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Stable Dropwise Condensation of Ethanol and Hexane on RationallyDesigned Ultra-Scalable Nanostructured Lubricant-Infused Surfaces Soumyadip Sett, Peter Sokalski, Kalyan Boyina, Longnan Li, Kazi Rabbi, Harpreet Auby, Thomas Foulkes, Allison Mahvi, George Barac, Leslie Bolton, and Nenad Miljkovic Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b01754 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 25, 2019

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Stable Dropwise Condensation of Ethanol and Hexane on Rationally-Designed Ultra-Scalable Nanostructured Lubricant-Infused Surfaces Soumyadip Sett1, Peter Sokalski1, Kalyan Boyina1, Longnan Li1, Kazi Fazle Rabbi1, Harpreet Auby1, Thomas Foulkes1,2, Allison Mahvi1, George Barac3, Leslie W. Bolton4, and Nenad Miljkovic*1,2,5,6, 1Department

of Mechanical Science and Engineering, University of Illinois, Urbana, Illinois 61801, USA of Electrical and Computer Engineering, University of Illinois, Urbana, Illinois 61801, USA 3BP International Limited, 150 W Warrenville Road, Naperville, Illinois 60563, USA 4BP plc, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN, UK 5Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA 6International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan 2Department

*Corresponding Author E-mail: [email protected]

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ABSTRACT Vapor condensation is a widely used industrial process for transferring heat and separating fluids. Despite progress in developing low surface energy hydrophobic and micro/nanostructured superhydrophobic coatings to enhance water vapor condensation, demonstration of stable dropwise condensation of low-surface-tension fluids has not been achieved. Here, we develop rationally designed nanoengineered lubricant-infused surfaces (LIS) having ultralow contact angle hysteresis ( 0, the lubricant will cloak the condensate droplets. The calculated spreading coefficient of two Krytox lubricants, namely Krytox 1525 (𝜇 = 496 mPa·s) and Krytox 16256 (𝜇 = 5216 mPa·s) on ethanol were -4.11 and -4.83, and on hexane were -2.54 and - 2.45, respectively. Hence the desired non-cloaking condition of 𝑆ol < 0 was achieved for the fluorinated Krytox lubricants with ethanol and hexane. In fact, considering both miscibility and cloaking, the fluorinated lubricants were the only suitable candidate for the design of stable LISs with ethanol and hexane, making it our lubricant choice for the condensation experiments conducted herein. In addition to Krytox, we selected a perfluorinated Fomblin lubricant which is also immiscible and non-cloaking with ethanol and hexane. Similar to Krytox in chemical composition, viscosity and surface tension, the Fomblin lubricant has linear perfluorinated molecules as compared to branched molecules present in Krytox. Thus, we chose vacuum grade Fomblin Y25/6 for designing stable LISs, whose physical properties are similar to Krytox 1525 (Table 1). Lubricant-infused surfaces were fabricated by dip coating the functionalized nanostructured CuO tubes in the lubricant of choice (Table 1) for 10 min. After dip-coating, the tubes were removed and left standing in the vertical orientation in ambient room conditions for approximately 24 hours to allow gravitational drainage of excess lubricant. The effect of drainage of excess lubricant was studied by visualizing water droplets on nanostructured CuO coupons infused with the three different lubricants at different times after coating (see Supporting Information, Section S3). After dip-coating and drainage for a period of 24 hours, excess lubricant was removed with presence of lubricant only within the CuO nanostructures. Contact angle measurements using 100 nL droplets on the functionalized smooth Cu (HP Cu) and different LISs are listed in Table 2 for ethanol and hexane and Table S1 for water (see Supporting Information) with exemplary images shown in Figure 1(c) and (d). 7 ACS Paragon Plus Environment

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Table 2. Sample wetting characteristics. Intrinsic advancing (𝜃a) and receding (𝜃r) contact angles, and contact angle hysteresis (∆𝜃 = 𝜃a ― 𝜃r) of ethanol and hexane droplets on hydrophobic Cu (HP Cu), Krytox 1525 (LIS K1525), Krytox 16256 (LIS K16256) and Fomblin Y25/6 (LIS F25/6) LISs. Sample HP Cu LIS K1525 LIS K16256 LIS F25/6

Ethanol

Hexane

𝜃a [°]

𝜃r [°]

∆𝜃 [°]

𝜃a [°]

𝜃r [°]

∆𝜃 [°]

36.5 ± 1.2 71.1 ± 2.4 67.8 ± 1.9 62.4 ± 2.3

23.4 ± 3.7 69.3 ± 2.6 65.2 ± 3.2 59.7 ± 2.6

13.1 ± 3.9 1.8 ± 3.5 2.6 ± 3.7 2.7 ± 3.5

12.6 ± 2.8 45.7 ± 2.3 44.1 ± 2.6 37.4 ± 3.2

≈0 42.1 ± 2.5 40.7 ± 2.8 34.6 ± 2.7

12.6 ± 2.8 3.6 ± 3.4 3.4 ± 3.8 2.8 ± 4.2

To determine the overall condensation heat transfer performance, the hydrophobic Cu (HP Cu) and LISs were tested in a chamber with a controlled environment (see Supporting Information, Section S4). Prior to the condensation experiments, a vapor generator filled with the test liquid was vigorously boiled, and the test chamber was evacuated to a pressure 𝑃 < 4 ± 2 Pa with a leak rate of 0.1 Pa/min after chamber isolation. Chamber pump down and fluid boiling were done primarily to eliminate the non-condensable gases (NCGs) which pose an additional diffusional resistance to condensation heat transfer41, 42. During the condensation experiments, the chamber pressure and vapor generator temperature were continuously monitored to ensure saturated conditions. The surface temperature of the tube sample was independently controlled with an external water cooling loop, with the inlet and outlet temperatures continuously measured using Class A resistance temperature detectors (RTDs) to determine the overall heat flux (see Supporting Information, Section S5). Typical inlet-to-outlet tube temperature differences ranged from 0.5 to 7.5°C depending on the tube sample, working fluid, and vapor pressure. For all experiments, the cooling water inlet temperature was kept constant at 6 ± 1°C with a flow rate of 11 ± 0.3 L/min, resulting in fully turbulent flow with Reynolds number, Re = 36000. Accordingly, condensation heat transfer performance was tested within the vapor pressure range of 3.5 < 𝑃v < 10 kPa for ethanol and 11 < 𝑃v < 15 kPa for hexane, which are common conditions for condensers used in industrial separation and distillation applications. For details regarding the experimental facility, design, and operation, please refer to Sections S4 and S5 of the Supporting Information.

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Figure 2. Photographs of condensation of (a, d) water, (b, e) ethanol, and (c, f) hexane vapor on a smooth hydrophobic Cu tube and functionalized nanostructured CuO tube, respectively. Photographs of condensation of (g) water, (h) ethanol, and (i) hexane vapor on a Krytox 1525 LIS. The LIS showed stable dropwise condensation for all fluids. Chamber vapor pressure was (a, d, g) 𝑃v = 4.5 kPa, (b, e, h) 𝑃v = 7 kPa, and (c, f, i) 𝑃v = 12 kPa. For details regarding the experimental facility, design, and operation, please refer to Sections S4 and S5 of the Supporting Information. The scale bar presented in (c) is the same for all images. Figures 2(a-c) and (d-f) show condensation of water, ethanol, and hexane on the smooth hydrophobic Cu (HP Cu) and functionalized nanostructured CuO tubes, respectively. Due to the intrinsic hydrophobicity of the tube surface, water vapor condensate formed discrete droplets on the outer tube surface, which grew in time before being removed by gravity, sustaining continuous dropwise condensation (Figure 2a, d). However, the solid-vapor interface of the HP Cu and CuO tubes did not have sufficiently low surface energy to prevent wetting by the low-surface-tension fluids (Figure 2b, c, e, f). As expected from the high contact angle hysteresis (Table 2), ethanol (Figure 2b) and hexane (Figure 2c) underwent filmwise condensation, limiting heat transfer due to the added thermal resistance of the thin condensate film43. In contrast, condensation on the Krytox 1525 LIS tube (LIS K1525) provided a liquid–liquid interface between the condensate droplets and the immiscible lubricant, resulting in negligible droplet pinning, low contact angle hysteresis25, 28, and easy droplet removal. Figure 2(g-i) show dropwise condensation of water 9 ACS Paragon Plus Environment

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(Figure 2g and Video S1), ethanol (Figure 2h and Video S2), and hexane (Figure 2i and Video S3) vapor on the LIS K1525 tube.

Figure 3. Experimental steady state log mean water to vapor temperature difference (∆𝑇LMTD) as a function of overall surface heat flux (𝑞″) for (a) ethanol and (b) hexane condensation on hydrophobic Cu (HP Cu, filmwise) and LIS (dropwise) surfaces. Rapid droplet removal due to dropwise condensation resulted in the highest heat fluxes for the LIS samples (for overall surface heat flux (𝑞″) as a function of saturated vapor pressure (𝑃v), see Figure S5 of the Supporting Information, Section S10). Experimental and theoretical steady state condensation coefficient (ℎc) as a function of saturated vapor pressure (𝑃v) for (c) ethanol and (d) hexane condensation on HP Cu (filmwise) and LIS (dropwise) surfaces. Dropwise condensation on the LIS impregnated with Krytox 1525 showed the highest ℎc. Error bars indicate the propagation of error associated with the fluid inlet and outlet temperatures (±0.25 K), pressure measurement (± 1%), and flow rate (± 1%). The theoretical predictions (dotted lines in c and d) were obtained from the classical Nusselt filmwise condensation model on tubes (for model derivation and parameters, see Supporting Information, Section S6, S7 and S8). 10 ACS Paragon Plus Environment

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For all condensation heat transfer experiments, the rate of condensation increased with increasing 𝑃v (see Supporting Information, Figure S6). Prior to utilizing ethanol and hexane, the heat transfer measurement method (see Supporting Information, Sections S6-S8) was benchmarked with steam as the working fluid. The measurements were in excellent agreement with prior steam condensation results for both dropwise and filmwise modes of condensation on horizontal tubes (see Supporting Information, Section S9). Figures 3(a) and 3(b) show the measured steady overall condensation heat flux (𝑞″) as a function of log mean vapor-to-liquid temperature difference (∆𝑇LMTD = [(𝑇v ― 𝑇in) ― (𝑇v ― 𝑇out)]/ln [(𝑇v ― 𝑇in)/(𝑇v ― 𝑇out)], where 𝑇v, 𝑇in, and 𝑇out are the vapor, cooling water inlet, and cooling water outlet temperatures) for ethanol and hexane, respectively. To maximize the tube internal heat transfer coefficient, the cooling water mass flow rate was held constant at 11 ± 0.3 L/min for all experiments (1.02 < 𝑆 ≤ 1.7, 7 < 𝑇s < 25°C, where S is the supersaturation and 𝑇s is the extrapolated tube surface temperature, see Supporting Information, Section S6). The overall heat transfer coefficient (HTC), 𝑈 = 𝑞"/∆𝑇LMTD was calculated from the measured values of condensation heat flux (𝑞″) and calculated ∆𝑇LMTD values. Knowing the thermal resistances of the internal tube single-phase forced convection and radial conduction through the copper tube wall, the steady-state condensation heat transfer coefficient at the tube outer surface, ℎc, was calculated (see Supporting Information, Sections S6, S7 and S8 for detailed calculations and error analysis). To validate our results, we modeled the filmwise condensation using the classical Nusselt theory for tube condensation33, 44. The filmwise condensation results (square symbols in Figures 3c, d) of ethanol and hexane on smooth hydrophobic Cu (HP Cu) surfaces were in excellent agreement with Nusselt theory (dotted line in Figures 3c, d). As expected, for ethanol and hexane, the HP Cu tube showed filmwise behavior with the lowest overall and condensation HTC (𝑈filmwise ≈ 2.87 ± 0.4 kW/m2K, ℎc,filmwise ≈ 3.38 ± 1.3 kW/m2K for ethanol and 𝑈filmwise ≈ 3.43 ± 0.5 kW/m2K, ℎc,filmwise ≈ 3.93 ± 1.07 kW/m2K for hexane) due to the thin condensate film acting as the dominant thermal resistance to heat transfer43. The steady filmwise condensation HTC (ℎc,filmwise) decreased with increasing 𝑃v (Figure 3c-d) due to the build-up of the condensate on the tube outer surface, increasing the overall thermal resistance. Meanwhile, condensation of both ethanol and hexane on the LISs exhibited steady 11 ACS Paragon Plus Environment

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dropwise condensation behavior. The heat transfer performance during dropwise condensation of ethanol and hexane on LIS tubes substantially exceeded that of filmwise condensation ( 𝑈dropwise ≈ 4.8 ± 0.4 kW/m2K, ℎc,dropwise ≈ 6.23 ± 0.7 kW/m2K for ethanol and 𝑈dropwise ≈ 6.9 ± 0.9 kW/m2K, ℎc,dropwise ≈ 9.4 ± 1.6 kW/m2K for hexane) as shown in Figure 3(c) and 3(d), respectively. The three different LISs infused with the three separate lubricants gave similar enhanced heat transfer performance for ethanol and hexane condensation, having a variance of ±11% and ±30% in 𝑈dropwise for ethanol and hexane, respectively, and ±13% and ±40% in ℎc,dropwise for ethanol and hexane, respectively. The dropwise condensation heat transfer enhancement was more pronounced at higher 𝑃v, where filmwise HTC decreases due to increasing condensate film thickness while dropwise HTC increases due to nucleation site activation at higher supersaturations. The enhanced heat transfer performance for dropwise condensation of hexane on all three LIS tubes compared to filmwise condensation on the smooth hydrophobic Cu tube was equally pronounced as that observed with ethanol. The Krytox 1525 infused LIS tube (LIS K1525) had the best performance. The ℎc,dropwise was higher than ℎc,filmwise by 150% for LIS K1525, 100% for LIS F25/6, and 50% for LIS K16256. Interestingly, although all three LIS tubes had a variance of ±13% in ℎc,dropwise for ethanol, statistically significant differences in ℎc,dropwise for hexane were observed (Figure 3d). To understand if the infused lubricant viscosity played a role in the heat transfer performance, we analyzed the droplet departure size of the condensate during our condensation experiments. Departure size characterization of condensate droplets from the three LIS tubes for water, ethanol, and hexane condensation revealed that the viscosity of the infused lubricant did not play a role in the droplet departure diameter (see Supporting Information, Table S3)29. To explain the variance in heat transfer coefficients, we examined both the lubricant and hexane thermophysical properties (Table 1) along with the contact angle hysteresis of condensate droplets on the LISs (Table 2). For ethanol, the intrinsic advancing contact angle (𝜃a) on the three LISs was 62.4º < 𝜃a < 71.1º, with a maximum contact angle hysteresis, ∆𝜃 = 𝜃a ― 𝜃r ≈ 2.7º (Table 2). Although previous works have pointed to the strong dependence of dropwise condensation heat transfer on contact-angle-dependent droplet conduction resistance45, minimal effects are expected in the narrow range observed here for ethanol on the three LISs. Furthermore, the low ∆𝜃 < 2.7º and moderate 𝜃a ≈ 65º ensure stable dropwise condensation for all three LIS with minimal 12 ACS Paragon Plus Environment

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expected heat transfer results46. However, for hexane, 𝜃a on the three LISs was 37.4º < 𝜃a < 45.7º, with a maximum ∆𝜃 ≈ 3.6º (Table 2), resulting in condensation close to the dropwise-to-filmwise transition. Dropwise condensation stability is governed by the ∆𝜃-mediated droplet shedding (sliding) length scale. For droplets having relatively low 𝜃a ( 270 mPa.s, 𝜈 > 140 cSt) remained stable for over 10 hours of operation29. Hence, for our aforementioned condensation experiments, the usage of higher viscosity Krytox 1525 (𝜇 = 496 mPa·s, 𝜈 = 250 cSt), Krytox 16256 (𝜇 = 5216 mPa·s, 𝜈 = 2560 15 ACS Paragon Plus Environment

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cSt), and Fomblin Y25/6 (𝜇 = 524 mPa·s, 𝜈 = 276 cSt), ensured prolonged dropwise condensation of ethanol and hexane. It should be noted that the optimal lubricant viscosity range for best heat transfer performance of the LIS surfaces depends on the working fluid and its properties. The findings reported here have important implications for the potential development of durable, scalable and robust surfaces for dropwise condensation of low-surface-tension fluids. In particular, condensation of refrigerants is a crucial industrial process4, 48. Current state-of-the-art strategies in the commercial and industrial sectors5 involve enhancement of filmwise condensation heat transfer of refrigerants and other low-surface-tension fluids by passive techniques such as fabrication of enhanced microscale surfaces composed of fins and channels on the condenser surface. However these techniques remain limited by the fundamental constraint of filmwise wetting behavior and could greatly benefit from dropwise condensation. The demonstration of stable dropwise condensation of ethanol and hexane shows that LISs may be a potential solution for creating refrigerant-repellant surfaces. Furthermore, the presence of 1 to 5% oil content in stateof-the-art (SOA) refrigeration systems stemming from compressor lubricant entrainment presents a unique opportunity to develop closed-cycle LISs condenser surfaces that can be replenished with compressor lubricant at steady state. Indeed, at the condenser inlet of an air-conditioning system, the refrigerant enters as a superheated vapor with entrained low vapor pressure compressor oil droplets which can deposit on the condenser surface, presenting an opportunity to convert a system penalty (oil entrainment) into a benefit. In addition to refrigeration, systems that use non-refrigerant low-surface-tension process fluids such as chemical plants49, natural gas production facilities50, biomass combustion units51, and the food industry52, stand to significantly benefit from dropwise condensation with respect to condenser size reduction and energy cost savings. Unlike previous studies, our work shows that rigorous sustainable dropwise condensation can be achieved for such low-surface-tension fluids, attaining 150% higher heat transfer coefficients compared to SOA filmwise condensation. A particular design criteria needing further attention when considering alternate LIS condensation applications is the freezing point of the lubricant. For the lubricants studied here, the freezing points range from -48ºC (Krytox 1525) to -15ºC (Krytox 16256), which are well below the working conditions for the majority of refrigeration and petrochemical separations applications. However, as the condensing temperature approaches that of the freeing point of the lubricant, we envision

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that the frozen lubricant will lose its atomic smoothness, resulting in increased contact line pinning and transition to filmwise condensation will ensue. Taking into consideration the consequences of lubricant drainage from LISs on the overall heat transfer performance, longer term durability tests of at least a month (> 1 month) are needed to determine lubricant loss from the LIS surfaces. Such condensation experiments will help to quantify the longevity of the lubricant layer, and determine whether the loss of heat transfer performance is gradual or sudden due to degradation of LIS surfaces. Besides condensate shear, mechanical damage and wear of condenser surfaces present a challenge to LIS implementation53. Presence of solid particles, contaminants, and high vapor face velocities (> 1 m/s) can erode both the lubricant and underlying structure. Although the ability of LIS to self-heal due to passive capillary forces stands to minimize erosion effects, future work is needed to quantify anti-erosion properties and to develop both structure and lubricant design guidelines for anti-erosion purposes. In addition to enhancing condensation heat transfer, the atomically smooth lubricant layer on LISs has also been shown to inhibit industrial scale biofouling and scale buildup54-56, a significant concern in many industrial heat transfer applications. The synergy offered by antifouling and enhanced heat transfer has significant potential in SOA applications to dramatically enhance both performance and longevity. Further work is needed to ensure that lubricant selection and structure design can achieve synergistic benefit of both enhanced heat transfer and anti-fouling function. Although both are heterogeneous nucleation mediated phenomena, fouling and condensation may present competing requirements for LIS design as liquid and solid deposition have differing growth mechanisms and participating media (vapor vs. liquid). Our results on enhanced heat transfer performance for dropwise condensation of ethanol and hexane on LISs layout promising design guidelines for fabricating next generation surfaces promoting dropwise condensation of low-surface-tension fluids. For developing stable LISs for enhanced condensation applications, proper lubricant selection is of utmost importance. The lubricant properties determine both the apparent advancing contact angle and the contact angle hysteresis, the two necessary factors for preventing condensate film formation and promoting rigorous dropwise condensation. Our dropwise condensation HTC results for ethanol and hexane show that for working fluids having surface tension higher than the infused lubricant in LIS, the heat transfer performance is independent of the lubricant viscosity. However, when the surface tension of the condensate fluid and the lubricant becomes comparable, lower viscosity LISs result 17 ACS Paragon Plus Environment

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in easier droplet shedding, leading to better heat transfer performance. It is important to note that lubricants having viscosities that are too low can be detrimental, since the surfaces degrade quickly through lubricant drainage. Thus, for best heat transfer results, we recommend using lubricants with viscosity 𝜇 ≈ 500 mPa·s for developing stable, robust and durable LISs. In summary, we demonstrated dropwise condensation of ethanol and hexane on ultrascalable nanostructured CuO based LISs. As a result, 100% and 150% higher condensation heat transfer coefficients were realized for ethanol and hexane condensation on all LISs, respectively, with a 200% enhancement for both fluids on LISs impregnated with Krytox 1525 (LIS K1525) as compared to filmwise condensation on smooth hydrophobic tubes. The rationally designed LISs developed here proved to be sufficiently robust and durable to maintain dropwise condensation heat transfer performance for both ethanol and hexane for seven hours, suggesting that they may be suitable candidates for industrial applications where such performance would need to be sustained over hundreds of hours. Our work points to the requirement for careful lubricant selection in order to both minimize contact angle hysteresis, achieve an appreciable apparent advancing contact angle to prevent transition to filmwise condensation, and utilize optimum viscosity lubricant fluids to minimize lubricant drainage while enabling rapid droplet shedding. These results not only provide guidelines for developing novel LISs for enhanced heat transfer during condensation of low-surface-tension fluids but also outline future directions towards achieving dropwise condensation of refrigerants.

ASSOCIATED CONTENT Supporting Information Three videos showing dropwise condensation of water, ethanol and hexane vapor on LIS K1525, as well as detailed information on surface fabrication, experimental setup/procedure, data collection methodology, condensation heat transfer calculations, error analysis and steam benchmarking results (PDF).

ACKNOWLEDGEMENTS This work was supported by the BP plc through the International Centre for Advanced Materials (ICAM), and the Office of Naval Research (ONR) (Grant No. N00014-16-1-2625). N.M. 18 ACS Paragon Plus Environment

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gratefully acknowledges funding support from the Air Conditioning and Refrigeration Center (ACRC), an NSF founded I/UCRC at UIUC and the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science and Technology. Scanning electron microscopy and focused ion beam milling was carried out in part in the Materials Research Laboratory Central Facilities, University of Illinois.

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