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1. Modeling pyramidal silicon nanopores with effective ion transport

   https://doi.org/10.1088/1361-6528/ac8c9c  

   Xiang, Feibin ; Dong, Ming ; Zhang, Wenchang ; Liang, Shengfa ; Guan, Weihua ( September 2022 , Nanotechnology)

   Abstract While the electrical models of the membrane-based solid-state nanopores have been well established, silicon-based pyramidal nanopores cannot apply these models due to two distinctive features. One is its 35.3° half cone angle, which brings additional resistance to the moving ions inside the nanopore. The other is its rectangular entrance, which makes calculating the access conductance challenging. Here, we proposed and validated an effective transport model (ETM) for silicon-based pyramidal nanopores by introducing effective conductivity. The impact of half cone angle can be described equivalently using a reduced diffusion coefficient (effective diffusion coefficient). Because the decrease of diffusion coefficient results in a smaller conductivity, effective conductivity is used for the calculation of bulk conductance in ETM. In the classical model, intrinsic conductivity is used. We used the top-down fabrication method for generating the pyramidal silicon nanopores to test the proposed model. Compared with the large error (≥25% in most cases) when using the classical model, the error of ETM in predicting conductance is less than 15%. We also found that the ETM is applicable when the ratio of excess ion concentration and bulk ion concentration is smaller than 0.2. At last, it is proved that ETM can estimate the tip size of pyramidal silicon nanopore. We believe the ETM would provide an improved method for evaluating the pyramidal silicon nanopores. 

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2. 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)$.

    

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3. Extension of the two-layer model to heat transfer coefficient predictions of nanoporous Si thin films

   https://doi.org/10.1063/5.0099312  

   Wang, Sien ; Chen, Qiyu ; Hao, Qing ( July 2022 , Applied Physics Letters)

   Heat exchange between a solid material and the gas environment is critical for the heat dissipation of miniature electronic devices. In this aspect, existing experimental studies focus on non-porous structures such as solid thin films, nanotubes, and wires. In this work, the proposed two-layer model for the heat transfer coefficient (HTC) between a solid sample and the surrounding air is extended to 70-nm-thick nanoporous Si thin films that are patterned with periodic rectangular nanopores having feature sizes of 100–400 nm. The HTC values are extracted using the 3[Formula: see text] method based on AC self-heating of a suspended sample with better accuracy than steady-state measurements in some studies. The dominance of air conduction in the measured HTCs is confirmed by comparing measurements with varied sample orientations. The two-layer model, developed for nanotubes, is still found to be accurate when the nanoporous film is simply treated as a solid film in the HTC evaluation along with the radiative mean beam length as the characteristic length of the nanoporous film. This finding indicates the potential of increasing HTC by introducing ultra-fine nanoporous patterns, as guided by the two-layer model. 

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4. Swelling, softening, and elastocapillary adhesion of cooked pasta

   https://doi.org/10.1063/5.0083696  

   Hwang, Jonghyun ; Ha, Jonghyun ; Siu, Ryan ; Kim, Yun Seong ; Tawfick, Sameh ( April 2022 , Physics of Fluids)

   The diverse chemical and physical reactions encountered during cooking connect us to science every day. Here, we theoretically and experimentally investigate the swelling and softening of pasta due to liquid imbibition as well as the elastic deformation and adhesion of pasta due to capillary force. As water diffuses into the pasta during cooking, it softens gradually from the outside inward as starch swells and relaxes. The softening follows three sequential regimes: regime I, the hard-glassy region, shows a slow decrease in modulus with cooking time; regime II, the glassy to rubbery transition region, or leathery region, is characterized by a very fast, several orders of magnitude drop in elastic modulus and regime III, the rubbery region, has an asymptotic modulus about four orders of magnitude lower than the raw pasta. We present experiments and theory to capture these regimes and relate them to the heterogeneous microstructure changes associated with swelling. Interestingly, we observe a modulus drop of two orders of magnitude within the range of “al dente” cooking duration, and we find the modulus to be extremely sensitive to the amount of salt added to the boiling water. While most chefs can gauge the pasta by tasting its texture, our proposed experiment, which only requires a measurement with a ruler, can precisely provide an optimal cooking time finely tuned for various kinds of pasta shapes.

    

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5. Evaporative Drying from Hydrophilic or Hydrophobic Homogeneous Porous Columns: Consequences of Wettability, Porous Structure and Hydraulic Connectivity

   https://doi.org/10.1007/s11242-022-01775-7  

   Chakraborty, Partha Pratim ; Ross, Molly ; Bindra, Hitesh ; Derby, Melanie M. ( April 2022 , Transport in porous media)

   Evaporative drying from porous media is influenced by wettability and porous structures; altering these parameters impacts capillary effects and hydraulic connectivity, thereby achieving slower or faster evaporation. In this study, water was evaporated from a homogeneous porous column created with ~1165 glass (i.e., hydrophilic) or Teflon (i.e., hydrophobic) 2.38-mm-diameter spheres with an applied heat flux of 1000 W/m2 supplied via a solar simulator; each experiment was replicated five times and lasted seven days. This study investigates the combination of altered wettability on evaporation with an imposed heat flux to drive evaporation, while deploying X-ray imaging to measure evaporation fronts. Initial evaporation rates were faster (i.e., ~1.5 times) in glass than in Teflon. Traditionally, evaporation from porous media is categorized into three periods: constant rate, subsequent falling rate and slower rate period. Due to homogeneous porous structure and similar characteristic pore size (i.e., 0.453 mm), capillary effects were limited, resulting in an insignificant constant evaporation rate period. A sharp decrease in evaporation rate (i.e., falling rate period) was observed, followed by the slower rate period characterized by Fick’s law of diffusion. Teflon samples entered the slower rate period after 70 hours compared to 90 hours in glass, and combined with X-ray visualization, implying a lower rate of liquid island formation in the Teflon samples than the glass samples. The evaporative drying front, visualized by X-rays, propagated faster in glass with a final depth (after seven days) of ~30 mm, compared to ~24 mm in Teflon. Permeability was modeled based on the geometry [e.g., 3.163E-9 m2 (Revil, Glover, Pezard, and Zamora model), 3.287E-9 m2 (Critical Path Analysis)] and experimentally measured for both glass (9.5E-10 m2) and Teflon (8.9E-10 m2) samples. Rayleigh numbers (Ra=2380) and Nusselt (Nu=4.1) numbers were calculated for quantifying natural evaporation of water from fully saturated porous media, Bond (Bo=193E-3) and Capillary (Ca=6.203E-8) numbers were calculated and compared with previous studies. 

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