Liquid Imbibition in Ceramic-Coated Carbon Nanotube Films

Highly stretchable and customizable microneedle electrode arrays for intramuscular electromyography

Stretchable three-dimensional (3D) penetrating microelectrode arrays have potential utility in various fields, including neuroscience, tissue engineering, and wearable bioelectronics. These 3D microelectrode arrays can penetrate and conform to dynamically deforming tissues, thereby facilitating targeted sensing and stimulation of interior regions in a minimally invasive manner. However, fabricating custom stretchable 3D microelectrode arrays presents material integration and patterning challenges. In this study, we present the design, fabrication, and applications of stretchable microneedle electrode arrays (SMNEAs) for sensing local intramuscular electromyography signals ex vivo. We use a unique hybrid fabrication scheme based on laser micromachining, microfabrication, and transfer printing to enable scalable fabrication of individually addressable SMNEA with high device stretchability (60 to 90%). The electrode geometries and recording regions, impedance, array layout, and length distribution are highly customizable. We demonstrate the use of SMNEAs as bioelectronic interfaces in recording intramuscular electromyography from various muscle groups in the buccal mass of Aplysia.

Intertube Aggregation-Dependent Convective Heat Transfer in Vertically Aligned Carbon Nanotube Channels

The heat transfer of carbon nanotube fin geometry has received considerable attention. However, the flow typically occurred over or around the pillars of nanotubes due to the greater flow resistance between the tubes. Here, we investigated the forced convective heat transfer of water through the interstitial space of vertically aligned multiwalled carbon nanotubes (VAMWNTs, intertube distance = 69 nm). The water flow provided significantly a greater Reynolds number (Re) and Nusselt number (Nu) than air flow due to the greater density, heat capacity, and thermal conductivity. However, it resulted in surface tension-induced nanotube aggregation after the flow and drying process, generating random voids in the nanotube channel. This increased permeability (1.27 × 10^-11 m²) and Re (2.83 × 10^-1) but decreased the heat transfer coefficient (h, 9900 W m^-2 K^-1) and Nu (53.77), demonstrating a trade-off relationship. The h (25,927 W m^-2 K^-1) and Nu (153.49) could be further increased, at an equivalent permeability or Re, by increasing nanotube areal density from 2.08 × 10¹⁰ to 1.04 × 10¹¹ cm^-2. The area-normalized thermal resistance of the densified and aggregated VAMWNTs was smaller than those of the Ni foam, Si microchannel, and carbon nanotube fin array, demonstrating excellent heat transfer characteristics.

Simplified Formula for the Critical Entry Pressure and a Comprehensive Insight into the Critical Velocity of Dislodgment of a Droplet in Crossflow Filtration

Produced water treatment remains a challenging issue for the oil production industry. Finding ways to effectively treat oily water systems without incurring higher operational costs is the struggle and focus of recent research work. The success in establishing a modeling approach to study the filtration of oily water systems is dependent upon our understanding of the fate of oil droplets at the membrane surface. It has been determined that four fates confront oil droplets at the membrane surface, namely, permeation, breakup, pinning, and rejection. Conditions for manifestation of any of these four fates depend on two operating conditions (transmembrane pressure and crossflow velocity) in comparison with two critical conditions (entry pressure and critical velocity of dislodgment). In this work, a new simplified formula for the critical entry pressure is introduced. It compares very well with the formula already existing in the literature. Furthermore, the complete model for the critical velocity of dislodgment in crossflow filtration is presented and highlighted. More investigations on the physical processes that are involved during the pinning of a droplet at a pore opening are presented. In addition, a thorough analysis of the forces that are involved during the permeation of a droplet that could lead to its breakup is presented. It is found that, once the droplet reaches the pore opening, the interfacial tension force and the pressure force continue to increase. Following the critical configuration, these forces continuously decline and the drag force due to the crossflow field, therefore, becomes sufficient to break up the droplet.

Vertically-Aligned Multi-Walled Carbon Nano Tube Pillars with Various Diameters under Compression: Pristine and NbTiN Coated

In this paper, the compressive stress of pristine and coated vertically-aligned (VA) multi-walled (MW) carbon nanotube (CNT) pillars were investigated using flat-punch nano-indentation. VA-MWCNT pillars of various diameters (30-150 µm) grown by low-pressure chemical vapor deposition on silicon wafer. A conformal brittle coating of niobium-titanium-nitride with high superconductivity temperature was deposited on the VA-MWCNT pillars using atomic layer deposition. The coating together with the pillars could form a superconductive vertical interconnect. The indentation tests showed foam-like behavior of pristine CNTs and ceramic-like fracture of conformal coated CNTs. The compressive strength and the elastic modulus for pristine CNTs could be divided into three regimes of linear elastic, oscillatory plateau, and exponential densification. The elastic modulus of pristine CNTs increased for a smaller pillar diameter. The response of the coated VA-MWCNTs depended on the diffusion depth of the coating in the pillar and their elastic modulus increased with pillar diameter due to the higher sidewall area. Tuning the material properties by conformal coating on various diameter pillars enhanced the mechanical performance and the vertical interconnect access (via) reliability. The results could be useful for quantum computing applications that require high-density superconducting vertical interconnects and reliable operation at reduced temperatures.

Dynamics of Liquid Transfer from Nanoporous Stamps in High-Resolution Flexographic Printing

Printing of ultrathin layers of polymeric and colloidal inks is critical for the manufacturing of electronics on nonconventional substrates such as paper and polymer films. Recently, we found that nanoporous stamps overcome key limitations of traditional polymer stamps in flexographic printing, namely, enabling the printing of ultrathin nanoparticle films with micron-scale lateral precision. Here, we study the dynamics of liquid transfer between nanoporous stamps and solid substrates. The stamps comprise forests of polymer-coated carbon nanotubes, and the surface mechanics and wettability of the stamps are engineered to imbibe colloidal inks and transfer the ink upon contact with the target substrate. By high-speed imaging during printing, we observe the dynamics of liquid spreading, which is mediated by progressing contact between the nanostructured stamp surface and by the substrate and imbibition within the stamp-substrate gap. From the final contact area, the volume of ink transfer is mediated by rupture of a capillary bridge; and, after rupture, liquid spreads to fill the area defined by a precursor film matching the stamp geometry with high precision. Via modeling of the liquid dynamics, and comparison with data, we elucidate the scale- and rate-limiting aspects of the process. Specifically, we find that the printed ink volume and resulting layer thickness are independent of contact pressure; and that printed layer thickness decreases with retraction speed. Under these conditions, nanoparticle films with controlled thickness in the <100 nm regime can be printed using nanoporous stamp flexography, at speeds commensurate with industrial printing equipment.

Synthetic Butterfly Scale Surfaces with Compliance-Tailored Anisotropic Drop Adhesion

Many natural surfaces such as butterfly wings, beetles' backs, and rice leaves exhibit anisotropic liquid adhesion; this is of fundamental interest and is important to applications including self-cleaning surfaces, microfluidics, and phase change energy conversion. Researchers have sought to mimic the anisotropic adhesion of butterfly wings using rigid surface textures, though natural butterfly scales are sufficiently compliant to be deflected by capillary forces exerted by drops. Here, inspired by the flexible scales of the Morpho aega butterfly wing, synthetic surfaces coated with flexible carbon nanotube (CNT) microscales with anisotropic drop adhesion properties are fabricated. The curved CNT scales are fabricated by a strain-engineered chemical vapor deposition technique, giving ≈5000 scales of ≈10 µm thickness in a 1 cm² area. Using various designed CNT scale arrays, it is demonstrated that the anisotropy of drop roll-off angle is influenced by the geometry, compliance, and hydrophobicity of the scales; and a maximum roll-off anisotropy of 6.2° is achieved. These findings are supported by a model that relates the adhesion anisotropy to the scale geometry, compliance, and wettability. The electrical conductivity and mechanical robustness of the CNTs, and the ability to fabricate complex multidirectional patterns, suggest further opportunities to create engineered synthetic scale surfaces.