A nanofluid is a fluid containing

nanometer 330px, Different lengths as in respect to the Molecule">molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm), or nanometer (American spelling Despite the va ...

-sized particles, called

nanoparticle A nanoparticle or ultrafine particle is a particle of matter 1 to 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 100 nm in only two directions. At ...

s. These fluids are engineered

colloidal suspensions A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...

of nanoparticles in a base

fluid In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...

. The nanoparticles used in nanofluids are typically made of metals, oxides, carbides, or

carbon nanotube A carbon nanotube (CNT) is a tube made of carbon with a diameter in the nanometre range ( nanoscale). They are one of the allotropes of carbon. Two broad classes of carbon nanotubes are recognized: * ''Single-walled carbon nanotubes'' (''S ...

s. Common base fluids include water,

ethylene glycol Ethylene glycol ( IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes: as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...

, and oil. Nanofluids have many potentially

heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...

applications, including microelectronics,

fuel cell A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen fuel, hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most bat ...

s, pharmaceutical processes, and hybrid-powered engines, 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 The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1. Heat transfer occurs at a lower rate in materials of low ...

and convective

heat transfer coefficient In thermodynamics, the heat transfer coefficient or film coefficient, or film effectiveness, is the Proportional (mathematics), proportionality constant between the heat flux and the thermodynamic driving force for the Heat transfer, flow of heat ...

compared to the base fluid. Knowledge of the

rheological Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied ...

behaviour of nanofluids is critical in deciding their suitability for convective heat transfer applications. Nanofluids also have special acoustical properties and in ultrasonic fields display shear-wave reconversion of an incident compressional wave; the effect becomes more pronounced as concentration increases. In

computational fluid dynamics Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid dynamics, fluid flows. Computers are used to perform the calculations required ...

(CFD), nanofluids can be assumed to be single phase fluids; however, almost all academic papers use a two-phase assumption. Classical theory of single phase fluids can be applied, where physical properties of nanofluid is taken as a function of properties of both constituents and their concentrations. An alternative approach simulates nanofluids using a two-component model. 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 surface chemistry, disjoining pressure (symbol ) according to an IUPAC definition arises from an attractive interaction between two surfaces. For two flat and parallel surfaces, the value of the disjoining pressure (i.e., the force per unit are ...

in the vicinity of the contact line. 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.

Properties

Thermal conductivity, viscosity, density, specific heat, and surface tension are significant thermophysical properties of nanofluids. Parameters such as nanoparticle type, size, shape, volume concentration, fluid temperature, and nanofluid preparation method affect thermophysical properties. *

Viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

*Density *

Thermal conductivity The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1. Heat transfer occurs at a lower rate in materials of low ...

Synthesis

Nanofluids are produced by several techniques: * Direct Evaporation (1 step) * Gas condensation/dispersion (2 step) * Chemical vapour condensation (1 step) * Chemical precipitation (1 step) * Bio-based (2 step) Base liquids include water, ethylene glycol, and oils have been used. Although stabilization can be a challenge, on-going research indicates that it is possible. Nano-materials used so far in nanofluid synthesis include

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

lic particles,

oxide An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...

particles,

graphene Graphene () is a carbon allotrope consisting of a Single-layer materials, single layer of atoms arranged in a hexagonal lattice, honeycomb planar nanostructure. The name "graphene" is derived from "graphite" and the suffix -ene, indicating ...

nano-flakes and ceramic particles.

Bio-based

A biologically-based, environmentally friendly approach for the covalent functionalization of multi-walled carbon nanotubes (MWCNTs) using clove buds was developed. No toxic/hazardous acids are typically used in common carbon nanomaterial functionalization procedures, as employed in this synthesis. The MWCNTs are functionalized in one pot using a

free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabolic disorders Metabolism ...

grafting reaction. The clove-functionalized MWCNTs are then dispersed in distilled water (DI water), producing a highly stable MWCNT aqueous suspension (MWCNTs Nanofluid).

Applications

Nanofluids are primarily used for their enhanced thermal properties as

coolant A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corr ...

s in heat transfer equipment such as heat exchangers, electronic cooling system(such as flat plate) and radiators. Heat transfer over flat plate has been analyzed by many researchers. However, they are also useful for their controlled optical properties. Graphene based nanofluid has been found to enhance

Polymerase chain reaction The polymerase chain reaction (PCR) is a method widely used to make millions to billions of copies of a specific DNA sample rapidly, allowing scientists to amplify a very small sample of DNA (or a part of it) sufficiently to enable detailed st ...

efficiency.

Nanofluids in solar collectors Nanofluid-based direct solar collectors are solar thermal collectors where nanoparticles in a liquid medium can scatter and Absorption (electromagnetic radiation), absorb solar radiation. They have recently received interest to efficiently distri ...

is another application where nanofluids are employed for their tunable optical properties. Nanofluids have also been explored to enhance thermal desalination technologies, by altering thermal conductivity and absorbing sunlight, but surface fouling of the nanofluids poses a major risk to those approaches. Researchers proposed nanofluids for electronics cooling. Nanofluids also can be used in machining.

Smart cooling

One project demonstrated a class of magnetically polarizable nanofluids with thermal conductivity enhanced up to 300%. Fatty-acid-capped magnetite nanoparticles of different sizes (3-10 nm) were synthesized. It showed that the thermal and rheological properties of such magnetic nanofluids are tunable by varying magnetic field strength and orientation with respect to the direction of heat flow. Such response stimuli fluids are reversible and have applications in miniature devices such as micro- and nano-electromechanical systems. A 2013 study considered the effect of an external magnetic field on the convective heat transfer coefficient of water-based magnetite nanofluid experimentally under laminar flow regime. It obtained up to 300% enhancement at Re=745 and magnetic field gradient of 32.5 mT/mm. The effect of the magnetic field on pressure was not as significant.

Sensing

A nanofluid-based ultrasensitive optical sensor changes its colour on exposure to low concentrations of toxic cations. 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 uses a magnetic nanofluid that consists of nano-droplets with magnetic grains suspended in water. In a fixed magnetic field, a light source illuminates the nanofluid, changing its colour 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 responsive nanofluids can detect and image defects in ferromagnetic components. The so-called photonic eye 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 could monitor structures such as rail tracks and pipelines.

Nanolubricants

Nanolubricants modify oils used for engine and machine lubrication. Materials including metals, oxides and allotropes of carbon have supplied nanoparticles for such applications. The nanofluid enhances thermal conductivity and anti-wear properties. Although MoS₂,

, and Cu-based fluids have been studied extensively, fundamental understanding of underlying mechanisms is absent. MoS₂ and graphene work as third body lubricants, essentially acting as ball bearings that reduce the friction between surfaces. This mechanism requires sufficient particles to be present at the contact interface. The beneficial effects diminish because sustained contac pushes away the third body lubricants. 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. Nanoborate additives as well as mechanical model descriptions of diamond-like carbon (DLC) coating formations have been developed. Companies such as TriboTEX provide commercial formulations of synthesized MSH nanomaterial coatings for vehicle engine and industrial applications.

Petroleum refining

Many researches claim that nanoparticles can be used to enhance crude oil recovery.

Photonic crystals

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

Flow battery

Nanoelectrofuel-based flow batteries ((NFB) have been claimed to store 15 to 25 times as much energy as traditional flow batteries. The Strategic Technology Office of the U.S.

Defense Advanced Research Projects Agency The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military. Originally known as the Adva ...

(DARPA) is exploring military’s deployment of NFB in place of conventional

lithium-ion batteries A lithium-ion or Li-ion battery is a type of rechargeable battery that uses the reversible intercalation of Li+ ions into electronically conducting solids to store energy. Li-ion batteries are characterized by higher specific energy, energy ...

. The nanofluid particles undergo

redox Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...

reactions at the

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...

. Particles are engineered to remain suspended indefinitely, comprising up to 80 percent of the liquid’s weight with the

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

of motor oil. The particles can be made from inexpensive minerals, such as

ferric oxide Iron(III) oxide or ferric oxide is the inorganic compound with the formula . It occurs in nature as the mineral hematite, which serves as the primary source of iron for the steel industry. It is also known as red iron oxide, especially when us ...

(anode) and gamma

manganese dioxide Manganese dioxide is the inorganic compound with the formula . This blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese and a component of manganese nodules. The principal use for is for dry-cel ...

(cathode). The nanofluids use a nonflammable aqueous suspension. As of 2024 DARPA-funded Influit claimed to be developing a battery with an

energy density In physics, energy density is the quotient between the amount of energy stored in a given system or contained in a given region of space and the volume of the system or region considered. Often only the ''useful'' or extractable energy is measure ...

of 550-850 wh/kg, higher than conventional lithium-ion batteries. A demonstration battery operated successfully between −40 °C and 80 °C. Discharged nanofluids could be recharged while in a vehicle or after removal at a service station. Costs are claimed to be comparable to lithium ion. An EV-battery sized fuel reservoir (80 gallons) was expected to provide range comparable to a conventional gasoline vehicle. Fluids that escape, e.g., following a crash, turn into a pastelike substance, which can be removed and reused safely. Flow batteries also produce less heat, reducing their thermal signature for military vehicles.

Nanoparticle migration

A 30-lab study reported that "no anomalous enhancement of thermal conductivity was observed in the limited set of nanofluids tested in this exercise". The

COST Cost is the value of money that has been used up to produce something or deliver a service, and hence is not available for use anymore. In business, the cost may be one of acquisition, in which case the amount of money expended to acquire it i ...

funded research programme, Nanouptake (COST Action CA15119) was conducted with the intention "develop and foster the use of nanofluids as advanced heat transfer/thermal storage materials to increase the efficiency of heat exchange and storage systems". One 5-lab study reported that "there are no anomalous or unexplainable effects". Despite these apparently conclusive experimental investigations theoretical papers continue to claim anomalous enhancement, particularly via Brownian and thermophoretic mechanisms. Brownian diffusion is due to the random drifting of suspended nanoparticles in the base fluid which originates from collisions between nanoparticles and liquid molecules.

Thermophoresis Thermophoresis (also thermomigration, thermodiffusion, the Soret effect, or the Ludwig–Soret effect) is a phenomenon observed in mixtures of mobile particles where the different particle types exhibit different responses to the force of a temper ...

induces nanoparticle migration from warmer to colder regions, again due to such collisions. A 2017 study considered the mismatch between experimental and theoretical results. It reported that Brownian motion and thermophoresis effects have no significant effects: their role is often amplified in theoretical studies due to the use of incorrect parameter values. Experimental validation of these assertions came in 2018 Brownian diffusion as a cause for enhanced heat transfer is dismissed in the discussion of the use of nanofluids in solar collectors.

References

External links

Magnetically responsive photonic crystals nanofluid (video)
produced by Nanos scientificae * {{Cite journal , last1=Khashi’ie , first1=Najiyah Safwa , last2=Md Arifin , first2=Norihan , last3=Nazar , first3=Roslinda , last4=Hafidzuddin , first4=Ezad Hafidz , last5=Wahi , first5=Nadihah , last6=Pop , first6=Ioan , date=January 2019 , title=A Stability Analysis for Magnetohydrodynamics Stagnation Point Flow with Zero Nanoparticles Flux Condition and Anisotropic Slip , journal=Energies , language=en , volume=12 , issue=7 , pages=1268 , doi=10.3390/en12071268 , doi-access=free , issn=1996-1073 European projects:
NanoHex is a European project developing industrial-class nanofluid coolants
Nanoparticles Fluid mechanics Heat transfer Nanomaterials