Abstract:
A system and method for performing heat dissipation is disclosed that includes contacting a heat transfer liquid with a heat exchange surface having raised hydrophilic nanoporous nanostructures disposed adjacent a central core upon a substrate. The heat transfer liquid forms a preselected contact angle when placed on the heat exchange surface. The raised nanoporous nanostructures define channels, interconnected pathways, and voids within the nanoporous nanostructures. The nanoporous nanostructures have additional surface irregularities upon the nanostructures themselves. The nanostructures are preferably formed by depositing metal oxides or other materials upon a substrate using a Microreactor Assisted Nanomaterial Deposition (MAND) process.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with United States Government support under Contract W911NF-07-2-0083 awarded by the Army Research Laboratory (ARL) through the University of Oregon to Battelle Memorial Institute. The Government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to heat dissipation systems and more particularly to methods and systems for enhanced heat dissipation. 
       BACKGROUND OF THE INVENTION 
       [0003]    Management of electronic system energy and cooling is gaining importance in development of future advanced lasers, radars, and power electronics. There is a general requirement to develop compact, light-weight, and low-cost thermal control and heat exchange systems. Requirements for such technologies and design techniques must dissipate ultra-high heat fluxes, reduce system energy usage, and increase system efficiencies. While a variety of combinations and various attempts have been made, new and better methods and systems are needed and desired. The present invention provides a new and improved system and process for meeting these needs. 
         [0004]    Additional advantages and novel features of the present invention will be set forth as follows and will be readily apparent from the descriptions and demonstrations set forth herein. Accordingly, the following descriptions of the present invention should be seen as illustrative of the invention and not as limiting in any way. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention is a system and method for performing heat dissipation characterized by contacting a heat transfer liquid with a heat exchange surface having raised hydrophilic nanoporous nanostructures disposed upon a substrate adjacent a central hydrophobic core. The heat transfer liquid forms a preselected contact angle when placed on the heat exchange surface. In preferred embodiments of the invention, the raised nanoporous nanostructures define interconnected voids and pathways within the nanoporous nanostructures, and have additional surface irregularities upon the nanostructures themselves. Various layers of these nanostructures can be constructed, resulting in, e.g., primary, secondary, tertiary, and other layers of the nanostructures. The structures are preferably formed by depositing metal oxides upon a substrate using a Microreactor-Assisted Nanomaterial Deposition process known as MAND™ (hereafter MAND). 
         [0006]    In one embodiment of the invention, the metal oxide material is ZnO, NiO, or a zeolite material and the underlying substrate contains Cu, Ni, Si, Ti, Al, AlN, stainless steel, inconel alloys, carbon-copper composites, and various combinations and alloys thereof. In various embodiments, the surface coating includes nanoporous nanostructures composed of a preselected material including, but not limited to, e.g., ZnO, NiO, Ni, Au, Ag, Pt, Sn, including combinations of these materials. The raised nanoporous nanostructures comprise vanes that are centrally arranged around a central core in a generally flower-like arrangement, forming nanopores, crevices, gaps, and fissures that serve as nucleation sites for boiling. The raised nanoporous nanostructures preferably extend at least 10 nm above the substrate. In embodiments where water is the heat transfer liquid, a contact angle on the surface of between 15° and 25° is preferred. Critical heat flux (CHF) values for these nanostructured surfaces are at least about 63 W/cm 2 . And, boiling heat transfer coefficients for these nanostructured surfaces are as high as ˜23,000 W/m 2 K. 
         [0007]    The nanoporous nanostructures are typically formed by mixing a preselected quantity of an aqueous solvent, a metal salt, and a complexing agent together to form an aqueous fluid. The aqueous fluid is flowed continuously across a surface of a substrate in a fluid reservoir at a preselected temperature below about 100° C. The aqueous fluid has a preselected residence time in contact with the substrate such that a plurality of nanoporous nanostructures form upon the substrate. The nanoporous nanostructures each have a plurality of surface protrusions with a plurality of random surface irregularities, forming nanopores, crevices, gaps, and fissures that serve as nucleation sites for boiling. The nucleation sites have a plurality of channels, interconnected voids, and passages that allow fluid to flow to the nucleation sites that allow for active boiling. In one embodiment, the surface protrusions have a height dimension measured from the surface between about 40 nm and about 50 nm. In other embodiments, the surface protrusions of the nanoporous nanostructures have a height dimension greater than or equal to about 50 nm. The nanoporous nanostructures include nanopores with a surface density in the range from about 30 pores per μm 2  to about 200 pores per μm 2 , and a pore density of from about 40 pores per μm 2  to about 100 pores per μm 2 . In various embodiments, the nanopores are of a size in the range from about 40 nm to about 100 nm. On average, the nanoporous nanostructures include nanopores with a mean diameter of about 50 nm. 
         [0008]    The nanostructures can include nucleation sites that include a hydrophobic surface portion. The nanostructures can further include a hydrophilic surface that surrounds the hydrophobic surface portion that includes nanopores that effectively entrain fluids within the nanopores and transfer the fluids via strong capillary forces to the nucleation sites where active boiling takes place. In one embodiment, the nanoporous nanostructures include nucleation sites having a hydrophobic surface portion with a mean pore diameter of about 1 μm, surrounded by hydrophilic surfaces with nanoscale dimensions that facilitate rapid and effective fluid transfer and migration to the hydrophobic surface portion. The nanostructures have an average root-mean-square roughness (height) in the range of 200-600 nm. Formation of the various hydrophilic and hydrophobic surfaces is sequentially and/or alternately performed using a lithographic masking technique, where a first hydrophobic surface portion is masked off and a hydrophilic portion is formed. Then, the formed hydrophilic surface portion is masked off while the hydrophobic surface portion is formed, or vice versa, thereby forming nanoporous nanostructures that include both hydrophobic and hydrophilic surface portions. The hydrophobic surface portions include a contact angle in the range from about 10° to 30°. A preferred contact angle for the hydrophobic surface portions is about 20°. The hydrophobic surface portions aid the nucleation sites of the nanoporous nanostructures to form bubbles when a fluid is introduced under suitable fluid conditions. The hydrophobic and hydrophilic surface conditions may be variously altered by a variety of MAND processing actions including, but not limited, to, e.g., altering temperature, residence time, and concentrations of constituents in the aqueous fluid that contact the substrates in the MAND fluid reactor. 
         [0009]    The purpose of the foregoing abstract is to enable the United States Patent and Trademark Office and the public generally, especially scientists, engineers, and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The abstract is neither intended to define the invention of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way. 
         [0010]    Various advantages and novel features of the present invention are described herein and will become further readily apparent to those skilled in this art from the following detailed description. In the preceding and following descriptions the preferred embodiment of the invention is shown and described by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of modification in various respects without departing from the invention. Accordingly, the drawings and description of the preferred embodiment set forth hereafter are to be regarded as illustrative in nature, and not as restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1 : shows various examples of nanoscale films prepared using Microreactor Assisted Nanomaterial Deposition (MAND) based on nanoscale building and assembly. 
           [0012]      FIG. 2  is a Scanning Electron Micrograph (SEM) of a nanostructured surface utilized in one embodiment of the present invention. 
           [0013]      FIG. 3   a  shows an aluminum (Al) surface with ZnO nanostructures, according to another embodiment of the invention. 
           [0014]      FIG. 3   b  shows a copper (Cu) surface with ZnO nanostructures, according to another embodiment of the invention. 
           [0015]      FIG. 4 : shows pool boiling curves for various nanostructured surfaces compared to neat Al and Si surfaces. 
           [0016]      FIG. 5 : Measured critical heat flux as a function of static contact angle for nanostructured (red-filled squares) and plain surfaces. 
           [0017]      FIG. 6 : Measured heat transfer coefficient as a function of nucleate boiling heat flux (log-log plot). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    In various embodiments of the present invention, various nanoporous nanostructures were created and tested with water as a heat transfer fluid. Results showed that the raised nanostructured materials provided enhanced boiling characteristics indicative of enhanced heat dissipation. Test results show that the present invention could be used in almost any application where transfer of very high heat fluxes (i.e., 200-1000 W/cm 2 ) across nearly isothermal material conditions are contemplated or required. 
         [0019]      FIG. 1  shows various nanostructures that can be prepared using a solution-based deposition process known as Microreactor Assisted Nanomaterial Deposition, or MAND. The MAND process is detailed in U.S. patent application Ser. No. 11/490,966 filed 21 Jul. 2006, now published as U.S. Publication Number 2007-0020400 A1 published 25 Jan. 2007; and U.S. patent application Ser. No. 11/897,998 filed 31 Aug. 2007, now published as U.S. Publication Number 2008-0108122 A1 published 8 May 2008, which references are incorporated herein in their entirety. ZnO nanostructures of the invention were prepared using this process. The MAND process is a preferred process due to its cost-effectiveness and controllability, but is not limited thereto. MAND deposition processes can be controlled to create nanostructured ZnO surfaces that are, e.g., hydrophobic (i.e., contact angles of ˜50°-60°) or superhydrophilic (i.e., contact angles &lt;20°) by changing MAND process parameters including, e.g., NaOH concentrations and residence times. More information regarding the preparation and use of the MAND process to create structures described in the present application is detailed by Jung et al. [Current Applied Physics, 8 (2008) 720-724], which reference is incorporated herein in its entirety. 
         [0020]    In the present application, formation of flower-like structures of ZnO on aluminum (Al) was obtained by using a 0.05M NaOH solution in a 70° C. water bath, which were deposited on the Al substrate using a 250° C. holding temperature, a deposition time of 10 minutes, at a rotating speed of 2500 rpm, and a flow rate of 8.28 mL/min. This resulted in the attachment of ZnO structures on the Al surface having a pore density of 30-100 square micrometers (μm 2 ) and an average pore size of 50-100 nm. The contact angle of the overall surface was 20°. These same parameters on a copper (Cu) surface resulted in the formation of ZnO structures-on-Cu that gave a contact angle of 30°. Other configurations were obtained by altering these MAND processing parameters. For example, a ZnO-on-Al surface was formed through a similar process using a 0.15M NaOH solution, a 200° C. holding temperature, a 5 minute deposition time, a rotating speed of 1500 rpm, and an 8.28 mL/min flow rate, which resulted in a contact angle of 96°. In another exemplary case, a seed layer was applied, followed by a MAND process where 0.10M NaOH was applied in a 70° C. water bath, a 250° C. holding temperature, a 10 minute deposition time, a rotating speed of 2500 rpm, and a flow rate of 8.28 mL/min, which resulted in a contact angle of 18°. A zeolite on silicon (Si) surface was formed by mixing tetra-propyl-ammonium hydroxide (TPAOH) 0.60; tetra-ethyl-ortho-silicate (TEOS); deionized water 165 (molar ratios) synthesized at 165° C. for 2 hours in an autoclave. This process resulted in a material having a contact angle of 14°. 
         [0021]      FIGS. 2   a - 2   b  are scanning electron micrographs (SEMs) that give examples of ZnO nanostructures on aluminum ( FIG. 2   a ) and copper substrates ( FIG. 2   b ), respectively. Atomic Force Microscopy (AFM) was used to evaluate the surface structures and topologies. Nanostructures of ZnO-on-Cu have an average roughness of 162.29 nm. Pore sizes for ZnO-on-Al nanostructures are in the range from about 50 nm to about 100 nm; pore densities are in the range from about 30/μm 2  to about 100/μm 2 ; the feature structures are typically about 40 nm tall. The features result in an overall surface having a preferred contact angle between 15°-25°, most preferably around 20°. While this preferred range is anticipated when water is the transfer fluid, other fluids and nano-textured surfaces may have other optimal contact angles. For example, nano-textured surfaces composed of other material combinations can provide critical contact angles having different ranges depending on the chemical composition that provides different fluid properties such as surface tension, heat of vaporization, and density. Thus, no limitations are intended. 
         [0022]    Contact angles are determined using a standard water droplet test. The water droplet, when placed on the surface, yields a contact angle that is measured as the inside angle the droplet contour surface makes with the planar surface it sits on at the point of contact (hence the term “contact angle”). Contact angle is a measure of the interfacial adhesion energy at the surface, a measure of the balance between the fluid surface tension forces on a surface, which is impacted by the presence of the nanostructures on the surface. This work has shown more particularly that the contact angle is a balance between fluid dynamic forces and bubble dynamic forces that occur during boiling at a selected surface. Hydrophilic properties of rough (e.g., nanostructured) surfaces can be characterized using Equation [1]: 
         [0000]      ( r  cos θ=cos γ)   [1]
 
         [0023]    Here, the roughness factor (r) is the ratio of total surface area to total projected surface area; (γ) and (θ) are contact angles on nanostructured (roughened) and smooth (non-structured or non-deposited) surfaces, respectively. Roughened surfaces (where r&gt;1) have a greater surface area. When a “wet” drop of fluid contacts a rough surface, the drop either wets the grooves (i.e., so-called “hydrophilic” state) or sits on the peaks of the rough surface (i.e., so-called “hydrophobic” state). Equation [1] predicts that textured surfaces of the invention become more hydrophilic as the surface area increases. The greater the surface density of surface features, the more hydrophilic the surface becomes. Nanostructures of the invention provide textured surfaces that increase the surface area, and hence, the hydrophilic character of the surface. 
         [0024]      FIGS. 2   a - 2   b  are SEM images that show various textured surfaces of the invention. These textured surfaces have: 1) porous microstructures and nanostructures that provide for control of hydrophobic and hydrophilic characteristics of the surfaces and allow in-flow of heated fluid to nucleation sites. “Nucleation sites” refers to locations in the nanoporous nanostructures where bubbles form when a heated fluid is introduced; 2) high pore densities that provide enhanced nucleation; and 3) features (i.e., protrusions) that protrude from the surface that provide an increase in the active boiling area and additional nucleation sites. Nanostructures of the invention have pore densities and nucleation site densities that are much greater than the “bare” substrates upon which the nanostructures are deposited. All of these characteristics affect the enhanced properties for heat transfer in a boiling fluid demonstrated by the invention. “Pool boiling” as defined herein refers to boiling that occurs at the heating surface under natural convection and nucleate boiling conditions, where the surface of interest is submerged in a large body of standing (i.e. “pooled”) liquid. The relative motion of bubbles in a liquid at a heating surface and the surrounding liquid is due primarily to buoyancy effects. “Flow boiling” as defined herein refers to boiling that occurs at the heating surface under conditions of a flowing fluid. 
         [0025]    At critical heat flux (CHF) conditions, a fluid (e.g., water) on a heated surface transitions from fully developed nucleate boiling (NB) in which discrete columns or groups of coalesced columns of bubbles are in the fluid to the condition where bubble columns become large and merge to form a continuous column (or film) of vapor (so-called “vapor column” or “vapor film”) between the fluid and the heated (or heater) surface. Thermal resistance at the surface then increases sharply at this juncture due to: 1) the presence of the vapor film and 2) the lower thermal conductivity of the vapor compared to the liquid (e.g., water). The combination of factors at the surface sets the maximum CHF value for the surface of interest as a practical operation limit. 
         [0026]    Conventional wisdom suggests that nanostructured surfaces will not improve heat transfer in a boiling fluid (so-called “boiling heat transfer”) because the bubble nucleation process is not expected to be enhanced by very small (i.e., nano-scale) cavities due principally to the large superheats needed for activation. Minimum cavity mouth radius (R c ) required for activation, is given by Equation [2], as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     c 
                   
                   = 
                   
                     
                       2 
                        
                       
                           
                       
                        
                       σ 
                        
                       
                           
                       
                        
                       
                         T 
                         sat 
                       
                     
                     
                       
                         ρ 
                         v 
                       
                        
                       
                         h 
                         fg 
                       
                        
                       Δ 
                        
                       
                           
                       
                        
                       
                         T 
                         s 
                       
                     
                   
                 
               
               
                 
                   [ 
                   2 
                   ] 
                 
               
             
           
         
       
     
         [0027]    Here, sigma (σ) is the surface tension; (T sat ) is the saturation temperature, density (ρ v ) is the vapor density [kg/m 3 ]; (h fg ) is the enthalpy of vaporization (J/kg); and (ΔT S ) is the surface or wall superheat temperature calculated as the difference between the temperature of the heated surface (T s ) and the saturation temperature (T sat ), i.e., (T s −T sat ), in units of [K]. The equation predicts that for a nanosized cavity of approximately 100 nm in a water environment at a saturation temperature (T sat ) of 100° C., the required superheat temperature (T surf −T sat ) will be 327 K (53.9° C.). Experiments were performed to test the predictions. The aim of these experiments was to obtain two key parameters: the wall super heat values (T s −T sat ) and the wall heat flux values (q″). Deionized water was used. Water was first boiled using, e.g., a microwave oven. Then, the water was sonicated for 20 minutes in an ultrasonic bath to remove dissolved gases. The sonicated water was then poured into a boiling chamber configured with two immersion heaters. Immersion heaters were then powered, reaching the water saturation temperature of 100° C. The water was degassed at this power level for about one hour. Boiling experiments were performed at atmospheric pressure and at the water saturation temperature. Experiments were carried out until a critical heat flux (CHF) was reached. At the onset of CHF, wall superheat values jumped to very high values. Experimental results were characterized using boiling curves that plotted the surface heat flux (q″) against the wall surface superheat values (T s −T sat ). Heat flux (q″) during nucleate boiling is given by Equation [3], as follows: 
         [0000]        q″=K (π( k   1   σC   p ) f ) 0.5   D   b   2   N   a   ΔT   w    [3]
 
         [0028]    Here, (k 1 ) is the liquid thermal conductivity; (σ) is the surface tension; and (C p ) is the specific heat. (K) is a constant that represents the bubble diameter of influence, which is independent of contact angle and physical properties of the fluid. (D b ) is the bubble diameter at the moment of departure; (f) is the vapor bubble departure frequency; (D b f) is the mean velocity of vapor bubble growth; and (N a ) is the nucleation site density. (ΔT w ) is the wall superheat value, defined previously above. 
         [0029]      FIG. 4  presents boiling curves for various nano-structured surfaces described herein against bare aluminum and copper surfaces in pool boiling tests conducted in water (T sat =100° C.) that show the dependence of dissipated heat flux (q″) on wall superheat (T s −T sat ) values. In the figure, ZnO nanostructured surfaces of the invention required a lower wall superheat for bubble nucleation, e.g., as observed at the onset (e.g., ˜10° C.) of nucleate boiling (ONB) compared to the bare Al surface. Partial nucleate boiling (PNB), where discrete bubbles are activated on the heater surface, also occurred at a lower superheat value for the ZnO nanostructured surfaces. Partial nucleate boiling (PNB) transitioned to fully-developed nucleate boiling (FNB) (e.g., at 20° C.-25° C.), where bubbles merged to form vapor columns, again at lower superheat values compared to the bare metal surfaces. At the critical heat flux (CHF) condition (e.g., ˜30° C.), bubbles were large and merged to form a continuous vapor film between the heater and the water (the heat transfer liquid). Each of the boiling conditions (ONB through CHF) was observed at a lower superheat value than that of the bare metal surface. In addition, the ZnO nanostructured surfaces exhibited enhanced boiling heat transfer (wall heat flux) values compared to the bare metal surfaces even at these lower superheat values. For example, a high CHF value of ˜82.5 W/cm 2  was observed for the ZnO nanostructures on Al compared to that for the bare Al surface, ˜23 W/cm 2 . In short, compared to the bare metal surfaces, the nano-structured surfaces on aluminum increased the wall heat flux, e.g., from ˜20 W/cm 2  to over 80 W/cm 2 . For nano-structured surfaces of ZnO-on-Cu (contact angle=30°), a CHF value of 63.5 W/cm 2  was observed at comparable low superheat values. TABLE 1 compares CHF values for various ZnO nanostructured surfaces of the invention and other surfaces measured in water as the boiling fluid. 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 CHF values for ZnO-structured surfaces. 
               
             
          
           
               
                   
                 Critical heat flux (CHF) 
               
               
                 SURFACE 
                 (W/cm 2 ) 
               
               
                   
               
               
                 Bare aluminum 
                 23 
               
               
                 Super-hydrophilic 
                 14 
               
               
                 Hydrophilic 
                 34 
               
               
                 ZnO nanostructures on Al (flower-like) 
                 80 
               
               
                 Unique structure 
                 79 
               
               
                 ZnO nanostructures on Cu (flower-like) 
                 63 
               
               
                   
               
             
          
         
       
     
         [0030]    In water, ZnO nanostructures on aluminum (Al) surfaces showed a pool boiling CHF value of from about 80 W/cm 2  to about 82.5 W/cm 2 . Bare Al, in contrast, gave a CHF of about 23.2 W/cm 2 . ZnO nanostructures on Al also showed a wall superheat reduction of from about 25° C. to about 38° C. for bubble nucleation compared to the bare Al surface. ZnO nanostructures prepared on a copper (Cu) surface also produced flower-like morphologies, giving a surface contact angle of about 30°. The ZnO nanostructures on copper (Cu) resulted in a CHF value of 63.5 W/cm 2  and a comparable reduction in superheat value for bubble nucleation compared to the bare Cu surface. 
         [0031]    Data presented in  FIG. 4  and in TABLE 1 contradict current understanding of bubble nucleation. As shown in the figure, for example, boiling curves for ZnO nano-structured surfaces display a unique staircase effect. And, wall superheat values increase steadily while heat flux remains constant. Then, a sudden increase in dissipated heat flux is observed concomitantly with increasing bubble nucleation population. Results are attributed to local higher superheat values that activate all of the smaller pore sizes surrounding a given nucleation site. In particular, ZnO nanostructured surfaces of the invention with higher contact angles (&gt;10°) exhibit a high density of bubble formation across the boiling surface throughout the bubble nucleation boiling regime [i.e., all boiling conditions after the onset of nucleate boiling (ONB) including partial nucleate boiling (PNB)] compared to the bare aluminum surfaces. However, nano-structured surfaces with low contact angles (&lt;10°) had lower bubble formation densities and had more difficulty forming bubbles uniformly across the boiling surface, as manifested by boiling curves with ‘high’ wall superheat values and low heat flux (CHF) values. 
         [0032]      FIG. 5  shows the dependence of critical heat flux (CHF) on the contact angle, and the measured CHF values, for plain silicon and aluminum surfaces. Current understanding of CHF demarcates two limits. One limit is a function of the surface morphology that is controlled over a range of contact angles. The second limit is hydrodynamically controlled and is relevant to well-wetted surfaces. In the figure, for flower-like morphologies, measured CHF values increased with decreasing contact angle. CHF maxima were also obtained at contact angles of approximately 18° to about 20°. Values decreased on either side of the curve. By comparison, a super hydrophilic surface ( FIG. 1 ) having a different morphology (carpet like) with a contact angle of 0° showed a low CHF value and also a high superheat value. 
         [0033]    Conventional theory predicts: 1) that bubble diameter decreases as a function of decreasing static contact angle, and 2) higher wall superheat values as a function of decreasing contact angles. Here, results show that critical boiling heat flux (q″) decreases as θ→0, but does not go to zero as conventional theory would suggest by Equation [3]. Results showing maxima for CHF dependence on contact angle are attributed to two competing effects: 1) CHF increases with decreasing contact angles down to 20°, but 2) a contrasting effect on CHF occurs where the critical heat flux value (q″) then decreases as (θ) decreases further to zero. With these nano-textured surfaces, the CHF dependence on contact angle is attributed to the balance between surface capillary fluid dynamics that brings fluid into the active nucleation sites, and the surface bubble dynamics that are governed by nucleation site densities and bubble diameters that ultimately lead to heat dissipation. Results suggest there is an optimum surface wettability condition that optimizes these two competing effects and causes the observed maxima in CHF as contact angle varies. The bubble nucleation frequency (1/f) is related to the bubble waiting period (t w ) and to the bubble-growth-time-to-departure value (t d ) by Equation [4], as follows: 
         [0000]      [(1/ f )= t   w   +t   d ]  [4]
 
         [0034]    Smaller contact angles (and therefore better wettability) are hypothesized to decrease the bubble waiting period (t w ), which increases the bubble nucleation frequency. As the contact angle gets smaller, capillary surface forces increase thereby bringing fluid to the active nucleation sites more effectively (thereby potentially increasing the bubble frequency). However, as the contact angle decreases, bubble diameter also decreases (by Equation [3]) and the number of active nucleation sites decreases, meaning CHF values are ultimately degraded when the contact angle becomes too small. These competing effects lead to the observed CHF maximum. In tests reported herein, contact angles were investigated from about 104° down to about 0°. A maximum CHF was discovered at about 20°. Nano-structured surfaces of the invention both a critical contact angle for CHF, and an enhanced heat flux augmentation. Results indicate there is a particular critical contact angle that maximizes CHF for various nano-textured surfaces, as described herein. 
         [0035]      FIG. 6  is a log-log plot that plots heat transfer coefficient (kW/m 2 K) vs nucleate boiling heat flux (kW/m 2 ) for selected surfaces. In the figure, curves are plotted only for the nucleate boiling regime. TABLE 2 compares Heat Transfer Coefficient (HTC) values of ZnO nanostructured surfaces and other selected surfaces. 
         [0000]    
       
         
               
             
               
               
             
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Comparison of Heat Transfer Coefficient (HTC) values 
               
               
                 for ZnO-nanostructured surfaces and other surfaces. 
               
             
          
           
               
                   
                 Heat transfer coefficient (HTC) 
               
               
                 SURFACE 
                 (kW/m 2  K) 
               
               
                   
               
             
          
           
               
                 Bare aluminum 
                 3.3 
               
               
                 Super-hydrophilic 
                 5.2 
               
               
                 Hydrophilic 
                 5.0 
               
               
                 ZnO nanostructures on Al (flower-like) 
                 23 
               
               
                 Unique structure 
                 14.5 
               
               
                 ZnO nanostructures on Cu (flower-like) 
                 20.7 
               
               
                   
               
             
          
         
       
     
         [0036]    As shown, ZnO nanostructured surfaces show almost an order of magnitude increase in HTC compared to a bare Al surface. CHF values (described previously) for ZnO nanostructured surfaces correspond to a boiling heat transfer coefficient as high as ˜23,000 W/m 2 K, representing an increase in CHF values for nano-textured surfaces of almost 4 times, which is contrary to conventional boiling heat transfer theory. Significant increases in both CHF and heat transfer coefficient in  FIG. 4  and  FIG. 6 , respectively, have important and far-reaching ramifications on the use of ZnO nanostructured surfaces for cooling of high heat fluxes in advanced power electronics, advanced high-power radar, and advanced laser systems. ZnO nanostructured surfaces and other nanostructured surfaces of the invention are expected to have significant future impacts on flow-boiling environments and configurations, which may exhibit surface heat fluxes in the range of hundreds of W/cm 2 . Such heat fluxes have important potential for enhancing electronic cooling in critical commercial and military instruments, devices, and systems. 
       CONCLUSIONS 
       [0037]    Pool-boiling experiments utilizing the method and system of the present invention on “bare” and nanostructured surfaces have demonstrated that nanostructured surfaces including, e.g., ZnO-on-Al and ZnO-on-Cu display superior heat transfer characteristics compared to bare Al and Cu substrates. A 10-fold improvement in heat transfer coefficients is observed for nanostructured surfaces compared with bare Al and Cu substrates. A 4-fold improvement in critical heat flux is also measured. 
         [0038]    While preferred embodiments of the present invention have been shown and described, it will be apparent to those of ordinary skill in the art that many changes and modifications may be made with various material combinations without departing from the invention in its true scope and broader aspects. The appended claims are therefore intended to cover all such changes and modifications as fall within the spirit and scope of the invention.