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1. On the generalized Beltramian motion of the bidirectional vortex in a right-cylindrical cyclone with a hollow core

   https://doi.org/10.1063/5.0087621  

   Cecil, Orie_M; Majdalani, Joseph (April 2022, Physics of Fluids)

   In this work, an exact inviscid solution is developed for the incompressible Euler equations in the context of a bidirectional, cyclonic flowfield in a right-cylindrical chamber with a hollow core. The presence of a hollow core confines the flow domain to an annular swirling region that extends into a toroid in three-dimensional space. The procedure that we follow is based on the Bragg–Hawthorne framework and a judicious assortment of boundary conditions that correspond to a wall-bounded cyclonic motion with a cylindrical core. At the outset, a self-similar stream function is obtained directly from the Bragg–Hawthorne equation under the premises of steady, axisymmetric, and inviscid conditions. The resulting formulation enables us to describe the bidirectional evolution of the so-called inner and outer vortex motions, including their fundamental properties, such as the interfacial layer known as the mantle; it also unravels compact analytical expressions for the velocity, pressure, and vorticity fields, with particular attention being devoted to their peak values and spatial excursions that accompany successive expansions of the core radius. By way of confirmation, it is shown that removal of the hollow core restores the well-established solution for a fully flowing cylindrical cyclone. Immediate applications of cyclonic flows include liquid and hybrid rocket engines, swirl-driven combustion devices, as well as a multitude of heat exchangers, centrifuges, cyclone separators, and flow separation devices that offer distinct advantages over conventional, non-swirling systems. 

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2. Additive Manufacturing and Functionalization of Hollow Polypropylene Sorbents for Water Remediation

   https://doi.org/10.1002/marc.202400879  

   Zheng, Jian; Robertson, Mark; Patil, Nikhil_A; Park, Jay_Hoon; Qiang, Zhe (January 2025, Macromolecular Rapid Communications)

   Abstract As the demand for clean water intensifies, developing effective methods for removing pollutants from contaminated sources becomes increasingly crucial. This work establishes a method for additive manufacturing of functional polymer sorbents with hollow porous features, designed to enhance interactions with organic micropollutants. Specifically, core–shell filaments are used as the starting materials, which contain polypropylene (PP) as the shell and poly(acrylonitrile‐co‐butadiene‐co‐styrene) as the core, to fabricate 3‐dimensional (3D) structures on‐demand via material extrusion. After 3D printing, the cores of the printed roads are removed through solvent extraction, creating hollow structures that increase accessible surface area for adsorption. Subsequently, a sulfonation‐induced crosslinking reaction installs sulfonic acid functionalities into the PP backbones, while enhancing their chemical stability. It is found that larger voids, and thinner polymer shells, enable improved structural retention during the sulfonation through limiting reaction‐induced stresses. The hollow sulfonated PP sorbents exhibit a strong affinity against cationic pollutants. Specifically, larger voids within these structures not only improve structural integrity but also result in accelerated adsorption kinetics by maximizing accessible surface area, thereby enhancing pollutant removal efficiency. This work provides a promising solution for advanced structured sorbent fabrication with hollow architectures, leading to more effective solutions for water contaminant removal in the future. 

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3. Thermo-optics of gilded hollow-core fibers

   https://doi.org/10.1039/d3nr05310e  

   Kolchanov, Denis S; Machnev, Andrey; Blank, Alexandra; Barhom, Hani; Zhu, Liangquan; Lin, Qijing; Inberg, Alexandra; Rusimova, Kristina R; Mikhailova, Mariia A; Gumennik, Alexander; et al (July 2024, Nanoscale)

   Gilded hollow-core fibers, embedded with gold nanoparticles, offer a unique combination of efficient optical transmission and surface heating, enabling applications in light-driven catalysis and improved laser-ignition in internal combustion engines. 

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4. A comparative study of porous and hollow carbon nanofibrous structures from electrospinning for supercapacitor electrode material development

   https://doi.org/10.1016/j.surfin.2021.101386  

   K Asare, MF Hasan (August 2021, Surfaces and interfaces)

   Co-axial electrospinning is an efficient technique to develop core-shell or hollow nanofibrous structures. In this study electrospun carbon nanofibers with three different morphologies, i.e. solid nanofibers with porous structure (P-ECNF), hollow nanofibers with solid wall (H-ECNF), and hollow nanofibers with porous wall (HP-ECNF) were developed through bicomponent electrospinning and co-axial electrospinning of polyacrylonitrile (PAN) and poly (methyl methacrylate) (PMMA) by varying proportion of the sacrificial PMMA. Through comparative electrochemical analyses, it is revealed that the primary factors for electrochemical performance, i.e. specific capacitance, of the electrospun carbon nanofibrous materials are mesopore volume and total pore volume. The hollow structure as well as ordered carbon structure and intact fiber structure also benefits electrolyte transfer and subsequent electrochemical performance but is secondary. Overall the porous carbon nanofibrous electrode material from electrospinning PAN/PMMA (50/50) solution (P-ECNF-50-50) outperformed those hollow and hollow-porous counterparts from co-axial electrospinning and demonstrated the largest specific capacitance due to the largest mesopore volume as well as the largest total pore volume. This electrode material also showed excellent cycling stability (without any loss of specific capacitance) after 3,000 cycles of charging and discharging. It even showed some increase of specific capacitance with cycling test due to its relatively large amount of micropores. 

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5. Bioinspired, Mechanically Robust Chemiresistor for Inline Volatile Organic Compounds Sensing

   https://doi.org/10.1002/admt.202000440  

   Xu, Weiheng; Ravichandran, Dharneedar; Jambhulkar, Sayli; Franklin, Rahul; Zhu, Yuxiang; Song, Kenan (August 2020, Advanced Materials Technologies)

   Abstract There are advantages to polymer/nanoparticle composite‐based volatile organic compounds (VOCs) sensors, such as high chemical and physical stability, operability under extreme conditions, flexible use in manufacturing, and low cost. Nevertheless, their lower limit of detection due to thickness‐dependent diffusion has constrained their application. Inspired by the metaxylem in vascular plants and its vertical conduits and horizontal pits that enable efficient transpiration, a polymer/nanoparticle composite‐based sensor is fabricated with a controllable, spontaneously formed, hollow core for inline VOCs transportation, and porous microstructure for radial direction diffusion. The hollow core is surrounded by an inner porous layer (thermoplastic polyurethane (TPU)), a middle sensing layer (TPU/graphene nanoplatelets/multiwalled carbon nanotubes), and an outer mechanically durable layer (TPU). This multilayered structure shows a 600% higher response rate compared to a single‐layered composite fiber sensor, with a low limit of detection (e.g., ≈15 ppm for xylene) and high selectivity based on the Flory–Huggins interaction parameter. This flexible and stretchable sensor also demonstrates a dual parameter sensing capability from VOC concentrations and uniaxial strain deformation. Via a one‐step fiber spinning procedure, this self‐induced hollow fiber offers a unique method of microstructural design, which enables the detection of low‐concentration VOCs by polymer/nanoparticle‐based sensors. 

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