PUBLICATIONS

Carvajal, M., Pahari, B. R., Ramesh, D., Oates, W. S., Quantifying Thermal Transport in 3D Printed Fractal Structures. ASME journal of heat and mass transfer. 2025, p.1-18

Heat transport through 3D printed fractal media is investigated by comparing a fractal diffusion model to infrared measurements using Bayesian uncertainty quantification. The Delayed Rejection Adaptive Metropolis (DRAM) algorithm, based on the Markov Chain Monte Carlo (MCMC) sampling technique, is used to infer parameter uncertainty, quantify parameter correlation, and compute error propagation of the temperature distributions. The results demonstrate that fractal operators improve modeling thermal transport through complex fractal structures and help understand fractal structure-property relationships. For example, correlations among fractal spatial and temporal scaling parameters, diffusion coefficients, and fractal dimensions are quantified. We find a scaling relationship between the diffusion coefficient D and the temporal fractal time derivative order α that scales nominally as D α eα based on constraints from the second law of thermodynamics. The results have implications for building a stronger understanding of heat transport in complex materials beyond random media and models based on Gaussian probability homogenization.

Ramesh, D., Stanisauskis, E., Consoliver-Zack, J., Pahari, B. R., Oates, W. S. (2022). Characterization and Uncertainty Analysis of Heat Transport in 3D Printed Multifractal Media. Proceedings of the ASME 2022 Conference on Smart Materials, Adaptive Structures and Intelligent Systems.

Fractal structures represent a broad class of multiscale material structures that can find applications in stimuli-responsive materials. Design and optimization of the complex adaptive structures require understanding and tuning of multiscale transport properties of materials. Studying spatial and temporal diffusion further increases the complexity and introduces uncertainty in quantifying the model prediction. In this work, a series of 3D fractal structures with varying fractal dimensions were constructed using the diffusion-limited aggregation (DLA) algorithm. The fractal structures were 3D printed with a fused filament fabrication process (FFF), and transient heat-transfer properties were analyzed using thermal imaging. The experimental results were analyzed to obtain the time constants and characteristic length scales of diffusion as a function of the average fractal dimension.

Ramesh, D., D'Souza, N. Experimental and computational investigation of PVDF-BaTiO3 interface for impact sensing and energy harvesting applications. SN Appl. Sci. 2(6), 1129 (2020). doi.org/10.1007/s42452-020-2788-y.

The investigation of the PVDF-BaTiO3 composite, utilizing DFT calculations, charge density and dielectric property analysis, not only unveils its enhanced electrical properties but also highlights the potential for advanced impact sensing applications due to improved interfacial interactions and increased piezoelectric response, contributing both scientifically and technologically to material science and sensor development.

Rizvi, H. R., D'Souza, N., Ayre, B., & Ramesh, D. (2019). Bioinspired cellular sheath-core electrospun non-woven mesh. Emergent Materials, 2(3), 1-14. https://doi.org/10.1007/s42247-019-00043-7.

 Fibers are valuable to biomedical applications. Used as sutures or meshes, there is an increased dual need to provide functionality such as drug delivery. Porosity represents a high surface area to volume architecture. Coaxial fibers with porous and non-porous layers offer a new design framework for fiber design that can resolve dual needs of mechanical robustness with transport phenomena. Using preferential solubility of a polymer in supercritical CO2, we develop a new architecture using biocompatible polymers based on porous core-sheath fiber fabrication technique. Polycaprolactone was selected as the CO2 miscible phase and Poly(butyrate adipate terephthalate)(PBAT) as the immiscible phase. The mechanical performance of the fibers was investigated using quasi static and dynamic loading. SEM images indicate no physical detachment of the two polymer surface after CO2 exposure indicating a successful amalgamation of polymers at the boundary of core and sheath. PCL as a sheath and as a core showed an increase of 650% and 468% in tensile strength compared to pristine PCL and PBAT. Introduction of porosity on the surface of coaxial fiber fPCL(cPBAT) further enhanced the yield strength increases by 40%. Dynamic mechanical analysis was used to analyze the viscoelastic properties of the fibers. The storage and loss modulus for coaxial fibers shows superior modulus throughout the glassy, glass transition and rubbery region as compared to the pristine PCL and PBAT, showing enhancement in both the elastic and viscous response of the material. The results indicate a new approach that is free of volatile organic solvents to manipulate the architecture of the cross-section of the electrospun fiber and tailor mechanical properties to the required application.

Ramesh, D., D'Souza, N. One-step fabrication of biomimetic PVDF-BaTiO3 nanofibrous composite using DoE. Materials Research Express 5(8), 85308. doi:10.1088/2053-1591/aad156

Dielectric polyvinylidene fluoride (PVDF) and Barium titanate (BaTiO3)-PVDF nano-fibrous composites were made using the electrospinning process based on a design of experiments approach. An ultrasonication process was optimized using 2 k factorial DoE approach to disperse BaTiO3 particles in PVDF solution in DMF. Scanning electron microscopy was used to characterize the microstructure of the fabricated mesh. The FT-IR and Raman analysis were carried out to investigate the crystal structure of the prepared mesh. Surface morphology contribution to the adhesive property of the composite was explained through contact angle measurements. The capacitance of the prepared PVDF-BaTiO3 nanofibrous mesh was a more than 40% increase over the pure PVDF nanofibers. The results obtained indicates that electrospinning offers a potential way to produce nanofibers with desired crystalline nature which was not observed in molded samples. In addition, BaTiO3 can be used to increase the capacitance, desired surface characteristics of the PVDF nanofibers which can find potential application as flexible piezoelectric sensor mimicking biological skin for use in structural health monitoring applications.

Ramesh, D., Ayre, B. G.,Webber, C. L., D'Souza,N. A. Dynamic mechanical analysis, surface chemistry and morphology of alkali and enzymatic retted kenaf fibers. Textile Research Journal 85 (19), 2059-2070. doi:10.1177/0040517515576322.

Bast fibers grow in the bark layer of many plants and have been used for textiles and cordage for over 6000 years. Bast fibers are expanding into new markets of non-woven fabrics and composite materials, and a comparative assessment of surface reactive groups and mechanical properties after different retting procedures is of value. Here, bast fiber of kenaf (Hibiscus cannabinus L., Malvaceae) were prepared by (1) alkali retting with 2% NaOH and (2) enzymatic retting with pectinase, and compared with commercially-available fiber retted by the natural microbe population in ocean water. Fiber structure was analyzed by fluorescence and electron microscopy; fiber chemistry was assessed by Raman and X-ray photoelectron spectroscopy, and by carbohydrate analysis; and mechanical properties were determined by dynamic mechanical analysis. Collectively, these show that enzymatic and microbial retting preserve the natural fiber structure and result in superior mechanical properties compared with alkali retting, which disrupts structure and degrades quality. The impacts of the retting procedure on fiber chemistry, morphology and mechanical properties are discussed.

Schlesinger, J., Rajander, J., Ihalainen, J. A., Ramesh, D., Eklund, P., Fagerholm, V., Solin, O. Isomerism of [64Cu-NOTA-Bn]-labeled radiotracers: separation of two complex isomers and determination of their interconversion energy barrier using ion pair chromatography. Inorganic Chemistry 50(10), 2059-20

The model complex [64Cu((S)-p-NH2-Bn-NOTA)]− ([64Cu]1) was used to study the isomerism of [64Cu-NOTA-Bn]-labeled radiotracers. Two complex isomers [64Cu]1a and [64Cu]1b, which were formed at a ratio of 1:9 during the complexation of [64Cu]Cu2+ with (S)-p-NH2-Bn-NOTA, were separated using ion pair chromatography. To study the interconversion, the nonradioactive complex isomers Cu1a and Cu1b were separated and thermally treated at 90 °C in both ammonium acetate solution and deionized water. A faster interconversion rate was observed for both isomers with lower concentrations of ammonium ions. At the end of reaction, the thermodynamic Cu1a to Cu1b equilibrium ratio was 6:94. The particular energy barriers of the interconversion for Cu1a and Cu1b were 130 kJ mol−1 and 140 kJ mol−1. Spectrophotometric measurements with Cu1a and Cu1b revealed two isomers adopting different geometrical configurations.