Essay: Introduction to Nanomaterials:

Materials in the micrometer scale mostly exhibit the same physical properties as that of bulk form while materials in the nanometer scale may exhibit physical properties distinctively different from that of bulk. Materials in this nanoscale size range exhibit some remarkable specific properties for example of gold which does not exhibit catalytic properties as bulk material, while Au (gold) nanocrystal demonstrates to be an excellent low temperature catalyst.1
Nanomaterials are considered as futuristic materials for potentially wide applications in physics, chemistry, life sciences, medicine and engineering. The bulk properties such as melting point and hardness are related to the enhanced surface interactions among nanoparticles. The size-tunable electronic properties are due to quantum confinement effects. Different types of nanomaterials have their uses in day to day life applications as cosmetics, industrial to advanced technologies in cancer diagnosis and therapy.2-6
Nanomaterials are the materials with morphological features in nanoscale dimensions where generally at least one of the dimensions is less than 100 nm. Due to their sizes, shape and morphology, these materials exhibit tremendous physico-chemical properties in comparison to the bulk materials.7
Nanomaterials can be categorised according to their dimensions as;
1. Nanomaterials with three dimensions and less than 100 nm for example nanoparticles, quantum dots.
2. Nanomaterials with two dimensions and less than 100 nm for example nanotubes, nanofibers, and nanowires.
3. Nanomaterials with one dimension and less than 100nm are thin fibers, layers and surface coatings.
The nanomaterials are finding usages in sunscreens, tooth pastes, sanitary ware coatings, textile fibre coatings, catalysts, gas sensors, fuel additives and even in preserving food products.8,9 More exciting applications are either expected or rapidly being developed in the areas such as biosensors, bio-labeling, drug targeting, gene delivery, hyperthermia therapy, microelectronics, solar cells, electroluminescent devices, catalysis, water purification, detergent, cosmetics and antimicrobial agent.10 This led to advances in synthesis of different classes of nanomaterials of controlled shapes, sizes with favorable physicochemical properties.
The nanomaterials can be categorized into two groups, e.g., fullerene and inorganic Nanoparticles.
Table 1.1 Classifications of nanomaterials11-14
Nanomaterials
Fullerenes Inorganic Nanomaterials
Carbon based Other (non-carbon) Metal nanoparticles (Ag, Au, Fe etc.), metal oxide nanoparticles (e.g. Al2O3, MgO, SiO2, CaO, TiO2, Fe3O4, ZnO, CuO), semiconductor nanoparticles other than metal oxides (e.g. ZnS, SnS, CdS, CdSe, CdTe, GaAs)
Buckyball clusters (C20, C60), nanotubes (CNTs), fullerite, graphene, graphites etc. Inorganic fullerenes (MoS2, WS2, TiS2 and NbS2 etc.)
The CNTs in the fullerene group has been one of the most exciting nanomaterials having wide applications.12 On the other hand metal and metal oxide nanoparticles are getting significant scientific attention due to their existing as well as potentially new applications due to their unique optical, magnetic, electrical properties. In addition the semiconductor nanoparticles exhibit size dependent electronic and optical properties having tremendous impact in the areas of biomedical and environmental applications.13, 14
Among different nanoparticles, ZnO nanoparticles are found to be versatile materials with wide range of applications as photocatalysts, sensors, piezoelectric transducers, energy materials as solar cells, electroluminescent devices, UV laser diodes, disinfecting agent.
Recently polymeric nanomaterials of the diameter in the range of 10-1000 nm also reported. For example polyvinyl alcohol, polycaprolactone, polylactic acid, polylactic co-glycolic acid, albumin, gelatin, dextran, alginates, pectinates, chitosan have attracted a great attention especially in the field of biomedical applications, including imaging, enzyme immobilization and drug delivery. 7, 15
1.2 Physicochemical properties of nanomaterials and nanofluids16
Nanomaterials have unique properties in comparison to bulk counterpart. These properties are responsible for mechanisms of various beneficial applications like in biomedicine, industrial, cosmetics etc. These physicochemical properties are originated from the nanomaterials size and surface area, composition, shapes and effects the nanofluids properties as classified below;
i. Size and surface area of the particles
ii. Particle shape and aspect ratio
iii. Surface charge on nanoparticles
iv. Chemical composition and crystalline structure of the nanoparticles
v. Aggregation and concentration of nanoparticles in nanofluids
vi. Surface coating and surface roughness
vii. Solvents or media effecting the nanofluids properties
Fig.1.1 (a) Ag & Au shows the size & shape depending colors17,
(b) Effect of size and shape on λabs18
In case of nanomaterials physicochemical properties such as size, surface area, surface chemistry, surface roughness, dispersion medium, and tendency to agglomerate is playing detrimental effect on their activities in various applications. So nanofluids, being used in the different forms in the market products for day to day use, required through study of their physicochemical properties.
1.3 Methods for nanomaterials synthesis
Different synthetic methods have different way to control on sizes. These methods can be classified into two main approaches,
1. “Bottom Up” and
2. “Top Down” approach.
Bottom up methods involve the assembly of atoms or molecules into nanostructured arrays. There are two type of bottom up technology i.e. chaotic and controlled.
The chaotic processes involve elevating the constituent atoms or molecules to a chaotic state and then suddenly changing the conditions to make that state unstable. Laser ablation, exploding wire, arc, flame pyrolysis, combustion, and precipitation synthesis techniques are the examples of chaotic process. On the other hand the controlled processes are self-limiting growth solution, self-limiting chemical vapor precipitation/deposition and shaped pulse femtosecond laser techniques, molecular beam epitaxy etc.
In the bottom-up approach, the structure of nanoparticles is constructed by atoms, molecules or clusters. In top-down approaches, a bulk piece of a required material is reduced to nanosize via cutting, grinding and etching processs.19
Table 1.2 Classification of different methods of nanomaterials synthesis19
Bottom-up or chemical methods Top-down or physical methods
Chemical reduction or precipitation method,
Microemulsion or colloidal techniques,
Sonochemical reduction method,
Electrochemical method,
Microwave-assisted synthesis,
Solvothermal or hydrothermal method,
Biological or biosynthesis techniques. Pulse laser ablation or deposition method,
Vacuum vapor deposition (PVD & CVD),
Pulsed wire discharge (PWD) method,
Mechanical/ball milling method,
Mechanochemical synthesis.
The co-precipitation or chemical methods have been the most promising one for the preparation of ferrite nanoparticles as it is economic, required no critical instrumental setups, and also have ecological values without zero environmental hazards. ZnO nanomaterials have several reported applications due its nanosize, easy, controlled and economic manufacturing methods, other activities (as photocatalytic activity) and the MNPs have special properties of superparamagnetism at quantum sizes. These properties of nanomaterials will also affect the activities of the base fluids in the nanofluids formation.
1.4 The crystal structures of metal oxides under study i.e. ZnO & Fe3O4:
Structurally, ZnO could exist as zinc blende, rock salt and wurtzite. In ambient pressure and temperature, ZnO crystallizes in wurtzite structure as shown in the Fig. below.20
Fig.1.2 Crystal structures of ZnO nanomaterial; rock salt, zinc blend and wurtzite.21, 22
Wurtzite is a hexagonal lattice comprising two interconnecting sub lattices of Zn2+ and O2-, where a zinc ion is tetrahedrally coordinated with O2- and vice versa. This tetrahedral coordination gives rise to polar symmetry along the hexagonal axis and it is this polarity which is responsible for various properties of ZnO nanomaterials.23
Further, it is a direct band gap semiconductor where the Fermi energy level lies at the top of the valance band. The electronic band structure of ZnO has been extensively studied by various groups. However, the properties of ZnO NPs are different from that of the bulk.
The sizes of the nanoparticles are between atomic or molecular scale and those of macroscopic solids, more towards the molecular dimensions. In such case quantum mechanical effect would be important to explain the characteristics of nanoparticles.
The electronic property depends on the edges of valence and conduction band which vary with size. The continuum of states in the bulk semiconductor gives rise to the valence and the conduction bands separated by a band gap.
On reducing the size of the material from bulk to nanoscale dimension, the shift in the band edges give rise to an increase in the band gap and the energy levels close to the edges become discrete and can also be shown by a blue shift in the UV absorption spectra.
When the sizes of the ZnO nanomaterials become comparable or less than the bohr radius of exciton in bulk material, then the excitons (electron-hole pair) are forced to reside within a structure of nanoscale dimension due to the quantam confinement rule and this will broaden the band gap and result to a blue shift to the absorption edge.24
Amongst the iron oxide family magnetite (Fe3O4) and maghemite (γ-Fe2O3) exhibiting excellent magnetic properties and reported to be good applicant for biomedical purposes. Trivalent state of the iron is the main characteristics properties of these oxide compounds along with low solubility and brilliant colors.25 Iron oxides are crystalline in most of the forms except schwertmannite and ferrihydrite which are poorly crystalline.
Table 1.3 Different forms of iron oxides having same crystallographic structures.25
Sr No Crystal forms Chemical formulas Structural Morphology Surface area activity
1 Ferrihydrite Fe5HO8.4H2O, Fe5(O4H3)3, 5Fe2O3.9H2O, Fe2O3.2FeOOH2.6H2O Spherical High surface area (100 – 700 m2/g)
2. Wustite FeO have 4 formula units in the cubic unit cell. Cubic Highly unstable
3. Goethite FeO(OH) exhibits an orthorhombic symmetry Acicular structure 8 – 200 m2/g
4. Akaganeiten Iron oxyhydroxide phase
β-FeOOH Long tunnel type structure Unstable
5. Lepidocrocite γ−FeO(OH) + double layers of Fe-octahedra Lath-like or tabular 15 – 260 m2/g
6. Magnetite (Fe3O4) FeO.Fe2O3 Octahedron and rhombodecahedron 4 – 100 m2/g
7. Hematite α-Fe2O3,
distorted octahedra FeO6. Isostructural with corundum, α-Al2O3, rhombohedral symmetry 10 – 90 m2/g
Magnetite (Fe3O4) is a black, ferromagnetic mineral containing both Fe2+ and Fe3+with stoichiometric magnetite Fe2+/Fe3+. Sometimes magnetite is non-stoichiometric too resulting in a cation deficient Fe3+ layer. The crystal structure of magnetite is inverse spinel AB2X4 with a unit cell consisting of 32 oxygen atoms in a face-centered cubic structure and a unit cell edge length of 0.839 nm.
Fig. 1.3 Inverse spinel AB2X4 structure of magnetite26
Iron oxides can be synthesized using wet chemical methods but the control over the particle size in nano range and morphology must require to be controlled. Variety of stabilizing agents and capping agents, in-vitro functionalization of MNPs had been used to control the agglomeration. Other chemical synthesis methods include chemical precipitation, sol-gel, hydrothermal, surfactant mediated-precipitation, emulsion-precipitation, microemulsion precipitation, electro-deposition, and micro wave assisted hydrothermal techniques etc.
1.5 Capping agents for the nanomaterials synthesis
Due to the ease of aggregation of nano range particles they have tendency to get agglomerated in the fluid because of the quantum effect on size. This agglomeration must be stopped or minimized introducing stabilizing or capping agents in the synthesis of nanofluids to enhance the surface area and other relative properties of dispersed nanomaterial. In the emerging field of tissue engineering, biodegradable polymers have the orthopedic applications, such as bone and joint replacement and other applications as biomedical.27, 29
Table 1.4 Classification of polymeric capping agents.27-29
Type of polymers utilized Name of the material or polymeric solution Physical appearance Applications Application mechanism
Biodegradable polymers Polysaccharides (pectin, starch etc.) polyglycolide, polylactide, polyhydroxobutyrate, chitosan, hyaluronic acid, and hydrogels etc. Solids, semisolid, gels or hydrogels fluids or liquids Catalytic, cell imaging, drug delivery, drug targeting etc in biomedical as coolant or lubricants in heat transfer Used as caaping or encapsulating agents etc.
Non-biodegradable polymers Silicone rubber, polyethylene, acrylic resins, polyurethane, polypropylene, and polymethylmethacrylate (PMMA) Most are solids Act as a biocompatible cement in the the healing and in rebuilding mechanism for bone reformation PLA is used to make screws and darts for meniscal repair
A biodegradable polymer must have the following properties to do encapsulation of nanomaterials when used as carrier application in the therapeutic agent for drug delivery etc.
1. A capping agent must be non-toxic in order to eliminate foreign body response;
2. Capping agent must not too long time for the polymer to degrade and have to be proportional to the time required for therapy;
3. The degraded products of the capping agents have not cytotoxic and have easy elimination pathways from the body system.
4. A Capping agent must be processed in such a way to avoid or lessen the mechanical properties during the application.
5. Stability enhancement by capping agent depends upon how, easily be sterilized during the reaction and
6. Shelf life of a capping agent has to be acceptable in the application medium.
Functionalizing the MNP is the other way to stabilize the ferrofluid. Nanofluids prepared from functional polymers have super-paramagnetic stability against agglomerization and have superb biomedical importance when using the biodegradable polymers for the aforesaid purposes.
1.6 Introduction to Nanofluids.
Choi et al. was the first group used the concept of nanofluids to obtain better thermal characteristics that include thermal conductivity for heat transfer application. Among all the nanoparticles used in the nanofluid, metal oxides, carbides or carbon nanotubes are most common.30 Nanofluids also known as the two phase colloidal systems in which nanomaterials play the one solid phase while the other one is liquid phase of the base fluid.31 Nanofluids can have the nanomaterials in the shape of nanoparticles, nanofibers, nanotubes, nanowires, nanorods, nanosheets or nanodroplets suspended in the base fluid.32
Nanofluids due to their unique properties being potentially useful in many applications such as heat transfer, engine cooling and vehicle thermal management, electronic devices, nuclear system, space, heat exchanger, electronics cooling system, fuel cell, solar collectors, domestic refrigerator.30
Nanofluids having the magnetic nano particles dispersed are called as magnetic nanofluids or ferrofluids. Ferrofluids have applications in a variety of fields such as bioengineering, targeting drug delivery using magnetic properties, magnetic fluids intracellular hyperthermia, MRI (magnetic resonance imaging) contrast enhancement agents, electronic packing, mechanical engineering tools, and aerospace field etc. Thermal properties and stability of nanofluids having applications in heat transfer studies are the current topics of discussions amongst researcher during the last few decades.33
Table 1.5 Study area with the role of dispersed nanomaterials.33
Sr. No. Nano-material Base fluid Capping agents Property enhancement Study areas or appplications
1. Fe3O4 Water SDBS Thermal conductivity Thermal conductivity, viscosity vs conc., shear rate, and magnetic field
2. Fe3O4 Water uncapped Thermal conductivity Thermal conductivity, and viscosity
3. MNPs Water alginate Colloidal stability and core-shell study Morphology by SEM/TEM, FT-IR
4. MNPs Water, transformer oil, volatile, nonpolar to polar inorganic solvent Surfactants Stability Physical properties like pH, size, viscosity
5. ZnO EG, PEG, PPG N.A. Viscosity Antibacterial and catalytic activity
6. ZnO EG, PEG, PPG N.A. Viscosity Viscosity and flow related study
7. TiO2 & ZnO Paints N.A. Surface textures Glow and smoothening
8. Au, TiO2 & ZnO antiseptic creams and in sun screen N.A. Antibacterial, antimicrobial effect Antibacterial, antimicrobial study
9. TiO2 & ZnO Reactive dye solutions doping Photodegradation efficiency Photodegradation applications
1.7 Rheological behavior of nanofluids
The experiments on the smooth muscle tissue animals like the rabbits or rats make the therapeutic system ready for human therapeutics. The literature review study find the application use of rats on a polycaprolactone/polylactide scaffold.39 Recently the government officials in several countries like US, Swedan have initiated to save lives of these poor animals. Due to these initiatives some of the countries started recently to ban the research on the animals or use of the lived bodies in the research purposes.
So the rheological study of flow related investigation helps the therapeutic systems easily acceptable for the human therapeutics. Nanofluids depending on the base fluid may be a newtonian fluid or may behave as non newtonian fluids depending on the morphology of the materials and molecular structure of the base fluids material.
Various non-newtonian behaviors can be classified as below:
Time-Independent behaviors:
The Properties of fluids are independent of time under shear. They are known as bingham plastic, pseudo-plastic or as dilatants fluids.
Bingham-plastic: Fluids showing resist a small shear stress but flow easily with larger shear stresses applications for example tooth-paste, jellies, and some slurries.
Pseudo-plastic: These are class of fluids of mostly non-Newtonian fluids. With these fluids viscosity value is decreases with increasing velocity gradient. polymer solutions, blood are some examples. Pseudoplastic fluids are also called as Shear thinning fluids because at low shear rates (du/dy) the shear thinning fluid is more viscous than the Newtonian fluid while at high shear rates it is less viscous.
Dilatant fluids: Viscosity of these fluids increases with increasing shear rate (velocity gradient). Suspensions of starch are fluids behave in this way. Dilatant fluids are also called as shear thickening fluids.
Time dependent behaviors:
Time dependent properties are depending on duration of shear. They also have the three forms as; thixotropic, rheopectic or viscoelastic fluids.
Thixotropic fluids: these fluids for which the dynamic viscosity decreases with the time on application of shearing forces. For example thixotropic jelly paints.
Rheopectic fluids: Dynamic viscosity of rheopectic fluids increases with the time of which shearing forces are applied. Gypsum suspension in water is the well known example of this class.
Fig. 1.4 Effect of change of shear rate on viscosity of time-dependent fluids
Visco-elastic fluids: Some fluids have elastic properties allowing them to spring back when a shear force is released for example the egg white.
The properties of dispersion nanofluids depends on the nanomaterials and the base fluids properties. Particle size, crystal structure and the orientation of the nano particles in the base fluids medium are responsible for the flow of the nanofluids. The nanofluids are characterized as newtonian or non-newtonian fluids.
The rheological study of a fluid have the classification of fluids as the flow related properties and properties related to elastic nature. Firstly the flow behavior study is useful in explaining the properties of fluids on that basis the fluid is characterized as
1. Ideally viscous (Newtonian)
2. Shear-thinning (pseudoplastic)
3. Shear-thickening (dilatant)
The yield point determination is also carried out from flow behavior by mathematical curve fitting of flow curves on a linear scale known as approximation, “regression”. Yield point is the condition at which the sample starts to flow not before the external forces exceed the network-of-forces in the internal structure. The flow study is helpful for the structure recovery, sagging of layers, curring, melting, softening, solidification, crystallization, gel formation. It is also helpful for many more viscosity dependent parameters like pour point etc. and nature of additives in the fluids.
Flow behavior study involved the following parameters studies;
a) Flow curve
 Shear stress vs shear strain profile
b) Viscosity curve
 Viscosity vs time profile
 Viscosity vs temperature profile
 Viscosity vs shear strain profile
Fig. 1.5 Rheology of the fluids; (a) Flow curve, (b) Viscosity curve
In many cases, the measurement of viscosity singly is not sufficient as the elastic effects are occurring. These effects are resulting in viscoelastic behavior of the fluid. Below the yield point there is elastic deformation only. The MCR-102 model of Anton Paar is used in our study to define the rheological properties of the nanofluid. Oscillatory mode gives the shear modulus studies as;
 G’ [Pa] is the storage modulus governed from elastic portion and describes the stored energy in nanofluids materials.
 G” [Pa] is loss modulus generated by viscous portion and describes the lost/dissipated or deformation in energy forms.
G’ & G” are two shear modulus generated due to the viscoelastic behavior and the tan δ = G”/ G’ is called as the loss factor or damping factor.
Table 1.6 Shear modulus study to describe the elastic nature of the nanofluid.
Elastic
Viscoelastic Viscous
G’>>G”
tan δ << 1 0 G’>G”
tan δ < 1 G’>G”
tan δ = 1 G”>G’
tan δ > 1 G”>>G’
tan δ >> 1
∞
Gel-like structure at the Gel point liquid-like structure
Fig. 1.6 Shear modulus study (A) Shear modulus vs shear strain, (B) Shear modulus vs shear stress.
This study helps in determination of the LVE range and the cross-over point to predict the viscoelastic nature of the nanofluids. The linear range provides the elastic region while the region beyond the LVE range is known as viscous region as the fluid starts flowing in the direction of the applied strain force.
1.8 Motivation and finding of gaps
Metal oxides have various applications in the day to day life to facilitate the different thermal management (heat transfer), catalytic, antibacterial and many more activities to enhance the scientific application in the field of materials science. The various metal oxides reported used in catalytic activity as photo-catalyst, in degradation activities, in antibacterial activities, in biomedical applications as bio-imaging by using nanomaterials as support or carrier materials, in drug loading applications, in drug targeting for the critical diseases like various kind of tumors in humans as well as in animals.
The mobility related aspects of the nanofluids have great emphasis on the prediction of nanofluids properties for various applications including the biomedical as well as industrial applications. Recently research communities tried to explain these properties of the nanofluids for some application as in thermal stability study, heat transfer studies etc. and their correlation with the flow of the nanofluids. The flow or mobilty related study of these properties for environmental and biomedical applications is not explored so much.
1.9 Aim and objective of the work
Keeping in mind the aforesaid discussion the objectives of the work is to investigate and study the flow related physicochemical properties of the nanofluids. My study will correlate the effect of size, shape and other morphological changes in nanomaterials to the physicochemical properties of the synthesized nanofluids by studying the mobility of different size nanoparticles in the base fluids.
This study will be helpful for predicting the role of physicochemical properties of nanofluids applications in the area of the environmental, biomedical (drug delivery, therapy, diagnostics etc.), industrial (coolant and lubricants), fuel additives, cosmetics (creams, pastes and sun screen lotions etc.), and many other applications. My aim is to prepare the nanofluids from different nanomaterials using different capping agent to minimize the agglomeration, and use of various polar to non polar, polymeric solutions as base fluids.
1.10 Applications of nanofluids
Zinc oxide is a material having several applications due to high surface area, good electrical, electrochemical and structural properties of nanostructured ZnO. The synthesis of ZnO nanoparticles and their structural/optical characterization can be seen in literature.34
Fig. 1.7 Paper sensor based on ZnO nanoparticles and cellulose fiber.35
Research groups in the field of magnetic nanoparticles in recent years found viral applications in biomedical sciences like detection of protein, tissue engineering, probing DNA structure, drug and gene delivery, heat distraction of tumor (hyperthermia), kinetic studies, separation and purification of bio molecule and cells, MRI contrast enhancement, biosensors, and design of proteins for electron transport.36
Fig. 1.8 Biomedical applications of MNPs.37, 38
Magnetic nanofluids due to their special properties of superparamagnetism gathered attention of research community in the recent years. For example Li et al. have experimentally investigated and reported the viscosity and the thermal conductivity of the ferrofluids in aqueous medium with the presence of the external magnetic field and without external magnetic field.33 The detail of various applications are in the reported literature review.
Chapter – Two
Literature review  
2.1. Synthesis of Nanofluids from nanomaterials
Research on nanofluids has been extended towards exploring the influence of different kinds of nanomaterials like metal, metal oxide, carbon nanotubes, graphene etc. on the different properties of the base fluids.40
Due to the very small size of nanomaterials the nanofluids are also facing the stability problems as in the case reported by Shuganthi et al.40 the nanomaterials tend to agglomerated and influences the temperature related changes in the ZnO-Water nanofluids as it can affect the relative viscosity of the nanofluids on changing the temperature.31
Shuganthi et al.31 had reported about the crucial cooling phenomena taking the Maxwell theoretical prediction, which states that ‘The mixture of solid particles and liquid moieties would result in dispersion with thermal conductivity between that of solid particles and the liquid’. Suspensions of micron-sized particles are prone to settling, and so undesirable for real-time applications. Use of nanometer sized particles over micrometer sized particles overcomes the settling of particles.31
Chung et al.41 have reported the effect of ultrasonication in preparation of ZnO dispersions. They had reported that the minimum achievable size of aggregates was 50—300 nm and the reduction in size of the aggregates was found to occur through fragmentation of aggregates to give size of up to 20 nm and 40-100 nm.
From the literature report suggested it is clear that the intrinsic defects in the synthesized ZnO NPs could be passivated by capping agents.31 The role of capping agent towards quenching the defects in ZnO NPs is not superficial. These defects are mainly attributed to oxygen vacancies or zinc vacancies42 and could act as trap centers of excitons.43 Therefore interactions between capping agents and the surface defects of ZnO NPs outcomes to the excitons, and consequently ROS generation and results to anti-microbial activity of the capped ZnO NPs.
Mercaptoacetic acid (HS-CH2-COOH) has two functional groups, i.e., —SH and —COOH, one is used for stabilizing NPs while the other can impart functionality.44 The water soluble polyethylene glycol (PEG) also shows antioxidant property and has been frequently used in various biomedical applications.45 Ascorbic acid is a natural sugar known for exhibiting excellent antioxidant properties is also reported to be used as stabilizing agent.
Polysorbate 80, also referred as tween 80, a lipophilic food additive is reported as non-toxic and acts as surfactant to be used as stabilizing agent.46
Fig. 2.1 Structure of the tween-80 surfactant used as capping agent.46
Sahu and Dutta47 reported synthesis of superparamagnetic magnetite nanoparticles (MNPs) and pectin are reported using crosslinking with Ca2+ ions to forms spherical calcium pectinate nanostructures, (MCPs). These nanostrucres of MCPs are in the range of 100—150 nm of size. The swelling behavior, with an average size of 400nm for the MCPs has been also reported using zeta sizer.
The literature on the stability and particle size distribution studies of the Fe3O4 nanoparticles and Fe3O4 nanoparticles with oleic acid coating, nanofluid in PEG base fluid had also reported by Maottar et al.33 the quantum effects on the absorption wavelength, such as band gap enlargement on particle size decreasing and also the effect of oleic acid capping is reported.33
Flow behavior and suspension structure of Fe3O4 in the PEGylated fluid have been determined by rheological properties. Viscosity study of Fe3O4-PEG nanofluid also reported as a function of temperature. The oleic acid coated Fe3O4 nanoparticles resulted to chain-like structure in the PEG base fluid, also verified by the magnetorheological experiments.33
Kole and Dey42, 43 formulated highly stable ZnO—ethylene glycol nanofluid through very long optimized duration of ultrasonication (60 h) without any surfactant. However prolonged ultrasonication beyond 60 h, have no significant effect on aggregate sizes.49
Similar phenomenon has also been observed for CuO—ethylene glycol and Al2O3—water nanofluids.44 About 40% enhancement in thermal conductivity with 3.75 % v/v concentration was reported for ZnO-EG nanofluid.49
Mohammed et al.34 reported in another work, the nanofluid preparation using PPG, PPG-water, PVP-water, and mixture of these two polymers as base fluids. The ZnO—PPG and ZnO—PPG—H2O nanofluids have been reported for viscosity values at temperatures 293.15, 298.15, 308.15 and 318.15 ̊K.34
Rashin et al.52 reported the novel nanofluid of zinc oxide in coconut oil has been synthesized via ultrasonically assisted two step method (sizes 26 nm). Viscosity vs temperature profiles and shear rates profile study provide the shear thinning of nanofluids.
Heris et al.53 reported the study of rheological behavior of zinc oxide nanolubricants forming turbine oil—zinc-oxide nanofluid. They reported that nanolubricants and the base fluid behave like Bingham fluid. The enhancement in viscosity of Nanolubricant as a function of nanoparticle volume fraction is predicted using Bachelor equation with modified volume fraction. The rheological data for reported work revealed that nanoparticles make aggregates in the turbine oil in diameter range of 3.60 nm diameter of a single nanoparticle.
Bhagat et al.30 describes preparation of zinc oxide (ZnO) based nanofluids in polymer matrix with rheological study for heat transfer application. The 30-40% increase was observed in the heat absorption capacity for the ZnO nanofluid. Various common solvents such as water, ethanol, Toluene and Hexane were tested with and without the bath of ZnO nanofluid. The literature showed that the presence of ZnO nanofluid bath were reduced the temperature propagation in a sonochemically heated system.
Kandpal et al.36 prepared the nanofluids from Fe3O4 via the copolymerization of MNPs in to the PDMS (poly-dimethyl siloxane) in the presence of carboxylic acids (acrylic acid and methacrylic acid). In the study oleic acid capped ultrasonically grafted MNP in to the polymeric solution give rise to stable ferrofluid and reported the ferrofluid with high thermal stability.
Table 2.1 Nanofluids prepared from different base fluids.30-34, 36, 40-56, 58, 59
Sr. No. Nanomaterial Base fluids used Work importance/properties
1. ZnO PEG Thermal stability
2. ZnO EG+water Thermal conductivity
3. ZnO ED Thermal stability via UV-VIS
4. ZnO Lubricant (PAO-6) Tribological behavior of lubricant
5. ZrO2 Lubricant (PAO-6) Tribological behavior
6. Y2O3 Ethyl alcohol Thermal conductivity
7. Spinal (MgAl2O4) Ethyl alcohol Thermal conductivity
8. Cu Water Thermal conductivity
9. CuO Water Thermal conductivity
10. Titanate nanotubes EG Heat transfer properties
11. Al2O3 Water Thermal conductivity
12. TiO2 Water Heat transfer properties
13. TiO2 EG Heat transfer properties
14. TiO2 Water, EG, PPG Rheological characterization
15. TiO2 Solvent free Photocatalytic activity
16. SnO2 EG Thermal conductivity
17. SiO2+ surfactant EG + water Heat transfer properties
18. SiO2+ surfactant EG + water Heat transfer properties of synthetic oil (Therminol-66)
19. SiO2 nanobeads Ethanol Aggregation & dislocation properties
20. SiO2 EG + water Viscosity & specific heat study
21. SiO2 Mineral oil AC breakdown voltage & viscosity
22. Ag EG, water Viscosity & electrical properties
23. SiC EG Rheology and thermal properties
24. Bimetallic (Pd-Ag) EG Electrical conductivity
25. MoS2 Solvent free Use of canopy and hydrothermal synthesis
26. MgAl2O4 DEG Viscosity & thermal conductivity
27. Mg-Fe layered clay (leponite) Double hydroxide layer Rheological properties
28. Graphite Oil Thermal conductivity
29. Graphene/GOs Water, acid, SDBS Thermophysical & Rheology
30. Fe3O4 Solvent free Rheological properties
31. Fe3O4 PVA Rheological properties
32. Fe3O4 PEG, PPG Rheology & thermal stability via UV-VIS spectroscopy
33. Fe3O4 Glycerol Rheological properties
34. Fe-Ni Oleyl amine, oleic acid Viscosity & yield stress study via rheology
35. CuO EG, coconut oil Zeta potential & rheology
36. Co3O4 EG, paraffin Rheological properties & thermal conductivity
37. CNTs Water, EG, glycerol Lubrication & heat transfer properties
38. MWCNT, CNBs, C60 Engine oil (SAE20W50) Thermal conductivity, viscosity, pour & flash point study
39. Al2O3 Glycol + water Thermal conductivity & heat transfer properties
40. CdS:Ho Dilute sulfuric acid Ultrasonic velocity and compressibility in EG
41. Pd/PVA/Ag EG Catalytic activities & flow properties
42. SiC EG Flow behavior (newtonian)
(PEG: polyethylene glycol, EG: ethylene glycol, ED: ethane-1, 2-diol, PAO: polyalphaolefins, PPG: polypropylene glycerol, DEG: diethylene glycol, SDBS: Sodium dodecyl benzene sulfonate, PVA: polyvinyl alcohol, CNTs: carbon nanotubes, CNBs: carbon nanobeads.)
Kandpal et al.36 used the grafting of MNPs in the polymeric PDMS solution using fast and green microwave based synthesis utilizing acrylates and oleic acids. The acrylic acid based polymers have high resistance towards UV radiation, restriction to hydrolyzation, good compatibility with additives and have excellent optical properties. Polyacrylic acid prevents segregation of the dopant cation by the coordination of carboxylic group with the metal ion, making the polymer a convenient matrix. In the study Kandpal et al. reported that oleic acid acts as a surfactant to stabilize MNPs via strong chemical bonds between carboxylic acid and nanoparticles.
In the Fe3O4 nanoparticles case, magnetization interference from domain wall is not expected especially for the particles of single magnetic domain in the matrix. In this regards work of the particles of size about 5—20nm diameter were reported. The superparamagnetic susceptibility, high Ms value, biocompatibility and non-toxicity and their simple method of synthesis by co-precipitation method are the reasons for MNPs to be used prominently for its wide application area.47
Heyhata et al. reported the laminar flow convective heat transfer characteristics of alumina-water nanofluid. They were reported up to 32% increase in the heat transfer coefficient of the nanofluid than the water and also showed its dependence with increasing the Reynolds number and particle concentrations.
2.2. Structural studies of nanofluids
The study for special class of fluids, describes the colloid structures and their property in suspensions. Rheological properties are also helpful to understand the properties of nanofluids for flow and other related applications.33
The nanofluid viscosity depends on several factors as the morphological characteristics of the nanoparticles, nanoparticle loading, method of formulation, nature and concentration of the surfactant used.54 The reported literature shows that the magnetic nanoparticles have high saturation magnetization and formed a highly stable ferrofluid under magnetic and gravitational field.55
At higher concentrations heat transfer coefficient growth stops and starts to decrease. There are several parameters other than nanoparticle concentration affecting the thermo physical properties of nanofluids.30 For example, increase in the zeta potential value of fluid, minimizes particle-particle interaction and so the lowering of the viscosity and thermal conductivity values of the suspension.56
Suganthi et al.40 had also reported for the preparation and characterization of ZnO-EG and ZnO-EG-water nanofluids for their use as coolants. ZnO—EG nanofluids with 4 % v/v nanoparticles enhanced the thermal conductivity up to 33.40% and reduced the viscosity by 39.20% at 27 ̊C. Similarly, 2 % v/v ZnO—EG—water nanofluids showed 17.26% of enhancement in thermal conductivity while reduction of viscosity by 17.34% at 27 ̊C. The change in extent of hydrogen bonding network of ethylene glycol by ZnO nanoparticles resulted in lowering of the viscosity.
The properties of ethylene glycol based nanofluids have great impact due to their use as coolants in automobiles. Sand—ethylene glycol—water dispersions synthesis is reported using stirred bead milling and ultrasonication and about 20 % enhancement in thermal conductivity was reported at a particle concentration of 1.80 % w/v.40
2.3. Physicochemical properties influencing the flow of nanofluids
Moosavi et al.58 reported in their work the physicochemical properties changes for the ZnO—ethylene glycol and ZnO—glycerol nanofluids. They found the Viscosity ratio and surface tension ratio were increase with increasing concentration of the nanomaterials to an extent.
Ojha et al.59 reported that ZnO—water nanofluid prepared without surfactant showed differences in viscosity at the same temperature during the heating and cooling cycles and this aspect is known as hysteresis in viscosity.44 This hysteresis was prevented using a surfactant for complete dispersion of ZnO nanoparticles.
Mohammed et al.34 reported the rheological behavior of nanofluids over nanoparticles concentrations (1.00, 0.50, 4.50%) and shear rates (0.01—1000 s-1) at 298.15 ̊K. The nanofluid exhibited a pseudoplastic flow behavior, indicating an existence of particle aggregations. Jalal et al. studied and synthesized ZnO nanoparticles via green solvent i.e. ionic liquid and use glycerol as a base fluid for the antibacterial activity of nanofluid suspensions. Thermal conductivity, viscosity and surface tension values of ZnO nanofluids have also been reported using same methodology.33
Yu et al. found that ZnO—EG nanofluid with low volume concentration shows Newtonian behaviors while a shear thinning behavior for higher volume concentrations. Lee et al. confirmed that all of the ZnO—EG nanofluids showed Newtonian behavior.
ZnO—ethylene glycol nanofluids containing 5 vol% of nanoparticles, prepared by 3-h ultrasonication reported to give 26.50% enhancement to the thermal conductivity. These nanofluids were newtonian at lower concentrations while non-newtonian (shear-thinning) at higher concentrations.60
Transient heat transfer experiments reported the ZnO—EG and ZnO—EG—water nanofluids with better heat absorption characteristics compared to the base fluids. This heat transfer enhancements were proportional to thermal conductivity enhancements. It was also reported that proportionality showed a superior thermal conductivity of nanofluids utilized for cooling applications. Thermal management and energy storage systems are the core areas of research in the fields such as automobile, industrial cooling, renewable energy utilization as evident from the recent literatures.40
2.4 Theoretical models to correlate the physicochemical properties of nanofluids
Some theories were also used to predict the concentration and viscosity relationships to describe the flow properties of the nanofluids. The nanofluids were generally showed a pseudoplastic flow behavior with an existence of particle aggregations in the suspensions.
The Eyring-NRTL and Eyring-mNRF models have been used to correlate the viscosity values of the nanofluids with temperature. The Einstein, Brinkman, Lundgren and Batchelor models have also reported in the prediction of viscosity values of ZnO—PPG nanofluid.34
Bingham plastic and Herschel—Bulkley models were reported for the shear stress—shear rate evaluations and also colloid yield stress. The colloid yield stress and concentration relationship was used to describe the suspension structure while Carreau—Yasuda model evaluated the shear rate vs viscosity changes at each concentration.