dynamic nanofin heat sinks

www.MaterialsViews.comFULL PAPERMicrofl uidics An effi cient heat transfer technique for

cooling microchips is developed using dynamic nanofi n heat sinks . CrO 2 na-noparticles are magnetically chained and docked onto hot spots, establishing high aspect ratio and fl exible nanofi ns to fa-cilitate the heat exchange between those spots and the liquid coolant. The system enhances the heat transfer and can of-fer a practical cooling solution for future electronics.

P. Yi,* K. Khoshmanesh,* A. F. Chrimes, J. L. Campbell, K. Ghorbani, S. Nahavandi, G. Rosengaten, K. Kalantar-zadeh* ........................... x–xx

Dynamic Nanofi n Heat Sinks

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1 . Introduction

Effi cient, compact and high powered heat removal is recog-nized as one of the main requirements for the performance advancement of electronics. [ 1–3 ] Cooling systems for micro-chips are conventionally based on fi n structures that are uti-lized to increase the contact surface area of circulating air or liquid coolants. However the ever-increasing transistor packing density in microchips, which is associated with more hot spots, is pushing the limits of such traditional cooling systems. To achieve more effective cooling, researchers are exploring alternative methods, particularly for high heat fl ux hot spots. Much research has been carried out on engineering the geometry of circuits and coolant channels to create two and three dimensional (2D and 3D) structures with enhanced

heat exchange. [ 3–5 ] It is also possible to increase heat transfer rates by the incor-poration of high aspect ratio microfi ns, which are permanently etched into the microchip substrates, by patterning highly thermal conductive carbon nano-tube (CNT) bundles, which are formed on the chip surfaces, [ 6,7 ] or even using active methods such as Peltier coolers. [ 8,9 ] All of the aforementioned approaches, however, are either costly (high aspect ratio silicon microchip etching), incom-patible with silicon industry standards (CNT growth at high temperatures dete-riorates silicon chips) or yet to exceed the desired effi ciencies (Peltier coolers). [ 10 ] Another recently explored approach is to

use thermally conductive liquid metal coolants driven by dig-ital electrowetting. [ 11,12 ] However, liquid metals are expensive, hazardous and there are serious technological issues associ-ated with handling them.

The use of highly thermally conductive nanoparticle sus-pensions in liquid coolants has also been widely suggested as a solution. [ 13–18 ] These nanoparticles increase the overall thermal conductivity of the liquid, making it more effective for heat removal and have been coined “nanofl uids”. [ 19 ] However, thus far it has been impossible to increase the concentration of nanoparticles in liquid coolants beyond 10% v/v, as above this ratio the liquid coolant viscosity increases to values impractical for pumping effi ciently. [ 20–24 ] Even at such high ratios the best enhancements in thermal conductivities remain under 50% of that of the liquid coolant itself. [ 25–27 ]

An advantage of dealing with suspended nanoparticles is that they can potentially move independently of the fl owing liquid. [ 28–30 ] This feature is favourable as, by applying a force the nanoparticles can be directed to desired locations (such as around a hot spot) and their localized concentrations can be tuned. As such, it can be hypothesised that by exerting the right forces, thermally conductive nanoparticles can be moved and placed around the hot spots to locally remove the heat. Ideally, these nanoparticles should remain in intimate contact with each other to allow an effi cient exchange of heat phonons between them and the liquid coolant. Additionally, after removing the force, the locally concentrated nanoparticles can be released, simultaneously removing the locally stored heat. Another important feature of suspended nanoparticles is that they can be aligned along the exerted force fi eld lines. [ 31 ] Hence, nanoparticles can self-assemble desired structure, such as micro/nanofi ns, and remain in intimate well-ordered con-tact with one another to promote the propagation of phonons

Dynamic Nanofi n Heat Sinks

Pyshar Yi ,* Khashayar Khoshmanesh ,* Adam F. Chrimes , Jos L. Campbell , Kamran Ghorbani , Saeid Nahavandi , Gary Rosengaten , and Kourosh Kalantar-zadeh*

The limitation of hot spot cooling in microchips represents an important hurdle for the electronics industry to overcome with coolers yet to exceed the effi ciencies required. Nanotechnology-enabled heat sinks that can be magne-tophoretically formed onto the hot spots within a microfl uidic environment are presented. CrO 2 nanoparticles, which are dynamically chained and docked onto the hot spots, establish tuneable high-aspect-ratio nanofi ns for the heat exchange between these hot spots and the liquid coolant. These nanofi ns can also be grown and released on demand, absorbing and releasing the heat from the hot spots into the microfl uidic system. It is shown that both high aspect ratio and fl exibility of the fi ns have a dramatic effect on increasing the heat sinking effi ciency. The system has the potential to offer a practical cooling solution for future electronics.

DOI: 10.1002/aenm201300537

P. Yi, Dr. K. Khoshmanesh, Dr. A. F. Chrimes, Dr. K. Ghorbani, Prof. K. Kalantar-zadehSchool of Electrical and Computer Engineering RMIT University , Melbourne , VIC 3000 , AustraliaE-mail: [email protected]; [email protected]; [email protected] J. L. CampbellSchool of Applied Sciences RMIT University , Melbourne , VIC 3000 , Australia Prof. S. NahavandiCentre for Intelligent Systems Research , Deakin University Waurn Ponds , VIC 3217 , Australia Prof. G. RosengatenSchool of Aerospace Mechanical & Manufacturing Engineering RMIT University , Melbourne , VIC 3000 , Australia

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Section). The experimental setup is schematically shown in Figure 1 b,c. The fl ow of 0.06% w/w (0.012% v/v) of CrO 2 nanoparticles suspension in Mili-Q water is provided through the microchannel via a syringe pump in withdrawal mode. The CrO 2 nanoparticles used are needle-shaped with a length of 100–400 nm and a diameter of 20–50 nm (Figure 1 d).

In the fi rst set of experiments, the fl ow rate is fi xed at 40 μ L min −1 , resulting in a laminar fl ow. The magnetic coil is energized with 0.6 A for 10 min to allow the system to reach a steady state condition (while maintaining the liquid tem-perature of the inlet to 22 ° C). Figure 2 a shows the schematic of microchannel plane in the z -axis, the orientation which is chosen for imaging. The inset of this fi gure shows the arrange-ment of these fi ns and their fl exibility that allows them to be aligned along the magnetic fi eld and also move with the fl ow. Figure 2 b illustrates the optical image of the trapped CrO 2 particles after 10 min along the side wall surface of the micro-channel. The evolution of the CrO 2 layer thickness in time is presented in Figure 2 b–d. The zoomed in image of Figure 2 a shows the confi guration of CrO 2 nanoparticles after 10 min. They form spire shape bundles of nanoparticles (nanofi ns) per-pendicular to the side wall surface near the magnetic coil.

The trapping mechanism of nanoparticles onto the side wall of the microfl uidic system is shown in Figure 2 e. This trapping mechanism can be divided into three steps according to our observations (see Supporting Information for the movies): the formation of chains along the magnetic lines, the attraction of chained or individual particles towards the magnetic coil, and the docking of the chains onto the side wall and their growth, forming spires.

The growth of nanofi ns is further explored by measuring the average length of chains on the side walls in time. The CrO 2 spires grow almost linear over the fi rst 5 min reaching an average length of ≈320 μ m. The growth decelerates thereafter and tapers off after 10 min, as shown in Figure 2 f–i. Interest-ingly when the magnetic fi eld is switched off, as can be seen in Figure 2 f-ii, the thickness of the layer reduces to just less than

with least scattering and the highest exchanges of heat with the liquid environment.

Here, we will investigate the possibility of forming dynamic cooling by means of microsized fi ns formed from nanopar-ticle suspensions (nanofi ns). Ferromagnetic chromium oxide (CrO 2 ) [ 32 ] nanoparticles are used for establishing the dynamic coolant nanofi n structures so that magnetic forces [ 33 ] can be employed for structuring them. The thermal conductivity of chromium oxides (10–32 W m −1 K −1 ) [ 34 ] is much higher than its other magnetic counterparts such as iron oxides (3–6 W m −1 K −1 ), [ 35 ] rendering it an excellent candidate for heat transfer applications. CNTs have thermal conductivities in the order of 2 kW m −1 K −1 or more [ 31 ] , however, they are not affected by magnetophoretic forces. Decoration of CNTs with magnetic nanoparticles such as iron oxides has been demonstrated [ 31 ] but this reduces the overall thermal conductivity to that of closer to the oxide. As such, CrO 2 remains an exellent candidate. In this paper, we show how to magnetically form dynamically docked chains of CrO 2 nanoparticles onto hot spots and demonstrate the increased heat transfer rates between the coolant fl uid and the hot spots via such structures. Furthermore, we demostate the effect of the fl exibility of the fi ns on the cooling effcienciny of the system.

2 . Results

2.1 . Characterization of CrO 2 Nanoparticles Trapping in the Microfl uidic System

The fabricated microfl uidic system with the embedded mag-netic coil is shown in Figure 1 a. A magnetic coil is embedded into the system (made of polydimethylsiloxane (PDMS)) that serves as both the heat and the magnetic force source. The substrate is a 100 μ m thick glass, which allows for the thermal imaging with minimal bullring effect (refer to Experimental

Figure 1. The images of the system, nanoparticles and experimental setup. a) The image of microfl uidic system formed by integrating a magnetic coil within a PDMS block bonded to a 100 μ m glass slide. b) Schematic of the experimental setup for characterisation of CrO 2 nanofi ns using inverted optical microscope or thermal infrared camera. c) The cross section of the set-up. d) A TEM image of CrO 2 nanoparticles. The scale bar is 200 nm.

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Subsequently, we carry out the study on the trapping of CrO 2 nanoparticles and chains at different DC currents of 0.2, 0.4 and 0.6 A (corresponding to magnetic fi elds of 8.37, 16.7 and 25.1 mT at the core of the coil), while the fl ow rate is set to 40 μ L min −1 . The layer thickness after 5 min is demonstrated in Figure 3 e–g. Figure 3 h clearly shows the impact of the current on the growth of CrO 2 particle chains, which is almost propor-tional to square root of the applied current. Obviously, the mag-netophoresis effect is gradually being tapered at larger currents.

2.2 . Thermal Imaging of Magnetophoretic System

In order to characterize the thermal performance of the mag-netophoretic platform, we obtain thermal images of the system using an infrared camera, as described in our previous work, [ 23 ] from the side of the 100 μ m thick glass substrate. The thick-ness of the substrate is one-fi fth of that of the microchannel, which results in relative similarity of the thermal images on both sides of the glass substrate. As such, the thermal image of the glass substrate is a good approximation for the tempera-ture at the bottom of the channel and its surrounding PDMS. The fl ow rate of the medium is set to 40 μ L min −1 , while we set the temperature of the liquid at the inlet of the microfl u-idic channel to 23 ° C. The thermal imaging is conducted for the cases of with and without nanoparticles suspension. The case without nanoparticles almost accurately represents the case of

100 μ m in less than 1.5 min (see the Supporting Information movies). At the same fl ow rate, the root of the layer remains, as it has a higher adhesion to the surface. As such, at this fl ow rate it is possible to juggle between 100 and 320 μ m thickness by switching on and off the magnetic fi eld. For clearing the root layer in a short time, fl ow rates as large as 200 μ L min −1 are needed. The growth cycle shown in Figure 2 f is repeated up to 5 times by switching on and off the magnetic fi eld without facing any issues regarding the agglomeration of the CrO 2 nanoparticles.

Further experiments are conducted to characterize the perfor-mance of our system at different fl ow rates as well as various DC currents applied to the coil. The trapping and docking of CrO 2 particles at fl ow rates of 10, 20 and 40 μ L min −1 , while the cur-rent of 0.6 A is applied (that produces a magnetic fi eld of 25.1 mT at the core of the coil), are investigated (see Figure 3 a–c). The thickness of the CrO 2 chains vs fl ow rates is demonstrated in Figure 3 d. The dynamics of CrO 2 nanoparticles and chain docking onto to the side wall are the result of a competition between two forces. Increasing the fl ow rate enhances the total number of CrO 2 particles passing through the micro-channel, therefore increases the amount of particles and forms chains that can be docked. However, increasing the fl ow rate also leads to the increase of hydrodynamic drag force and therefore decreases the likelihood of trapping of further CrO 2 nanoparticles. It seems that the latter is the governing factor dominant in this case.

Figure 2. Characterization of CrO 2 nanoparticles trapping and docking onto the microfl uidic channel side wall near the heat source. a) Trapped CrO 2 onto the microchannel side wall after 10 min, at the current of 0.6 A and a fl ow rate of 40 μ L min −1 . The inset demonstrates the zoomed in image of the nanofi ns layer. b–d) The growth of CrO 2 layer after 2, 5 and 10 min. e) A schematic of trapping mechanism of CrO 2 nanoparticles and the formation of chains under the infl uence of the magnetic fi eld. f–i) The thickness of the CrO 2 layer as a function of time over 10 min. f-ii) The dynamic response of the system when the magnetic fi eld is switched off after 5 min. Scale bars of 1 mm and 300 μ m are used from (a–d) and for the inset of (a), respec-tively. The same time scale is used in (f-i,ii).


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comparison to 40 μ L min −1 case, which is expected due to the reduced convection effect.

3 . Discussion

The heat transfer enhancement by nanofi ns is seen in Figure 4 a,b. The convective heat transfer at the surface of the glass in the presence of nanofi ns (the dashed rectangular region shown in Figure 4 c) is approximately 18.5 mW higher than the no fi n scenario, showing a 75.5% increase (calculation details are given in the Supporting Information energy storage calculation section).

For comparison, we have conducted a series of computational fl uid dynamics (CFD) simulations in order to assess the validity of the measurements and determine the thermal performance of nanofi ns (see Figure 5 ). In these simulations, the tempera-ture contours are obtained for the case of no fi ns (Figure 5 a) and the case of a CrO 2 region of fi n bundles with dimensions similar to that observed in the measurements (Figure 5 b; simulations details are presented in the Supporting Informa-tion simulations section). Our simulations with microfi ns pre-dict an increased heat transfer of 12.3 mW, corresponding to a 48.4% enhancement. The measured temperature drop (for nanofi ns case) at the centre of the heater is smaller than that of the measurement (2.4 vs 1.7 ° C). This difference is due to the nanofi ns being porous in the experiments, compare to the simulations for which the fi ns are impermeable. Another pos-sible factor in the enhancement of heat transfer can be the fl ex-ibility of the nanofi ns (they can be bent by liquid fl ow as is seen in the Supporting Information videos). This allows smoother passage of liquid surrounding the nanofi ns and subsequently more effi cient convective heat transfer, with minimized pres-sure drop within the microchannel. The fi ns are fl exible and extremely porous, so liquid can pass through them with

liquid with nanoparticles suspension of 0.012% v/v without the effect of magnetic fi eld. In this situation, the thermal conduc-tivity of the liquid is only 0.06% more than that of water, which has a negligible effect on the temperature contours.

Figure 4 a,b show the temperature contours of the surface of the substrate with and without CrO 2 nanostructured layers at 40 μ L min −1 . For a more discernible comparison, the tem-perature profi les along the x axis are shown for both cases in Figure 4 d,e. The profi les are plotted 1.5 mm above and below the side wall of the channel near the coil. Interestingly, while the temperature reaches a peak value of 56.7 ° C at the location of magnetic coil for the system without nanofi ns, the maximum temperature is signifi cantly lower (54.3 ° C) for the system with fi ns.

In the microfl uidic system without nanofi ns, the propa-gation of heat is almost symmetric within the PDMS section (Figure 4 a). A portion of the heat is conducted into the micro-fl uidic channel, increasing the temperature of the fl owing liquid to a maximum of 46.4 ° C along the wall of the channel near the coil and a maximum of 39.2 ° C along the other side of the microchannel. The temperature profi les within the micro-channel are presented in Figure 4 d. The temperature profi le of the channel with the nanofi ns formed on the wall near the hot spot is quite different (Figure 4 e). The temperature of the liquid is now much higher than the scenario without nanofi ns: the maximum temperatures are 50.9 ° C and 44.4 ° C on the side wall near the coil and the opposite wall, respectively. Visibly, the heat is more effi ciently conducted into the microfl uidic channel in the presence of the nanofi ns. The temperatures contours are also further stretched into the right side in comparison to the without fi n condition (Figure 4 b). The nanofi ns effi ciently take the heat from the hot spots and transfer it into the liquid. For comparison, the profi les for lower fl ow rate of 20 μ L min −1 are also presented in Supporting Information Figure S4. Obviously, the temperature peaks are less stretched to the right side in

Figure 3. Effect of fl ow rate and current on CrO 2 layer thickness after 5 min. Image of the CrO 2 layer at a fl ow rate of: a) 10 μ L min −1 , b) 20 μ L min −1 and c) 40 μ L min −1 . d) Thickness of the layer vs fl ow rate: Images of CrO 2 layer formed at a current of e) 0.2 A, f) 0.4 A and g) 0.6 A. h) Thickness of the layer vs current. Scale bars of 500 μ m are used from (a–g).


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Figure 4. Variations of temperature along the glass slide, obtained by infrared camera at a fl ow rate of 40 μ L min −1 . Thermal images: a) tempera-ture profi le of the microchannel without nanofi ns and b) temperature profi le of the microchannel with the CrO 2 nanofi ns. c) The dimensions of the microchannel for comparison. d) Plots of temperature profi les for with and without nanofi ns cases along the microchannel from the sidewall near the magnetic coil to the other sidewall (18 of them for each case). e) Plots of temperature profi les for with and without nanofi ns cases from the sidewall near the magnetic coil 1.5 mm down towards the coil. A scale bar of 1.5 mm is used in (a,b).

Figure 5. Simulation results for heat transfer within the magnetophoretic system. The fl ow rate of the outlet is 40 μ L min −1 while the temperature of the liquid entering the channel is set to 23 ° C. The system exchanges heat with the environment via free convection. The convection coeffi cient is 6–12 W m −2 K −1 across different surfaces while the ambient temperature is taken as 23 ° C. a) Temperature contours for the case of water (without fi ns) across the glass surface. b) Temperature contours for the case of CrO 2 fi ns with k = 21 W m −1 K −1 thermal conductivity – parallel plates of trapezoids are implemented due to computational limitations. c,d) Cross sections of the channels shown in (a,b), respectively, for without and with fi n cases.


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by a factor of 10 to reach 400 μ L min −1 to enhance the convec-tive heat transfer via the channel. Simulations were conducted in three cases: i) no fi ns, ii) fi xed fi ns by assuming a no-slip boundary condition at the surface of the fi ns, and iii) fl exible fi ns by assuming a zero shear stress at the surface of the fi ns.

Figure 6 a–c show the temperature contours along the middle plane of the channel ( z = 250 μ m) for the cases of no fi ns, fi xed fi ns and fl exible fi ns, respectively. For the case of no fi ns, the tem-perature reaches a peak value of 262.4 ° C at the coil (Figure 6 a). Incorporation of fi xed fi ns improves the heat transfer via the channel, as evidenced by proceeding of the red and orange contours towards the channel core while slight receding of the blue contours along the channel axis. This reduces the peak temperature to 244 ° C (Figure 6 b) that is 18 ° C less than that of the case of no fi ns. Interstingly, the incorporation of fl exible fi ns signifi cantly improves the heat transfer via the channel, as evidenced by spreading of the red and orange contours along the channel axis while hampering of the blue contours. This reduces the peak temperature to 174 ° C (Figure 6 c) that is 98 ° C less than that of the case of no fi ns. This reduced tem-perature is completely within the range that can be tolerated by silicon chips.

Figure 6 d–f show the velocity contours along the middle plane of the coil ( x = 19 mm) for the cases of no fi ns, fi xed fi ns and fl exible fi ns, respectively. For the case of no fi ns, a parabolic velocity profi le forms along the cross section of the channel with a peak velocity of 16.5 mm s −1 at the channel core. By incorporation of fi xed fi ns, the fl ow bypasses the fi ns and is squeezed along a narrower region formed between the tips of the fi ns and the opposite side wall. The fl ow still fol-lows a parabolic profi le with a peak velocity of 26.5 mm s −1 at the core of the narrow region. Alternatively, incorporation of fl exible fi ns leads to complete change of the velocity profi le. In this case, the fl ow passes through the fi ns more smoothly and exhibits a peak velocity of 26.8 mm s −1 across the narrow gap between the fi ns. This signifi cantly enhances the con-vective heat transfer between the fl ow and the hot surface of the fi ns, as evidenced in Figure 6 c. Such a phenomenon is not achievable by rigid fi ns with fi xed walls, as evidenced in Figure 6 b.

4 . Conclusion

This work demonstrates the creation of dynamically controlled fl exible heat sink concept by utilizing magnetophoretically pat-terned CrO 2 nanoparticles within a microfl uidic system. Upon the application of magnetic fi eld, the CrO 2 nanoparticles form a nanofi n structure along the side wall of the channel. The high thermal conductivity of nanofi ns leads to enhancement of convective heat transfer with the fl ow and reduces the hot spot temperatures. The formed nanofi ns are fl exible, enabling the smooth passage of the fl ow between them, which in turn sig-nifi cantly increases the heat exchange with the nearby passing fl uid. This is a very unique feature of nanofi n that cannot be achieved using conventional microfabricated rigid fi ns. More-over, the nanofi ns have the capacity for storing and releasing the thermal energy to serve as a dynamic heat sink with a response time in the order of minutes.

minimal disruption. Obviously, if the concentration of assem-bled nanoparticles is too high, or the channel is too narrow, then the fl ow of fl uid in the microchannel will be hindered. In this case, the coil should remain switched off for longer dura-tions to avoid any disruption in fl ow.

Another important issue, which is also clearly observed from the CFD images of Figure 5 a,b, is that when nanofi n layer is heated it stores the thermal energy along the side wall near the heat source (approximately 101.5 mJ more heat than the no fi n condition). Effi cient thermal energy can fl ow directionally via the rod-shape CrO 2 , which is used in this work, from the hot spots into the liquid of the microfl uidic channel. In comparison to other morphologies of nanoparticles, rod-shape CrO 2 can increase the phonon scattering at their boundaries, which limits the thermal conduction. [ 36 ] Furthermore, the response time of the system is less than 10 min for forming the CrO 2 nanostruc-tured layer and shorter for removing it. After switching off the magnetic force source, the released nanofi ns take this stored heat, and carry them away from the hot spots via the microfl u-idic channel. As such, the system can also be used dynamically for cooling hot spots by turning the magnetic force on and off.

Longer nanofi ns increase the heat exchange between the liquid in the microfl uidics and hot spots. Obviously, to generate stronger magnetic fi eld that results in assembly of longer nano-fi ns a larger electrical current is required. Additionally, higher fl ow rate produces a better heat exchange but inversely reduces the length of nanofi ns (as seen in Figure 3 d) by producing forces against their assembly. To compensate for this effect higher magnetic fi elds are required, which leads to larger elec-trical currents. These increases in current will have the adverse effect of more heat generated by the coil itself. Such effects should certainly be addressed in the design of any nanofi ns cooling system and an optimum condition for fl ow rate, dimen-sions of the microfl uidic channels and the strength of magnetic fi eld should be implemented.

While we have shown the proof of concept, clearly for making such cooling applications practical, the magnetic system itself should generate minimal heat in comparison to the heat gener-ated by electronic circuits at the hot spots. Moreover, the loca-tion of the magnetic coil should be carefully selected to min-imize the adverse effect of additional heat to the system and at the same time maximize the magnetic fi eld strength along the microchannel. Certainly to avoid overheating, effi cient designs of the optimum magnetic coil core and dimensions are needed. It is also possible to use permanent magnets which generally produce large magnetic fi elds and do not require any applied electrical current that avoids heating. In this case, fl ux switching procedures can be used for dynamically altering the nanofi ns. [ 37 ]

To further explore the effectiveness of nanofi n heat sink for practical applications, we numerically investigated the perfor-mance of the system in a case similar to the cooling of micro-chips made of silicon. The amount of heat produced by the coil was increased by a factor of 10 to reach 4.2 W (2.85 × 10 8 W m −3 for the coil). A square silicon block with a thermal conductivity of 150 W m −1 K −1 [ 38 ] was inserted around the coil to direct the heat produced by the coil towards the channel. The bottom sur-face of the system was insulated to further increase the heat exchange via the channel. Finally, the fl ow rate was increased


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The solution (2.5 times) was further diluted in DI water. This reduced the fi nal concentration (to 0.06% w/w (0.012% v/v)). Transmission electron microscopy imaging (TEM, Jeol 1010 TEM, USA) was conducted to verify the size and shape of the CrO 2 nanoparticles.

Microfl uidics : The detail of the fabrication procedure for the microfl uidic system is presented in Supporting Information Figure S1. It is very important to mention that a thin glass substrate (with a thickness of 100 μ m) was used as the substrate to minimize the heat dissipation within the substrate to avoid obtaining blurry thermal images. Such a thin glass substrate was fragile and could easily buckle under the weight of the PDMS block and the inlet/outlet tubes. To resolve this issue, the magnetophoretic platform was clamped between two 3 mm PMMA sheets. [ 39 ] A groove (with a width of 5 mm) was milled along the middle line of both PMMA sheets to facilitate the access of inlet/outlet tubes as well as to ensure that the glass substrate is exposed to the camera. The infrared camera was placed (at a distance of 0.3 m) from the magnetophoretic system. The temperature contours were recorded in 90 s frames and the images were extracted from the last frame.

Apparatus : The syringe pump used was Harvard (PHD 2000) Infusion. The syringe pump was activated in refi ll mode to supply suction, avoiding the leakage and generation of bubbles within the microchannel. The magnetic coil was energized via a DC power supply (Gw Instek, GPS-X303 series, Taiwan). Before each experiment, to prevent the adhesion of CrO 2 particles to the glass and PDMS surface,

Future compact electronic elements require more effi -cient and microfl uidic friendly heat exchange. Hence, we believe that this work represents a framework for a new gen-eration of microfl uidic-based cooling systems and offers a great potential for further development and integration into microchips.

5 . Experimental Section Magnetic Nanoparticles : CrO 2 nanoparticles with dimensions shown in

Figure 1 d were purchased from Sigma-Aldrich, Australia. The dispersion of the CrO 2 nanoparticles was stabilized with trisodium citrate dehydrate (Na 3 C 6 H 5 O 7 , 2H 2 O) at room temperature to avoid aggregation. To achieve this, trisodium citrate dehydrate (300 μ L at 5 mg mL −1 ) was added to Mili-Q water (15 mL), and then the nanoparticles were added. This solution was sonicated (for 30 min) to reduce aggregates. Next, the solution was placed in a thermo-mixer (at 70 ° C with a speed of 600 rpm for 8 h) to functionalize the surface of the CrO 2 nanoparticles with citric acid. The solution was then centrifuged (at 8000 rpm for 15 min) and washed with Milli-Q water. This process was repeated (3 times), and the particles were re-dispersed (in 15 mL Milli-Q water). A new concentration of suspended CrO 2 nanoparticles in Mili-Q water was then achieved (which at this point was 0.15% w/w).

Figure 6. Numerical characterization of the magnetophoretic nanofi n heat sink with an embedded silicon block around the coil (the example mimic the cooling conditions for a silicon chip). The heat produced by the coil is increased to 4.2 W while the fl ow rate of the coolant is increased to 400 μ L min −1 . Temperature contours at the middle plane of the channel ( z = 250 μ m) shown for the a) case of no fi ns, b) case of fi xed fi ns, and c) case of fl exible (zero shear) fi ns. Velocity contours at the middle plane of the coil ( x = 19 mm) for the d) case of no fi ns, e) case of fi xed fi ns, and f) case of fl exible fi ns, corresponding to (a,b,c), respectively.


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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements K.K. received the Discovery Early Career Researcher Award (DECRA) under project DE- 120101402 and acknowledges the Australian Research Council for funding. The authors acknowledge the facilities and the technical assistance of the Australian Microscopy & Microanalysis Research Facility at the RMIT Microscopy and Microanalysis Facility of RMIT University.

Received: May 16, 2013 Revised: July 31, 2013

Published online:

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