ablation fluence threshold and laser pulse duration is investigated with three different extrapolation methods. The results indicate that no matter which extrapolation method is employed, the ablation fluence thresholds predicted by the presented model agree with the published data.

This paper presents the design and characteristics of a new two-dimensional nonresonant tertiary motion generator which is based on the flextensional structure. A tool holder connects two perpendicularly placed flextensional actuators with flexure hinges which decouple the motion outputs from the two actuators. Piezoelectric stacks, which are preloaded through precision screws, are used to provide input displacements. By balancing the requirements of driving current, stiffness, and the displacement amplification ratio, the proposed design is targeted to operate at above 10 kHz with two-dimensional vibrations amplitude of 10 μ m in each direction. Technical difficulties in driving a nonresonant mode piezoelectric actuator at a high frequency are discussed. The solutions and optimization procedures are presented in this paper. The design is optimized by finite-element simulation; and the results are presented and verified by our prototype design.

In spite of the great progress made toward addressing the challenge of particle contamination in nanomanufacturing, its deleterious effect on yield is still not negligible. This is particularly true for nanofabrication processes that involve close proximity or contact between two or more surfaces. One such process is Jet-and-Flash Imprint Lithography (J-FIL™), which involves the formation of a nanoscale liquid film between a patterned template and a substrate. In this process, the presence of any frontside particle taller than the liquid film thickness, which is typically sub-25 nm, can not only disrupt the continuity of this liquid film but also damage the expensive template upon contact. The detection of these particles has typically relied on the use of subwavelength optical techniques such as scatterometry that can suffer from low throughput for nanoscale particles. In this paper, a novel mechanics-based method has been proposed as an alternative to these techniques. It can provide a nearly 1000 × amplification of the particle size, thereby allowing for optical microscopy based detection. This technique has been supported by an experimentally validated multiphysics model which also allows for estimation of the loss in yield and potential contact-related template damage because of the particle encounter. Also, finer inspection of template damage needs to be carried out over a much smaller area, thereby increasing throughput of the overall process. This technique also has the potential for inline integration, thereby circumventing the need for separate tooling for subwavelength optical inspection of substrates.

A computational model to investigate the flushing of electric discharge machining (EDM) debris from the interelectrode gap during the spray-EDM process is developed. Spray-EDM differs from conventional EDM in that an atomized dielectric spray is used to generate a thin film that penetrates the interelectrode gap. The debris flushing in spray-EDM is investigated by developing models for three processes, viz., dielectric spray formation, film formation, and debris flushing. The range of spray system parameters including gas pressure and impingement angle that ensure formation of dielectric film on the surface is identified followed by the determination of dielectric film thickness and velocity. The debris flushing in conventional EDM with stationary dielectric and spray-EDM processes is then compared. It is observed that the dielectric film thickness and velocity play a significant role in removing the debris particles from the machining region. The model is used to determine the spray conditions that result in enhanced debris flushing with spray-EDM.

In micro electro-discharge machining (micro-EDM), it is believed that electrical conductivity of the dielectric modified by additives plays an important role in discharge initiation and electrical breakdown, thereby affecting the process characteristics including process accuracy, material removal rate (MRR), and surface finish. However, there has been a lack of systematic efforts to evaluate the effect of dielectric conductivity in micro-EDM. This paper investigates the role of electrical conductivity of the dielectric on the breakdown, plasma characteristics, and material removal in micro-EDM via modeling and experimentation. Experiments have been carried out at four levels of electrical conductivity of saline water, i.e., 4 μ S/cm, 362 μ S/cm, 1106 μ S/cm, and 4116 μ S/cm, to study electrical breakdown of the dielectric and resulting craters. A global modeling approach is employed to model the micro-EDM plasma in saline water and predict the effect of dielectric conductivity on electron density, plasma temperature, heat flux to anode, plasma resistance, and discharge energy. It is found from both experiments and model-based simulations that increase in the dielectric conductivity facilitates the electrical breakdown of the dielectric by lowering the minimum breakdown potential at a given interelectrode gap. Experimental results also show increase in the volume of material removed per discharge when dielectric conductivity is increased, which is attributed to the increase in anode heat flux predicted by the micro-EDM plasma model. The model also predicts increase in electron density, decrease in plasma resistance, and decrease in discharge energy as the dielectric conductivity increases.

In spite of its applications in macromanufacturing processes, water jet processing has not been extensively applied to the field of micromanufacturing owing to its poor tolerance and lack of effective control of the jet impingement position. This paper investigates the phenomenon of liquid dielectrophoresis (LDEP) using a localized nonuniform static electric field to deflect and control the jet's trajectory at the microscale for a water jet in air. A new analytical modeling approach has been attempted by representing the stable length of a water jet as a deformable solid dielectric beam to solve for the deflection of the jet under the action of the electric field. This method bypasses the complicated flow analysis of the water jet in air and focuses specially on the effect of the electric field on the trajectory of a laminar water jet within the working length. The numerical analysis of the phenomena for this electrode configuration was carried out using comsol . Preliminary proof-of-concept experiments were conducted on a 350 μ m diameter sized water jet flowing at 0.6 m/s using a pin plate electrode configuration where a deflection of around 10 deg was observed at 2000 V. The results from the simulation are in good agreement with the results obtained in the preliminary experiments. This novel approach of modeling the water jet as a deformable dielectric beam might be useful in numerous applications involving precise control of the water jet's trajectory particularly in microwater jet material processing.

Just as in conventional injection molding of plastics, process simulations are an effective and interesting tool in the area of micro-injection molding. They can be applied in order to optimize and assist the design of the microplastic part, the mold, and the actual process. Available simulation software is however actually made for macroscopic injection molding. By means of the correct implementation and careful modeling strategy though, it can also be applied to microplastic parts, as it is shown in the present work. Process simulations were applied to two microfluidic devices (a microfluidic distributor and a mixer). The paper describes how the two devices were meshed in the simulations software to obtain a proper simulation model and where the challenges arose. One of the main goals of the simulations was the investigation of the filling of the parts. Great emphasis was also on the optimization of selected gate designs for both plastic parts. Subsequently, the simulation results were used to answer the question which gate design was the most appropriate with regard to the process window, polymer flow, and part quality. This finally led to an optimization of the design and the realization of this design in practice as actual steel mold. Additionally, the simulation results were critically discussed and possible improvements and limitations of the gained results and the deployed software were described. Ultimately, the simulation results were validated by cross-checking the flow front behavior of the polymer flow predicted by the simulation with the actual flow front at different time steps. These were realized by molding short shots with the realized molds and were compared to the simulations at the global, i.e., part level and at the local, i.e. feature level.

In electrodischarge machining (EDM), the thermal energy causing material removal at the electrodes is given by the electrical energy supplied to the discharge. This electrical energy, also known as the discharge energy, can be obtained from time-transient voltage and current waveforms across the electrodes during a discharge. However, in micro-EDM, the interelectrode gaps are shorter causing the plasma resistance to be significantly smaller than other impedances in the circuit. As a result, the voltage and current waveforms obtained by a direct measurement may include voltage drop across the stray impedances in the circuit and may not accurately represent the exact voltage drop across micro-EDM plasma alone. Therefore, a model-based approach is presented in this paper to predict time-transient electrical characteristics of a micro-EDM discharge, such as plasma resistance, voltage, current, and discharge energy. A global modeling approach is employed to solve equations of mass and energy conservations, dynamics of the plasma growth, and the plasma current equation for obtaining a complete temporal description of the plasma during the discharge duration. The model is validated against single-discharge micro-EDM experiments and then used to study the effect of applied open gap voltage and interelectrode gap distance on the plasma resistance, voltage, current, and discharge energy. For open gap voltage in the range of 100–300 V and gap distance in the range of 0.5–6 μ m, the model predicts the use of a higher open gap voltage and a higher gap distance to achieve a higher discharge energy.

This paper presents a computational approach for simulating the motion of nanofibers during fiber-filled composites processing. A finite element-based Brownian dynamics simulation (BDS) is proposed to solve for the motion of nanofibers suspended within a viscous fluid. We employ a Langevin approach to account for both hydrodynamic and Brownian effects. The finite element method (FEM) is used to compute the hydrodynamic force and torque exerted from the surrounding fluid. The Brownian force and torque are regarded as the random thermal disturbing effects which are modeled as a Gaussian process. Our approach seeks solutions using an iterative Newton–Raphson method for a fiber's linear and angular velocities such that the net forces and torques, including both hydrodynamic and Brownian effects, acting on the fiber are zero. In the Newton–Raphson method, the analytical Jacobian matrix is derived from our finite element model. Fiber motion is then computed with a Runge–Kutta method to update fiber position and orientation as a function of time. Instead of remeshing the fluid domain as a fiber migrates, the essential boundary condition is transformed on the boundary of the fluid domain, so the tedious process of updating the stiffness matrix of finite element model is avoided. Since the Brownian disturbance from the surrounding fluid molecules is a stochastic process, Monte Carlo simulation is used to evaluate a large quantity of motions of a single fiber associated with different random Brownian forces and torques. The final fiber motion is obtained by averaging numerous fiber motion paths. Examples of fiber motions with various Péclet numbers are presented in this paper. The proposed computational methodology may be used to gain insight on how to control fiber orientation in micro- and nanopolymer composite suspensions in order to obtain the best engineered products.

A new three-dimensional (3D) printing process designated as shockwave-induced freeform technique (SWIFT) is explored for fabricating microparts from nanopowders. SWIFT consists of generating shockwaves using a laser beam, applying these shocks to pressure sinter nanoparticles at room temperature, and creating structures and devices by the traditional layer-by-layer formation. Shockwave cold compaction of nanoscale powders has the capability to overcome limitations, such as shrinkage, porosity, rough surface, and wide tolerance, normally encountered in hot sintering processes, such as selective laser sintering. In this study, the window of operating parameters and the underlying physics of SWIFT were investigated using a high-energy Q-switched Nd: YAG laser and nanodiamond (ND) powders. Results indicate the potential of SWIFT for fabricating high-performance diamond microtools with high aspect ratios, smooth surfaces, and sharp edges. The drawback is that the SWIFT process does not work for micro-sized powders.

The scale-up of microcontact printing (μCP) to a roll-to-roll technique for large-scale surface patterning requires scalable tooling for continuous pattern printing with μm-scale features (e.g., 1–50 μm). Here, we examine the process of creating such a tool using an optical direct-write or “maskless” method working on a rotating cylindrical substrate. A predictive model of pattern formation is presented along with experimental results to examine the key control factors for this process. It is shown that factors can be modulated to vary the cross-sectional shape in addition to feature height and width. This feature can then be exploited to improve the robustness of the final printing process.

This paper presents a five-axis ball-end milling force model that is specifically tailored to microscale machining. A composite cutting force is generated by combining two force contributions from a shearing/ploughing slip-line (SL) field model and a quasi-static indentation (ID) model. To fully capture the features of microscale five-axis machining, a unique chip thickness algorithm based on the velocity kinematics of a ball-end mill is proposed. This formulation captures intricate tool trajectories as well as readily allows the integration of runout and elastic recovery effects. A workpiece updating algorithm has also been developed to identify tool–workpiece engagement. As a dual purpose, historical elastic recovery is stored locally on the meshed workpiece surface in vector form so that the directionality of elastic recovery is preserved for future time increments. The model has been validated through a comparison with five-axis end mill force data. Simulation results show reasonably accurate replication of end milling cutting forces with minimal experimental data fitting.

Whirling is applied to machining of microscrews on thin wires. A micro whirling machine has been developed for this. In order to suppress the vibration of the workpiece, the wire is inserted in polyurethane tubes clamped on a metal bar. Frequency analyses have been conducted by loading impulse forces at the center of the wire. The dynamic response is improved with reducing the vibration in the clamping force by the developed clamping system. Thirty micrometers microgrooves have been machined on 0.3 mm diameter stainless steel wires with fine surface finish, with the developed machine tool.

Atomization-based cutting fluid systems (ACFs) are increasingly being used in micromachining applications to provide cooling and lubrication to the tool–chip interface. In this research, a shielding nozzle design is presented. A computational fluid dynamic model is developed to perform parameter analysis of the design. The numerical simulations were accomplished using the Eulerian approach to the continuous phase and a Lagrangian approach for droplet tracking. Based on the results of the simulations it is determined that the shielding nozzle is effective at providing droplets to the cutting surface at an appropriate speed and size to create a lubricating microfilm.