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Research Papers

J. Micro Nano-Manuf. 2013;1(2):021001-021001-8. doi:10.1115/1.4023754.

This work describes the centrifugal casting and fast curing of double-sided, polydimethylsiloxane (PDMS)-based components with microfeatures. Centrifugal casting permits simultaneous patterning of multiple sides of a component and allows control of the thickness of the part in an enclosed mold without entrapment of bubbles. Spinning molds filled with PDMS at thousands of revolutions per minute for several minutes causes entrapped bubbles within the PDMS to migrate toward the axis of rotation or dissolve into solution. To cure the parts quickly (<10 min), active elements heat and cool cavities filled with PDMS after the completion of spinning. Microfluidic channels produced from the process have a low coefficient of variation (<2% for the height and width of channels measured in 20 parts). This process is also capable of molding functional channels in opposite sides of a part as demonstrated through a device with a system of valves typical to multilayer soft lithography.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2013;1(2):021002-021002-10. doi:10.1115/1.4024097.

Hot embossing replica are characterized by the quality of the molded structures and the uniformity of the residual layer. In particular, the even distribution of the residual layer thickness (RLT) is an important issue in hot embossing and the related process of thermal nanoimprint lithography, as variations in the RLT may affect the functionality or further processing of replicated parts. In this context, the paper presents an experimental and simulation study on the influence of three process factors, namely the molding temperature, the embossing force, and the holding time, on the residual layer homogeneity achieved when processing 2 mm thick PMMA sheets with hot embossing. The uniformity of the RLT was assessed for different experimental conditions by calculating the standard deviation of thickness measurements at different set locations over the surface of each embossed sample. It was observed that the selected values of the studied parameters have an effect on the resulting RLT of the PMMA replica. In particular, the difference between the largest and lowest RLT standard deviation between samples was 18 μm, which was higher than the accuracy of the instrument used to carry out the thickness measurements. In addition, the comparison between the obtained experimental and simulation results suggests that approximately 12% of the RLT uniformity was affected by the local deflections of the mold. Besides, polymer expansion after release of the embossing load was estimated to contribute to 8% of the RLT nonuniformity. It is essential to understand the effects of the process parameters on the resulting homogeneity of the residual layer in hot embossing. In this research, the best RLT uniformity could be reached by using the highest considered settings for the temperature and holding time and the lowest studied value of embossing force. Finally, the analysis of the obtained results also shows that, across the range of processing values studied, the considered three parameters have a relatively equal influence on the RLT distribution. However, when examining narrower ranges of processing values, it is apparent that the most influential process parameter depends on the levels considered. In particular, the holding time had the most effect on the RLT uniformity when embossing with the lower values of process parameters while, with higher processing settings, the molding temperature became the most influential factor.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2013;1(2):021003-021003-7. doi:10.1115/1.4024098.

Low-adhesive superhydrophobic and superamphiphobic (both superhydrophobic and superoleophobic) surfaces with a liquid contact angle larger than 150 deg and rolling angle less than 10 deg have attracted great interest for fundamental research and potential application. However, the existing methods to fabricate the aforementioned surfaces are contaminative, dangerous, expensive, and time-consuming. Low-adhesive superhydrophobic surfaces on aluminum substrates and steel substrates were fabricated via electrochemical etching method and electrochemical deposition method, respectively. Low-adhesive superamphiphobic surfaces on magnesium alloy substrates were fabricated via one-step electrochemical etching method. The sample surfaces were investigated using electron microscopy, energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectrophotometry (FTIR), X-ray diffraction (XRD), optical contact angle measurements, and digital roughness and microhardness measurements. The SEM results show that the hierarchical rough structures composed of micrometer-scale pits, protrusions, rectangular-shaped plateaus, and smaller step-like structures and particles are present on the aluminum surfaces after electrochemical etching; meanwhile, the hierarchical micro/nanometer-scale rough structures composed of micrometer-scale globular structures and nanometer-scale SiO2 particles are present on the steel surfaces. After being modified with a low surface energy material, superhydrophobic surfaces on aluminum substrates with 167.0 deg water contact angle and 2 deg rolling angle and superhydrophobic surfaces on steel substrates with 172.9 deg water contact angle and 1 deg rolling angle are obtained. For magnesium alloy, the hierarchical micro/nanometer-scale rough structures composed of micrometer-scale, grain-like structures, protrusions, pits, globular structures, lump-like structures, and nanometer-scale sheets and needles are present on the magnesium alloy surfaces. After obtaining the hierarchical micro/nanometer-scale rough structures, the magnesium alloy surfaces directly show a superamphiphobicity without any chemical modification. The hierarchical rough structures are essential to fabricate superhydrophobic surfaces. In addition, the re-entrant structures are important to fabricate superamphiphobic surfaces. Furthermore, the proposed electrochemical machining method is simple, economic, and highly effective.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2013;1(2):021004-021004-11. doi:10.1115/1.4024002.

Superabrasive microgrinding wheels are used for jig grinding of microstructures using various grinding approaches. The desire for increased final geometric accuracy in microgrinding leads to the need for improved process modeling and understanding. An improved understanding of the source of wheel topography characteristics leads to better knowledge of the interaction between the individual grits on the wheel and the grinding workpiece. Analytic stochastic modeling of the abrasives in a general grinding wheel is presented as a method to stochastically predict the wheel topography. The approach predicts the probability of the number of grits within a grind wheel, the individual grit locations within a given wheel structure, and the static grit density within the wheel. The stochastic model is compared to numerical simulations that imitate both the assumptions of the analytic model where grits are allowed to overlap and the more realistic scenario of a grind wheel where grits cannot overlap. A new technique of grit relocation through collective rearrangement is used to limit grit overlap. The results show that the stochastic model can accurately predict the probability of the static grit density while providing results two orders of magnitude faster than the numerical simulation techniques. It is also seen that grit overlap does not significantly impact the static grit density allowing for the simpler, faster analytic model to be utilized without sacrificing accuracy.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2013;1(2):021005-021005-10. doi:10.1115/1.4024099.

This paper examines the formation of burrs in micromilling of a thin nickel–titanium alloy (nitinol) foil used in implantable biomedical device applications. The paper analyzes the effects of key micromilling process parameters such as spindle speed, feed, tool wear, backing material, and adhesive used to attach the foil to the backing material on the burr height. It is found that burr height is larger on the downmilling side for grooves cut with a worn tool at high feeds, low spindle speeds with a softer backing material, and a weaker adhesive bond. Some important interaction effects of these factors are also studied. The study also shows that the mechanics of burr formation in such thin materials depends on whether the mode of cutting is dominated by tearing or chip formation, which is a function of the feed rate. A kinematic model to predict burr widths is developed and verified through experiments.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2013;1(2):021006-021006-9. doi:10.1115/1.4024266.

A dynamic model for analyzing the wire transport system of micro w-EDM (wire electronic discharge machining) is proposed. Based on the model, two mechanisms are proposed to stabilize the wire tension. The first mechanism is the active wire feed apparatus where the wire spool is fed by a motor actively, instead of passively pulled by the windup motor. Hence, the inertia loading of the wire spool can be isolated from the system. The second mechanism is mounting a multilayer damped vibration absorber (MDVA) on the system. As the wire tension variation occurs, the MDVA oscillates to attenuate the wire tension variation. The performances of both mechanisms on the wire tension variation are theoretically investigated and experimentally validated through corner cutting on the 1.0 mm thickness tungsten carbide. Results show that the wire tension variation can be reduced from 10.3 gf to 3.3 gf after mounting the active wire feed apparatus and the oscillation frequency is increased from 13 Hz to 21 Hz. The wire tension variation can be further reduced to 1.9 gf after mounting the MDVA on the system and the high frequency perturbation is significantly attenuated. The 30-deg corner cutting shows that the corner error are significantly reduced from 26.0 μm to 12.0 μm; the standard deviation of kerf is reduced from 4.34 μm to 0.96 μm, and the surface roughness Ra is reduced from 1.15 μm to 0.63 μm after employing both developed mechanisms.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Micro Nano-Manuf. 2013;1(2):024501-024501-4. doi:10.1115/1.4024082.

Fine thermoelectric elements were fabricated on electrode chips by welding together the tips of thin 5 μm diameter Pt and W wires by Joule heat welding. The Pt/W junction was heated by bringing it into contact with a wire carrying a current, thus generating a voltage due to the Seebeck effect in the circuit containing the junction. The Pt/W junctions of two thermoelectric elements in separate circuits were brought into contact with each other. Current was supplied to one of the thermoelectric elements, while the temperature was measured using the other element as a thermocouple. The temperature, which is due to the Peltier effect, was found to depend on the direction of current supply.

Commentary by Dr. Valentin Fuster

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