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1. Understanding microwave-assisted extraction of phenolic compounds from diverse food waste feedstocks

   https://doi.org/10.1016/j.cep.2024.109870  

   Gupta, Yagya; Barrett, Brian; Vlachos, Dionisios G (September 2024, Chemical Engineering and Processing - Process Intensification)

   Microwave-assisted extraction (MAE) of natural antioxidants from food waste (FW) offers an economically appealing waste management strategy. Here, we characterize five single waste streams (apple, coffee, olive, tomato, and potato peel waste) and study the MAE of phenolic acids from select feedstocks and mixtures. This library of materials enables us to unravel the relationship of the FW composition and physical properties with dielectric properties, heating, and extractive yields. For example, the protein, ash, and moisture contents affect dielectric properties the most. Our study unveils the significance of moisture in free and bound states on FW dielectric properties, heating, and target acid yields. The microwaves primarily heat the solvent (dimethylformamide) due to its superior dielectric properties compared to FW (dry and moist, single and mixtures) at ≤ 0.05 solid-to-liquid ratio. High moisture content provides higher phenolic yields at lower temperatures and shorter times due to enhanced heat and mass transfer by the microwaves. We recommend extraction before drying waste streams. Further, our data indicates significant interactions between components of mixed FW that drive 2-3x higher yields than those predicted from a simple additive model from single component results. Our work provides new insights for developing versatile MAE strategies to treat complex mixed FW feedstocks. 

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2. Computational design strategy to improve RF heating uniformity

   https://doi.org/10.1108/RPJ-08-2021-0193  

   Allison, Jared; Pearce, John; Beaman, Joseph; Seepersad, Carolyn (March 2022, Rapid Prototyping Journal)

   Purpose Recent work has demonstrated the possibility of selectively sintering polymer powders with radio frequency (RF) radiation as a means of rapid, volumetric additive manufacturing. Although RF radiation can be used as a volumetric energy source, non-uniform heating resulting from the sample geometry and electrode configuration can lead to adverse effects in RF-treated samples. This paper aims to address these heating uniformity issues by implementing a computational design strategy for doped polymer powder beds to improve the RF heating uniformity. Design/methodology/approach Two approaches for improving the RF heating uniformity are presented with the goal of developing an RF-assisted additive manufacturing process. Both techniques use COMSOL Multiphysics® to predict the temperature rise during simulated RF exposure for different geometries. The effectiveness of each approach is evaluated by calculating the uniformity index, which provides an objective metric for comparing the heating uniformity between simulations. The first method implements an iterative heuristic tuning strategy to functionally grade the electrical conductivity within the sample. The second method involves reorienting the electrodes during the heating stage such that the electric field is applied in two directions. Findings Both approaches are shown to improve the heating uniformity and predicted part geometry for several test cases when applied independently. However, the greatest improvement in heating uniformity is demonstrated by combining the approaches and using multiple electrode orientations while functionally grading the samples. Originality/value This work presents an innovative approach for overcoming RF heating uniformity issues to improve the resulting part geometry in an RF-assisted, volumetric additive manufacturing method. 

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3. Development of a Raman spectroscopy system for in situ monitoring of microwave‐assisted inorganic transformations

   https://doi.org/10.1002/jrs.6478  

   Jamboretz, John; Reitz, Andreas; Birkel, Christina_S (December 2022, Journal of Raman Spectroscopy)

   Abstract Microwave heating methods offer unique advantages in preparations of inorganic solids due to the high heating rates, potentially selective heating, and time/energy reductions. Understanding of these enhancements as well as involved mechanisms is poor due to the lack of available and easily applicable in situ monitoring methods, particularly for samples in the solid state. Existing in situ studies typically rely on access to beamline facilities as well as custom‐built microwave systems, which is in the best case inconvenient and in the worst case not achievable. In situ Raman spectroscopy is an ideal technique as it provides rapid and unambiguous phase identification by a noncontact method. Further, the instrument components are simple and compact, facilitating use in the typical synthetic laboratory. Only a few reports on using Raman spectroscopy for in situ measurements during microwave heating exist, and they all utilize specialized custom reactor setups. In this work, a new Raman measurement system designed to observe inorganic transformations in situ that is readily deployable in a standard, commercially available laboratory scale microwave reactor is described. As a simple demonstration, the anatase‐to‐rutile phase transition in TiO₂is monitored under both microwave and conventional furnace heating. The excellent time resolution achieved demonstrates the utility of the system in understanding microwave‐assisted methods for the preparation of inorganic compounds. The simplicity will encourage integration by the non‐specialist to understand microwave heating for synthetic preparations and promote wider application of the technique. 

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4. Pristine carbon nanotubes are efficient absorbers at radio frequencies

   https://doi.org/10.1088/1361-6528/ac6cf8  

   Rommelfanger, Nicholas J; Brinson, Kenneth; Bailey, John E; Bancroft, Analiese M; Ou, Zihao; Hong, Guosong (June 2022, Nanotechnology)

   Abstract Radio frequency ablation and microwave hyperthermia are powerful tools for destroying dysfunctional biological tissues. However, wireless application of these techniques is hindered by their inability to focus the electromagnetic energy to small targets. The use of locally injected radio frequency- or microwave-absorbing nanomaterials can help to overcome this challenge by confining heat production to the injected region. Previous theoretical work suggests that high-aspect-ratio conducting nanomaterials, such as carbon nanotubes, offer powerful radio frequency and microwave absorption. While carbon nanotubes have been demonstrated as radiothermal agents, common solubilization methods may reduce their absorption efficiency, yielding undesirable nonspecific heating in the biological tissue background. In this manuscript, we hypothesize that pristine carbon nanotubes can act as efficient absorbers at radio frequencies, thus providing differential heating over the tissue background. Specifically, we use a sonication-free preparation technique to preserve both the high aspect ratio and local concentration of pristine carbon nanotubes. We validate the differential heating of these samples by 4.5-fold at 2 GHz compared to the heating of saline at a physiological concentration using infrared thermography. In addition, we successfully achieved local heating of pristine carbon nanotubes within a three-dimensional biological tissue phantom. Numerical simulations further aid in producing a temperature map within the phantom and confirming localized heating. Due to their significant differential and local heating, we believe that pristine carbon nanotubes may facilitate region-specific radio frequency ablation and microwave hyperthermia while keeping nonspecific heating to a low level in the normal tissue background. 

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5. Modeling the evaporation and drying of two-component slurry droplets

   https://doi.org/10.1063/5.0224897  

   Bhattacharjee, Anurag; Gnanaskandan, Aswin (August 2024, Physics of Fluids)

   In this paper, we present a mathematical model and numerical simulation for the evaporation and drying of a liquid droplet containing suspended solids, relevant to processes such as spray drying and spray pyrolysis in the food and pharmaceutical industries. The model comprises three stages: first is the evaporation of the liquid droplet consisting of solid particles, followed by the second stage starting with the formation of a porous crust around a wet-core region and, finally, the third stage with sensible heating of the dry particle. Using a finite difference method with a moving grid, we account for the moving interface between the crust and wet core. Our model incorporates spatial temperature variations and is validated against experimental data on colloidal silica droplet drying, showing good agreement. We examine model assumptions and analyze the impact of drying conditions on drying rate and final particle morphology. Along with the temperature and velocity of the drying gas, we also find that the shape of suspended solid particles inside the droplet and assuming continuum flow of vapor through the crust influence drying quality. Finally, we develop a regime map to predict whether the final particle will be solid or hollow based on operating conditions. 

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