1.10 The Ways to Enhance the Stability of Nano fluids

1.10.1 Surfactants Used in Nanofluids

Surfactants used in nano fluids are also called dispersants. Adding dispersants in the two-phase systems is an easy and economic method to enhance the stability of nano fluids. Dispersants can markedly affect the surface characteristics of a system in small quantity. Dispersants consists of a hydrophobic tail portion, usually a long-chain hydrocarbon, and a hydrophilic polar head group.

 Dispersants are employed to increase the contact of two materials, sometimes known as wet ability. In a two-phase system, a dispersant tends to locate at the interface of the two phases, where it introduces a degree of continuity between the nano particles and fluids. 

According to the composition of the head, surfactants are divided into four classes: non-ionic surfactants without charge groups in its head (include polyethylene oxide, alcohols, and other polar groups), anionic surfactants with negatively charged head groups (anionic head groups include long-chain fatty acids, sulfosuccinates, alkyl sulfates, phosphates, and sulfonates), cationic surfactants with positively charged head groups (cationic surfactants may be protonated long-chain amines and long-chain quaternary ammonium compounds), and amphoteric surfactants with zwitterionic head groups (charge depends on pH. The class of amphoteric surfactants is represented by betaines and certain lecithins). 

How to select suitable dispersants is a key issue. In general, when the base fluid of nano fluids is polar solvent, we should select water-soluble surfactants; otherwise, we will select oil-soluble ones. For non-ionic surfactants, we can evaluate the solubility through the term hydrophilic/lipophilic balance (HLB) value. The lower the HLB number, the more oil-soluble the surfactants, and in turn, the higher the HLB number, the more water-soluble the surfactants is. The HLB value can be obtained easily by many handbooks. Although surfactant addition is an effective way to enhance the dispersibility of nano particles, surfactants might cause several problems. 

For example, the addition of surfactants may contaminate the heat transfer media. Surfactants may produce foams when heating, while heating and cooling are routine processes in heat exchange systems. Furthermore, surfactant molecules attaching on the surfaces of nano particles may enlarge the thermal resistance between the nano particles and the base fluid, which may limit the enhancement of the effective thermal conductivity.

1.11 Surface Modification Techniques: Surfactant-Free Method

Use of functionalized nano particles is a promising approach to achieve long-term stability of nano fluid. It represents the surfactant-free technique. Yang and Liu presented a work on the synthesis of functionalized silica (SiO₂) nano particles by grafting silanes directly to the surface of silica nano particles in original nano particle solutions. 

One of the unique characteristics of the nano fluids was that no deposition layer formed on the heated surface after a pool boiling process. Hwang et al. introduced hydrophilic functional groups on the surface of the nano tubes by mechano chemical reaction. The prepared nanofluids, with no contamination to medium, good fluidity, low viscosity, high stability, and high thermal conductivity, would have potential applications as coolants in advanced thermal systems. A wet mechano chemical reaction was applied to prepare surfactant-free nano fluids containing double- and single-walled CNTs. 

Results from the infrared spectrum and zeta potential measurements showed that the hydroxyl groups had been introduced onto the treated CNT surfaces. The chemical modification to functionalize the surface of carbon nano tubes is a common method to enhance the stability of carbon nano tubes in solvents. 

Here, we present a review about the surface modification of carbon nano tubes. Plasma treatment was used to modify the surface characteristics of diamond nano particles . Through plasma treatment using gas mixtures of methane and oxygen, various polar groups were imparted on the surface of the diamond nano particles, improving their dispersion property in water. A stable dispersion of titania nano particles in an organic solvent of diethylene glycol dimethylether (diglyme) was successfully prepared using a ball milling process. 

In order to enhance dispersion stability of the solution, surface modification of dispersed titania particles was carried out during the centrifugal bead mill process. Surface modification was utilized with silane coupling agents, (3-acryl-oxypropyl) trime thoxysilane and trime thoxypropylsilane. Zinc oxide nano particles could be modified by polymethacrylic acid (PMAA) in aqueous system. 

The hydroxyl groups of nano-ZnO particle surface could interact with carboxyl groups of PMAA and form poly (zinc methacrylate) complex on the surface of nano-ZnO. PMAA enhanced the dispersibility of nano-ZnO particles in water. The modification did not alter the crystalline structure of the ZnO nano particles.