2.1. Preparation of nanofluids

1. Introduction

62 Microfluidics and Nanofluidics

the workpiece [3–5].

grinding wheel and workpiece [6–8].

pressure gases are quite limited [12–14].

The most significant features of grinding process are high grinding wheel speed and high energy consumption. As abrasive grains on grinding wheel surface in grinding process are usually cut at negative rake, the energy consumed for grinding process to remove unit volume of materials is far greater than other machining forms and most of the energy will be converted into heat quantity [1, 2]. And more than 90% of the heat quantity will be transferred inside grinding wheel and workpiece, reducing the life of the grinding wheel and surface quality of

Cooling and lubrication grinding processing method applied to industrial production at the earliest is flood grinding, namely, injecting a large quantity of grinding fluid in grinding zone in the form of continuous fluid supply. However, the application of a large amount of grinding fluids results in high cost, and moreover, "air barrier" phenomenon generated surrounding the grinding wheel, preventing grinding fluids from entering the wedge-shaped zone between

As a kind of green and environmentally friendly processing technology appearing at the earliest, dry processing technology has abolished the application of cutting fluids on the precondition that machining precision and tool service life are guaranteed. However, it is difficult for heat quantity generated during the dry grinding process to be taken away by debris and the main heat quantity will still be transferred into grinding wheel and workpiece. Minimum quantity lubrication (MQL) grinding processing technology is another green machining technology [9–11]. It refers to a grinding processing technology in which minimum quantity lubricant is mixed and atomized with gases under a certain pressure and then sprayed into grinding zone so as to exert cooling and lubricating effects. However, study shows that heat quantity generated by the grinding process and brought away by high-

In view of flood grinding and cooling properties of MQL, researchers have been anxious to seek for a new cooling and lubricating form to apply to the grinding process. Relevant theories about heat transfer enhancement show that the heat transfer capability of solid is greater than that of liquid and gas [15–17]. On this basis, researchers have put forward nanofluid minimum quantity lubrication cooling (NMQLC) technique which adds a certain amount of nanoscale solid particles into degradable minimum quantity lubricant to generate nanofluids, and then nanofluids will be atomized through high-pressure gas and transferred into grinding zone in way of jet flow [18– 22]. Sheikholeslami [23, 24] simulated the natural convection of water-based CuO nanofluid considering Brownian motion. Results showed that the Nusselt number decreases with the increase of Hartmann number and increases with the increase of volume fraction and Rayleigh number of nanofluids. NMQLC grinding process has integrated all advantages of MQL grinding process, solved heat transfer problems of MQL grinding, significantly improved workpiece surface quality and burning phenomenon, lengthened service life of grinding wheel and improved working environment. Therefore, it is a green and environmentally friendly, high-

efficiency, and low-energy-consumption grinding processing technology [25, 26].

Preparation of nanofluids is the precondition for NMQLC grinding and high-quality nanofluids can obtain favorable cooling and lubricating effects. Nowadays, preparation method of nanofluids can be divided into two types: single-step method and two-step method. Single-step nanofluid preparation method is to disperse nanoparticles in base fluid while preparing them, which then saves the problems like nanoparticle collection and storage and can effectively avoid the oxidizing reaction of metal nanoparticles in atmosphere. This method is of high cost and small preparation quantity, and it is not suitable for actual batch application in production [6].

Two-step nanofluid preparation method is to add a certain proportion of nanoparticles in base fluid and select corresponding surface dispersant together with supersonic vibration according to physical and chemical properties so that nanoparticles can be distributed uniformly and stably in base fluid and nanofluid with suspension stability will be formed [27]. The two-step nanofluid preparation method is of simple operation and extensive application and it is nearly applicable to the preparation of all kinds of nanofluids.
