Editorial for the Special Issue “New Trends and Processes in Nanofluids and Carbon-Based Nanoparticles”

1. Introduction

The accelerating advancements in energy technologies, thermal management systems, and electrochemical processes have intensified efforts to develop high-performance materials that outperform conventional heat transfer media and electrolyte solutions [1]. Nanofluids (colloidal systems engineered by dispersing nanoscale particles) have emerged as versatile solutions, offering superior thermal conductivity, enhanced electrical performance and tunable physical properties [2]. Within the broad range of investigated nanostructures, carbon-based materials stand out for their exceptional thermal and electrical conductivities, wide aspect ratio spectrum, and ability to create interconnected networks. These unique features make them primary candidates for innovative formulations aimed at enhancing energy efficiency and device performance [3]. Recent research has increasingly explored strategies involving unconventional base fluids, incorporation of hybrid nanoparticles [4], or advanced surface functionalization strategies [5]. A notable example is the use of ionic liquids, which confer unique properties such as wide electrochemical stability or excellent thermal behavior [6]. Parallel developments in the synthesis of carbon nanostructures such as graphene derivatives, carbon nanotubes, carbon dots, and carbon–metal hybrid systems have further expanded the design space for high-performance nanofluids [7]. At the same time, computational modeling and machine learning-assisted predictive tools are becoming an integral part of understanding transport mechanisms and driving recipe optimization.

Although carbon-based nanofluids show great promise, significant challenges still require further investigation. Many nanofluids still exhibit stability issues, which limit their industrial applications [8]. Theoretical models often fail to accurately represent the physical properties of nanofluids, leading to discrepancies between experimental results and model predictions [9]. Furthermore, the lack of standardized protocols for nanofluid preparation and measurement methodologies hinders comparisons between studies. Additional experimental research is necessary to prove nanofluid reliability for industrial use, as well as to confirm theoretical models under practical conditions. Similarly, numerical simulations are essential for elucidating the complex mechanisms governing these materials and optimizing formulation parameters. Finally, comprehensive review articles also play a pivotal role in consolidating the rapidly expanding literature and identifying persistent scientific gaps that may guide future research.

2. Six Articles, Six Angles on the Same Problem

The six contributions to this Special Issue collectively address key areas of the field, ranging from theoretical insights and modeling approaches to the practical challenges involved in material design, while also exploring their prospective impact on real-world devices.

“Nanofluids and Ionic Fluids as Liquid Electrodes: An Overview on Their Properties and Potential Applications”

Pereira et al. [10] provide a comprehensive review of electro-active nanofluids, ionanofluids, and ionic liquids as liquid electrodes, highlighting their electrochemical properties and technological potential. These innovative materials are attracting increasing attention for applications in energy storage and conversion systems, including redox flow batteries, dye-sensitized solar cells, and electrochemical sensors. The article explains the mechanisms behind their superior performance, which result from the synergistic combination of the pathways and electric double layers created by nanoparticle dispersions, together with the broader electrochemical stability and thermal resilience provided by ionic liquids. The review underscores the role of carbon-based nanostructures (such as graphene and carbon nanotubes) and hybrid nanoadditives in enhancing the multifunctionality of developed materials.

“Thermophysical and Electrical Properties of Ethylene Glycol-Based Nanofluids Containing CaCO₃”

Traciak et al. [11] experimentally investigated ethylene glycol-based nanofluids loaded with calcium carbonate (CaCO₃) as eco-friendly alternatives for thermal management. Dispersions with CaCO₃ mass concentrations ranging from 1 to 3 wt% were prepared and characterized in terms of density, heat capacity, thermal conductivity, viscosity, surface tension, and electrical properties. Results revealed improvements in thermal conductivity of up to 14.8% accompanied by moderate viscosity increases (below 7%), which led to Mouromtseff number ratios >1 in both laminar and turbulent regimes. Electrical conductivity and dielectric permittivity also rose with both nanoparticle loading and temperature, likely due to electric double-layer effects and additional charge carriers introduced by CaCO₃.

“Why Carbon Nanotubes Improve Aqueous Nanofluid Thermal Conductivity: A Qualitative Model Critical Review”

Khoswan et al. [12] presented a qualitative model to elucidate the mechanisms responsible for the remarkable thermal conductivity enhancements observed when carbon nanotubes (CNTs) are dispersed in water. This literature survey intends to integrate existing theories (such as Brownian motion, interfacial nanolayer formation, and percolation networks) into a unified framework. Authors identified several critical parameters (including CNT morphology, length-to-diameter aspect ratio, and interfacial resistance) as primary drivers of heat transfer improvement. By correlating these factors with experimental findings, the study offers practical guidelines for selecting CNT types, adjusting concentrations, and improving colloidal stability to maximize thermal performance while minimizing viscosity-related limitations.

“Heat Transfer and Entropy Generation for Mixed Convection of Al₂O₃–Water Nanofluid in a Lid-Driven Square Cavity with a Concentric Square Blockage”

Korukçu [13] numerically analyzed the mixed convection in a lid-driven square cavity containing an isothermally heated square blockage at its center, occupying one quarter of the cavity height. The system was filled with Al₂O₃–water dispersions, and heat transfer and entropy generation were evaluated for various Richardson numbers (Ri = 0.01–100) and nanoparticle volume concentrations 0–5%, while maintaining a constant Grashof number (Gr = 100). The findings showed that increasing nanoparticle loading and reducing Ri number significantly enhanced the average Nusselt number on the blockage surface. Conversely, entropy generation increases as particle concentration rises and Ri values decreases. The research derived predictive correlations for heat transfer and entropy generation, offering practical guidance to improve thermal efficiency in industrial engineering systems.

“Synthesis of Cellulose-Based Fluorescent Carbon Dots for the Detection of Fe(III) in Aqueous Solutions”

Magagula et al. [14] reported a rapid, one-step microwave-assisted method to produce nitrogen-doped carbon quantum dots (N-CQDs) from cellulose nanocrystals and urea. The synthesized N-CQDs displayed strong fluorescence, high sensitivity to Fe³⁺ ions, and pH-dependent stability. Additionally, the study introduced empirical models for fluorescence quenching, achieving remarkable performance with a detection limit as low as 75 nM. The study proved the versatility of carbon-based nanoparticles for environmental sensing, combining green nanotechnology with advanced optical properties to enable efficient quality monitoring of water.

“An Experimental Investigation of the Stability and Thermophysical Properties of MWCNT Nanofluids in a Water–Ethylene Glycol Mixture”

Cardenas Contreras et al. [15] conducted an experimental study on multi-walled carbon nanotube (MWCNT) nanofluids, formulated using an ethylene glycol/water mixture (50:50%) as a base fluid. Dispersions with volume concentrations ranging from 0.025 to 0.1% were prepared through a two-step approach combining ultrasonication and high-pressure homogenization. Stability assays confirmed no visible sedimentation and only a 10% reduction in relative concentration over 30 days. Thermal conductivity increased by up to 6.4%, while viscosity rose by a maximum of 11%, with both transport properties strongly influenced by temperature and nanoparticle content. Fitting correlations were developed to describe experimental thermal conductivity and viscosity data with average deviations below 1.5% and 3%, respectively.

3. Conclusions

Across the six studies, a common idea stands out: nanofluids can significantly enhance the thermal and electrochemical performance of practical applications, as long as their composition and operating conditions are carefully optimized. Carbon-based nanostructures can create interconnected conductive pathways, significantly enhancing thermal conductivity with only moderate rises in dynamic viscosity. In electrochemical systems, these advanced fluids can also provide notable benefits, including improved ionic conductivity, wider electrochemical stability, and faster redox reaction kinetics.

Despite these promising findings, key obstacles still need to be addressed before such tailored thermal fluids can achieve widespread industrial adoption:

• Ensuring stability throughout thermal cycles and under mechanical stress is critical since even small-scale aggregation can compromise performance or obstruct circulation.
• Uniform measurement practices and comprehensive property databases, particularly for dielectric behavior and thermal performance under varying conditions, are highly necessary to eliminate uncertainty in engineering decisions.
• Real-word performance also requires assessing flow properties and pressure drop loads to ensure manageable pumping requirements.
• Material integrity and safety of different system parts must be confirmed, as well as compliance with environmental and health standards.
• Practical adoption also relies on achieving cost efficiency and guaranteeing uniform properties during the preparation of industrial amounts of nanofluids.