Incorporation of micro- and nanostructures on metals can improve thermal performance in a variety of applications. In this work, we demonstrate two independent highly scalable and cost-effective methods to generate micro- and nanostructures on copper and stainless steel, two widely used metals in energy and thermal applications. The performance of the developed structures, fabricated using scalable chemical etching techniques, is compared against their respective base metals. Our results demonstrate significant flow boiling heat transfer coefficient improvements up to 89% for etched copper and 104% for etched stainless steel. Mercury porosimetry is used to demonstrate that the varying pore-size distributions and presence of micro/nanoscale channels help to regulate heat transfer mechanisms, such as nucleate and convective flow boiling. Furthermore, structure integrity after 7-day flow boiling tests demonstrate surface structure resiliency to damage, a key challenge to implementation. This work combines advances in thermal performance with surface structure durability to provide guidelines for broader application of similar chemical etching methods to scalably create micro- and nanosculptured surfaces.

Integration of microstructures and nanostructures for enhanced performance of devices has received considerable attention in the past.1–6 In particular, boiling enables highly efficient heat transfer facilitated by energy release through latent heat of vaporization7 and is thus widely utilized in a plethora of industrial applications.8–14 Further augmentation of boiling performance through surface modification or sculpturing can be realized by increasing the nucleation site density,15–17 designing cavities of the correct size,18 tuning wetting characteristics of the surface,19,20 and enhancing capillary driven evaporation.21,22

Among previous efforts aimed at enhancing flow boiling performance, superhydrophobic surfaces have demonstrated improved phase change heat transfer characteristics due to their ability to trap air and reduce nucleation energy barriers.23 However, superhydrophobic surfaces suffer from the limitation of not being applicable to highly wetting low-surface tension fluids, such as refrigerants and dielectric liquids. Considering superhydrophilic surfaces, nanowires24,25 and micro pin-fin fences26,27 are two specific structure classes that have recently gained prominence due to their ability to demonstrate increased heat transfer performance by combining many of the desirable characteristics described above. However, a vast majority of such structures have so far only been designed on silicon substrates. Metallic heat sinks are widely used in a variety of applications with copper-based devices playing a prominent role in electronics cooling,28 automotive and off-road vehicles,29 and building energy systems,30 due to their high thermal conductivity and ease of manufacturability when compared to silicon. While fabrication of micro- and nanoscale structures on copper has been demonstrated,31 scalability and cost-effectiveness of metal-based fabrication methods remains a challenge and relatively few studies evaluate structure durability,32 making industrial-scale implementation difficult. In addition, even though stainless steel (SS) based heat exchangers are widely used due to their excellent anti-corrosion performance,33 studies focused on surface structuring of stainless steel for enhanced thermal performance are limited due to the difficulty in structuring such an inert and chemically stable metal. Thus, a dearth of facile, cost-effective fabrication techniques applicable to copper and stainless steel materials warrants further research in this area.

In this study, we develop nanofabrication methods for scalably generating micro- and nanostructures on arbitrarily shaped copper and stainless steel for enhanced boiling heat transfer. Since chemical etching has previously been shown to improve heat transfer performance with aluminum as the base metal,32 this study examines the applicability of the etching approach on copper and stainless steel. Commercially available copper and stainless steel tubes are used in these experiments, with inner diameters of Din = 4.6 mm and lengths of L = 90 cm. The grade of copper was 122, while the grade of stainless steel was 304. Microstructuring techniques with varying pore size distribution were custom developed for both metals to determine their effects on heat transfer performance. See Sec. S1 of the supplementary material for fabrication procedure. Detailed surface characterization studies are performed for all three structured surfaces. The scanning electron microscopy (SEM) analysis reveals that all three structured surfaces display both nanoscale and micrometer scale cavities with the etched copper surface displaying the largest micrometer scale cavities [Figs. 1(A)–1(C)]. The average structure heights of 1.5 μm for etched copper, 1 μm for CuO, and 1 μm for etched stainless steel are obtained through analysis of FIB images [Figs. 1(D)–1(F)]. See Secs. S2 and S3 of the supplementary material for confocal microscopy, x-ray photoelectron spectroscopy (XPS) images, and characterization equipment details.

FIG. 1.

SEM of (A) etched Cu, (B) CuO, and (C) etched stainless steel (SS) structures, along with the corresponding FIB milled cross-sectional SEM images of (D) etched Cu, (E) CuO, and (F) etched SS structures.

FIG. 1.

SEM of (A) etched Cu, (B) CuO, and (C) etched stainless steel (SS) structures, along with the corresponding FIB milled cross-sectional SEM images of (D) etched Cu, (E) CuO, and (F) etched SS structures.

Close modal

After conducting surface characterization of the three fabricated samples, we quantified the heat transfer performance by performing flow-boiling tests in a custom-built experimental facility. See Secs. S4 and S5 of the supplementary material for details on the experimental facility and data reduction. While a majority of prior flow boiling structured surface work has focused on water, low surface tension fluids, such as dielectric fluids, are increasingly gaining prominence in the electronics cooling industry.34 In addition, there remain many applications where water cannot be used as the working fluid due to undesirable thermophysical properties. Hence, we focus on our study on the low surface tension fluid R515B, recently developed as an alternative to the widely used R134a refrigerant (see Table S4 of the supplementary material for thermophysical properties of R515B). The refrigerant R515B has significantly lower global warming potential (GWP) value when compared to R134a in addition to being classified in the lowest toxicity and lowest flammability categories.

Figures 2(A)–2(D) display variation of effective flow boiling heat transfer coefficient (h) as a function of heat flux (q) for all surfaces considered in this study. Heat flux dependence for all surfaces can be seen by the increased heat transfer coefficients for incremental changes in heat flux until partial dry-out occurs where the heat transfer coefficient flattens out or starts to drop. During flow boiling, nucleate boiling dominance occurs when both low vapor quality and low mass flux is present. The combination of these two conditions ensures that low vapor velocities exist, thus favoring bubble nucleation. The observed increasing trends in heat transfer coefficients in this study are attributed to the combined effects of nucleation (at low mass flux values) and convective evaporation (at higher mass flux values). Among the copper surfaces, etched copper demonstrates the greatest enhancements while CuO showcases a marginal increase in heat transfer coefficient when compared with bare copper.

FIG. 2.

Effective heat transfer coefficients (h) of etched Cu, CuO, and unmodified Cu as a function of heat flux (q) at a saturated pressure of Psat = 560 kPa for (A) G = 100 kg/(m2 s) and (B) G = 200 kg/(m2 s). Effective heat transfer coefficients of etched SS and unmodified SS at Psat = 560 kPa for (C) G = 100 kg/(m2 s) and (D) G = 200 kg/(m2 s). Enhancement ratios of etched Cu, etched SS, and CuO at Psat = 560 kPa for (E) G = 100 kg/(m2 s) and (F) G = 200 kg/(m2 s).

FIG. 2.

Effective heat transfer coefficients (h) of etched Cu, CuO, and unmodified Cu as a function of heat flux (q) at a saturated pressure of Psat = 560 kPa for (A) G = 100 kg/(m2 s) and (B) G = 200 kg/(m2 s). Effective heat transfer coefficients of etched SS and unmodified SS at Psat = 560 kPa for (C) G = 100 kg/(m2 s) and (D) G = 200 kg/(m2 s). Enhancement ratios of etched Cu, etched SS, and CuO at Psat = 560 kPa for (E) G = 100 kg/(m2 s) and (F) G = 200 kg/(m2 s).

Close modal

The average effective heat transfer coefficient (h) for the microstructured etched copper surface increases from 62% at a mass flux of G = 100 kg/(m2 s) [Fig. 2(A)] to 73% at a mass flux of G = 200 kg/(m2 s) [Fig. 2(B)] due to an increase in convective evaporation with increasing mass flux. See Sec. S6 of the supplementary material for results at G = 150 kg/(m2 s). For the microstructured etched stainless steel surface, similar effects of increased convective contribution with an increase in mass flux with heat transfer coefficient enhancement ranging from 64% at G = 100 kg/(m2 s) [Fig. 2(C)] to 87% at G = 200 kg/(m2 s) [Fig. 2(D)]. Similar mass flux effect comparisons for the CuO surface demonstrate reduced impact with average heat transfer coefficient increases ranging between 26% and 28% for the two extreme mass flux cases [Figs. 2(A) and 2(B)].

To analyze the mechanisms governing the observed enhancements and to quantify differences in thermal performance for the fabricated microstructures, we plot the enhancement ratios (ER) as a function of the exit vapor quality (x). Enhancement ratios in this study refer to a ratio between the structured surface heat transfer coefficient and unstructured smooth surface heat transfer coefficient. A clear distinction in trends can be seen when comparing the enhancement ratios for etched copper at the low and high mass flux cases. While a decreasing trend in ER for etched copper is seen at the lower mass flux case of G = 100 kg/(m2 s) as vapor quality increases [Fig. 2(E)], an increasing trend is observed at the higher mass flux of G = 200 kg/(m2 s) [Fig. 2(F)]. Since nucleate boiling plays a greater role under low mass flux and low vapor quality conditions, the highest increases in heat transfer coefficients for G = 100 kg/(m2 s) occur at low exit vapor quality (x < 0.25). Progressive decline in ER at higher exit vapor qualities is attributed to suppression of nucleate boiling as more of the test section is occupied by higher vapor quality fluid as the heat flux increases and thin film evaporation effects begin to dominate.32 Conversely, the opposite trend is observed for higher mass flux of G = 200 kg/(m2 s). The highest increase in heat transfer coefficient occurs at the highest exit vapor quality of x = 0.8. In addition, declining nucleation effects with increasing mass flux can be seen in the enhancement ratio for etched copper dropping from ER = 85% at G = 100 kg/(m2 s) to ER = 66% at G = 200 kg/(m2 s) at low exit vapor qualities. Further increases in ER at higher exit vapor qualities for the high mass flux case is thus primarily attributed to enhanced evaporation. Interestingly, trends for the CuO surface display nearly identical enhancement ratios at low mass flux contrary to the decreasing trend observed at low mass flux for the etched copper surface. A general trend of increased enhancement ratios at both mass fluxes is observed for the microstructured stainless surface [Figs. 2(E) and 2(F)] with one difference from the etched Cu trends: enhancement ratios increase with increasing mass flux even at low exit vapor qualities. This points to the microstructured stainless steel surface displaying the greatest convective evaporation associated enhancements. Thus, despite similar structure heights for the three structured surfaces and similar average enhancements for the etched copper and etched stainless surfaces, it is clear that nucleation effects and convective evaporation have varying levels of impact on these surfaces.

Nucleation characteristics in boiling are highly dependent on the cavity size, number of nucleation sites, and cavity depth. Since cavity depth for all three surfaces have already been shown to be similar, we focus our attention on the remaining two features. While many previous studies rely on SEM images to determine cavity size,35 such an approach conveys cavity size information only for the visible top layer ignoring complex interconnected structures within. Hence, to quantify cavity size, we employ mercury porosimetry (see Sec. S7 of the supplemental material for more information) to characterize the pore size distribution for all three structured surfaces. Mercury porosimetry utilizes mercury, a highly non-wetting liquid, to penetrate into pores through the application of external pressure. Applied pressure (P) is inversely related to pore diameter (D) as D=4σcosθm/P, where σ is the liquid–vapor surface tension, and θm is the apparent advancing contact angle of mercury on a smooth surface composed of the porous material of interest. Thus, mercury porosimetry enables determination of pore size distribution throughout the surface and overcomes limitations from previous SEM approaches. Figure 3(A) shows that micro- and nanoscale cavities are found in all three structured surfaces. However, a difference in pore volume is observed with the etched copper displaying 6× the pore volume for micrometer sized pores ranging in size between 4.5 and 6 μm when compared to the etched SS and CuO in the same cavity distribution. Considering the effects of nucleation described earlier where nucleate boiling was found to be more important for the etched copper surface at low mass flux values when compared to copper oxide and etched stainless steel, the presence of an increased number of micrometer scale cavities is one of the distinguishing features for etched copper enabling such behavior. This inference is consistent with previous findings where micrometer scale cavities have been shown to be activated prior to nanoscale cavities.36 Additionally, the lack of nucleation effects observed in the enhancement ratio for etched stainless steel can be attributed to the dearth of nanoscale cavities in the desirable size range (cavities are smaller than 7 nm). Cavity radii smaller than a certain threshold (estimated to be 35 nm from Hsu's model37) can prove to be insufficiently small for vapor embryos to grow, thus playing a minimal role in nucleate boiling.

FIG. 3.

(A) Experimentally measured cumulative pore intrusion volume (Vp) as a function of pore diameter (dp) for etched Cu, CuO and etched SS. R515B contact angle images on (B) etched Cu, (C) CuO, and (D) etched SS. Film thickness variation (δ) and average film thickness comparison (δav, dotted horizontal lines) for (E) etched Cu and unmodified Cu at G = 200 kg/(m2 s) and (F) etched SS and unmodified SS at G = 200 kg/(m2 s). Average film thickness (δav) comparison at heat fluxes q = 17 kW/m2 and q = 27 kW/m2 and mass flux G = 200 kg/(m2 s) for (G) etched Cu and unmodified Cu and (H) etched SS and unmodified SS.

FIG. 3.

(A) Experimentally measured cumulative pore intrusion volume (Vp) as a function of pore diameter (dp) for etched Cu, CuO and etched SS. R515B contact angle images on (B) etched Cu, (C) CuO, and (D) etched SS. Film thickness variation (δ) and average film thickness comparison (δav, dotted horizontal lines) for (E) etched Cu and unmodified Cu at G = 200 kg/(m2 s) and (F) etched SS and unmodified SS at G = 200 kg/(m2 s). Average film thickness (δav) comparison at heat fluxes q = 17 kW/m2 and q = 27 kW/m2 and mass flux G = 200 kg/(m2 s) for (G) etched Cu and unmodified Cu and (H) etched SS and unmodified SS.

Close modal

The number of potential nucleation sites was determined through porosity measurements and etched copper was found to have the highest number of cavities: 58% higher when compared to CuO. The CuO surface in turn has 21% more cavities when compared to the etched stainless steel surface. Thus, a higher number of appropriately sized nucleation sites results in etched copper demonstrating more nucleation effects when compared with CuO. Conversely, the low number of nucleation sites lead to etched stainless steel displaying negligible nucleation effects. Lowering the apparent contact angle of the working fluid (θ) on structured surfaces can contribute to increased nucleation through reduced bubble departure diameter (Dd) as Dd scales as Ddθ[σ/(σ/g(ρlρv)]1/2, where g is the gravitational constant and ρl and ρv are the liquid and vapor densities, respectively. Contact angle measurements on flat surfaces shown in Figs. 3(B)–3(D) demonstrate the reduction in contact angle for the structured copper surfaces (see Sec. S8 of the supplementary material for more details). Reduced bubble departure diameters resulting from higher wettability in turn lead to increased bubble departure frequencies since departure diameter and frequency are inversely related.38,39 The microlayer evaporation contribution40 for structured surfaces would also be higher due to film thinning between the surface and bubble periphery during bubble growth. Due to the inherent roughness associated with the plain stainless steel surface (see Sec. S9 of the supplementary material), liquid completely wets both plain stainless steel and etched stainless steel. Thus, no nucleation enhancements can be attributed to contact angle effects for stainless steel, which has already been shown to be least dependent on nucleation effects amongst the three surfaces tested. Based on these results, CuO is expected to exhibit maximum bubble departure frequency and thus the number of bubbles that emerge from a single nucleation site is expected to be larger for CuO when compared to etched copper. However, since the difference in contact angle between the two structured copper surfaces is minimal (∼3°) while the number of nucleation sites is significantly larger for the etched copper surface (∼58%), we conclude that cavity sizing and number of nucleation sites play a much more prominent role in nucleate boiling when compared to contact angle reduction effects for the surfaces considered in this study.

Considering convection effects on the structured surfaces next, we quantify and base our analysis on the liquid film thickness surrounding the tube wall obtained through an adiabatic glass visualization section (see Sec. S10 of the supplementary material for film thickness determination methodology). Liquid film thickness plays a major role in convection dominated flow regimes with conduction resistance across the film treated as the limiting factor for heat transfer.41 Film thickness measurements shown in Figs. 3(E) and 3(F) demonstrate lower conduction thermal resistance for etched copper and etched stainless steel in comparison to their respective control surfaces under the same working conditions, through reduced thickness values. Film thinning observed in etched copper is attributed to lower apparent contact angle when compared to plain Cu and increased evaporation in the micro/nanoscale pores. Etched stainless steel on the other hand exhibits film thinning due to the presence of sub-micron scale channels (visible in FIB images) formed through the etching process. These distinct sub-micron channels facilitate thin film evaporation leading to higher heat transfer coefficients when compared to the etched copper surface. Capillary driven wicking through these channels present in structured surfaces also contributes to enhancements, with mercury porosimetry demonstrating that the etched stainless-steel surface has a large number of pores with diameters smaller than 7 nm. Due to marginal increases in heat transfer coefficient for CuO surfaces when compared to plain copper, similar thickness values are obtained for these two surfaces. See Sec. S11 of the supplementary material for CuO film thickness comparison. Additionally, the average film thickness reduces as the heat flux increases due to increased evaporation facilitated by lower conduction thermal resistance for both the plain and structured surfaces [Figs. 3(G) and 3(H)]. The effects of micro/nanoscale channels present in etched stainless steel can also be observed when comparing the relative average film thickness reduction for both etched surfaces. The etched stainless steel surface outperforms the etched copper surface under both heat flux conditions with a 170% greater thickness reduction at lower heat flux and a 76% greater thickness reduction at higher heat flux. Thus, the porous nature of the etched structured surfaces clearly enables enhanced convective evaporation which in turn contributes to enhanced heat transfer coefficient.

To characterize short-term durability of the structured surfaces, surface characterization was performed by axially cutting open each internally structured tube through electric discharge machining after conducting 7-day duration flow boiling tests at a mass flux of 100 kg/(m2 s). The presence of microstructures after conducting the test is confirmed through FIB images, thus indicating that the structures survive (see Sec. S13 of the supplementary material). The results with the etched copper and etched stainless steel surfaces are expected since the structures are made of the same metal as the base metal and hence can be expected to persist during long term durability experiments as well. On the other hand, oxide-based structures have dissimilar materials at the interface (metal and the oxide). These structures can delaminate at the weak interface due to varying thermal coefficients of expansions and have previously been shown to be less durable.42 Thus, durability tests over a longer period of time are warranted for oxide-based structures.

In summary, we develop scalable etching techniques to generate internal micro-/nanostructures on copper and stainless steel substrates. Enhanced flow boiling heat transfer coefficients up to 89% for etched copper and 104% for etched stainless steel are demonstrated through combined effects of increased nucleation and enhanced convective evaporation. Our results indicate that micro- to nanoscale cavities generated by etching the base metal are critical to improving thermal performance. Robustness of the generated structures is also showcased through short-term 7-day durability tests, indicating potential applicability of the demonstrated fabrication procedures for industrial use. In addition to demonstrating improved thermal performance, this work also provides guidelines for future geometry optimization of structured surfaces for a variety of applications.43–46 

See the supplementary material for information on the fabrication procedure, surface characterization, experimental rig details, data reduction methods, contact angle measurement, film thickness measurement methodology, and durability tests.

The authors gratefully acknowledge funding support from the Office of Naval Research (ONR) under Grant No. N00014-21-1-2089. All authors gratefully acknowledge funding support from the Air Conditioning and Refrigeration Center (ACRC). N.M. gratefully acknowledges funding support from the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science and Technology. Surface characterization studies were performed at Materials Research Laboratory Central Facilities at the University of Illinois at Urbana-Champaign.

The authors have no conflicts to disclose.

Kazi Fazle Rabbi and Alireza Bakhshi contributed equally to this work.

Nithin Vinod Upot: Data curation (equal); Formal analysis (equal); Investigation (lead); Methodology (lead); Software (equal); Validation (equal); Visualization (equal); Writing – original draft (lead); Writing – review & editing (equal). Kazi Fazle Rabbi: Formal analysis (equal); Software (equal); Validation (equal); Writing – review & editing (equal). Alireza Bakhshi: Data curation (equal); Software (equal); Validation (equal); Writing – review & editing (equal). Johannes Kohler Mendizabal: Formal analysis (equal); Software (equal); Writing – review & editing (equal). Anthony Jacobi: Funding acquisition (equal); Project administration (equal); Writing – review & editing (equal). Nenad Miljkovic: Conceptualization (equal); Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article and its supplementary material.

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Supplementary Material