Lubricant-Infused Surfaces for Low-Surface-Tension Fluids: Promise

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Lubricant-Infused Surfaces for Low Surface Tension Fluids: Promise vs Reality Soumyadip Sett, Xiao Yan, George Barac, Leslie Bolton, and Nenad Miljkovic ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10756 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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ACS Applied Materials & Interfaces

Lubricant-Infused Surfaces for Low Surface Tension Fluids: Promise vs Reality Soumyadip Sett1, Xiao Yan1, George Barac2, Leslie W. Bolton3, Nenad Miljkovic1,4,5,* 1

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

*Corresponding author email: [email protected]

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Abstract The past few decades have seen substantial effort for the design and manufacturing of hydrophobic structured surfaces for enhanced steam condensation in water based applications. Such surfaces promote dropwise condensation and easy droplet removal. However, less priority has been given to applications utilizing low surface tension fluids as the condensate. LubricantInfused Surfaces (LIS) or Slippery Liquid-Infused Porous Surfaces (SLIPS) have recently been developed, where the atomically-smooth, defect free slippery surface leads to reduced pinning of water droplets, and omniphobic characteristics. The remarkable results of LIS and SLIPS with a range of working fluid droplets gives hope of their viability with low surface tension condensates. However the presence of the additional liquid in the form of the lubricant brings other issues to consider. Here, in an effort to study the dropwise condensation potential of LIS and SLIPS, we investigate the miscibility of a range of low surface tension fluids with widely used lubricants in LIS and SLIPS design. We consider a wide range of condensate surface tensions (12 to 73 mN/m) and different categories of lubricants with varied viscosities (5 – 2700 cSt), namely fluorinated Krytox oils, hydrocarbon silicone oils, mineral oil, and ionic liquids. In addition, we use both theory and pendant drop experiments to predict the cloaking behavior of the lubricants and immiscible condensate working fluid pairs. Our work not only shows that careful attention must be paid to lubricant-condensate selection to create long lasting LIS or SLIPS, it develops lubricant selection design guidelines for stable LIS and SLIPS for enhanced condensation in applications utilizing low surface tension working fluids.

Keywords: LIS, SLIPS, Lubricants, Miscibility, Cloaking, Low Surface Tension

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1. Introduction Liquid repellent surfaces are an integral part in our daily life as well as in numerous industrial processes. Over the last few decades, inspired from natural surfaces like lotus leaves 3

strider legs , Namib Desert beetle

4-5

1-2

, water

6

, and geckos’ feet , significant work has been done in

developing water repellent or hydrophobic structured surfaces. The techniques range from fabricating nanostructured functional surfaces to spray coating low surface energy materials. Hydrophobic surfaces have also found utility for condensation, a phenomenon ubiquitous in nature and industry. The low surface energy leads to higher contact angles of water droplets, lower contact angle hysteresis, and hence easy droplet removal, promoting dropwise condensation and enhancing condensation heat transfer by up to 1000% when compared to their low surface energy counterparts 7-9. Recently, a new class of nanotextured surfaces, Lubricant-Infused Surfaces (LIS) or Slippery Liquid-Infused Porous Surfaces (SLIPS) have been introduced

10-11

. In such slippery surfaces,

liquid lubricant is stabilized in a porous or nanotextured solid by capillary forces

10, 12-14

.

Lubricants with low surface energy and vapor pressure form the most stable SLIPS or LIS such as fluorinated Krytox oils 13, 20

10-11, 15-17

, Silicone oils

13, 17-19

16

13-14

, mineral oils , and ionic liquids

,

11,

. The lubricants chosen have low vapor pressure to minimize their losses due to evaporation.

The infused lubricant creates a chemically homogeneous and atomically-smooth interface for liquid droplets, leading to extremely low contact angle hysteresis. The absence of defects leads to easy droplet shedding

13, 16

and self-cleaning

much potential in enhanced condensation

10, 21

11, 22

. Applications of LIS and SLIPS coatings has

, anti-icing 23, anti-fouling 24-26, and self-healing 10

applications. Furthermore, condensation of water vapor on LIS surfaces has been shown to lead to rapid clearing of condensate liquid for the re-establishment of nucleation sites, promoting dropwise condensation and enhancing condensation heat transfer

11, 22

. Although the lubricant

layer prevents contact line pinning (immobility of the three-phase contact line), it can lead to adverse effects like lubricant encapsulation of condensate droplets resulting in coating of the condensate liquid with a thin layer of lubricant, termed as lubricant cloaking 14. A recent study 27 discusses in details the various parameters and constraints that need to be taken care of while designing stable LIS surfaces. During condensation, cloaking of the condensate droplets creates a vapor diffusion barrier reducing direct condensation and the droplet coalescence rate resulting in reduced droplet density

11

16, 20

,

. It is also interesting to note that lubricant cloaking is 3



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governed by the interfacial energies of the lubricant and condensate liquids, and that the surface has no role to play 13. Though significant work has been done in the field of steam condensation and enhanced surface wettability for water

8, 28-30

, the same cannot be said for other, lower surface tension

liquids. A broad spectrum of these fluids such as hydrocarbons, alcohols, and refrigerants are used in a number of industrial applications like chemical plants, refineries, production

32

, biomass combustion

33

, food industry

33

31

, natural gas

, and building energy technology

34-35

.

Dropwise condensation of low surface tension fluids has the potential to significantly enhance condensation heat transfer. Recently, a four to eight-fold increase in heat transfer coefficient was achieved by dropwise condensation as compared to filmwise condensation of such low surface tension fluids on LIS

15

. However, care must be taken when choosing the lubricant for low

surface tension condensates. The primary condition for stable LIS and SLIPS is that the lubricant and the working fluid need to be immiscible. The similar surface tension and molecular structure of the two group of liquids, along with polarity of the molecules limits the choices. Furthermore, for immiscible pairs, the lubricant may cloak the condensate droplets, resulting in gradual depletion of the lubricant from the surface. In this work, we utilize theory and experiments to determine suitable LIS and SLIPS lubricants for condensation of low surface tension fluids. A wide variety of lubricants having various viscosities (5 - 2700 cSt), that are immiscible with water, were chosen for the tests. The working fluids investigated have surface tensions ranging from 12 to 73 mN/m and comprise alcohols, alkanes, toluene and perfluorohexane. Initially, we tested the miscibility of the lubricants with the working fluids. Despite their immiscibility with water, many of the lubricants (like silicone oils) were miscible with most of the working fluids, making them inappropriate for designing SLIPS and LIS. Furthermore, for immiscible lubricant-working fluid pairs, we calculated the possibility of cloaking of the working fluid droplets by the lubricants. Through experimental miscibility results of each lubricant-working fluid pair, coupled with theory and pendant drop experiments to determine the spreading coefficients and cloaking parameters, we were able to integrate the miscibility and cloaking results to determine suitable pairs of lubricantworking fluid. Using the results, we develop guidelines for the future design of SLIPS and LIS surfaces for enhanced condensation of low surface tension fluids.


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2. Experimental Methods Miscibility tests were performed for a wide variety of lubricants and low surface tension fluids. The lubricants were chosen based on their immiscibility with water, industrial utilization, and use in previous studies of SLIPS and LIS 11, 13-14, 18. Furthermore, to provide a range of interfacial parameters, fluorinated and mineral oils, hydrocarbons, and ionic liquids were selected. For our tests, we used three different viscosity fluorinated Krytox oils: Krytox-1506, 1525, and 165256 (Chemours). Hydrocarbon silicone oils with viscosities 5, 100, 1000 cSt (Clearco Products) and 500 cSt (Sigma Aldrich) were also tested. The mineral oil and ionic liquid tested were Carnation oil (Sonneborn LLC) and 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIm, Sigma Aldrich), respectively. Table 1 lists the physical properties of the tested lubricants. The low surface tension fluids (working fluids) were chosen such that they cover a wide range of surface tension (12 to 73 mN/m) and class of chemicals, namely alcohols, alkanes, ethylene glycol, toluene (Sigma Aldrich), and perfluorohexane (Fluorinert FC-72, 3M). For reference tests, water (1 cSt) was used as the working fluid. Table 1 lists the physical properties of the different working fluids. For miscibility tests, we added colored dyes to the working fluids where possible to distinguish them from the lubricants. Water-soluble dye Pylaklor Deep Red S-438 (Pylam Products) was added to water and ethylene glycol, resulting in red colored solutions. Similarly, alcohol soluble Brilliant Green dye (Pylam Products) was added to ethanol and isopropanol, resulting in green colored solutions. Prior to mixing the two fluids (working fluid and lubricant), the dyes were added to the lubricants to ensure their insolubility in the lubricants (Figure 1a). No suitable dye was available that was soluble in alkanes, toluene, and perfluoro-hexane while being insoluble in the lubricants. As a means of initial assessment, the significant difference in the viscosities of these working fluids and the lubricants were sufficient to visualize the interface between the two and hence determine the miscibility between the two sets of fluids. Initially, 1 mL of the working fluid (dyed for water, ethylene glycol, ethanol, and isopropanol) and the lubricant oil were added to a 3 mL glass vial. For proper mixing of the two fluids, the vial was vigorously shaken for two minutes, after which the resulting mixture solution was placed on an optical table and kept undisturbed for 24 h. Snapshots were taken at regular 120 second time intervals, allowing time dependent visualization of the separation of the two fluids. The observations were categorized into three classes. In the first case, the two liquids 5 ACS Paragon Plus Environment


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instantly separated. Although potential for partial miscibility exists even in separated fluid pairs, for the purposes of our study, as means of initial assessment, condensate-lubricant pairs which separated almost instantaneously were deemed immiscible (Figure 1b). In the second case, though initially difficult to distinguish between the two liquids in the solution, separation occurred after 1 to 3 hours (Figure 1c). These were deemed to be delayed separated solutions, suggesting some degree of partial miscibility. The third category comprised cases where the two liquids could not be distinguished after 24 hours of mixing, indicating that these pairs are miscible (Figure 1d). Along with miscibility, the other important parameter to consider is the possibility of encapsulation (or cloaking) of the working fluid droplets by the impregnated lubricants on the surfaces. The propensity for cloaking was determined from the spreading coefficient of oil on the condensate, and required determination of the liquid-vapor surface tensions of the working fluid and the lubricant, as well as the interfacial tension between the lubricant and fluid. The liquidvapor surface tensions of the working fluids, and the lubricant oils, were readily available in literature

11, 16, 36-37

. To determine the interfacial tension between the two fluids, was

measured using the pendant drop method (KSV Instruments, CAM 200). Denser liquid droplets were formed at the tip of a 20 µL (inner orifice diameter of 0.5 mm) pipette inside the liquid bath of the lighter fluid contained in a 5 mL rectangular transparent glass cuvette (Figure 2). The pendant drop images were then analyzed using ImageJ software38 to calculate the interfacial tension between the working fluids and the lubricants.

3. Results and Discussion 3.1

Miscibility

Table 2 summarizes the results of the miscibility test for all lubricants and working fluids outlined in Table 1. As expected, all tested lubricants were indeed found to be immiscible with water. The Krytox fluorinated oils are immiscible with alcohols, alkanes, ethylene glycol, and toluene. The molecular structure of perfluorohexane, being similar to that of the Krytox oils, both containing fluorinated carbon bonds, makes them miscible. The other lubricants did not behave in a similar way to the Krytox oils. Carnation oil, a mineral oil, was found to be immiscible with ethylene glycol, alcohols, and perfluorohexane and miscible with alkanes and toluene. Of the lubricants tested, the fluorinated Krytox oils, Silicone oils and the Carnation 6 ACS Paragon Plus Environment

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mineral oil are non-polar in nature, while the ionic liquid BMIm is polar in nature. In line with previous studies

11, 39

, we found BMIm to be immiscible with water. However, BMIm was

miscible with the alcohols (ethanol, isopropanol), ethylene glycol, and toluene. The highly nonpolar liquids pentane, hexane, and perfluorohexane were found to be immiscible with the ionic liquid BMIm. The worst among all the lubricants tested in terms of miscibility with the low surface tension liquids were the Silicone oils. Regardless of the viscosity of the silicone oil, they were found to be miscible with all of the test working fluids, except perfluorohexane and water, despite being one of the most widely used lubricants for fabricating LIS and SLIPS

13, 17-19

. The

miscibility test results indicate that no single lubricant is suitable for all of the working fluids studied here. Experimental observations suggest that Krytox should be the preferred lubricant for most of the working fluids studied here, with the exception of perfluorohexane. Carnation mineral oil might be suitable for the polar organic compounds, like ethylene glycol, ethanol and perfluorohexane. Similarly, ionic liquid BMIm may be suitable for alkanes and silicone oils for perfluorohexane based applications.

3.2 Spreading Coefficient and Cloaking A secondary problem that might arise among the immiscible fluids is cloaking. The lubricant can spread and encapsulate a working fluid drop, forming a cloak surrounding the lubricant droplet. Shedding of cloaked drops from the condensing surface results in depletion of the lubricant. Furthermore, during condensation, cloaked droplets inhibit droplet growth and nucleation site density

11

. The possibility of encapsulation (or cloaking) of the working fluid droplets by the

impregnated lubricants on LIS or SLIPS is determined by calculating the spreading coefficient of the oil on the droplets, given by 40: = − − ,

(1)

where , , and are the liquid-vapor surface tensions of condensate and lubricant, and interfacial tension between the lubricant and condensate, respectively. The spreading coefficient > 0 implies the lubricants on the lubricant impregnated surface will cloak the condensate.

Hence for effective selection of lubricant in the design of LIS and SLIPS, < 0 is desired 11, 13.

To determine the interfacial tensions between the working fluids and the lubricants ( ), we used the statistical thermodynamic model of van Oss, Chaudhury and Good (vOCG) 7 ACS Paragon Plus Environment

36, 41

. The


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vOCG method expresses the surface tension in terms of two principle components, Lifshitz-van der Waals interactions (LW) and Lewis acid-base components (AB): = + .

(2)

The Lewis acid-base component is given as 36, 41: = 2( ). ,

(3)

where is the Lewis acid parameter and the Lewis base parameter. For non-polar

molecules, = = 0, since these interact only through Lifshitz-van der Waals interactions.

Among the lubricants studied here, Krytox (fluorinated) oils, silicone oils and carnation mineral are non-polar. The working fluids studied here are widely used for various industrial and research applications, hence their Lifshitz-van der Waals and Lewis acid-base components of the surface tension were readily found in literature

36, 42

. After obtaining the interfacial tension

parameters, the vOCG method allows for the calculations of the interfacial tension between the two liquids as: = + − 2

.

.

(4)

Note, in equation (4), the Lewis acid-base pair interaction between the two liquids was not considered since for all cases considered here, one of the liquids (lubricant) was non-polar. The surface tension components for the ionic liquid BMIm were not available, therefore the interfacial tension between BMIm and the working fluids could not be calculated using the vOCG method. Table 3 lists the spreading coefficients ( ) calculated using the interfacial tension values from the vOCG method. It should be noted that for our calculations, we considered the possibility of cloaking only for immiscible working fluids and lubricant oils. Hence, only the Krytox fluorinated oils, the Carnation mineral oil (with ethylene glycol, ethanol, and perfluorohexane), and Silicone oils (with perfluorohexane alone) were considered as lubricants which could potentially cloak the low surface tension working fluid droplets. To verify the theoretical calculations using the vOCG method, we utilized the pendant drop method to experimentally determine the interfacial tension between two fluids

11, 23, 43-44

. The

pendant drop method consists in forming a droplet of the heavier liquid at the tip of a pipette submerged inside the lighter liquid. The interfacial tension between the two liquids determines


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the shape of the droplet. Measuring the shape of the pendant droplet allows the interfacial tension to be calculated as 43, 45: =

(∆) ,

(5)

where is the density difference between the two liquids, ! is the radius of curvature at the tip of the drop and is the shape factor given by 45:

() () () . = 0.12836 − 0.7577 + 1.7713 + , − 0.5426 + , , (* (* (*

(6)

where (* is the largest diameter of the pendant droplet and () is the diameter at distance (* from the drop apex (Figure 2). Table 3 lists the spreading coefficient calculated using Eq. (1) and the measured interfacial tension values. Note that the pendant drop of the heavier liquid could be formed inside the bulk of the lighter liquid only when the two liquids were immiscible. Hence, measurements could only be done for the combinations shown as immiscible in Table 2. Furthermore, the pendant drop method is useful for measuring large interfacial tension values, in other words between two fluids having large difference in their surface tensions. However, when the surface tensions of the two fluids are very similar, their interfacial tension is very small. This results in the fluid dripping continually from the pipette rather than forming droplets, as was found to be the case for all combinations involving perfluorohexane. The spreading coefficients of the lubricants on the working fluids determined using the vOCG (theory) and pendant drop (experiment) methods were found to be in excellent agreement, as shown in Tables 3 and 4. Even though the values obtained from the two methods are not identical, their signs are same (for maximum cases), which is the main requirement for determining the cloaking behavior of the lubricant. In case of conflict, we chose the spreading coefficient calculated from the measured value of interfacial tension using the pendant drop method. It is apparent from Eq. (1) that the higher the surface tension of the working fluids, , the higher the probability of > 0, and chances of the lubricant to spread on or cloak the

droplet. As seen in Tables 3 and 4, the spreading coefficients of both the Krytox oils and silicone oils on water are positive ( > 0). Despite their wide usage for fabricating LIS and SLIPS, it has been well established that these oils can cloak water droplets 11, 13-14. While cloaking of water is a contra-indicator for using Krytox and silicone oils for LIS and SLIPS, it perhaps does not 9 ACS Paragon Plus Environment


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rule them out completely – whereas miscibility does. An analogous problem potentially exists with using LIS and SLIPS for ethylene glycol or toluene based applications. Though the surface tensions of ethylene glycol (48 mN/m) and toluene (28 mN/m) are lower than that of water, they are higher than the alcohols and alkanes, resulting in > 0 for all the lubricants tested (Krytox, silicone, and Carnation oil). The immiscible pairs of lubricant and alcohol/alkane working fluids yielded more favorable results. All viscosities of fluorinated Krytox oils had < 0 for ethanol, isopropanol, pentane, and hexane, making them the strongest candidates to be used as lubricants for LIS and SLIPS. High viscosity silicone oils (100 – 1000 cSt) and Carnation mineral oil are also candidates for use as lubricants for ethanol based applications, however their miscibility with isopropanol, pentane, and hexane restricts wider utilization. The ionic liquid BMIm also showed < 0 with pentane and hexane, making it a suitable choice as lubricant for alkane working fluids. Perfluorohexane, despite its immiscibility with silicone oils and Carnation mineral oil, showed > 0 for all lubricants. 3.3

Choice of lubricant for designing stable LIS and SLIPS

In order to ensure that a lubricant-working fluid combination is appropriate for an LIS or SLIPS application, the miscibility criterion must be met, but it is preferable that the cloaking criterion should also for applications requiring long term stability and use. Table 5 lists the combined results of miscibility (Table 2) and cloaking (Tables 3 and 4) for the different lubricants tested with the low surface tension working fluids, as outlined in Figure 3. Even for condensation of water on LIS and SLIPS, the ionic liquid BMIm and Carnation oil appear to be the only two suitable lubricants for designing stable surfaces. Despite testing of a wide variety of lubricants, none appear suitable for LIS/SLIPS surface design for ethylene glycol, toluene, and perfluorohexane applications. Among the working fluids tested, the results show that all the different lubricants (except low viscosity Silicone oil SO-5 and ionic liquid BMIm) could be used for designing stable LIS/SLIPS only for ethanol based applications. For isopropyl alcohol though, Krytox-based LIS/SLIPS would be best suited, with the latter being immiscible and having < 0. In fact most lubricants are miscible with the working fluids tested, making Krytox the only good candidate for design of stable LIS/SLIPS. The latter, along with ionic liquid BMIm, are also good candidates for applications of pentane and hexane.


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3.4

Discussion

The rising demand of energy, emission of greenhouse gases and consumption of water have motivated the need to maximize thermoelectric power plant efficiency. Though the steam cycle, on which most power plants depend, comprises many components, its overall efficiency is limited by the condenser. Typically, steam condenses on metals as a film that creates a thermal barrier to heat transfer. The past few years have seen significant work done to promote dropwise condensation on superhydrophobic surfaces instead of filmwise, leading to a tenfold increase in heat transfer

46-47

. However, during prolonged condensation, droplets nucleating within textures

of the solid can make such surfaces lose their non-wetting properties. By contrast, lubricant infused surfaces not only promote dropwise condensation

11, 15

, but also, due to the lubricant

surface layer, prevent nucleation within the texture. Additionally, the critical size of droplets to shed from superhydrophobic surface is on the order of few millimeters, whereas droplets as small as 20 µm are mobile on LIS/SLIPS 11, 14. Indeed, condensation heat transfer of LIS/SLIPS has been shown to be twice that of conventional hydrophobic and superhydrophobic surfaces under industrial condenser conditions 22. The enhanced droplet shedding and high condensation heat transfer with water indicate the potential of LIS/SLIPS for condensation of low surface tension fluids. The ease of droplet removal from LIS/SLIPS allows such surfaces to find use in several other applications. In particular, LIS/SLIPS have been shown to reduced ice adhesion, which is a primary concern for the transportation, agriculture, energy, and construction sectors. Though superhydrophobic surfaces have been explored extensively for anti-icing, recent studies

48

have

shown that frost formed on these surfaces leads to strong ice adhesion, in contrast to LIS/SLIPS, which have been shown to reduce ice adhesion and accumulation on the surface

14, 23

.

Furthermore, the lubricant layer on LIS/SLIPS makes them an ideal choice for self-cleaning. Traditionally, superhydrophilic surfaces relying on filmwise flow, or superhydrophobic surfaces with low contact angle hysteresis

49

, have been used for self-cleaning applications.

Unfortunately, superhydrophilic and superhydrophobic surfaces find limited use due to their inability to handle a wide range of fluids 14. Lubricant infused surfaces have been shown to have an extremely low contact angle hysteresis (~ 1°) and ability to repel a wide variety of liquids 10, 12, 25

, making them ideal candidates for self-cleaning applications. In addition, recent studies have

shown reduced bacterial accumulation and overall adhesion of fouling films on LIS/SLIPS 10, 26. 11 ACS Paragon Plus Environment


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Finally, fouling and corrosion are of major concern in industries associated with heat exchangers, oil and gas pipelines and power generation systems. In this respect, lubricant infused surfaces can result in ten times lower scale formation rates as compared to untreated surfaces 50-51. The aforementioned applications indicate the strong potential of LIS/SLIPS as novel materials or surfaces. It should be noted, however, that for applications covering a wide range of fluids, the liquid/liquid interface between the lubricant and the working fluids is of utmost importance and can limit the performance of LIS/SLIPS. Although the lubricant-working fluid design criteria developed here should ensure long lasting LIS/SLIPS, additional failure modes exist which need to be investigated in the future. Though the existence of an interface between the lubricant-working fluid pair indicates immiscibility, partial miscibility may be present, a phenomena very common with organic fluids 52-53. Even small levels of partial miscibility could lead to depletion of the lubricant from LIS/SLIPS. The miscibility tests done in this study are only a means of initial assessment. For promising combinations of lubricant-condensate fluids, future studies should utilize more quantifiable analytical techniques such as nuclear magnetic resonance

spectroscopy

(NMR),

gas

chromatography

mass

spectrometry

(GC-MS),

thermogravimetric analysis (TGA), or Fourier transform infrared spectroscopy (FTIR) to determine the definitive extent of miscibility. Additionally, the drainage of lubricants from LIS/SLIPS via cloaking may not be the only mechanism of lubricant loss. Shear induced drainage during condensate shedding may deplete the lubricant even for immiscible and non-cloaking condensate-lubricant pairs. Indeed, a recent study of water condensation on Krytox-based LIS showed that surfaces impregnated with lowviscosity lubricants (12 cSt) degraded quickly via lubricant drainage, whereas those surfaces with higher viscosity (140 cSt) remained stable for more than 10 hours of operation

16

.

Furthermore, the use of an immiscible and non-cloaking lubricant (Carnation oil) with water also showed lubricant drainage, indicating the presence of shear based lubricant loss

16

. Lubricant

drainage into a growing network of nanoicicles also degraded the anti-icing performance of the lubricant nanotextured surfaces during frosting-defrosting cycles

54

. Recent studies have

demonstrated LIS/SLIPS with fluorinated lubricants to have the best anti-icing capability in terms of lubricant retention for repetitive icing/deicing cycles

55

. Moreover, lubricated

nanostructured fabrics, similar to LIS/SLIPS have shown depletion of oils from them after physical contact with other objects 56. 12 ACS Paragon Plus Environment

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The widespread utilization of refrigerants as thermal systems working fluids indicates that additional studies are needed to prove the utility of LIS/SLIPS in such applications. The results presented here for perfluorohexane, coupled with previous observation of fluorinated compressor lubricant absorption into refrigerants, indicate that LIS/SLIPS may not be suitable coatings for refrigerants due to the likelihood of rapid lubricant depletion. Furthermore, the recent adoption of next-generation low-GWP refrigerants such as R-513A, R-1234ze, R-514A, and R-1233zd, in external condensation applications necessitates further testing prior to practical adoption of LIS/SLIPS. The limited availability of suitable lubricants, potential shear induced drainage during condensation, as well as poor potential for ultra-low surface tension refrigerants, points to a need to be able to attach the lubricant to the substrate. In the future, it would be interesting to test the stability of slippery omniphobic covalently attached liquids (SOCAL) for enhanced low surface tension fluid condensation applications

57

. Unlike LIS/SLIPS, SOCAL surfaces are fabricated

through acid-catalyzed graft polycondensation of dimethyldimethoxysilane, and result in extremely smooth, chemically homogeneous coating with ultra-low contact angle hysteresis.

4. Conclusions In this study, we tested the viability of various lubricants for designing stable LIS/SLIPS for low surface tension working fluids. We experimentally tested the miscibility and cloaking behavior of a number of working fluids having a wide range of surface tensions (12 – 73 mN/m) with different viscosities (5 - 2700 cSt) and chemical nature of lubricants. Most of the lubricants were found to be either miscible or capable of cloaking the droplets of low surface tension fluids. The results indicate that few lubricant options exist for designing stable LIS/SLIPS for low surface tension working fluid applications. This work not only provides a framework for the selection of candidate lubricants for stable LIS and SLIPS coatings, it outlines future directions for the development of coating-enabled enhanced condensation of low surface tension fluids.



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Acknowledgements The authors gratefully acknowledge the help of Dr. Daniel J. Preston for suggesting the use of the vOCG method and the pendant drop technique. This work was supported by the BP plc through the International Centre for Adv. Mater. (ICAM), and the Office of Naval Research (ONR) with Dr. Mark Spector as the program manager (Grant No. N00014-16-1-2625). N. Miljkovic gratefully acknowledges the support of the International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science and Technology.


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swollen Polymeric Films for Omniphobic Personal Protective Gear. ACS Appl. Mater. Interfaces 2015, 7 (7), 4224-4232. (57) Wang, L.; McCarthy, T. J. Covalently Attached Liquids: Instant Omniphobic Surfaces with Unprecedented Repellency. Angew. Chem. 2016, 128 (1), 252-256.


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Table 1. Physical properties of the tested lubricants and low surface tension fluids at 20°C. Liquid Density, ρ [kg/m3] Lubricant Krytox – 1506 1880 Krytox – 1525 1900 Krytox – 16256 1920 Carnation Oil 810 Silicone Oil SO – 5 918 Silicone Oil SO – 100 970 Silicone Oil SO – 500 971 Silicone Oil SO - 1000 971 Ionic liquid BMIm 1430 Working Fluid Water 1000 Ethylene glycol Ethanol Isopropanol Pentane Hexane Toluene Perfluorohexane

1113 789 786 626 655 867 1680

Liquid-Vapor Surface Tension, γ [mN/m] 17 19 19 28 19 21 21 21 34

Vapor Pressure, 0123 [kPa] 5 x 10-8 1.3 x 10-8 4 x 10-15 10-2 0.7 0.7 0.7 0.7 Not Available

Dynamic Viscosity, μ [mPa·s] 113 496 5216 9.7 4.6 97 486 971 64

72.7

2.33 7.5 x 10-3

0.89 16.2

5.83 4.4 57.9 16.2 2.93 25

1.095 2.38 0.24 0.297 0.55 0.64

48.4 24.8 21.4 15.1 18 27.9 12



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Page 20 of 26

Table 2. Miscibility test results. Green corresponds to immiscible (I), yellow corresponds to delayed separation (DS) and red corresponds to completely miscible (M) liquids. Isopropanol is labeled as IPA. Water

Ethylene Glycol

Ethanol

IPA

Pentane

Hexane

Krytox 1506

I

DS

I

I

I

I

I

M

Krytox 1525

I

DS

I

I

I

I

I

M

Krytox 16256

I

I

I

I

I

I

I

M

Carnation

I

I

I

M

M

M

M

I

DS

DS

M

M

M

M

M

I

SO – 100

I

DS

DS

M

M

M

M

I

SO – 500

I

DS

DS

M

M

M

M

I

SO – 1000

I

DS

DS

M

M

M

M

I

BMIm

I

M

M

M

I

I

M

I

SO – 5


Toluene Perfluorohexane

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Table 3. Spreading coefficient, of lubricants on the working fluids calculated using vOCG method, where n/a corresponds to miscible liquids (Table 2). Note, due to unavailability of the polar surface tension components, calculations could not be performed for lubricant BMIm with the working fluids. Red shading corresponds to cloaking ( > 0) and green shading corresponds to non-cloaking ( < 0). Isopropanol is labeled as IPA. Water

Ethylene Glycol

Ethanol

IPA

Pentane

Hexane

Toluene

Perfluorohexane

Krytox 1506

4.502

10.407

1.180

1.76

-5.079

-0.964

1.372

n/a

Krytox 1525

2.704

8.947

-0.809

-0.2

-7.426

-3.074

-0.605

n/a

Krytox 16256

2.704

8.947

-0.809

-0.2

-7.426

-3.074

-0.605

n/a

Carnation

-6.587

0.991

-10.851

n/a

n/a

n/a

n/a

0.498

SO - 5

2.047

8.404

n/a

n/a

n/a

n/a

n/a

7.990

SO - 100

0.891

7.438

-2.793

n/a

n/a

n/a

n/a

7.012

SO - 500

0.694

7.273

-3.007

n/a

n/a

n/a

n/a

6.845

SO - 1000

0.596

7.190

-3.114

n/a

n/a

n/a

n/a

6.761

-

-

-

-

-

-

-

-

BMIm



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Page 22 of 26

Table 4. Spreading coefficient, of the lubricants on the working fluids calculated from measured interfacial tension between the two fluids using the pendant drop method, where n/a corresponds to miscible liquids (Table 2). Red shading corresponds to cloaking ( > 0) and green shading corresponds to non-cloaking ( < 0). Due to the low surface tension of perfluorohexane and vanishing interfacial tension with the lubricants, measurements were not possible using the pendant drop method. Isopropanol is labeled as IPA. Water

Ethylene Glycol

Ethanol

IPA

Pentane

Hexane

Toluene

Perfluoro -hexane

Krytox 1506

3.643

6.439

-3.702

2.85

-4.282

-1.216

5.422

n/a

Krytox 1525

3.185

6.310

-4.106

-1.72

-5.478

-2.547

1.046

n/a

Krytox 16256

3.271

7.279

-4.832

-2.16

-5.830

-2.450

1.634

n/a

Carnation

-5.664

3.125

-8.324

n/a

n/a

n/a

n/a

-

SO – 5

3.114

7.429

n/a

n/a

n/a

n/a

n/a

-

SO – 100

2.947

7.816

-1.646

n/a

n/a

n/a

n/a

-

SO – 500

3.016

8.452

-2.014

n/a

n/a

n/a

n/a

-

SO – 1000

2.518

8.063

-1.983

n/a

n/a

n/a

n/a

-

BMIm

-4.760

n/a

n/a

n/a

-24.127

-20.863

n/a

-


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Table 5. Combined miscibility and cloaking results. Green cells labeled PASS correspond to immiscible and non-cloaking lubricant-fluid pairs, and red cells labeled FAIL corresponds to either miscible, or cloaking, or both lubricant-fluid pairs. Isopropanol is labeled as IPA. Water

Ethylene Glycol

Ethanol

IPA

Pentane

Hexane

Krytox 1506

FAIL

FAIL

PASS

FAIL

PASS

PASS

FAIL

FAIL

Krytox 1525

FAIL

FAIL

PASS

PASS

PASS

PASS

FAIL

FAIL

Krytox 16256

FAIL

FAIL

PASS

PASS

PASS

PASS

FAIL

FAIL

Carnation

PASS

FAIL

PASS

FAIL

FAIL

FAIL

FAIL

FAIL

SO - 5

FAIL

FAIL

FAIL

FAIL

FAIL

FAIL

FAIL

FAIL

SO - 100

FAIL

FAIL

PASS

FAIL

FAIL

FAIL

FAIL

FAIL

SO - 500

FAIL

FAIL

PASS

FAIL

FAIL

FAIL

FAIL

FAIL

SO - 1000

FAIL

FAIL

PASS

FAIL

FAIL

FAIL

FAIL

FAIL

BMIm

PASS

FAIL

FAIL

FAIL

PASS

PASS

FAIL

-


Toluene Perfluoro -hexane


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Page 24 of 26

Figure 1. Optical images of the miscibility tests of lubricants with different low surface tension fluids. (a) Insolubility of the dyes used with lubricants – Pylaklor Deep Red in Krytox 1525, Brilliant Green in Krytox 1525, Pylaklor Deep Red in Silicone oil – 500, and Brilliant Green in Silicone oil – 500. Immiscibility of (b) water, (c) ethanol, and (d) hexane with lubricant Krytox 1525. The difference in viscosity enabled the visualization of the lubricant and working fluid separately for cases with no dye. Lubricant Krytox 1525 with (e) ethylene glycol (delayed separation), and (f) completely miscible hexane. In figures (b) – (f), the four different snapshots correspond to the two liquids before mixing, just after mixing, 5 min after mixing and 1 h after mixing, respectively.


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Figure 2. Pendant drop images during interfacial tension measurement between (a) water and Krytox 1506, (b) pentane and ionic liquid BMIm, and (c) ethylene glycol and Carnation oil. The measured diameters for calculating the shape factor, (Eq. 6) are shown in (a).

Figure 3. Conditions for effective selection of lubricant in designing stable LIS/SLIPS.



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TOC 214x103mm (150 x 150 DPI)


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Lubricant-Infused Surfaces for Low-Surface-Tension Fluids: Promise

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