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This use of the term "nanofluid" is a typical modern use of a hype prefix, "nano," to add import to a topic that otherwise is well known and well established. Suspensions and dispersions of nanoparticles in liquids date back to 1847. Michael Faraday was the first to report what later came to be called metallic nanoparticles. In 1847 he discovered that the optical properties of gold colloids differed from those of the corresponding bulk metal. This was probably the first reported observation of a quantum size effect, and might be considered to be the birth of nanoscience (Michael Faraday#cite note-38). These and other of Faraday's discoveries are memorialized in The Faraday Museum housed in the Royal Institution in London. Suspensions of nanosize silver particles known as Carey Lea silver (Mathew Carey Lea) were used in imaging from the mid-1800s and in color photographic films in the mid-1900s and later.

A dispersion (dispersion (chemistry)) is a suspension of a discontinuous phase in a continuous phase. If both phases are liquids, immiscible with respect to each other, such a dispersion is knonw as an emulsion or nanoemulsion. A microemulsion is a single-phase thermodynamically. If the discontinuous phase is a solid and the continuous phase is a liquid, such a suspension is a liquid dispersion. A nanofluid is a fluid containing nanometer-sized particles, called nanoparticles.[1] These fluids are engineered colloidal suspensions of nanoparticles in a base fluid.[2][3] The nanoparticles used in nanofluids are typically made of metals, oxides, carbides, or carbon nanotubes.[4] Common base fluids include water, ethylene glycol[5] and oil. The usage of the term nanofluid should be abandoned because it is a poor replacement for the well established chemical term, dispersion.

Nanofluids have novel properties that make them potentially useful in many applications in heat transfer,[6] including microelectronics, fuel cells, pharmaceutical processes, and hybrid-powered engines,[7] engine cooling/vehicle thermal management, domestic refrigerator, chiller, heat exchanger, in grinding, machining and in boiler flue gas temperature reduction. They exhibit enhanced thermal conductivity and the convective heat transfer coefficient compared to the base fluid.[8] Knowledge of the rheological behaviour of nanofluids is found to be critical in deciding their suitability for convective heat transfer applications.[9][10] Nanofluids also have special acoustical properties and in ultrasonic fields display additional shear-wave reconversion of an incident compressional wave; the effect becomes more pronounced as concentration increases.[11]

The spreading of a nanofluid droplet is enhanced by the solid-like ordering structure of nanoparticles assembled near the contact line by diffusion, which gives rise to a structural disjoining pressure in the vicinity of the contact line.[12] However, such enhancement is not observed for small droplets with diameter of nanometer scale, because the wetting time scale is much smaller than the diffusion time scale.[13]

Contents

SynthesisEdit

Nanofluids are produced by several techniques:

  1. Eco-friendly synthesis (1 step)
  2. Direct Evaporation (1 step)
  3. Gas condensation/dispersion (2 step)
  4. Chemical vapour condensation (1 step)
  5. Chemical precipitation (1 step)

Several liquids including water, ethylene glycol, and oils have been used as base fluids. Although stabilization can be a challenge, on-going research indicates that it is possible. Nano-materials used so far in nanofluid synthesis include metallic particles, oxide particles, carbon nanotubes, graphene nano-flakes and ceramic particles.[14][15]

An innovative, bio-based, environmentally friendly approach for the covalent functionalization of multi-walled carbon nanotubes (MWCNTs) using clove buds was developed.[16][17] This approach is innovative because no any toxic and hazardous acids which are typically used in common carbon nanomaterial functionalization procedures, employed in this synthesis. The MWCNTs are functionalized in one pot (one step) using a free radical grafting reaction. The clove-functionalized MWCNTs are then dispersed in distilled water (DI water), producing a highly stable CMWCNT aqueous suspension (MWCNTs Nanofluid).

Smart cooling (nanofluids) dispersionsEdit

Realizing the modest thermal conductivity enhancement in conventional nanofluids, a team of researchers at Indira Gandhi Centre for Atomic Research Centre, Kalpakkam developed a new class of magnetically polarizable nanofluids where the thermal conductivity enhancement up to 300% of basefluids is demonstrated. Fatty-acid-capped magnetite nanoparticles of different sizes (3-10 nm) have been synthesized for this purpose. It has been shown that both the thermal and rheological properties of such magnetic nanofluids are tunable by varying the magnetic field strength and orientation with respect to the direction of heat flow.[18][19][20] Such response stimuli fluids are reversibly switchable and have applications in miniature devices such as micro- and nano-electromechanical systems.[21][22] In 2013, Azizian et al. considered the effect of an external magnetic field on the convective heat transfer coefficient of water-based magnetite nanofluid experimentally under laminar flow regime. Up to 300% enhancement obtained at Re=745 and magnetic field gradient of 32.5 mT/mm. The effect of the magnetic field on the pressure drop was not as significant.[23]

Nanoparticle migrationEdit

In nanofluids, it is recognized that nanoparticles do not follow the fluid streamlines passively. In fact, there are some reasons that induce a slip velocity between the nanoparticles and the base fluid. Movements of nanoparticles has significant impact on rheological and thermophysical properties of the nanofluids. Therefore, investigating the nanoparticles motion is critical for evaluating the performance of nanoparticles inclusion to the base fluid as a heat transfer medium. Since the nanoparticles are very small ( 100 nm), Brownian and thermophoretic diffusivities are the main slip mechanisms in nanofluids, as Buongiorno[3] declared. Brownian diffusion is due to random drifting of suspended nanoparticles in the base fluid which originates from continuous collisions among the nanoparticles and liquid molecules. Thermophoresis induces nanoparticle migration from warmer to colder region (in opposite direction of the temperature gradient), making a non-uniform nanoparticle volume fraction distribution.[24][25]

In fact, theoretical models estimated that nanoparticles are non-homogeneously distributed. The level of non-uniformity is completely depend on thermal boundary conditions, the nanoparticle size, shape, and material. Rigorous readers encouraged to find more interesting results in open literature.[26][27][28][29][30][31][32]

Stimuli-responsive dispersions (nanofluids) for sensing applicationsEdit

Researchers have invented a nanofluid-based ultrasensitive optical sensor that changes its colour on exposure to extremely low concentrations of toxic cations.[33] The sensor is useful in detecting minute traces of cations in industrial and environmental samples. Existing techniques for monitoring cations levels in industrial and environmental samples are expensive, complex and time-consuming. The sensor is designed with a magnetic nanofluid that consists of nano-droplets with magnetic grains suspended in water. At a fixed magnetic field, a light source illuminates the nanofluid where the colour of the nanofluid changes depending on the cation concentration. This color change occurs within a second after exposure to cations, much faster than other existing cation sensing methods.

Such response stimulus nanofluids are also used to detect and image defects in ferromagnetic components. The photonic eye, as it has been called, is based on a magnetically polarizable nano-emulsion that changes colour when it comes into contact with a defective region in a sample. The device might be used to monitor structures such as rail tracks and pipelines.[34][35]


Magnetically responsive photonic crystals nanofluidsEdit

Magnetic nanoparticle clusters or magnetic nanobeads with the size 80–150 nanometers form ordered structures along the direction of the external magnetic field with a regular interparticle spacing on the order of hundreds of nanometers resulting in strong diffraction of visible light in suspension.[36][37]

NanolubricantsEdit

Another word used to describe nanoparticle based suspensions is Nanolubricants.[38] They are mainly prepared using oils used for engine and machine lubrication. So far several materials including metals, oxides and allotropes of carbon have been used to formulate nanolubricants. The addition of nanomaterials mainly enhances the thermal conductivity and anti-wear property of base oils. Although MoS2, graphene, Cu based fluids have been studied extensively, the fundamental understanding of underlying mechanisms is still needed.

Molybdenum disulfide (MoS2) and graphene work as third body lubricants, essentially becoming tiny microscopic ball bearings, which reduce the friction between two contacting surfaces.[39][40] This mechanism is beneficial if a sufficient supply of these particles are present at the contact interface. The beneficial effects are diminished as the rubbing mechanism pushes out the third body lubricants. Changing the lubricant, like-wise, will null the effects of the nanolubricants drained with the oil.

Other nanolubricant approaches, such as Magnesium Silicate Hydroxides (MSH) rely on nanoparticle coatings by synthesizing nanomaterials with adhesive and lubricating functionalities. Research into nanolubricant coatings has been conducted in both the academic and industrial spaces.[41][42] Nanoborate additives as well as mechanical model descriptions of diamond-like carbon (DLC) coating formations have been developed by Ali Erdemir at Argonne National Labs.[43] Companies such as TriboTEX provide consumer formulations of synthesized MSH nanomaterial coatings for vehicle engines and industrial applications.[44][39]

Dispersions (nanofluids) in petroleum refining processEdit

Many researches claim that nanoparticles can be used to enhance crude oil recovery.[45] It is evident that development of dispersions (nanofluids) for oil and gas industry has a great practical aspects.

ApplicationsEdit

Nanofluids are primarily used for their enhanced thermal properties as coolants in heat transfer equipment such as heat exchangers, electronic cooling system(such as flat plate) and radiators.[46] Heat transfer over flat plate has been analyzed by many researchers.[47] However, they are also useful for their controlled optical properties.[48][49][50][51] Graphene based nanofluid has been found to enhance Polymerase chain reaction[52] efficiency. Nanofluids in solar collectors is another application where nanofluids are employed for their tunable optical properties.[53][54] In 2017, an environmentally friendly, facile and cost effective procedure was developed for synthesizing a highly dispersed functionalized graphene nanofluids for use as a heat transfer fluids (Coolants).[55] Unlike the conventional synthesizing methods which typically involve toxic and corrosive inorganic acids (e.g. Nitric Acid and Sulfuric Acid), that are harmful to environment and human health, In this approach, graphene nanoparticles were functionalized covalently with Gallic Acid (green agent) in a one-pot method. This green synthesized nanofluid can be a suitable alternative for use as a heat transfer fluid in terms of energy saving and overall thermal performances.

Thermophysical properties of dispersions (nanofluids)Edit

See alsoEdit

ReferencesEdit

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External linksEdit