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Nano thermal fluid and transport characteristics of swimming

Author: FuHuaiLiang
Tutor: GaoLei
School: Suzhou University
Course: Condensed Matter Physics
Keywords: nanofluids photophoresis thermophoresis magnetic nanofluids anisotropy interface resistance nanolayer
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Type: PhD thesis
Year: 2013
Downloads: 121
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Abstract

The metal or non-metal nanoparticles dispersed into such matrix solvents as water,ethylene glycol, oil, etc. will form a suspension liquid with good dispersion, high stabilityand strong heat conduction, which is called nanofluids. As a newly emerging frontier area,nanofluids will have innovative research applications of nanotechnology in the traditionalareas of thermal engineering. Due to its enhanced thermal conductivity, it is regarded as"the coolant of future" and it will have a wide application prospect and potential economicvalue in the energy, chemical industry, automobile, building, microelectronics, medicalinformation, and other fields. In recent years, while studying nanofluids’ enhancing heattransfer, researchers have begun to pay attention to the study on manipulation of micronanofluids on the basis of magnetic field, thermophoresis and photophoresis.According to the thermo-physical properties and phoretic transport properties ofnanofluids, this paper focuses study on the thermo-physical properties of magneticnanofluids, the characteristics of thermophoresis and photophoresis and particle propertiesdepending on the particle size, mainly considering the effect of magnetic field, anisotropy,size effect, interface layer, interfacial thermal resistance and other factors. And the mainwork is as follows:With the applied magnetic field, magnetic nanofluid is anisotropic with adjustablethermo-physical properties. In view of the experimental conclusion, we establish a researchmodel about the anisotropic thermo-physical properties of magnetic nanofluids. With boththe fluid properties and magnetic properties of the material, magnetic nanofluids is a newtype of functional material. By considering the geometrical anisotropy and physicalanisotropy, we adopt the self-consistent anisotropic effective medium theory to study thesignificantly enhanced anisotropic thermal conductivity of magnetic nanofluids. Geometricanisotropy arises from the aggregation of magnetic nanoparticles along the magnetic fielddirection. And physical anisotropy is due to the anisotropy of effective medium. For magnetic nanofluids with spherical nanoparticles randomly dispersed, in the absence ofmagnetic field, the system is isotropic, and we use Maxwell Garnett theory to study thethermal conductivity of system. Under the applied magnetic field, we extend the forecastsystem of thermal conductivity using anisotropic effective medium formula. We found thatthe effective thermal conductivity non-monotonically depends on the applied magneticfield, and the effective thermal conductivity both parallel and perpendicular to themagnetic field direction agrees well with the experimental results.In the nanofluids, nanoparticles will have thermophoretic motion under temperaturegradients. In recent years, with the development of optical micro-manipulation technology,researchers have begun to focus on particle thermophoresis research in the liquid andmanipulate nanoparticles, DNA, etc. by using thermophoresis. However, in the aspect oftheoretical research, the thermophoresis mechanism of liquid particles has not yet beenfully understood. With considering interfacial layer, the interaction between nanoparticlesand matrix molecules, we investigate particle thermophoresis properties of nanofluidssystem. According to the steady state heat transfer equations, we first infer temperaturefield of nanofluids suspending the nanoparticles with gradient nanolayers. then determinethe fluid velocity field distribution near the particle surface according to the Navier-Stokesequation, and then get particle thermophoresis mobility. We find that the nanoparticlesthermophoretic mobilityDT non-monotonically depends on the particle radiusrp. Thereexists a critical radiusR0.Whilerp <R0, thermophoretic mobility increases with increasingparticle radius; and whilerp> R0, thermophoretic mobility decreases with increasingparticle radius. When the particle radius is large enough, the thermophoretic mobility willbe independent of particle size in agreement with the current experimental results. At thesame time, we also studied the effect of particles on the thermal conductivity, systemtemperature on particle thermophoresis. And results show that the particle thermophoreticmobility decreases with the increase of thermal conductivity, and increases linearly withthe increase of temperature. Light can change the motion of microscopic particles. Under the irradiation of light,positive or negative photophoretic motion will happen with particles in fluids. Because theincident light absorbed by the articles is not uniform and asymmetry, resulting intemperature gradients near the particle surface, which causes the non-uniform andnon-symmetric force on particles and form driving force, photophoretic motion appear atlast. Generally, if optical absorption in particle is on the welcoming light side, positivephotophoresis happens; If optical absorption in particle is on the shade side, negativephotophoresis will happen. Research has shown that particles with high absorption ratewill have positive photophoretic motion and particles with low absorption rate will havenegative photophoretic motion. In fact, particle photophoresis features relate with not onlyparticle absorption coefficient, but also the real part of the refractive index, the wavelengthof incident light, thermo-physical properties of particle and matrix and interfacial thermalresistance and other factors.We study the photophoretic motion by considering the interfacial thermal resistanceand the physical characteristics of particles. We first determine the internal electromagneticfield distribution according to Mie scattering theory. And then the fluid temperature fielddistribution is obtained by solving the steady-state heat transfer equation, and according tothe thermophoretic mechanism and the slip-flow boundary conditions, particles’photophoresis velocity is determined by solving the Navier-Stokes equation. We discussthe relation between refractive index and thermo-physical properties of particle, interfacialthermal resistance, particle size, particle parameters, and have the following conclusions:Generally, particles with the larger size parametersα, high absorption coefficientκ pandlow refractivityn p, positive photophoretic motion happens. And negative photophoresismovement will happen in the opposite case. At the same time, we also find higherphotophoretic motion will happen for particles with low thermal conductivity and lowinterfacial resistance.With the continuous development of micro-manipulation technology, light trapping or manipulation technology has been developed rapidly. Because light manipulation has suchcharacteristics as high accuracy, strong control force, no contact and no damage, etc., it hasa wide application prospect in medical, chemistry, physics, biology and other fields. Ourstudy on the photophoresis characteristics of particles will provide more reliable theoreticalbasis for light trapping or manipulation.As the newly emerging interdisciplinary in1990s, nanofluids still does not have awidely understood and unified theory about its thermo-physical properties and phoretictransport mechanism. In this paper, we study the thermo-physical properties of magneticnanofluids, the characteristics of particle thermophoresis and photophoresis, and it willhave certain academic value to the research and application to manipulate particle innanofluids by magnetic field, temperature gradient and light. And it will also furtherpromote the development of nanometer material science and lay a theoretical basis for theapplication of nanometer materials in the industries.

Nano thermal fluid and transport characteristics of swimming

Abstract

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