Comparison of nanoemulsion fluids and emulsions
Abstract
Cooling is one of the most important technique challenges faced by a range of diverse industries and military needs. There is an urgent need for innovative heat transfer fluids with improved thermal properties over currently available ones. This chapter discusses the development and characterization of nanoemulsion heat transfer fluids with phase changeable nanodroplets to increase the thermophysical properties and the heat transfer rate of the fluid. Nanoemulsion heat transfer fluids can be formed by dispersing one fluid into another immiscible fluid as nanosized structures such as droplets and tubes, in which those nanostructures are swollen reverse micelles with the dispersed phase and stabilized by the surfactant molecules. In addition to the enhancement of thermophysical properties such as thermal conductivity by mixing another liquid of higher thermal conductivity, an even larger amount of heat can be absorbed or released when these nanodroplets undergo phase transition from liquid to gas or vice versa, and thus enhancing the heat transfer rate. Three types of nanoemulsion heat transfer fluids are introduced: alcohol-in-polyalphaolefin, water-in-FC-72, and water-in-polyalphaolefin. Structural and property characterizations of these nanoemulsion heat-transfer fluids are the two main aspects of this chapter. This chapter also identifies several critical issues in the nanoemulsion heat transfer fluids to be solved in the future.
Keywords
- Nanoemulsion
- Thermophysical Property
- Heat Transfer
- Small Angle Neutron Scattering (SANS)
1. Introduction
Cooling is one of the most important technical challenges faced by a range of diverse industries: microelectronics, optoelectronics, and, especially, power electronics [1–8]. This technology gap is the result of the higher currents, switching frequencies, and component densities of today’s electronics and power electronics. The advances in semiconductor materials and more precise fabrication techniques have the unfortunate side effect of generating higher amounts of waste heat within a smaller volume. Today, it is not unusual to see heat fluxes of 200 W/cm2 in a power module, a figure that is expected to increase over 1000 W/cm2 in the near future.
Thermal management of such high flux is becoming the bottleneck to improvements in electronics and power electronics. Existing cooling systems is striving to meet the ever-increasing demand in higher computational power and smaller footprint. It is important that a cooling system with significantly improved heat transfer systems and their kernel components can be developed; in particular the cooling fluid used inside many heat transfer systems but yet has received little attention. The heat transfer fluids used in these heat transfer systems, including the coolants, lubricants, oils, and other fluids, limit the capacity and compactness of the heat exchangers that use these fluids due to their inherently poor heat transfer properties. The heat transfer capability of the heat exchangers can be easily amplified if fluids with better thermal properties are used. Therefore, development of innovative heat transfer fluids with improved thermal properties over those currently available is urgently needed.
The strategy of adding solid, highly conductive particles to improve thermal conductivity of fluids has been pursued since Maxwell’s theoretical work was first published more than 100 years ago [9]. Early-stage studies have been confined to millimeter- or micrometer-sized solid particles dispersed in fluids. In the past decade, researchers have focused on suspensions of nanometer-sized solid particles, known as nanofluids [2, 8]. Many reviews and introductory reports on nanofluids have already been published [1, 2, 4, 10–14].
In this paper, it is intended to introduce some recent developments in another type of engineered heat transfer fluids, in which phase changeable nanodroplets are added to increase the thermophysical properties and heat transfer rate of the base fluids [15–22]. This chapter starts with the introduction of nanoemulsion fluids with potential application in thermal fluids. It is followed by the discussion on structural and thermophysical characterization techniques for nanoemulsion fluids [15–19, 21]. Then, three groups of nanoemulsion fluids and their properties are discussed. This chapter is not intended to serve as a complete description of all nanoemulsion fluids available for heat transfer applications. The selection of the coverage was influenced by the research focus of the authors and reflects their assessment of the field.
2. Nanoemulsion heat transfer fluids
One fluid is dispersed into another immiscible fluid as nanosized structures such as droplets and tubes to create a “nanoemulsion fluid.” Those nanosized structures of the dispersed phase are micelles stabilized by the surfactant molecules on the outside. Nanoemulsion fluids are part of a broad class of multiphase colloidal dispersions [15–19, 21]. Different from the preparation of the nanofluids and emulsions [23–31], the nanoemulsion fluids are spontaneously generated by self-assembly which does not require external shear force. Thus the nanoemulsion fluids are thermodynamically stable [16–19, 23, 32–44]. Table 1 is the comparison between self-assembled nanoemulsion fluids and conventional emulsions. Nanoemulsion fluids made of specific fluids are suited for thermal management applications. Figure 1 shows a picture of ethanol-in-polyalphaolefin (PAO) nanoemulsion heat transfer fluids: both PAO and PAO-based nanoemulsion fluids are transparent but the nanoemulsion exhibits the Tyndall effect [15, 16, 19, 21, 45, 46].
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1 | Appearance | Transparent | Turbid | |
2 | Interfacial tension | Ultra low (usually <<1 mN/m) |
Low | |
3 | Droplet size | <50 nm | >500 nm | |
4 | Stability | Thermodynamically stable, long shelf life | Thermodynamically unstable | |
5 | Preparation | Self-assembly | N of external shear | |
6 | Viscosity | Newtonian | Non-Newtonian |
2.1. Formation of self-assembled nanoemulsion fluids
Self-assembled nanoemulsion fluids are thermodynamically stable, and the formation of these fluids can be explained using the classical thermodynamic theory [23, 34–39, 42, 43, 47–49]. The nanoemulsion fluid consists of one oil phase, one water phase, and certain surfactants. The adding of surfactant lowers the surface tension of the oil–water interface and the change in free energy of the system is given by Equation 1,
where
Figure 2 shows the typical phase behavior diagram of a ternary system that contains two immiscible so-called oil and water phases and an amphiphilic surfactant component. The term “water” refers to a polar phase while “oil” is used for an apolar organic phase. When a system has a composition that lies in the shaded areas, a nanoemulsion fluid, either oil-in-water or water-in-oil, can be formed through self-assembly.
2.2. Structure characterization methods
Similar to other multiphase colloidal dispersions, the microstructure of nanoemulsion fluids is sensitive to many factors, including the different dispersed liquid, surfactants and base fluid, and molar ratio of dispersed liquid to surfactant [34]. In addition to that, temperature, pH-value, and salinity also play an important role in the microstructure [23]. So accurate characterization of the microstructure of nanoemulsion fluids is important to understand the nanoemulsion fluids and yet challenging and costly to perform. In the past, small angle X-ray scattering (SAXS), small angle neutron scattering (SANS), dynamic or laser-light scattering (DLS), transmission electron microscopy (TEM), and nuclear magnetic resonance (NMR) have been used for the structure characterization [23]. In addition, the measurement of nanoemulsion properties, such as viscosity, electric conductance, thermal conductivity, dielectric permittivity, electrophoretic birefringence, ultrasonic interferometry, and ultrasonic absorption, can also provide information on the internal microstructure.
SANS allows the characterization of the structures inside the material on the nanometer (10-9 m) to micrometer (10-6 m) scale [51]. Many advanced engineering materials obtain unique performance because of their engineered structures on this length scale. For example, the toughness of high-impact plastics depends on the admixture of stiff and flexible segments of polymer molecules on the nano-to-micro scale, as well as, many biological processes in cells: from the storage of information on magnetic disks, to the hardness of steels and superalloys, to the conduction of current in superconductors, and many other materials properties.
Among all methods currently available for characterizing the microstructure of nanoemulsion fluids, small-angle neutron scattering (SANS) provides a unique approach to probe structure in liquids thanks to the distinctive penetrating power of neutron. Unlike the conventional dynamic light-scattering method using laser or X-rays, it can be applied to “concentrated” colloidal suspensions (e.g., >1 % volume fraction) and can penetrate through a container [51–57]. Another advantage of SANS method is the deuteration method, in which deuterium labeled components in the sample in order to enhance their contrast that it can probe specific molecules or structure inside the sample with the deuteration technique. This unique method allows SANS to measure density fluctuations and composition (or concentration) fluctuations, which is very important to understand the structure inside nanoemulsion fluids [23].
2.3. Thermophysical properties characterization methods
2.3.1. Thermal conductivity
Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids that are required in many industrial applications. Conventional heat transfer fluids have relatively poor thermal conductivity compared to metals [9]. It has been reported that the dispersed liquid nanodroplets could alter thermal conductivity of the base fluids [1, 2, 4, 58–61]. However, because of the absence of a theory for the thermal conductivity of nanoemulsion heat transfer fluids, an investigation of the effect of nanodroplets on the thermal conductivity will be conducted.
There are two widely used methods to measure the thermal conductivity of nanoemulsion fluids which includes (1) the transient hot-wire technique and (2) 3ω-wire method [62]. In the transient hot-wire method, thermal conductivity value is determined from the heating power and the slope of temperature change versus logarithmic time. The 3ω-wire method is used to measure the fluid thermal conductivity [19, 21, 62, 63]. This method is actually a combination of the transient hot-wire method and the 3ω-wire method, in which a metal wire is suspended to a liquid acting as both heater and thermometer. One advantage of this 3ω-wire method is that the temperature oscillation can be kept low enough: it is usually below 1 K as compared to about 5 K for the hot-wire method. It greatly helps to retain constant liquid properties of test liquid during measurement. Calibration experiments were performed for hydrocarbon (oil), fluorocarbon, and water at atmospheric pressure before each measurement.
2.3.2. Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or tensile stress. Viscosity is another macroscopically observable parameter that characterizes a nanoemulsion fluid, and it may range anywhere between a low viscous fluid and a gel state. It is an important quantity for many practical applications of nanoemulsion fluids, especially those used for hydraulic fluids. For instance, pumping such systems might be of interest in their application, and here viscosity plays an important role. Viscosity can be determined from the equation below:
where
Viscosity of a nanoemulsion fluid depends largely on its microstructure, that is, the type of aggregates that are present, on their interactions, and on the concentration of the system. So the viscosity can be used to monitor structural changes in the nanoemulsion system. In order to do so, one has to compare the experimental data to theoretical expressions that give the viscosity expected for certain model systems.
2.3.3. Specific heat measurement
The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius. A differential scanning calorimeter (DSC) is usually used to measure a material’s specific heat. In DSC measurement, it compares the differential heat flow (heat/time) between the measured material and the empty reference pan by adjusting the heat flux into a pan containing the sample with the heat flux into an empty pan while keeping both the measured sample and reference sample at nearly the same temperature. The difference in the amount of heat supplied to the sample and the reference is recorded as a function of temperature (or time), and the positive or negative peaks in the relationship correspond to exothermic or endothermic reactions in the sample, respectively. In order to determine the sample heat capacity, three measurements are usually carried out: for the sample, for the baseline, and for a standard. The baseline is subtracted from the sample measurement to obtain absolute values of the heat flow to the sample. The heat capacity is to be determined by the heat flow, the temperature rise, and the sample mass.
3. Nanoemulsion heat transfer fluids
3.1. Ethanol-in-Polyalphaolefin (PAO) nanoemulsion fluids
Polyalphaolefin (PAO) has been widely used as dielectric heat transfer fluids and lubricants due to its chemical stability within a wide temperature range. However, its thermal properties are relatively poor compared to other heat transfer fluids [64]. Ethanol-in-PAO is a liquid–vapor phase change nanoemulsion fluid, in which over 90 % consist of PAO to keep its chemical stability very close to pure PAO. Meanwhile, the ethanol nanodroplets could evaporate explosively and thus enhance the heat-transfer rate of the base fluid PAO [16]. The microstructure and thermophysical properties of the ethanol-in-PAO nanoemulsion fluids are discussed below.
3.1.1. Microstructure of ethanol-in-PAO nanoemulsion fluids
The microstructure of ethanol-in-PAO nanoemulsion fluids are on the NG-3 (30 m) beamline at the NIST Center for Neutron Research (NCNR) in Gaithersburg, MD. Samples are loaded into 2-mm quartz cells. Figure 3 shows the SANS data, the scattering intensity I versus the scattering vector q = 4π sin(θ/2)/λ, where λ is the wavelength of the incident neutrons, and θ is the scattering angle. The approximation q = 2πθ/λ is used for SANS (due to the small angle θ). The analysis of the SANS data suggests that the inner cores of the swollen micelles, that is, the ethanol droplets, are spherical and have a radius of about 0.8 nm for 9 vol. %.
3.1.2. Thermal conductivity of ethanol-in-PAO nanoemulsion fluids
The relative thermal conductivity in ethanol-in-PAO nanoemulsion fluids measured using the 3ω-wire method along with the prediction by the Maxwell model (one model based on effective medium theory [EMT]) is shown in Figure 4. The relative thermal conductivity is defined as
where
It can be seen in Figure 4 that the relative thermal conductivity of ethanol-in-PAO nanoemulsion fluids is rather moderate (e.g. 2.3 % increase for 9 vol. % (
3.1.3. Viscosity of ethanol-in-PAO nanoemulsion fluids
Figure 5 shows the relative dynamic viscosity,
where
3.2. Water-in-FC-72 nanoemulsion fluids
Water-in-FC-72 nanoemulsion fluids are another group of nanoemulsion fluids designed for heat transfer purpose, in which water could undergo liquid–solid transition and thus increase heat transfer rate of the base fluid FC-72. FC-72 is one of the lines of Fluorinert™ Electronic Liquids developed by 3M™, which is used as the cooling fluids in liquid-cooled thermal management systems due to its low boiling point and excellent dielectric properties [66]. However, its heat transfer properties such as thermal conductivity and heat capacity are much inferior, compared to other fluids such as water.
3.2.1. Microstructure of water-in-FC-72 nanoemulsion fluids
Water-in-FC-72 nanoemulsion fluids are generated by emulsifying deionized water into FC-72 with a small amount of perfluorinated amphiphiles. Figure 6 (a) shows the picture of the water-in-FC-72 nanoemulsion fluids and the pure FC-72. The autocorrelation function of the scattered light for the 12 vol. % water-in-FC-72 nanoemulsion fluids is plotted in Figure 6 (b), in which the curve shows a typical exponential decay of the correlation function versus time [19, 21]. The Brownian diffusivity and effective hydrodynamic radius of the nanodroplets are found to be 3.5×10-7 cm2/s and 9.8 nm at
3.2.2. Thermal conductivity of water-in-FC-72 nanoemulsion fluids
Thermal conductivity of the water-in-FC-72 nanoemulsion is measured for different water loadings, and the results are shown in Figure 7. The 3ω -wire method is used to measure the fluid thermal conductivity. In water-in-FC-72 nanoemulsion fluids, the water phase has a thermal conductivity much higher than that of the base liquid FC-72. Water’s thermal conductivity is 0.609 W/(mK) at 300 K and FC-72’s thermal conductivity is much smaller, about 0.066 W/mK [66]. The addition of water is expected to improve the effective thermal conductivity of FC-72.
A very large increase in thermal conductivity (up to 52 % for water-in-FC-72 nanoemulsion of 12 vol. % water) can be seen in Figure 7. The observed enhancement in thermal conductivity is much larger than that predicted by the EMT with assumption of spherical droplets [67]. This suggests that the water droplets are column-like with high aspect ratio of length to radius, which leads to a higher thermal conductivity enhancement in nanoemulsion fluids than the spherical droplets.
3.2.3. Viscosity of water-in-FC-72 nanoemulsion fluids
The dynamic viscosity of water-in-FC-72 nanoemulsions of different nanodroplet concentrations is measured using a Brookfield viscometer at room temperature. The results have been normalized to the viscosity of pure FC-72 and are shown in Figure 8. The measured viscosity increase is nonlinear with the higher concentration of water added inside which agrees well with the nonlinear increase in thermal conductivity. This nonlinear increase in viscosity is common in colloidal systems, and has been interpreted by the aggregation of nanodroplets, that is, formation of column-like microstructure. Similar to the ethanol-in-PAO nanoemulsion fluids discussed previously, the Einstein equation significantly underpredicts the viscosity increase in the water-in-FC-72 nanoemulsion fluids at relatively high water loadings, as can be seen in Figure 8.
3.2.4. Effective heat capacity of water-in-FC-72 nanoemulsion fluids
Another significant thermal property enhancement can be achieved here using the phase change of water nanodroplets formed inside the water-in-FC-72 nanoemulsion fluids. In water-in-FC-72 nanoemulsion fluids, the fluid’s heat capacity can be increased by the high specific heat of water(the volumetric heat capacity of water is about 4.18 J/ml K, and is over two times the heat capacity of PAO (1.74 J/ml K) [65]) and/or the latent heat of water is the highest among common heat transfer fluids (
The measured and calculated heat capacities of the water-in-FC-72 nanoemulsion fluids using a TA-CC100 DSC are shown in Figure 9. It can be seen that over 15 % increase in heat capacity can be achieved for a water volumetric fraction of 12 %, in which the measured
The use of phase-changeable nanodroplets (e.g. water nanodroplets) provides another way to simultaneously increase the effective specific heat and thermal conductivity of conventional heat-transfer fluids.
3.3. Water-in-PAO nanoemulsion fluids
3.3.1. Microstructure of water-in-PAO nanoemulsion fluids
Figure 10 shows the SANS data for water-in-PAO nanoemulsion fluids with water volumetric concentration covering 1.8 vol. % to 10.3 vol.%.
It is clear from the SANS data that intensity curves of water-in-PAO nanoemulsion fluids gradually change the shape with increasing water loading. They can be further classified into three ranges (marked using three different colors): the 1.8–4.5 vol. % water-in-PAO nanoemulsion fluids with a smooth and gradually increasing scattering intensity for low q range (less than 0.1
The hard sphere model (one typical simple correlation length model) fits well for low water concentrations (i.e. 1.8 % to 4.5 % volume fractions), and nanodroplet radii are found to be 13.2
Thus, a more comprehensive fitting model must be used to take into account of the structure change inside the water-in-PAO nanoemulsion fluids. Here, the three-region Guinier–Porod empirical model is used to accommodate the structural changes inside the system by fitting curves that are shown in Figure 12 [51, 54, 56, 68].
Generally, the scattering intensity is given by two contributions in the Guinier–Porod model:
Here
Based on the fitting curves using the three-region Guinier–Porod model, there are two dimensionality parameters
3.3.2. Thermal conductivity of water-in-PAO nanoemulsion fluids
Figure 13 shows the thermal conductivity enhancement in water-in-PAO nanoemulsion fluids as a function of the loading of water from 0.47 % to 8.6 vol. %, in which the thermal conductivity linearly increases with higher water volume fraction and reaches a maximum of 16 % increase at 8.6 vol. % water.
3.3.3. Viscosity of water-in-PAO nanoemulsion fluids
The dynamic viscosity of water-in-PAO nanoemulsion fluids with different water volumetric concentrations is shown in Figure 14. All the water-in-PAO nanoemulsion fluids exhibit a shear-independent characteristic of Newtonian fluids. One unique phenomenon that can be seen in Figure 14 is that there is a maximum value in viscosity: it first increases with water concentration, reaches a maximum at 5.3 vol. %, and then decreases. This trend is different from the thermal conductivity shown in Figure 13 and the viscosity trend as observed in other nanoemulsion fluid systems. The maximum in viscosity can be attributed to the attraction force between droplets within the nanoemulsion fluids. The surfactant molecules become hydrated when more water is added inside and their counter ions are released into water which makes surfactants molecules and droplets charged oppositely so that the interdroplet attraction keeps increasing until the hydration process is complete. This may lead to a maximum viscosity in water-in-PAO nanoemulsion fluids as shown in Figure 14. It also coincides with the nonlinear inner structure change with increasing water concentration as seen in Figure 10.
3.3.4. Effective heat capacity of water-in-PAO nanoemulsion fluids
Similarly, the heat capacity of water-in-PAO nanoemulsion fluids was also investigated (shown in Figure 15). As shown in water-in-FC-72 nanoemulsion fluids, the water inside the water-in-PAO nanoemulsion fluids can increase the system’s effective heat capacity through the high specific heat of water (i.e.
DSC cyclic curves of water-in-PAO nanoemulsion fluids under different water loadings are shown in Figure 15. During the heating and cooling cycles, water nanodroplets undergo a melting–freezing transition in the nanoemulsion fluids. Interestingly, the presence of a single freezing peak in Figure 15 indicates a correspondence of the structural change with increasing water concentration (or water to surfactant molar ratio) as observed in our previous SANS measurement result: there is no obvious melting/freezing peak for water concentrations less than 4.5 vol. %, while the exothermic crystallization peak starts at around −20 oC when water concentration is higher or equal to 4.5 vol. %. When the water concentration is increased further above 8.6 vol. %, the freezing peak shifts to effect lower supercooling and peak values.
To gain further insight into that, the specific heat of each sample is also calculated and summarized here in Table 2. The calculated
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10.3 | 28.97 | –18.88 |
8.6 | 34.17 | –23.97 |
7.8 | 31.27 | –22.74 |
5.3 | 26.72 | –20.32 |
4.5 | 9.808 | –20.63 |
3.6 | 2.48 | -45 |
1.8 | 2.18 | 0 |
In addition to that, a total maximum heat capacity increase of 88 % is obtained in the 8.6 vol. % water/PAO nanoemulsion fluids: for a temperature increase from −20 to 0 oC, 1 ml PAO absorbs 37.6 joules heat, and for the nanoemulsion containing 8.6 % water nanodroplets, the melting of ice nanoparticle absorbs 34 joules heat, it means that upon the melting of the ice nanoparticles in the nanoemulsion, the heat capacity of PAO has increased up by about 76 % in addition to the enhancement in heat capacity caused only by the addition of water without phase change (~12%).
4. Conclusions
The use of adding another material into thermal fluids has been emerged in recent years as a way to enhance the heat capacity and thermal conductivity of the base fluids simultaneously. A new type of nanostructured heat transfer fluids: nanoemulsion fluids are discussed in this chapter, such as ethanol-in-PAO, water-in-FC-72, and water-in-PAO nanoemulsion fluids. Many interesting properties have been reported in these nanoemulsion heat transfer fluids recently. The nanoemulsion heat transfer fluids can be formed by self-assembly and are thermodynamically stable. The self-assembled nanostructures have a significant effect on its macroscopic thermophysical properties which coincides with the structural characteristics measured using SANS. In addition to that, the effective heat capacity of base fluid can also be greatly enhanced when those phase changeable nanodroplets undergo phase transition: the effective heat capacity of FC-72 by more than 200 % when those droplets undergo liquid–solid phase transition in water-in-FC-72 nanoemulsion fluids, and the effective heat capacity of PAO is increased by 80 % in water-in-PAO nanoemulsion fluids. The use of nanoemulsion fluids provides a means to increase the fluid conductivity and heat capacity simultaneously in the base fluids and their application in a wide variety of applications appears promising, but several critical issues remain to be solved in the future, for example, large subcooling or superheating during phase change due to lack of nucleation sites, and large viscosity increase due to the dispersed nanodroplets inside.
Acknowledgments
The authors would like to thank Dr. Boualem Hammouda at Center for Neutron Research (National Institute of Standards and Technology, Gaithersburg, MD) for helping to conduct the SANS experiments and having a constructive discussion on SANS data post processing.
This study is financially supported by National Science Foundation (CBET-0730963). The SANS measurements performed at the NIST-CNR are supported in part by the National Science Foundation under Agreement No. DMR-0944772.
The identification of commercial products does not imply endorsement by the National Institute of Standards and Technology nor does it imply that these are the best for the purpose.
References
- 1.
Eastman, J. A., Phillpot, S. R., Choi, S. U. S., and Keblinski, P., 2004, "Thermal transport in nanofluids," Annual Review of Materials Research, 34, pp. 219–246. - 2.
Eastman, L. J., Choi, S. U. S., LI, S., and Thompson, L. J., 1997, "Enhanced thermal conductivity through development of nanofluids," Nanocrystalline and Nanocomposite Materials II. - 3.
Inaba, H., 2000, "New challenge in advanced thermal energy transportation using functionally thermal fluids," International Journal of Thermal Sciences, 39(9–11), pp. 991–1003. - 4.
Buongiorno, J., Venerus, D. C., Prabhat, N., McKrell, T., Townsend, J., Christianson, R., Tolmachev, Y. V., Keblinski, P., Hu, L.-W., Alvarado, J. L., Bang, I. C., Bishnoi, S. W., Bonetti, M., Botz, F., Cecere, A., Chang, Y., Chen, G., Chen, H., Chung, S. J., Chyu, M. K., Das, S. K., Di Paola, R., Ding, Y., Dubois, F., Dzido, G., Eapen, J., Escher, W., Funfschilling, D., Galand, Q., Gao, J., Gharagozloo, P. E., Goodson, K. E., Gutierrez, J. G., Hong, H., Horton, M., Hwang, K. S., Iorio, C. S., Jang, S. P., Jarzebski, A. B., Jiang, Y., Jin, L., Kabelac, S., Kamath, A., Kedzierski, M. A., Kieng, L. G., Kim, C., Kim, J.-H., Kim, S., Lee, S. H., Leong, K. C., Manna, I., Michel, B., Ni, R., Patel, H. E., Philip, J., Poulikakos, D., Reynaud, C., Savino, R., Singh, P. K., Song, P., Sundararajan, T., Timofeeva, E., Tritcak, T., Turanov, A. N., Van Vaerenbergh, S., Wen, D., Witharana, S., Yang, C., Yeh, W.-H., Zhao, X.-Z., and Zhou, S.-Q., 2009, "A benchmark study on the thermal conductivity of nanofluids," Journal of Applied Physics, 106(9). - 5.
Zimparov, V., 2002, "Energy conservation through heat transfer enhancement techniques," International Journal of Energy Research, 26(7), pp. 675–696. - 6.
Dewan, A., Mahanta, P., Raju, K. S., and Kumar, P. S., 2004, "Review of passive heat transfer augmentation techniques," Proceedings of the Institution of Mechanical Engineers Part A-Journal of Power and Energy, 218(A7), pp. 509–527. - 7.
Boyd, R. D., 1985, "Subcooled flow boiling critical heat-flux (CHF) and its application to fusion energy components. 1. A review of fudamentals of CHF and related data-base," Fusion Technology, 7(1), pp. 7–30. - 8.
Choi, S. U. S., Zhang, Z. G., Yu, W., Lockwood, F. E., and Grulke, E. A., 2001, "Anomalous thermal conductivity enhancement in nanotube suspensions," Applied Physics Letters, 79(14), pp. 2252–2254. - 9.
Maxwell, J. C., 1904, "A treatise on electricity and magnetism," Oxford University Press, Cambridge, UK. - 10.
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 nanofluids containing copper nanoparticles," Applied Physics Letters, 78(6), pp. 718–720. - 11.
Keblinski, P., Phillpot, S. R., Choi, S. U. S., and Eastman, J. A., 2002, "Mechanisms of heat flow in suspensions of nano-sized particles (nanofluids)," International Journal of Heat and Mass Transfer, 45(4), pp. 855–863. - 12.
Prasher, R., Song, D., Wang, J. L., and Phelan, P., 2006, "Measurements of nanofluid viscosity and its implications for thermal applications," Applied Physics Letters, 89(13). - 13.
Wang, X. Q., and Mujumdar, A. S., 2008, "A review on nanofluids - Part II: Experiments and applications," Brazilian Journal of Chemical Engineering, 25(4), pp. 631–648. - 14.
Wang, X. Q., and Mujumdar, A. S., 2008, "A review on nanaofluids - Part I: Theoretical and numerical investigations," Brazilian Journal of Chemical Engineering, 25(4), pp. 613–630. - 15.
Xu, J. J., Wu, C. W., and Yang, B., 2010, "Thermal- and phase-change characteristics of self-assembled ethanol/polyalphaolefin nanoemulsion fluids," Journal of Thermophysics and Heat Transfer, 24(1), pp. 208–211. - 16.
Xu, J., Yang, B., and Hammouda, B., 2011, "Thermal conductivity and viscosity of self-assembled alcohol/polyalphaolefin nanoemulsion fluids," Nanoscale Research Letters, 6. - 17.
Xu, J., Hammouda, B., and Yang, B., 2012, "Thermophysical properties and pool boiling characteristics of water in polyalphaolefin nanoemulsion fluids," ASME, Proceedings of ASME Micro/Nanoscale Heat & Mass Transfer International Conference 2012. - 18.
Xu, J., and Yang, B., 2012, "Novel heat transfer fluids: Self-assembled nanoemulsion fluids," Nanotechnology, D. J. N. Govil, ed., Studium Press LLC. - 19.
Yang, B., and Han, Z. H., 2006, "Thermal conductivity enhancement in water-in-FC72 nanoemulsion fluids," Applied Physics Letters, 88(26). - 20.
Han, Z. H., Cao, F. Y., and Yang, B., 2008, "Synthesis and thermal characterization of phase-changeable indium/polyalphaolefin nanofluids," Applied Physics Letters, 92(24). - 21.
Han, Z. H., and Yang, B., 2008, "Thermophysical characteristics of water-in-FC72 nanoemulsion fluids," Applied Physics Letters, 92(1). - 22.
Han, Z. H., Yang, B., Qi, Y., and Cumings, J., 2011, "Synthesis of low-melting-point metallic nanoparticles with an ultrasonic nanoemulsion method," Ultrasonics, 51(4), pp. 485–488. - 23.
Kumar, P., and Mittal, K. L., 1999, Handbook of microemulsion science and technology, New York: Marcel Dekker. - 24.
Rosele, M. L., "Boiling of dilute emulsions," PhD Dissertation, University of Minnesota. - 25.
Bulanov, N. V., Skripov, V. P., and Khmylnin, V. A., 1984, "Heat transfer to emulsion with superheating of its disperse phase," Journal of Engineering Physics, pp. 1–3. - 26.
Bulanov, N. V., Skripov, V. P., and Khmylnin, V. A., 1993, "Heat transfer to tmulsion with a low-boiling disperse phase," Heat Transfer Research, pp. 786–789. - 27.
Bulanov, N. V., 2001, "An analysis of the heat flux density under conditions of boiling internal phase of emulsion," High Temperature, 39(3), pp. 462–469. - 28.
Bulanov, N. V., and Gasanov, B. M., 2005, "Experimental setup for studying the chain activation of low-temperature boiling sites in superheated liquid droplets," Colloid Journal, 67(5), pp. 531–536. - 29.
Bulanov, N. V., Gasanov, B. M., and Turchaninova, E. A., 2006, "Results of experimental investigation of heat transfer with emulsions with low-boiling disperse phase," High Temperature, 44(2), pp. 267–282. - 30.
Bulanov, N. V., and Gasanov, B. M., 2008, "Peculiarities of boiling of emulsions with a low-boiling disperse phase," International Journal of Heat and Mass Transfer, 51(7–8), pp. 1628–1632. - 31.
Lunde, D. M., 2011, "Boiling dilute emulsions on a heated strip," MS thesis, University of Minnesota. - 32.
Chen, S. J., Evans, D. F., and Ninham, B. W., 1984, "Properties and structure of 3-component ionic microemulsions," Journal of Physical Chemistry, 88(8), pp. 1631–1634. - 33.
Ruckenstein, E., 1986, "The surface of tension, the natural radius, and the interfacial-tension in the thermodynamics of microemulsions," Journal of Colloid and Interface Science, 114(1), pp. 173–179. - 34.
Siano, D. B., Bock, J., Myer, P., and Russel, W. B., 1987, "Thermodynamics and hydrodynamics of a nonionic microemulsion," Colloids and Surfaces, 26, pp. 171–190. - 35.
Rosano, H. L., Cavallo, J. L., Chang, D. L., and Whittam, J. H., 1988, "Microemulsions - A commentary on their preparation," Journal of the Society of Cosmetic Chemists, 39(3), pp. 201–209. - 36.
Chen, Z. Q., Chen, L. D., Hao, C., and Zhang, C. Z., 1990, "Thermodynamics of microemulsion. 1. The effect of alkyl chain-length of alkyl aromatics," Acta Chimica Sinica, 48(6), pp. 528–533. - 37.
Moulik, S. P., Das, M. L., Bhattacharya, P. K., and Das, A. R., 1992, "Thermodynamics of microemulsion formation. 1. Enthalpy of solution of water in binary (triton-X 100 + butanol) and ternary (heptane + triton-X 100 + butanol) mixtures and heat-capacity of the resulting systems," Langmuir, 8(9), pp. 2135–2139. - 38.
Moulik, S. P., and Ray, S., 1994, "Thermodynamics of clustering of droplets in water/AOT/heptane microemulsion," Pure and Applied Chemistry, 66(3), pp. 521–525. - 39.
Ray, S., Bisal, S. R., and Moulik, S. P., 1994, "Thermodynamics of microemulsion formation. 2. Enthalpy of solution of water in binary-mixtures of aerosol-OT and heptane and heat-capacity of the resulting systems," Langmuir, 10(8), pp. 2507–2510. - 40.
Strey, R., 1994, "Microemulsion and interfacial curvature," Colloid and Polymer Science, 272(8), pp. 1005–1019. - 41.
Bergenholtz, J., Romagnoli, A. A., and Wagner, N. J., 1995, "Viscosity, microstructure, and interparticle potential of AOT/H2O/N-decane inverse microemulsions," Langmuir, 11(5), pp. 1559–1570. - 42.
Mukherjee, K., Mukherjee, D. C., and Moulik, S. P., 1997, "Thermodynamics of microemulsion formation. 3. Enthalpies of solution of water in chloroform as well as chloroform in water aided by cationic, anionic, and nonionic surfactants," Journal of Colloid and Interface Science, 187(2), pp. 327–333. - 43.
Talegaonkar, S., Azeem, A., Ahmad, F. J., Khar, R. K., Pathan, S. A., and Khan, Z. I., 2008, "Microemulsions: a novel approach to enhanced drug delivery," Recent Patents on Drug Delivery & Formulation, 2(3), pp. 238–257. - 44.
Wu, C., Cho, T. J., Xu, J., Lee, D., Yang, B., and Zachariah, M. R., 2010, "Effect of nanoparticle clustering on the effective thermal conductivity of concentrated silica colloids," Physical Review E, 81(1). - 45.
Tyndall, J., 1868, "On the blue colour of the sky, the polarization of sky-light, and on the polarization of light by cloudy matter generally," Proceedings of the Royal Society of London, p. 223. - 46.
He, G. S., Qin, H.-Y., and Zheng, Q., 2009, "Rayleigh, Mie, and Tyndall scatterings of polystyrene microspheres in water: Wavelength, size, and angle dependences," Journal of Applied Physics, 105(2). - 47.
Haque, O., and Scamehorn, J. F., 1986, "Thermodynamics of microemulsion formation by mixtures of anionic and nonionic surfactants," Journal of Dispersion Science and Technology, 7(2), pp. 129–157. - 48.
Mukhopadhyay, L., Mitra, N., Bhattacharya, P. K., and Moulik, S. P., 1997, "Thermodynamics of formation of biological microemulsion (with cinnamic alcohol, aerosol OT, Tween 20, and water) and kinetics of alkaline fading of crystal violet in them," Journal of Colloid and Interface Science, 186(1), pp. 1–8. - 49.
De, M., Bhattacharya, S. C., Panda, A. K., and Moulik, S. P., 2009, "Interfacial behavior, structure, and thermodynamics of water in oil microemulsion formation in relation to the variation of surfactant head group and cosurfactant," Journal of Dispersion Science and Technology, 30(9), pp. 1262–1272. - 50.
Moulik, S. P., and Paul, B. K., 1998, "Structure, dynamics and transport properties of microemulsions," Advances in Colloid and Interface Science, 78(2), pp. 99–195. - 51.
Hammouda, B., Krueger, S., and Glinka, C. J., 1993, "Small-angle neutron-scattering at the national-institute-of-standards-and-technology," Journal of Research of the National Institute of Standards and Technology, 98(1), pp. 31–46. - 52.
Chen, S. H., 1986, "Small-angle neutron-scattering studies of the structure and interaction in micellar and microemulsion systems," Annual Review of Physical Chemistry, 37, pp. 351–399. - 53.
Gradzielski, M., and Langevin, D., 1996, "Small-angle neutron scattering experiments on microemulsion droplets: Relation to the bending elasticity of the amphiphilic film," Journal of Molecular Structure, 383(1–3), pp. 145–156. - 54.
Hammouda, B., 2010, "SANS from polymers-review of the recent literature," Polymer Reviews, 50(1), pp. 14–39. - 55.
Howe, A. M., Toprakcioglu, C., Dore, J. C., and Robinson, B. H., 1986, "Small-angle neutron-scattering studies of microemulsions stabilized by aerosel-OT. 3. The effect of addictives on phase-stability and droplet structure," Journal of the Chemical Society-Faraday Transactions I, 82, pp. 2411–2422. - 56.
Marszalek, J., Pojman, J. A., and Page, K. A., 2008, "Neutron scattering study of the structural change induced by photopolymerization of AOT/D(2)O/sodecyl acrylate inverse microemulsions," Langmuir, 24(23), pp. 13694–13700. - 57.
Nagao, M., Seto, H., Shibayama, M., and Yamada, N. L., 2003, "Small-angle neutron scattering study of droplet density dependence of the water-in-oil droplet structure in a ternary microemulsion," Journal of Applied Crystallography, 36, pp. 602–606. - 58.
Wang, X.-Q., and Mujumdar, A. S., 2007, "Heat transfer characteristics of nanofluids: a review," International Journal of Thermal Sciences, 46(1), pp. 1–19. - 59.
Wang, L. Q., and Wei, X. H., 2009, "Nanofluids: synthesis, heat conduction, and extension," Journal of Heat Transfer-Transactions of the Asme, 131(3). - 60.
Ozerinc, S., Kakac, S., and Yazicioglu, A. G., 2010, "Enhanced thermal conductivity of nanofluids: a state-of-the-art review," Microfluidics and Nanofluidics, 8(2), pp. 145–170. - 61.
Philip, J., and Shima, P. D., 2012, "Thermal properties of nanofluids," Advances in Colloid and Interface Science, 183, pp. 30–45. - 62.
Cahill, D. G., 1990, "Thermal-conductivity measurement from 30-K to 750-K - The 3-omega method," Review of Scientific Instruments, 61(2), pp. 802–808. - 63.
Yang, B., 2008, "Thermal conductivity equations based on Brownian motion in suspensions of nanoparticles (nanofluids)," Journal of Heat Transfer-Transactions of the Asme, 130(4). - 64.
Touloukian, Y. S., Liley, P. E., and Saxena, S. C., 1970, "Thermal conductivity for nonmetallic liquids & gases," Washiongton: IFI/PLENUM, Thermophysical Properties of Matters. - 65.
2002, "Synfluid PAO databook," Chveron Philips Chemical LP, Synfluid PAO Databook . - 66.
3M, "Fluorinert™ electronic liquid FC-72 product information." - 67.
Evans, W., Fish, J., and Keblinski, P., 2006, "Role of Brownian motion hydrodynamics on nanofluid thermal conductivity," Applied Physics Letters, 88(9). - 68.
Hammouda, B., 2010, "A new Guinier-Porod model," Journal of Applied Crystallography, 43, pp. 716–719. - 69.
Mulligan, J. C., Colvin, D. P., and Bryant, Y. G., 1996, "Microencapsulated phase-change material suspensions for heat transfer in spacecraft thermal systems," Journal of Spacecraft and Rockets, 33(2), pp. 278–284.