1.INTRODUCTION

          Heat transfer can be enhanced by employing various techniques and methodologies, such as increasing either the heat transfer surface or the heat transfer coefficient between the fluid and the surface that allow high heat transfer rates in a small volume. Cooling is one of the most important technical challenges facing many diverse industries, including microelectronics, transportation, solid-state lighting, and manufacturing.

There is, therefore, an urgent need for new and innovative coolants with improved performance. The addition of micrometer- or millimetre-sized solid metal or metal oxide particles to the base fluids shows an increment in the thermal conductivity of resultant fluids. But the presence of milli- or microsized particles in a fluid poses a number of problems. They do not form a stable solution and tend to settle down. Apart from the application in the field of heat transfer, nano fluids (nano meter particles in a fluid) can also be synthesized for unique magnetic, electrical, chemical, and biological applications. They also cause erosion and clogging of the heat transfer channels.

The novel concept of “nano fluids” has been proposed as a route to surpassing the performance of heat transfer fluids currently available. A very small amount of nano particles, when dispersed uniformly and suspended stably in base fluids, can provide impressive improvements in the thermal properties of base fluids. Nano fluids, which are a colloidal mixture of nano particles (1–100 nm) and a base liquid (nano particle fluid suspensions), is the term first coined by Choi in 1995 at the Argonne National Laboratory to describe the new class of nanotechnology-based heat transfer fluids that exhibit thermal properties superior to those of their base fluids or conventional particle fluid suspensions.

Several investigations have revealed that the thermal conductivity of the fluid containing nano particles could be increased by more than 20% for the case of very low nano particles concentrations. Nowadays a fast growth of research activity in this heat transfer area has arisen. In fact, the exponential increase in the number of research articles dedicated to this subject thus far shows a noticeable growth and the importance of heat transfer enhancement technology in general.

 Just to give some data in table is given the number of papers from 1993 to 2010 (up to April) found in SCOPUS under “Nano fluids” and the other two columns are the papers found under “Nano fluids AND Heat Transfer” and “Nano fluids AND Properties”. Moreover, in SCOPUS under “Nano fluids and Review” about 34 papers were given as that result. This indicates a the high interest in nano fluids activity research and the potential market for nano fluids for heat transfer applications is estimated by the CEA in 2007 to be over 2 billion dollars per year worldwide, with prospect of further growth in the next 5–10 year.

The aim of this special issue is to collect basic, application and review articles of the most recent developments and research efforts in this field, with the purpose to provide guidelines for future research directions. The order of the papers is given presenting a possible range of applications, a review on specific heat capacity, and an experimental study to evaluate the effects of particle species, surface charge, concentration, preparation technique, and base fluid on thermal transport capability of nano fluids. A survey on heat transfer in nano fluids is summarized in order to analyze the theories regarding heat transfer mechanisms in nano fluids and to discuss the effects of clustering on thermal conductivity. 

After some considerations to address whether the heat transfer in nano fluids still satisfies the classical energy equation are theoretically examined by the macro scale manifestation of the micro scale physics in nano fluids, an experimental investigation on natural convection heat transfer characteristics in nano fluids in an enclosure and a numerical study on turbulent forced convection flow of nano fluids in a circular tube subjected are presented in the last two papers.

In the dimensional scale a nanometre is a billionth of a meter. Nano scale science and engineering has revolutionized the scientific and technological developments in nano particles, no structured materials, nano devices and systems. National Science Foundation (2004) defines nanotechnology as the creation and utilization of functional materials, devices, and systems with novel properties and functions that are achieved through the control of matter, atom-by-atom, and molecule by molecule or at the macro molecular level. 

A unique challenge exists in restructuring teaching at all levels to include nano scale science and engineering concepts and nurturing the scientific and technical workforce of the future.This is because nanoparticles are usually used at very low concentrations and manometer sizes. These properties prevent the sedimentation in the flow that may clog the channel. From these points of view, there have been some previous studies conducted on the heat transfer of nanoparticles in suspension. Since Choi wrote the first review article on nanofluids [1],The advances in nanotechnology have resulted in the development of a category of fluids termed nano fluids, first used by a group at the Argonne National Laboratory in 1995 (Choi 19952).

Nano fluids are suspensions containing particles that are significantly smaller than 100 nm (Wen and Ding 2004), and have a bulk solids thermal conductivity of orders of magnitudes higher than the base liquids. Experimental studies conducted have shown   Wanget .al.,1999, Lee et.al 1999,Keblinski et.al 2002) that the effective thermal conductivity increases under macroscopically stationary conditions. Lee and Choy (1996), under laminar flow conditions, nano fluids in micro channels have shown a twofold reduction in thermal resistance (Lee and Choi, 1996) and dissipate heat power three times more than that of pure water. 

Studies conducted using water-Cu nano fluids (Xian and Li, 2003) of concentrations approximately 2% by volume was shown to have a heat transfer coefficient 60% higher than when pure water was used. Such advances must have a broader impact culminating in promoting teaching, training and learning. Dissemination of research results will enhance the scientific and technological understanding of nanotechnology.

This effort aims at bringing nanotechnology to the undergraduate level, especially at the applied level in engineering and technology curricula. The focus is to incorporate nanotechnology into existing course curricula such as heat transfer and fluid mechanics. The intention of the work described here is to introduce a simple experimental procedure in a heat transfer course to facilitate the understanding of the convective heat transfer behaviour of nano fluids.

         

1.1 HEAT  EXCHANGER

A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air.

There are two primary classifications of heat exchangers according to their flow arrangement. In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side. In counter-flow heat exchangers the fluids enter the exchanger from opposite ends. The counter current design is most efficient, in that it can transfer the most heat from the heat (transfer) medium. See counter current exchange. In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger.

For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing resistance to fluid flow through the exchanger. The exchanger's performance can also be affected by the addition of fins or corrugations in one or both directions, which increase surface area and may channel fluid flow or induce turbulence.

The driving temperature across the heat transfer surface varies with position, but an appropriate mean temperature can be defined. In most simple systems this is the "log mean temperature difference" (LMTD). Sometimes direct knowledge of the LMTD is not available and the NTU method is used.

The transfer of thermal energy between fluids is one of the most important and frequently used processes in engineering.   The  transfer  of  heat  is  usually  accomplished  by  means  of  a  device known as a heat exchanger.  Common applications of heat exchangers in the nuclear field include boilers, fan coolers, cooling water heat exchangers, and condensers. The basic design of a heat exchanger normally has two fluids of different temperatures separated by some conducting medium.   The most common design has one fluid flowing through metal tubes and the other fluid flowing around the tubes.  On either side of the tube, heat is transferred by convection.   Heat is transferred through the tube wall by conduction. 

Heat exchangers may be divided into several categories or classifications.  In the most commonly used type of heat exchanger, two fluids of different temperature flow in spaces separated by a tube wall.   They transfer heat by convection and by conduction through the wall.   This type is referred  to  as  an  "ordinary  heat  exchanger,"  as  compared  to  the  other  two  types  classified  as “regenerators" and "cooling towers."An ordinary heat exchanger is single-phase or two-phase.  In a single-phase heat exchanger, both of the fluids (cooled and heated) remain in their initial gaseous or liquid states.   In two-phase exchangers,  either  of  the  fluids  may  change  its  phase  during  the  heat  exchange  process.   The steam  generator  and  main  condenser  of  nuclear  facilities  are  of  the  two-phase,  ordinary  heat exchanger classification

1.1.1 REQUIREMENTS OF HEAT EXCHANGERS

1. High thermal effectiveness

2. Pressure drop as low as possible

3. Reliability and life expectancy

4. High-quality product and safe operation

5. Material compatibility with the process fluids

6. Convenient size, easy for installation, reliable in use

7. Easy for maintenance and servicing

8. Light in weight but strong in construction to withstand the operational pressures

9. Simplicity of manufacture

10. Low cost

11. Possibility of effecting repair to maintenance problems The heat exchanger must meet normal process requirements specified through problem specification and service conditions for combinations of the clean and fouled conditions, and un corroded and corroded conditions.

 The exchanger must bemaintainable, which usually means choosing a configuration that permits cleaning as required and replacement of tubes, gaskets, and any other components that are damaged by corrosion, erosion, vibration, or aging. This requirement may also place limitations on space for tube bundle pulling, to carry out maintenance around it, lifting requirements for heat exchanger components, and adaptability for in-service inspection and monitorin

1.2 TYPES OF HEAT EXCHANGER

1.2.1 Parallel Flow Heat Exchangers

            In general, parallel flow heat exchangers considered less efficient than counter flow heat exchangers in terms of transferring heat from one fluid to another. However, there are applications where parallel flow has its benefits, such as when limiting the transfer of heat is recommended.

Another advantage if parallel flow heat exchangers are used is that outlet temperature of the fluid being cooled can reach a limiting temperature. If water is kept above 32 deg F, freezing can be avoided.

While parallel flow arrangement can be beneficial, under certain conditions that reduce the limiting temperature, channelling problems can occur or freeze may be caused at shutdown.

          Thus, parallel flow in heat exchangers minimizes the chance of freezing or channelling, but does not eliminate the possibility of either. Adding supplemental heat is recommended to solve these problems.

Figure.1(Parallel Flow Heat Exchangers)

1.2.2 COUNTER FLOW

Counter flow, as illustrated  the  above  exists  when the two fluids flow in opposite directions. Each of the fluids enters the heat exchanger at opposite ends. Because the cooler fluid exits the counter flow heat exchanger at the end where the hot fluid enters the heat exchanger, the cooler fluid will approach the inlet temperature of the hot fluid. Counter flow heat exchangers are the most efficient of the three types. In contrast to the parallel flow heat exchanger, the counter flow heat exchanger can have the hottest cold fluid temperature greater than the coldest hot-fluid temperatue.

Figure.2 (counter flow)

1.2.3 CROSS FLOW

Cross flow, as illustrated below, exists when one fluid flows perpendicular to the second fluid; that is, one fluid flows through tubes and the second fluid passes around the tubes at 90 angle. Cross flow heat exchangers are usually found in applications where one of the fluids changes state (2-phase flow). An example is a steam system's condenser, in which the steam exiting the turbine enters the condenser shell side, and the cool water flowing in the tubes absorbs the heat from the steam, condensing it into water. Large volumes of vapor may be condensed using this type of heat exchanger flow.

                                     Figure.3 (cross flow)

1.3.1 Double pipe heat exchangers

A double-pipe heat exchanger consists of two concentric pipes or tubes. The outer tube is called the annulus. In one of the pipes a warmer fluid flows and in the other a colder one. Due to the temperature difference between the fluids heat is transferred. By the word ‘fluid’ all substances that can ‘flow’ is meant. So the word fluid means not only liquids but also gases. In this part there will be looked at a double-pipe heat exchanger with parallel flow. This means that the hot fluid and the cold fluid flow in the same directions. There are also counter flow heat exchangers. In this situation the hot fluid and the cold fluid flow in opposite directions.

Counter and parallel flow heat exchanger temperature profiles are as shown below. From this easily can be concluded that the counter flow is in any case more efficient than the parallel flow since the pipe fluid gets further cooled using this counter flow. While the temperatures T (of the cooled fluid) and t (of the warmed fluid) in the parallel flow heat exchanger can only approach each other, they can pass each other in the counter flow (Tout < tout) and in this case there has to be more heat been transferred. 

Figure.4 (double pipe heat exchanger)

This explains why in practice only counter flow will be seen in case of the double pipe heat exchangers. But there is one other advantage for the counter flow, since the maximum temperature differences between the two flows are much smaller, they suffer less thermal forces. Double pipe exchangers are mostly built of common water tubing. The use of two single flow areas leads to relatively low flow rates and moderate temperature differences. 

A straight double pipe heat exchanger as seen in the diagrams will not appear in practice. Most common are U-type or hairpin constructions. Due to the need of a removable bundle construction and the need for the ability to handle differential thermal expansions the exchanger is implemented in two parts.  The fluids enter and leave the exchanger by the four nozzles on the right while the exchanger can freely expand to the left which makes the of expansion joints to the other machinery superfluous and makes demounti.

1.4  DEFINITIONS

Nano = 10^-9

1nm  =10^-9 meter

Nano particle = particle with a size between 0.1-1000 nm

Nano fluid =nano particle mixed in a conventional fluid

Conventional Fluid = water, oil, ethylene glycol

Heating Element = converts electricity into heat

Convection = heat transfer between a solid and conventional fluid

Steady State = temperatures remain constant with time

Laminar = dominated by diffusion and velocity profile is nearly linear

Turbulent = dominated by turbulent mixing

Reynolds Number = transition between laminar and turbulent boundaries       

  

Criteria	Microparticles	Nanoparticles
stability	Settle	Stable(remainin suspention almost indefinitely)
Surface/volume ratio	1	1000 times larger than that of micro   particles
conductivity	Low	High
Clog in    microchannel	Yes	No
Erosion	Yes	No
Pumping          Power	Large	Small
Table-1              Nanofluid heat transfer enhancement                        ·        Thermal conductivity enhancement                        ·        Convective heat transfer enhancement                        ·        Critical heat flux enhancement.

1.5 ASSUMPTIONS

It is important to assume that some of the nano particles will deposit on the copper tubing and will not deposit on the rubber tubing.  It is important to assume the nano particles will not deposit on the rubber tubing.  It is also important to assume that there will be no heat loss as the nano fluid travels from the tank through the rubber tubing. 

Experiments will be conducted using the same procedures and equipment and will have the same factors monitored.  Temperature and flow rates were measured at steady state  indicated by the temperatures steady for a ten minute period.

1.6 NANO FLUIDS

Nano fluids, the fluid suspensions of nonmaterial’s, have shown many interesting properties, and the distinctive features offer unprecedented potential for many applications. The recent progress on the study of nano fluids, such as the preparation methods, the evaluation methods for the stability of nano fluids, and the ways to enhance the stability for nano fluids, the stability mechanisms of nano fluids, and presents the broad range of current and future applications in various fields including energy and mechanical and biomedical fields. At last, the paper identifies the opportunities for future research.

Nano fluids are a new class of fluids engineered by dispersing nanometer-sized materials (nano particles, nano fibers, nano tubes, nano wires, nano rods, nano sheet, or droplets) in base fluids. In other words, nanofluids are nanoscale colloidal suspensions containing condensed nano materials. They are two-phase systems with one phase (solid phase) in another (liquid phase). Nano fluids have been found to possess enhanced thermo physical properties such as thermal conductivity, thermal diffusivity, viscosity, and convective heat transfer coefficients compared to those of base fluids like oil or water. It has demonstrated great potential applications in many fields.

For a two-phase system, there are some important issues we have to face. One of the most important issues is the stability of nano fluids, and it remains a big challenge to achieve desired stability of nano fluids. In this paper, we will review the new progress in the methods for preparing stable nano fluids and summarize the stability mechanisms.

In recent years, nano fluids have attracted more and more attention. The main driving force for nano fluids research lies in a wide range of applications. Although some review articles involving the progress of nano fluid investigation were published in the past several years , most of the reviews are concerned of the experimental and theoretical studies of the thermo physical properties or the convective heat transfer of nano fluids. The purpose of this paper will focuses on the new preparation methods and stability mechanisms, especially the new application trends for nano fluids in addition to the heat transfer properties of nano fluids. We will try to find some challenging issues that need to be solved for future research based on the review on these aspects of nano fluids.

1.7 Preparation methods for nano fluids

1.7.1 Two-Step Method

Two-step method is the most widely used method for preparing nano fluids. Nano particles, nano fibers, nano tubes, or other nano materials used in this method are first produced as dry powders by chemical or physical methods. 

Then, the nano sized powder will be dispersed into a fluid in the second processing step with the help of intensive magnetic force agitation, ultrasonic agitation, high-shear mixing, homogenizing, and ball milling. Two-step method is the most economic method to produce nano fluids in large scale, because nano powder synthesis techniques have already been scaled up to industrial production levels. 

Due to the high surface area and surface activity, nano particles have the tendency to aggregate. The important technique to enhance the stability of nano particles in fluids is the use of surfactants. However, the functionality of the surfactants under high temperature is also a big concern, especially for high-temperature applications.

Due to the difficulty in preparing stable nano fluids by two-step method, several advanced techniques are developed to produce nano fluids, including one-step method. In the following part, we will introduce one-step method in detail.

1.7.2  One-Step Method

To reduce the agglomeration of nano particles, Eastmanetal. developed a one-step physical vapour condensation method to prepare Cu/ethylene glycol nano fluids . The one-step process consists of simultaneously making and dispersing the particles in the fluid. In this method, the processes of drying, storage, transportation, and dispersion of nano particles are avoided, so the agglomeration of nano particles is minimized, and the stability of fluids is increased . 

The one-step processes can prepare uniformly dispersed nano particles, and the particles can be stably suspended in the base fluid. The vacuum-SANSS (submerged arc nano particle synthesis system) is another efficient method to prepare nano fluids using different dielectric liquids. The different morphologies are mainly influenced and determined by various thermal conductivity properties of the dielectric liquids. 

The nano particles prepared exhibit needle-like, polygonal, square, and circular morphological shapes. The method avoids the undesired particle aggregation fairly well.

One-step physical method cannot synthesize nano fluids in large scale, and the cost is also high, so the one-step chemical method is developing rapidly. Zhuetal. presented a novel one-step chemical method for preparing copper nano fluids by reducing C uSO4⋅ 5H2O with NaH2PO2⋅ H2O in ethylene glycol under microwave irradiation. 

Well-dispersed and stably suspended copper nano fluids were obtained. Mineral oil-based nano fluids containing silver nano particles with a narrow-size distribution were also prepared by this method. The particles could be stabilized by K orating, which coordinated to the silver particle surfaces via two oxygen atoms forming a dense layer around the particles. The silver nano particle suspensions were stable for about 1 month. Stable ethanol-based nano fluids containing silver nano particles could be prepared by microwave-assisted one-step method .

 In the method, polyvinyl pyrrolidone (PVP) was employed as the stabilizer of colloidal silver and reducing agent for silver in solution. The cationic surfactant octadecylamine (ODA) is also an efficient phase-transfer agent to synthesize silver colloids . The phase transfer of the silver nano particles arises due to coupling of the silver nano particles with the ODA molecules present in organic phase via either coordination bond formation or weak covalent interaction. Phase transfer method has been developed for preparing homogeneous and stable graphene oxide colloids.

However, there are some disadvantages for one-step method. The most important one is that the residual reactants are left in the nano fluids due to incomplete reaction or stabilization. It is difficult to elucidate the nano particle effect without eliminating this impurity effect.

by adjusting synthesis parameters such as temperature, acidity, ultrasonic and microwave irradiation, types and concentrations of reactants and additives, and the order in which the additives are added to the solution.

1.7.4 The stability of nano fluid

The agglomeration of nano particles results in not only the settlement and clogging of micro channels but also the decreasing of thermal conductivity of nano fluids. 

So, the investigation on stability is also a key issue that influences the properties of nano fluids for application, and it is necessary to study and analyze influencing factors to the dispersion stability of nano fluids. This section will contain (a) the stability evaluation methods for nano fluids, (b) the ways to enhance the stability of nano fluids, and (c) the stability mechanisms of nano fluids.

1.7.5 The Stability Evaluation Methods for Nano fluids

 Sedimentation and Centrifugation Methods

Many methods have been developed to evaluate the stability of nano fluids. The simplest method is sedimentation method. The sediment weight or the sediment volume of nano particles in a nano fluid under an external force field is an indication of the stability of the characterized nano fluid. 

The variation of concentration or particle size of supernatant particle with sediment time can be obtained by special apparatus. The nano fluids are considered to be stable when the concentration or particle size of supernatant particles keeps constant. Sedimentation photograph of nano fluids in test tubes taken by a camera is also a usual method for observing the stability of nano fluids . 

Zhuetal. used a sedimentation balance method to measure the stability of the graphite suspension. The tray of sedimentation balance immerged in the fresh graphite suspension. The weight of sediment nano particles during a certain period was measured. The suspension fraction of graphite nano particles at a certain time could be calculated. For the sedimentation method, long period for observation is the defect. Therefore, centrifugation method is developed to evaluate the stability of nano fluids. Singhetal. 

applied the centrifugation method to observe the stability of silver nano fluids prepared by the microwave synthesis in ethanol by reduction of Ag NO₃ with PVP as stabilizing agent. It has been found that the obtained nano fluids are stable for more than 1 month in the stationary state and more than 10 h under centrifugation at 3,000 rpm without sedimentation. Excellent stability of the obtained nano fluid is due to the protective role of PVP, as it retards the growth and agglomeration of nano particles by steric effect. Li prepared the aqueous  polyaniline  colloids and used the centrifugation method to evaluate the stability of the colloids . Electrostatic repulsive forces between nano fibers enabled the long-term stability of the colloids.

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.