Evolving opportunities in nanotechnology include nanoscale devices and systems, identification and understanding of fundamental nanoscale phenomena and processes, nanomaterials, development of new major research facilities, societal dimensions, instrumentation research, and nanomanufacturing. Meanwhile, evolving activities in nanomedicine are characterized by challenging issues related to this multidisciplinary effort. The following examples will illustrate some applications and roles of heat and mass transfer at the nanoscale in nanotechnologies.
Transport phenomena in micro- and nanoscales
Transport phenomena at dimensions between 1 and 100 μm are different from those at larger scales. At these scales, phenomena that are negligible at larger scales become dominant, but the macroscopic transport theory is still valid. One example of these phenomena in multiphase systemss is surface tension. In larger scale systems, the hydrostatic pressure or dynamic pressure effects may dominate over pressure drops caused by surface tension, but at smaller scales, the pressure drops caused by surface tension can dominate over hydrostatic and dynamic pressure effects. Transport phenomena in these scales are still regarded as macroscale, because classical theory of transport theory is still valid.
Main article: Transport phenomena in micro- and nanoscales.
Ultrashort Pulse Laser Melting and Resolidification of a Thin Metal Film
When the laser pulse is reduced to a nanosecond (10-9 sec) or less, the heat flux of the laser beam can be as high as 1012 W/m2 when the metal surface is molten. When a femtosecond pulse laser is used, the laser intensity can be up to 1021 W/m2. Compared to long pulsed laser processing, short-pulsed laser processing enables users to precisely control the size of the heat-affected zone, the heat rate, and the interfacial velocity. During laser-metal interaction, the laser energy is first deposited into electrons on the metal surface. The excited electrons move into deeper parts of the metal, by ballistic motion, with velocity close to the Fermi velocity (~106 m/s). Meanwhile, those hot electrons are diffused into a deeper part of the electron gas at a speed (<104 m/s) much lower than that of the ballistic motion. The hot electrons also collide with lattices – referred to as electron-lattice coupling – and transfer energy to the lattices (Hohlfeld et al., 2000). If the laser pulse width is shorter than the time required for the electron and lattice to achieve thermal equilibrium (thermalization time), the electrons and lattices can no longer be treated as being in thermal equilibrium (Grigoropoulos and Ye, 2000). The energy equations for the electrons and lattice must be specified separately and coupled through a coupling factor. In this case, the motion of the solid-liquid interface is no longer governed by the energy balance at the interface; instead, it is governed by the nonequilibrium kinetics of phase change. The solid phase during melting can be significantly overheated, while the liquid temperature during the resolidification stage can be significantly under-cooled.
Explosive Boiling during Ultrafast Laser-Materials Interaction
Material removal can be achieved by liquid-vapor phase change by using an ultrafast laser at intensity higher than that discussed above. The material removal can be achieved by (Chen and Beraun, 2003): (a) normal evaporation, (b) normal boiling, or (c) explosive boiling (or phase explosion). Normal evaporation occurs on the liquid surface without nucleation and is significant only for long-pulsed lasers (pulse width greater than 1 ns). Normal boiling requires higher laser fluence and a sufficiently long laser pulse (> 100 ns) to allow heterogeneous bubble nucleation of the vapor bubble. For an ultrashort pulsed laser, a melted material on and underneath the laser-irradiated surface cannot boil because the time scale does not allow the necessary heterogamous nuclei to form. Instead, the liquid is superheated to a degree past the normal saturation temperature and approaching the thermodynamic critical temperature, Tcr. At a temperature close to the critical temperature, homogeneous vapor bubble nucleation takes place at an extremely high rate, which in turn results in the near-surface region of the irradiated materials being ejected explosively. This process is referred to as explosive boiling or phase explosion. The explosion creates waves on the liquid surface, and the waves can be “frozen” upon rapid cooling of the surface(Craciun et al., 2002).
Heat and Mass Transfer in Nanofluid
While most research on heat transfer focuses on enhancement of heat transfer using various techniques, very few people pay attention to the inherently low thermal conductivity of the working fluid. It was demonstrated that dispersion of a tiny amount of nanoparticles in traditional fluids, which results in nanofluids, dramatically increases their thermal conductivities. For example, a small amount (less than 1% volume fraction) of copper nanoparticles or carbon nanotubes dispersed in ethylene glycol or oil can increase their inherently poor thermal conductivity by 40% and 150%, respectively (Eastman et al., 2001; Choi et al., 2001).
Keblinski et al. (2002) explored the possible factors influencing the heat transport capability of nanofluids that include (1) Brownian motion of nanoparticles, (2) molecular-level layering of the liquid at the nanoparticle surface, (3) nature of heat transport in nanoparticles, and (4) the effects of nanoparticles clustering. An order of magnitude analysis showed that the Brownian motion of nanoparticles is too slow to transport a significant amount of heat through a nanofluid; this conclusion was also supported by their results of Molecular Dynamics simulation. On the contrary, several empirical correlations to predict nanofluid thermal conductivity have been developed based on Brownian motion of nanoparticles (e.g., Jiang and Choi, 2004; Prasher et al., 2005). The viewpoints about the role of Brownian motion on the thermal conductivity of the nanofluids are still controversial at this time.
Xue and Xu (2005) proposed a model of thermal conductivity of nanofluids with interfacial shells by considering the temperature distribution and liquid layering. They predicted the thermal conductivities of Al2O3 /water, CuO/water, and CuO/ethylene glycol nanofluids using a 3-nm interfacial shell thickness. Good agreement with experiments was obtained. The shortcoming of the above model is that the liquid layering thickness cannot be determined by these models and must be obtained by matching the experimental data. On the other hand, Xue et al. (2004) suggested that the liquid layering was not responsible for the large enhancement of the nanofluid thermal conductivity. The role of liquid layering at the nanoparticle surface on the heat transfer enhancement is still debatable.
The carriers of heat in the crystalline solid are phonons, i.e., propagation of lattice vibrations. The commonly used diffusive heat transport theory is valid only if the mean free path of the phonons is much less than the size of the crystalline solid. If the mean free path of the crystalline solid is comparable to or greater than the size of the crystalline solid, which is the case for heat conduction in nanoparticles, the diffusive heat transport mechanism is no longer valid and ballistic transport is more realistic. The role of ballistic phonon motion on the enhancement of thermal conductivity has received scant attention.
Another possible mechanism for enhancement of thermal conductivity is clustering of nanoparticles in the nanofluids, which could occur if the nanoparticles are not finely dispersed in the base fluid. Xuan et al. (2003) pointed out that during stochastic motion of the suspended nanoparticles, aggregation and dispersion may occur among nanoparticle clusters and individual nanoparticles. The role of nanoparticle clustering on the enhancement of the thermal conductivity needs further investigation.
Ma et al. (2006) charged nanofluids (HPLC grade water containing 1.0 vol.% 5 – 50 nm of diamond nanoparticles) in a closed-loop copper pulsating heat pipe (PHP) and found that nanofluids significantly enhance the heat transport capability in the PHP. When the power input added on the evaporator is 100 W, the temperature difference between the evaporator and condenser can be reduced from 42 °C to 25 °C by addition of nanoparticles. The synthesis, heat transfer mechanism, and applications of nanofluids have been thoroughly reviewed by Das et al. (2006).
Near-field thermal radiation
Near field optics refers to the situation when the geometric features or distances are smaller than the characteristic wavelength. In the near-field regime, conventional radiative transfer approaches are often not applicable. For very small objects such as particles with dimensions less than the wavelength, its interaction with the electromagnetic (EM) wave field is very different and cannot be modeled with geometric optics. Consider a solid sphere of diameter D in air. When a monochromatic EM plane wave passes by, the particle can scatter or absorb the energy of the EM wave. As it turns out, when the wavelength (λ) is much smaller than the diameter, λ << D, the absorbed radiation cannot exceed the product, G×Ac, where G is the irradiance (energy flux) of the incident field and Ac is the cross-sectional area. The situation is very different when λ is comparable with or larger than D. The interaction between the particle and the EM field is well described by Maxwell’s equations. In such case, the particle can absorb more energy than a “blackbody”. Here, the concept of blackbody breaks down. The same is true when dealing with other nanostructures such as nanoslits or nanoapertures, where the energy transmittance can exceed unity due to diffraction effect. Near-field effect can also affect the far-field properties of nanostructured surfaces or objects. In the far-field, no matter how complex the structure is, the emissivity and transmittance cannot exceed unity. However, unique spectral- and angular-dependent radiative properties can be achieved by engineering nano/microstructures. Surface waves and photonic band structures are often utilized to enable unique optical properties of nano/microstructures.
Main article: Near-field thermal radiation
Chen, J.K. and Beraun, J.E., 2003, “Modeling of Ultrashort Laser Ablation of Gold Films in Vacuum,” Journal of Optics. A: Pure Applied Optics, Vol. 5, pp. 168-173.
Craciun, V., Bassim, N., Singh, R. K., Craciun, D., Hermann, J., Boulmer-Leborgne, C., 2002, “Laser-Induced Explosive Boiling During Nanosecond Laser Ablation of Silicon,” Applied Surface Science, Vol. 186, pp. 288-292.
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Eastman, J. A., Choi, S. U. S., Li, S., Yu, W. and Thompson, L. J., 2001, “Anomalously Increased Effective Thermal Conductivities of Ethylene Glycol-Based Nano-Fluids Containing Copper Nano-Particles,” Applied Physics Letters, Vol. 78, pp. 718-720.
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Jiang, S.P., and Choi, S.U.S., 2004, “Role of Brownian Motion in the Enhanced Thermal Conductivity of Nanofluids,” Applied Physics Letters, Vol. 84, pp. 4316-4318.
Keblinksi, P., Phillpot, S.R., Choi, S.U.S., and Eastman, J.A., 2002, “Mechanisms of Heat Flow in Suspensions of Nano-sized Particles (Nanofluids), Int. J. Heat Mass Transfer, Vol. 45, pp. 855-863.
Ma, H. B., Bogmeyer, B., Wison, C, Park, H, Yu, Q, Tirumala, M., Choi, S., 2006, “Nanofluid Effect on the Heat Transport Capability in an Oscillating Heat Pipe,” Applied Physical Letters, Vol. 88, pp. 143116(1-3)
Prasher, R., Bhattacharya, P., and Phelan, P.E., 2005, “Thermal Conductivity of Nanoscale Colloidal Solutions (Nanofluids),” Physical Review Letters, Vol. 94, pp. 025901-1–025901-4.
Xuan, Y., Li, Q., and Hu, W. 2003, “Aggregation Structure and Thermal Conductivity of Nanofluids,” AIChE Journal, Vol. 49, pp. 1038 – 1043.
Xue, L., Keblinski, P., Phillpot, S.R., Choi, S.U.S., and Eastman, J.A., 2004, “Effect of Liquid Layering at the Liquid-Solid Interface on Thermal Transport,” Int. J. Heat Mass Transfer, Vol. 47, pp. 4277-4284.
Xue, Q., and Xu, W. M., 2005, “A Model of Thermal Conductivity of Nanofluids with Interfacial Shells,” Materials Chemistry and Physics, Vol. 90, pp. 298-301.