Choi, in 1995, noticed that adding nanoparticles to ordinary fluids significantly enhanced their thermal conductivity over several modes of heat transfer [1] which can have significant implications to the thermal performance of many applications. Following this, numerous studies have shown that nanoparticles can greatly improve the heat transfer properties of conventional fluids such as water, oil, refrigerants, and alcohol [2, 3]. The efficiency of nanofluids (NFs) in heat transfer applications is closely linked to their stability [4,5,6,7,8,9]. Ensuring optimal dispersion of nanoparticles in base fluids is challenging due to their higher densities, leading to sedimentation and agglomeration. This necessitates preventing sedimentation and agglomeration, which can destabilize mixtures through gravitational and electrostatic forces (e.g., Van der Waals forces). Stability is influenced by preparation methods, base fluid and nanoparticle types, and the properties of the mixtures, e.g., control of pH or addition of surfactants.
Al2O3-H2O NFs are considered in the current study due to the high thermal conductivity of the Al2O3 nanoparticles [10], cost-effectiveness and practical applicability in heat transfer systems. The thermal conductivity of Al2O3 has been reported in the range of 36-40 W/m·K at 300 K [11, 12], significantly higher than that of water, which is approximately 0.613 W/m·K at 300 K [1]. Al2O3 is also one of the most economically viable nanoparticles, with reported costs as low as 1.87 EUR·g−1, making it more affordable than other nanomaterials such as silver or carbon-based alternatives [13]. Al2O3 nanoparticles provide a good balance of performance and stability [14], while water has been chosen as a base fluid due to its high-specific-heat capacity [11]. This is the reason behind their widespread use in the literature [15, 16].
Changes in pH modify the surface charge of nanoparticles, which modulates the electrostatic interactions between them. More specifically, changing the pH affects the degree of repulsive or attractive forces between nanoparticles, impacting their dispersion and tendency to agglomerate. For example, Sharma et al. [17] indicated that pH, particularly when deviating from the isoelectric point (IEP), affects NF stability as it controls the particle-to-particle electrostatic interactions and aggregation behavior. Zeta potential, which quantifies the electrostatic repulsion between particles, is commonly used to determine colloidal stability, with values beyond ± 30 mV indicating stable suspensions as reported by Cacua et al. and Chakraborty et al. [18, 19]. Several studies have investigated the influence of pH on nanoparticle stability. For example, Ji et al. [20] examined the effect of pH on the stability of γ-Al2O3 NFs. They found that at pH levels below the point of zero charge, γ-Al2O3 particles exhibited positive charge, leading to high zeta potential and preventing particle aggregation. Similarly, Choudhary et al. [21] demonstrated that modifying the pH affects both the electrostatic repulsion and particle size distribution. Other studies, such as Cacua et al. [19], have shown that pH adjustment, combined with surfactant use, can further optimize stability by controlling particle-surface interactions. These studies provide representative examples of how pH can influence electrostatic interactions and particle behavior in Al2O3-based NFs.
While some studies have examined the impact of volume concentrations on Al2O3-H2O NF stability, they have not thoroughly considered pH effects or optimal stabilization parameters given different preparation methodologies [20,21,22,23,24,25,26,27,28,29]. Increasing the nanoparticle volume fraction often enhances thermal conductivity but can also promote agglomeration, leading to instability [28]. Some studies suggest that low nanoparticle concentrations are more stable due to reduced particle-particle interactions [29, 30]. Therefore, the present study focuses on a concentration range of 0.01-0.05 vol.%, which has been frequently used in Al2O3 NF research [23]. However, its long-term stability—particularly under surfactant-free conditions and varying pH—remains insufficiently characterized, motivating the present investigation. In addition, this study applies multiple characterization techniques to the same surfactant-free samples over an extended period, a combination not commonly addressed in prior literature.
To better understand the factors affecting NF stability, a review of previous studies was conducted. Several researchers have explored different stability evaluation methods, focusing on techniques such as zeta potential, SEM, TEM, DLS, and sedimentation imaging.
Table 1 provides a comparison of recent published research investigating stability assessment approaches, along with key findings related to NF dispersion and aggregation behavior.
Table 1 demonstrates that pH adjustment, ultrasonication duration, and the use of surfactants are critical parameters influencing NF stability. Studies consistently indicate that pH values away from IEP enhance stability, with zeta potential values above ± 30 mV correlate with improved dispersion. Optimized ultrasonication (30-180 min) improves nanoparticle dispersion, though excessive sonication can alter nanoparticle properties.
While some studies have examined NF stability over extended periods, most used surfactants for stabilization. Although surfactants enhance nanoparticle dispersion, their long-term reliability is limited, as they degrade at elevated temperatures and can eventually lead to unexpected nanoparticle aggregation. This raises concerns about their effectiveness in high-temperature applications and highlights the need for alternative stabilization strategies.
On the other hand, studies that avoided surfactants often assessed stability over short durations and relied on limited characterization techniques. This makes it difficult to determine the long-term behavior of NFs when stabilization depends solely on electrostatic repulsion and preparation conditions. A comprehensive long-term evaluation using multiple characterization methods remains limited in the literature.
The current study addresses these gaps by conducting a systematic, multi-parameter characterization of Al2O3-H2O NF stability over two months, evaluating the effects of pH, preparation methods, and nanoparticle concentration. A key distinction is that no surfactants were used, ensuring that observed stability trends result purely from electrostatic stabilization mechanisms. Surfactants degrade over time, especially at elevated temperatures, which can lead to unexpected and detrimental effects on colloidal stability.