## 1 Introduction
According to certain studies, when only a small amount of spherical, pseudo-spherical, or amorphous nanoparticles is used as a lubricant additive, hence composing a so-called \"nanolubricant\", it is possible to significantly improve the performance of the base lubricating oil [12], resulting in significant advantages such as friction reduction, anti-wear properties, remarkable self-healing capabilities, reduced energy consumption, and enhanced environmental protection [11; 13]. Researchers have been concentrating on nanolubricants for a variety of reasons, such as their capability to lower friction and wear, but particle suspension is the primary challenge to the creation of novel nanolubricants [14]. Preventing nanoparticle aggregation is essential for efficiently improving lubrication in a contact area and granting a uniformity of action of these lubricants from the beginning to the end of their use [15]. For the development of innovative lubricants and their formulation, the achievement of dispersion principles is fundamental [14]. To achieve optimal dispersion, a variety of chemical-physical processes, including ultrasonication, homogenization, and particle surface modification, is applied [16]. The good dispersion of nanolubricants can be probed using a variety of methods, each of which provides a different perspective regarding the way they function. Some of these techniques include:Visual Inspection: the dispersion is directly inspected visually in order to detect any observable (macro-)accumulation or deposition of nanoparticles [17; 18; 19; 20; 21]. * Dynamic light scattering (DLS): by measuring the hydrodynamic radius of nanoparticles, it is possible to discern the occurrence of accumulation [22]. * UV-visible spectroscopy (UV-Vis): by analysing light absorption patterns, it is feasible to discover the tendency of nanoparticles to accumulate and precipitate [20; 21; 23; 24]. * Zeta Potential: the dispersion stability of nanolubricants is measured via zeta potential (electric potential at the slipping plane) analysis, where higher zeta potential values (positive or negative) indicate better stability [16]. * Fourier Transform Infrared Spectroscopy (FTIR): this technique is used to indicate stable physical interactions between the dispersed nanoparticle and the base lubricant oil, providing insights into chemical compatibility and bonding [25]. [PERSON] et al. [26] evaluated the stability of TiO\({}_{2}\) and SiO\({}_{2}\) nanoparticles in Polyvinyl ether (PVE) lubricant using visual observation, UV-Vis spectrophotometric analysis, and zeta potential measurements. The concentration ratio of the TiO\({}_{2}\)/PVE nanolubricant remained at 95% for a duration of 30 days during the study. After 7 h of sonication, the TiO\({}_{2}\)/PVE, SiO\({}_{2}\)/PVE, and SiO\({}_{2}\)-TiO\({}_{2}\)/PVE samples showed zeta potential values of 203.1 mV, 224.2 mV, and 105.3 mV, respectively. [PERSON] et al. [21] employed sedimentation photography and UV-Vis spectrophotometry to evaluate the stability of SiO\({}_{2}\)/Polyalkylene Glycol (PAG) nanolubricant. The results indicated a low level of sedimentation seen throughout a one-month timeframe. The UV-Vis test provided additional confirmation of the stability of the studied nanolubricant, as it maintained a relative concentration for nanoparticles of over 70% compared to its initial concentration. [PERSON] et al. [20] showed that nano-TiO\({}_{2}\)/SiO\({}_{2}\) exhibited significant dispersion properties, particularly at a concentration of 0.75 wt%, even in the absence of a surfactant. The homogenous dispersion of the substance not only led to a stable suspension, but also demonstrated its effectiveness in reducing friction and wear, as compared to a blank palm trimethylolpropane (TMP) ester. The rheological behaviour and tribological characteristics of a nano-lubricant comprising SiO\({}_{2}\) nanoparticles in an SAE40 engine oil were studied by [PERSON] et al. According to their research, adding SiO\({}_{2}\) nanoparticles significantly improved a number of important features.