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IN THIS ISSUE

Research Papers

J. Micro Nano-Manuf. 2014;2(2):021001-021001-9. doi:10.1115/1.4026545.

Superabrasive grind wheels are used for the machining of brittle materials such as tungsten carbide. Stochastic modeling of the wheel topography can allow for statistical bounding of the grind force characteristics allowing improved surface quality without sacrificing productivity. This study utilizes a machine vision method to measure the wheel topography of diamond microgrinding wheels. The results showed that there are large variances in wheel specifications from the manufacturer and that microgrinding wheels suffer from statistical scaling effects that increase wheel-to-wheel variability in the topography. Analysis of the static grit density values measured on the microgrinding wheels showed that the distributions provided by both analytic stochastic and numerical simulation models accurately predicted the static grit density within a significance level of 5%. Utilizing only manufacturer-supplied specifications caused the models to predict the static grit density with errors as large as 25.3% of the predicted value leading to a need for improved wheel tolerancing and in situ wheel measurement. The spacings between the grits on the wheel surface were shown to be independent of direction and can best be described by a loglogistic distribution.

Topics: Wheels , Density
Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(2):021002-021002-11. doi:10.1115/1.4026884.

During the electrical-assisted forming process, a significant decrease in the flow stress of the metal is beneficial to reduce the required force for the deformation with high-density electrical current introduced through the materials. It is an alternative manufacturing process of traditional hot forming to improve the formability without the undesirable effects caused by elevated temperature, such as surface oxidation. In this study, tension tests and electrical-assisted embossing process (EAEP) experiments were performed to study the electroplastic (EP) effect with high-density pulse current applied to the specimen and demonstrate the advantage of EAEP. In the first section of this study, specimens with various grain sizes were well prepared and an experimental setup was established to study the flow stress of SS316L sheet in the electroplastic tensile test. Extra cooling system was developed and the temperature increase caused by resistive heating was controlled. Thermal influence caused by resistive heating was thereby reduced. The impacts of the pulse current parameters on the flow stress were investigated. It was observed that the flow stress of the SS316L specimens was significantly reduced by the electroplastic effect. In the second section, the EAEP was proposed to fabricate microchannel feature on metal workpiece. Experiments were conducted to demonstrate the feasibility and advantage of the novel process. The protrusion feature height and microstructure of the grain deformation were measured to investigate the effect of the process parameters, such as the current density, the die geometric dimension, and the grain size of the specimen. Larger feature height was measured owing to the higher density current, which meant the electroplastic effects were helpful in EAEP.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(2):021003-021003-10. doi:10.1115/1.4027094.

A recent development in cooling and lubrication technology for micromachining processes is the use of atomized spray cooling systems. These systems have been shown to be more effective than traditional methods of cooling and lubrication for extending tool life in micromachining. Typical nozzle systems for atomization spray cooling incorporate the mixing of high-speed gas and an atomized fluid carried by a gas stream. In a two-phase atomization spray cooling system, the atomized fluid can easily access the tool–workpiece interface, removing heat through evaporation and lubricating the region by the spreading of oil micro-droplets. The success of the system is determined in a large part by the nozzle design, which determines the atomized droplet's behavior at the cutting zone. In this study, computational fluid dynamics are used to investigate the effect of nozzle design on droplet delivery to the tool. An eccentric-angle nozzle design is evaluated through droplet flow modeling. A design of simulations methodology is used to study the design parameters of initial droplet velocity, high-speed gas velocity, and the angle change between the two inlets. The system is modeled as a steady-state multiphase system without phase change, and droplet interaction with the continuous phase is dictated in the model by drag forces and fluid surface tension. The Lagrangian method, with a one-way coupling approach, is used to analyze droplet delivery at the cutting zone. Following a factorial experimental design, deionized water droplets and a semisynthetic cutting fluid are evaluated through model simulations. Statistical analysis of responses (droplet velocity at tool, spray thickness, and droplet density at tool) show that droplet velocity is crucial for the nozzle design and that modifying the studied parameters does not change droplet density in the cutting zone.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(2):021004-021004-6. doi:10.1115/1.4027127.

We demonstrate a fast dispersive laser scanning system by using MEMS digital micro-mirror arrays technology. The proposed technique utilizes real-time dispersive imaging system, which captures spectrally encoded images with a single photodetector at pulse repetition rate via space-to-time mapping technology. Wide area scanning capability is introduced by using individually addressable micro-mirror arrays as a beam deflector. Experimentally, we scanned ∼20 mm2 at scan rate of 5 kHz with ∼150 μm lateral and ∼160 μm vertical resolution that can be controlled by using 1024 × 768 mirror arrays. With the current state of art MEMS technology, fast scanning with <30 $μ$s and resolution down to single mirror pitch size of 10.8 $μ$m is also achievable.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(2):021005-021005-12. doi:10.1115/1.4027099.

Near-field electrohydrodynamic jet (E-jet) printing has recently gained significant interest within the manufacturing research community because of its ability to produce micro/submicron-scale droplets using a wide variety of inks and substrates. However, the process currently operates in open-loop and as a result suffers from unpredictable printing quality. The use of physics-based, control-oriented process models is expected to enable closed-loop control of this printing technique. The objective of this research is to perform a fundamental study of the substrate-side droplet shape-evolution in near-field E-jet printing and to develop a physics-based model of the same that links input parameters such as voltage magnitude and ink properties to the height and diameter of the printed droplet. In order to achieve this objective, a synchronized high-speed imaging and substrate-side current-detection system is implemented to enable a correlation between the droplet shape parameters and the measured current signal. The experimental data reveals characteristic process signatures and droplet spreading regimes. The results of these studies served as the basis for a model that uses the measured current signal as its input to predict the final droplet diameter and height. A unique scaling factor based on the measured current signal is used in this model instead of relying on empirical scaling laws found in prior E-jet literature. For each of the three inks tested in this study, the average error in the model predictions is under 10% for both the diameter and the height of the steady-state droplet. While printing under nonconducive ambient conditions of low relative humidity and high temperature, the use of the environmental correction factor in the model is seen to result in a 17% reduction in the model prediction error.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(2):021006-021006-13. doi:10.1115/1.4027121.

Excimer laser ablation is a versatile technique that can be used for a variety of different materials. Excimer laser ablation overcomes limitations of conventional two-dimensional (2D) microfabrication techniques and facilitates three-dimensional (3D) micromanufacturing. Previously, we have reported a characterization study on 248 nm KrF excimer laser micromachining. This paper extends the study to 193 nm ArF excimer laser micromachining on five representative micro-electro-mechanical systems (MEMS) materials (Si, soda-lime glass, SU-8, polydimethylsiloxane (PDMS), and polyimide). Relations between laser parameters (fluence, frequency and number of laser pulses) and etch performances (etch rates, aspect ratio, and surface quality) were investigated. Etch rate per shot was proportional to laser fluence but inversely proportional to number of laser pulses. Laser frequency did not show a notable impact on etch rates. Aspect ratio was also proportional to laser fluence and number of laser pulses but was not affected by laser frequency. Materials absorbance spectrum was found to have important influence on etch rates. Thermal modeling was conducted as well using numerical simulation to investigate how the photothermal ablation mechanism affects the etching results. Thermal properties of material, primarily thermal conductivity, were proved to have significant influence on etching results. Physical deformation in laser machined sites was also investigated using scanning electron microscopy (SEM) imaging. Element composition of redeposited materials around ablation site was analyzed using energy dispersive x-ray spectroscopy (EDXS) analysis. Combined with our previous report on KrF excimer laser micromachining, this comprehensive characterization study provides guidelines to identify optimized laser ablation parameters for desired microscale structures on MEMS materials. In order to demonstrate the 3D microfabrication capability of ArF excimer laser, cutting and local removal of insulation for a novel floating braided neural probe made of polyimide and nichrome was conducted successfully using the optimized laser ablation parameters obtained in the current study.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(2):021007-021007-6. doi:10.1115/1.4027368.

The method of ultrashort pulse filamentation induced refractive index modification is employed to inscribe fiber Bragg grating (FBG) in single-mode optical fiber (SMF). Line-by-line index inscription technique is used to write refractive index modulation in the core of SMF. The proposed pulse filamentation based index modification enables controlled and flexible writing of FBGs in optical fibers. Performance analysis of the fabricated FBG has been carried out for temperature, contact force, pressure, and axial strain sensing. The in-fiber FBG exhibits sensing performance very similar to FBGs commercially available to date. Then, the written FBG is engineered to demonstrate highly sensitive contact force sensor.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(2):021008-021008-11. doi:10.1115/1.4027339.

The property of nanocomposites is crucially affected by nanoparticle dispersion. Transmission electron microscopy (TEM) is the “golden standard” in nanoparticle dispersion characterization. A TEM Micrograph is a two-dimensional (2D) projection of a three-dimensional (3D) ultra-thin specimen (50–100 nm thick) along the optic axis. Existing dispersion quantification methods assume complete spatial randomness (CSR) or equivalently the homogeneous Poisson process as the distribution of the centroids of nanoparticles under which nanoparticles are randomly distributed. Under the CSR assumption, absolute magnitudes of dispersion quantification metrics are used to compare the dispersion quality across samples. However, as hard nanoparticles do not overlap in 3D, centroids of nanoparticles cannot be completely randomly distributed. In this paper, we propose to use the projection of the exact 3D hardcore process, instead of assuming CSR in 2D, to firstly account for the projection effect of a hardcore process in TEM micrographs. By employing the exact 3D hardcore process, the thickness of the ultra-thin specimen, overlooked in previous research, is identified as an important factor that quantifies how far the assumption of Poisson process in 2D deviates from the projection of a hardcore process. The paper shows that the Poisson process can only be seen as the limit of the hardcore process as the specimen thickness tends to infinity. As a result, blindly using the Poisson process with limited specimen thickness may generate misleading results. Moreover, because the specimen thickness is difficult to be accurately measured, the paper also provides robust analysis of various dispersion metrics to the error of the claimed specimen thickness. It is found that the quadrat skewness and the K-function are relatively more robust to the misspecification of the specimen thickness than other metrics. Furthermore, analysis of detection power against various clustering degrees is also conducted for these two selected robust dispersion metrics. We find that dispersion metrics based on the K-function is relatively more powerful than the quadrat skewness. Finally, an application to real TEM micrographs is used to illustrate the implementation procedures and the effectiveness of the method.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(2):021009-021009-9. doi:10.1115/1.4027384.

In recent years, high quality and accuracy surface are needed for the microchannels such as to be used as micro-total analysis systems (micro-TAS) chips. On the other hand, we demonstrated that the use of smaller size machine tools is one of the effective methods to improve the environmental impact in small parts manufacturing fields. Then, in the present report, we focus on magnetic polishing for microchannels with a ball nose shaped tool, integrating end milling, and polishing processes with a desktop-sized machine tool. In the case of nonmagnetic material, the method had an effective for the bottom of microchannels.

Topics: Polishing
Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(2):021010-021010-8. doi:10.1115/1.4027433.

The objective of this paper is to define and derive a dimensionless number as a function of material properties and process parameters to quantify the extent (magnitude) of thermocapillary flow in pulsed laser micropolishing (PLμP). Experimental work has shown that thermocapillary flow can tremendously reduce surface roughness (smoothing effect) although it inevitably introduces additional surface features (roughening effect) at the same time. Both the smoothing and roughening effects increase as the extent of thermocapillary flow increases. The extent of thermocapillary flow is the bridge from the available information (i.e., initial surface profile, material properties, and process parameters) to the polished surface profile to be predicted. A dimensionless number, called the normalized average displacement of a liquid particle in a single laser pulse, is proposed and derived via analytical heat transfer and fluid flow equations. The calculated normalized displacement is found to be proportional to the measured slope of the introduced features on Ti6Al4V surface polished with various process parameters, which indicates that the dimensionless number successfully describes the extent of thermocapillary flow. The normalized average displacement will be very useful for prediction of polished surface profile and hence parameter selection and process optimization in the future.

Commentary by Dr. Valentin Fuster