Nanofluid is a fluid containing nanometer-sized particles, called nanoparticles. These are having suspensions of nano scaled particles in a base fluid. The nanoparticles used in nanofluids are typically made of metals, oxides, carbides, or carbon nanotubes. Water, ethylene glycol and oil are commonly used as base fluids . Size of nanoparticles is commonly varied from 20nm to 100 nm. The smallest nanoparticles of few nanometers of diameter may contain thousand atoms. The properties that the nanoparticles can possess are significantly different from their parent materials and nano scaled particles may interact differently within their molecular bond with the base fluids than the microparticles and respond differently for mass-energy transfer applications. Nanofluids are basically belongs to a two-phase systems, in which solid phase is dispersed in liquid phase. The thermo-physical properties such as thermal conductivity, convective heat transfer coefficients, thermal diffusivity and viscosity have been found improved in case of nanofluids as compared to base fluids like oil or water. This improvement in thermo-physical demonstrated great potential applications in many fields. 
1. High specific surface area and therefore more heat transfer surface between particles and fluids.
2. The suspended nanoparticles in base fluid results enhancement in thermal conductivity which results improves the efficiency of heat transfer systems.
3. Reduced pumping power as compared to pure liquid to achieve equivalent heat transfer intensification which results system minimization.
4. Lesser energy requirements due to system minimization and improved heat transfer and heat carrying capacities.
5. Properties like thermal conductivity and surface wettability are also adjustable by varying particle concentrations in base fluid, to suit different applications.
6. Particle clogging can be reduced as compared to conventional slurries, hence it promotes system miniaturization.
1.2 Problems with Nanofluids
There are some important issues which have been faced for this two-phase system. Stability of nanofluid is one of the major problems and it remains a big challenge to achieve desired stability of nanofluids. Major problems include:-
1. Agglomeration - It is kind of cluster formation of nanoparticles after a period of time. It may be before or after the mixing in base fluid. Particles dispersed in base fluid may adhere together and form aggregates of increased size which may settle down due to gravity action. This agglomeration not only results settlement and clogging but thermal conductivity of nanofluid also decreased. When nanoparticles get agglomerated, they often lose their high-surface area due to grain growth so heat transfer rate will reduce.
2. Viscosity enhancement - Addition of nanoparticles in base fluid also enhance viscosity of mixture or nanofluid. Due to this pressure drop will increase in flow applications like in refrigeration hence pumping work will increase. So the optimum quantity of nanoparticles should be estimated.
3. Abrasion and erosion ' Due to high velocity of particles in pipe flow the effect of erosion will come into picture. This may result wear and tear of piping system.
4. Sedimentation ' Settling down of particles due to high density. If particle size is large then it will settle down and properties of nanofluid will be affected.
1.3 Ways to enhance the stability of Nanofluids
As already discussed, the agglomeration and clustering of nanoparticles in the nanofluid takes place before and after the nanofluid production and during its application. So depending on different factors following stability methods are used:-
1. Use of Surfactants ' These are also known as dispersants. Use of surfactants in the nanofluids is an easy and economical method to enhance the nanofluid stability. Surfactants basically affect the surface characteristics of a system. They consists of a hydrophobic tail portion which is usually a long-chain hydrocarbon and a hydrophilic polar head group. Dispersants are employed to increase the wettability between two phases. In a nanofluid, a surfactant tends to locate at the interface of the two phases, so basically it introduces a degree of continuity between the base fluid and nanoparticles. However for high temperature applications, the functionality of the surfactants is also a big concern. According to the composition, dispersants are classified into four classes: nonionic surfactants without charge groups (include alcohols, polyethylene oxide, and other polar groups), anionic surfactants with negatively charged head groups (include long-chain fatty acids, alkyl sulfates, sulfosuccinates, sulfonates and phosphates), cationic surfactants with positively charged head groups (include protonated long-chain amines and long-chain quaternary ammonium compounds), and amphoteric surfactants with zwitterionic head groups charge depends on pH (include betaines and certain lecithins). 
2. Surface Modification Techniques - Surfactant-Free Method - Use of functionalized nanoparticles is a promising approach to achieve long-term stability of nanofluid, this technique represents the surfactant free approach. Yang et al. presented a work regarding the synthesis of functionalized Silica (SiO2) nanoparticles by grafting silanes directly to nanoparticles surface in original nanoparticle solutions. By doing this a unique characteristics of the nanofluids was found that after a pool boiling process no deposition layer formed on the heated surface. Researchers also introduced hydrophilic functional groups on the nanotubes surface by mechanochemical reaction. The prepared nanofluids have no contamination to medium, low viscosity, enhanced stability, good fluidity and high thermal conductivity and also have potential to be used as coolants in thermal systems. This chemical modification to functionalize the surface of carbon nanotubes is a common method which results more stability of carbon nanotubes in solvents. One other method is used to modify the surface characteristics of Diamond nanoparticles is Plasma treatment. Plasma treatment uses gas mixtures of methane and oxygen, various polar groups are imparted on the surface of the diamond nanoparticles, which improves their dispersion property in water. 
3. Ultrasonic vibration ' Use of ultrasonic bath, processor and homogenizer are very effective techniques to break down the agglomerations. Ultrasonic vibrations are used to separate the particles. In this method nanoparticles are separated with strong and irregular ultrasonic shock inside the interaction chamber. This helps to get homogeneous suspensions of nanoparticles in base fluid with fewer aggregated particles at high-pressure. This procedure can be repeated for number of times (generally three times) to achieve the required homogeneous distribution of particles in the base fluids. This is an effective and simple method to eliminate agglomeration. 
4. Other stability mechanisms ' The term stability refers that the particles do not aggregate at a notable rate. The rate of aggregation is determined by the collisions frequency and the cohesion probability of during collision. Derjaguin, Verway, Landau and Overbeek (DVLO) developed a theory to achieve colloidal stability. DLVO theory propose that the particle stability in solution (nanofluid) is determined by the sum of attractive and electrical double layer repulsive forces of van der Waals, which exist between particles. If the force of attraction is greater than the force of repulsion, than two particles will collide, and the suspension is unstable. If a sufficient high repulsion is there between particles, the suspensions will be more stable. To have stable nanofluids, the repulsive forces between nanoparticles must be dominant. According to the types of repulsion, there are two fundamental mechanisms that affect solution stability, which are known as steric repulsion, and electrostatic repulsion. In steric stabilization, suspension system involves polymers and they will adsorb onto the nanoparticles surface, this results an additional steric repulsive force, which makes the nanofluid more stable. For example, if Zinc oxide (ZnO) nanoparticles can be modified by polymethacrylic acid (PMAA), this will result good compatibility with polar solvents. Silver nanofluids are found very stable when surface is modified with polyvinylpyrrolidone (PVP), which retards the growth and rate of agglomeration of nanoparticles will be reduced by steric effect. The Graphite suspension can also become more stable with the use of PVP. In electrostatic stabilization, to generate repulsive forces the surface charge will be developed on particles. It may be obtained through following mechanisms:
1. Preferential adsorption of ions.
2. Dissociation of surface charged species.
3. Accumulation or depletion of electrons at the surface.
4. Physical adsorption of charged species onto the surface.
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