More Like this

1. Slot-Die Coating Operability Window for Nanoparticle Bed Deposition in a Microscale Selective Laser Sintering Tool

   https://doi.org/10.1115/1.4049668  

   Behera, Dipankar ; Liao, Daniel ; Cullinan, Michael A. ( December 2020 , Journal of Micro and Nano-Manufacturing)

   Abstract This work seeks to develop a fundamental understanding of slot-die coating as a nanoparticle bed deposition mechanism for a microscale selective laser sintering (μ-SLS) process. The specific requirements of the μ-SLS process to deposit uniform sub-5 μm metal nanoparticle films while enabling high throughput fabrication make the slot-die coating process a strong candidate for layer-by-layer deposition. The key challenges of a coating system are to enable uniform nanoparticle ink deposition in an intermittent layer-by-layer manner. Identifying the experimental parameters to achieve this using a slot-die coating process is difficult. Therefore, the main contribution of this study is to develop a framework to predict the wet film thickness and onset of coating defects by simulating the experimental conditions of the μ-SLS process. The single-layer deposition characteristics and the operational window for the slot-die coating setup have been investigated through experiments and two-dimensional computational fluid dynamics simulations. The effect of coating parameters such as inlet speed, coating speed, and coating gap on the wet film thickness has been analyzed. For inlet speeds higher than the coating speed, it was found that the meniscus was susceptible to high instabilities leading to coating defects. Additionally, the study outlines the conditions for which the stabilitymore »of the menisci upstream and downstream of the slot-die coater can affect the uniformity and thickness range of the coating.« less
2. Spatiotemporal wicking dynamics: The effects of pillar height, density, and anisotropic geometries

   Bal, John R ; Manoharan S. R., Krishnan ; Putnam, Shawn A ( June 2021 , ITherm 2021 The Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems June 1 - June 4, 2021)

   Asymmetric microstructures are of particular interest to many technical fields. Such structures can produce anisotropic flow-fields, which, for example, can be used to control heat and mass transport processes. Anisotropic wicking structures can now be systematically engineered with unique micro-pillar geometries and spatial pillar-placement distributions. Such asymmetric wicking structure designs are of particular interest to the thermal management community due to need to cool heterogeneous materials with specific heat load configurations. In this study, asymmetric half-conical micropillars have been fabricated utilizing two-photon polymerization. Macroscopic characterization of anisotropic flow-field velocities is performed via high-speed videography. High-speed thin-film interferometry and microscopic side-angle videography are also used to characterize the microscale evolution of meniscus curvature during inter-pillar wicking. The wicking velocity is observed to be directly proportional to both the meniscus curvature and the cross-sectional area of the micro-pillars (normal to the flow). An anisotropic hemiwicking model is also described with comparisons to experimental data. The hemiwicking model predicts the macroscopic wicking behavior (within 20% or less) for the relatively broad range of pillar geometries and pillar spacing configurations. These anisotropic flow-field predictions can help engineers design the next-generation of micro-structured heat sinks, fluid-based sensors and chemical harvesting systems.
3. A simple analytic model for predicting the wicking velocity in micropillar arrays

   https://doi.org/10.1038/s41598-019-56361-7  

   Krishnan, Siva Rama ; Bal, John ; Putnam, Shawn A. ( December 2019 , Scientific Reports)

   Hemiwicking is the phenomena where a liquid wets a textured surface beyond its intrinsic wetting length due to capillary action and imbibition. In this work, we derive a simple analytical model for hemiwicking in micropillar arrays. The model is based on the combined effects of capillary action dictated by interfacial and intermolecular pressures gradients within the curved liquid meniscus and fluid drag from the pillars at ultra-low Reynolds numbers$${\boldsymbol{(}}{{\bf{10}}}^{{\boldsymbol{-}}{\bf{7}}}{\boldsymbol{\lesssim }}{\bf{Re}}{\boldsymbol{\lesssim }}{{\bf{10}}}^{{\boldsymbol{-}}{\bf{3}}}{\boldsymbol{)}}$$$\left({10}^{-7}\lesssim \mathrm{Re}\lesssim {10}^{-3}\right)$. Fluid drag is conceptualized via a critical Reynolds number:$${\bf{Re}}{\boldsymbol{=}}\frac{{{\bf{v}}}_{{\bf{0}}}{{\bf{x}}}_{{\bf{0}}}}{{\boldsymbol{\nu }}}$$$\mathrm{Re}=\frac{v_0{x}_0}{\nu }$, wherev₀corresponds to the maximum wetting speed on a flat, dry surface andx₀is the extension length of the liquid meniscus that drives the bulk fluid toward the adsorbed thin-film region. The model is validated with wicking experiments on different hemiwicking surfaces in conjunction withv₀andx₀measurements using Water$${\boldsymbol{(}}{{\bf{v}}}_{{\bf{0}}}{\boldsymbol{\approx }}{\bf{2}}\,{\bf{m}}{\boldsymbol{/}}{\bf{s}}{\boldsymbol{,}}\,{\bf{25}}\,{\boldsymbol{\mu }}{\bf{m}}{\boldsymbol{\lesssim }}{{\bf{x}}}_{{\bf{0}}}{\boldsymbol{\lesssim }}{\bf{28}}\,{\boldsymbol{\mu }}{\bf{m}}{\boldsymbol{)}}$$$\left(v_0\approx 2m/s,25\mu m\lesssim x_0\lesssim 28\mu m\right)$, viscous FC-70$${\boldsymbol{(}}{{\boldsymbol{v}}}_{{\bf{0}}}{\boldsymbol{\approx }}{\bf{0.3}}\,{\bf{m}}{\boldsymbol{/}}{\bf{s}}{\boldsymbol{,}}\,{\bf{18.6}}\,{\boldsymbol{\mu }}{\bf{m}}{\boldsymbol{\lesssim }}{{\boldsymbol{x}}}_{{\bf{0}}}{\boldsymbol{\lesssim }}{\bf{38.6}}\,{\boldsymbol{\mu }}{\bf{m}}{\boldsymbol{)}}$$$\left(v_0\approx 0.3m/s,18.6\mu m\lesssim x_0\lesssim 38.6\mu m\right)$and lower viscosity Ethanol$${\boldsymbol{(}}{{\boldsymbol{v}}}_{{\bf{0}}}{\boldsymbol{\approx }}{\bf{1.2}}\,{\bf{m}}{\boldsymbol{/}}{\bf{s}}{\boldsymbol{,}}\,{\bf{11.8}}\,{\boldsymbol{\mu }}{\bf{m}}{\boldsymbol{\lesssim }}{{\bf{x}}}_{{\bf{0}}}{\boldsymbol{\lesssim }}{\bf{33.3}}\,{\boldsymbol{\mu }}{\bf{m}}{\boldsymbol{)}}$$$\left(v_0\approx 1.2m/s,11.8\mu m\lesssim x_0\lesssim 33.3\mu m\right)$.
4. A Study of the Effect of Electrochemical Roughening of Platinum on the Sensitivity and Selectivity of Glutamate Biosensors

   https://doi.org/10.1149/1945-7111/ac5ad5  

   Tan, Chao ; Yin, Haocheng ; Brooks, Victoria ; Arumugam, Prabhu U. ; Siddiqui, Shabnam ( March 2022 , Journal of The Electrochemical Society)

   A systematic study of electrochemically roughened (ECR) thin film platinum (Pt) microelectrodes for glutamate, GLU (a major excitatory neurotransmitter) detection is presented. Scanning electron microscopy, energy dispersive spectroscopy, surface profilometry, electrochemical impedance spectroscopy and amperometry techniques were applied to investigate the effect of high-frequency electrical pulses on Pt microelectrode roughness, electroactive area, charge transfer resistance, and sensitivity and selectivity to hydrogen peroxide, a by-product of enzymatic biosensors and GLU. An increase in the mean surface roughness from 9.0 ± 0.5 to 116.3 ± 7.4 nm (n = 3) was observed which resulted in a 55 ± 2% (n = 3) increase in the electroactive area. An ECR microelectrode treated at +1.4 V and coated with a selective coating produced a GLU selectivity value of 342 ± 34 (n = 3) vs ascorbic acid and the highest GLU sensitivity of 642 ± 45 nAμM⁻¹cm⁻²(n = 3) when compared to other surface-treated Pt microelectrodes reported in the literature. An impedance model was created to elucidate the microstructural and electrochemical property changes to the ECR microelectrodes. The ECR surface comprises of uniformly distributed homogenous pores with very low impedance, which is ∼6-times lower when compared to a methanol cleaned electrode. The model couldmore »lay a foundation for the rational designing of biosensors for enhanced neurotransmitter detection.

   « less
5. Fabrication of Q-Carbon Nanostructures, Diamond and Their Composites with Wafer-Scale Integration

   https://doi.org/10.3390/cryst12050615  

   Khosla, Nayna ; Narayan, Jagdish ( May 2022 , Crystals)

   We report the formation of Q-carbon nanolayers, Q-carbon nanoballs, nanodiamonds, microdiamonds, and their composites by controlling laser and substrate variables. The choice of these parameters is guided by the SLIM (simulation of laser interactions with materials) computer modeling. For a constant film thickness and initial sp3 content, we obtain different microstructures with increasing pulse energy density as a result of different quenching rate and undercooling. This is related to decreasing undercooling with increasing pulse energy density. The structure of thin film Q-carbon evolves into Q-carbon nanoballs with the increase in laser annealing energy density. These Q-carbon nanoballs interestingly self-organize in the form of rings with embedded nanodiamonds to form Q-carbon nanoballs/diamond composites. We form high quality, epitaxial nano, and micro diamond films at a higher energy density and discuss a model showing undercooling and quenching rate generating a pressure pulse, which may play a critical role in a direct conversion of amorphous carbon into Q-carbon or diamond or their composites. This ability to selectively tune between diamond or Q-carbon or their composites on a single substrate is highly desirable for a variety of applications ranging from protective coatings to nanosensing and field emission to targeted drug delivery. Furthermore, Q-carbon nanoballsmore »and nanodiamonds are utilized as seeds to grow microdiamond films by HFCVD. It is observed that the Q-carbon nanoballs contain diamond nuclei of critical size, which provide available nucleation sites for diamond growth, leading to stress-free, adherent, and denser films, which are needed for a variety of coating applications.« less