## 3 Stability of Nanofluids  
The low stability of nanofluids over prolonged periods and their proneness to agglomeration have made their use on a large scale difficult. The nanofluids can lose their capacity to transfer heat and have their thermophysical properties altered due to the sedimentation [52]. The fast sedimentation of these colloidal suspensions of nanoparticles in a base fluid can also cause problems related to viscosity and flow in channels with complex geometries, mainly in microchannel systems [53]. Traditional techniques are used to solve the problems related to the stability and sedimentation of nanofluids. The most common are: the addition of surfactants, ultrasonic vibration, and controlling the pH value of the suspensions. In addition, new techniques have been presented to improve the stability of nanofluids and increase heat transfer [54]. In this work, the authors compare two distinct fluids; one formed by a traditional NF containing Al\({}_{2}\)O\({}_{3}\) nanoparticles, and the second a novel NF formed by polyacrylamide acid (PAA)-coated iron oxide nanoparticles (Fe\({}_{3}\)O\({}_{4}\)@PAA), obtained from a hydrothermal synthesis route. The results showed levels higher than the stability for the NF produced by the hydrothermal synthesis process.  
The option of adding surfactants to the nanofluids can be seen in several literature works [55; 56; 57]. The stabilization of nanofluids is achieved by reducing surface tension and increasing the immersion of particles in the base fluid [52]. However, the use of surfactants has some disadvantages. It interferes with the thermal properties of nanofluids compromised by increasing thermal conductivity, creating thermal resistance during the flow [58; 59; 60]. When heated at high temperatures, it can generate foams [58; 59], which when used during the nanoparticles' synthesis prevents their clustering only for a certain period of time [2; 61].  
Ultrasonic vibration is an efficient method to break down agglomerates of nanoparticles, making the mixtures more homogeneous. The dispersion level of nanoparticles in the base fluids will depend on the duration of the sonication process. Another important effect is that the size of the nanoparticles can change with the sonification time, and for this reason, more studies are needed. For example, using Au-water nanofluids, [PERSON] and [PERSON] [62] evaluated the effect of sonification time (from 10-60 min) on the particles size present in the nanofluids. The results obtained showed a decrease in particle size up to 45 min after beginning the process, while after that, no reduction was observed. The thermal conductivity is also affected by the dispersion of nanofluids and consequently has a direct impact on heat transfer. In the work of [PERSON] et al. [2], they revealed that the nanofluids produced more recently exhibit slightly higher thermal conductivities than those stored up to two months.  
The effect of pH on the properties of nanofluids is directly related to the electrostatic forces of interaction between particles dispersed in the base fluid. If the repulsion force between the particles is great, the colloidal mixture tends to stabilize through the high surface charge density generated [58]. In addition, when repulsive forces increase, particles can move freely due to Brownian motion, thus facilitating thermal transport [63]. However, if the electrostatic force is weak, the particles are attracted to each other, causing them to agglutinate and become sediment. Figure 1 shows several parameters that may affect the stability of nanofluids.