Mechanism of Frost Formation on Lubricant-Impregnated Surfaces

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Mechanism of Frost Formation on Lubricant-Impregnated Surfaces Konrad Rykaczewski, Sushant Anand, Srinivas Bengaluru Subramanyam, and Kripa K Varanasi Langmuir, Just Accepted Manuscript • DOI: 10.1021/la400801s • Publication Date (Web): 08 Apr 2013 Downloaded from http://pubs.acs.org on April 21, 2013

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Mechanism of Frost Formation on LubricantImpregnated Surfaces Konrad Rykaczewski,1* Sushant Anand,1 Srinivas Bengaluru Subramanyam,2 and Kripa K. Varanasi1* 1. Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139 2. Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 *[email protected] and [email protected]

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

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ABSTRACT

Frost formation is a major problem affecting variety of industries including transportation, power generation, construction, and agriculture. Currently used active chemical, thermal, and mechanical techniques of ice removal are time consuming and costly in operation. Use of nanotextured coatings infused with perfluorinated oil has been recently proposed as a simple passive anti-frosting and anti-icing method. However, we demonstrate that the process of freezing of subcooled condensate and frost formation on such Lubricant-Impregnated Surfaces is accompanied by migration of the lubricant from the wetting ridge and from within the textured substrate to the surface of frozen droplets. For practical applications this mechanism can compromise self-healing and frost repelling characteristics of Lubricant Impregnated-Surfaces, irrelevant of the underlying substrate’s topography. Thus, further research is necessary to develop liquid-texture pairs which will provide a sustainable frost suppression method.

KEYWORDS Lubricant Impregnated Surfaces, ESEM, frost, cryo-FIB/SEM


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INTRODUCTION Frost has long been a problem for human society. Norse mythology describes frost giants, also known as hrimthurs, as primitive beings who oppose the rule of Gods and create havoc and hassle. In modern society the effects of frosting can be equally dramatic: downed power lines, damaged crops, and stalled aircrafts.1-4 Moreover, frost and ice accumulation significantly decreases the performance of ships, wind turbines,5 and HVAC systems.1,

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active chemical, thermal, and mechanical methods of ice removal are time consuming and costly in operation. Therefore development of passive methods preventing frost and ice accretion is highly desirable. Hydrophobic surfaces have a high energy barrier for ice nucleation1,

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low ice adhesion strength,3-4, 20-25 and, if properly roughened on the nano- and/or micro-scales, can repel impact of supercooled water droplets.22-23, 26-38 However, the anti-icing properties of hydrophobic as well as superhydrophobic surfaces are negated once the surfaces are frosted.1, 33, 39-40

Frost formation and ice adhesion can also be reduced by addition of a liquid or grease onto

the working surface. For example, ice adhesion to aircraft surfaces is significantly reduced through application of silicone grease2-4 and frost formation can be prevented on exterior of freezers and heat exchangers coated with a 100 µm porous layer infused with propylene glycol antifreeze.6, 41 However, in both of these cases the non-solid phases are sacrificial3-4, 6 and can leak into the surroundings causing significant environmental problems.21 Recently developed textured solids impregnated with liquids are to circumvent these durability issues through use of surface chemistry and topography to lock-in place an intermediate liquid,4246

which itself is immiscible with water. These surfaces have been recently demonstrated to have

omniphobic,44 self-cleaning,43 and self-healing44 properties. Additionally, it was recently reported that nanotextured polymer coating infused with perfluorinated oil have superior anti-


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frosting and anti-icing properties as compared to superhydrophobic surfaces.47 However, the hybrid solid-liquid surfaces previously used in literature47 had a thick excess oil film, which can drain due to gravity and other perturbations such as air and sample motion.45-46, 48 In our recent studies on Lubricant-Impregnated Surfaces (LIS) we demonstrated that such temporary excess lubricant layer can mask the underlying complexities of physicochemical principles of water drop mobility.45-46 Furthermore, we also demonstrated that lubricants such as perfluorinated oils have a positive spreading coefficient on water that significantly alters microscale condensation dynamics by ‘cloaking’ drops.45 The presence of such ‘cloaking’ mechanism may play a crucial role in suppression of frost growth on LIS. Thus, in contrast to the mechanism previously described in literature,47 we expect frost growth on LIS to be strongly affected by the oil-water and oil-ice interactions as well as the underlying solid texture. In this work we perform a multiscale investigation of condensation frosting on superhydrophobic surfaces (SHS) and on LIS prepared using systematic dip-coating procedure, which avoids formation of the temporary excess oil film.49 Using light microscopy, we quantify the macroscale anti-frosting performance of variety of nanostructured and microstructured SHS and LIS. Using static50 and dynamic51-52 electron microscopy, we reveal the microscale mechanism of frost growth on LIS and its implication on self-healing and frost repelling characteristics of these surfaces.

EXPERIMENTAL METHODS Nanostructured and Microstructured Sample Fabrication Nanostructured SHS based on vapor phase deposited alumina nanoparticles and silicon nanowires as well as microstructured SHS based on silicon micro-posts were used in this study. The SHS consisting of alumina nanoparticles (NP-SHS) were fabricated on top thin flexible


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copper foil using RPX-540 manufactured by Integrated Surface Technologies.53 Specifically, the copper surface was conformally coated by nanoparticles synthesized in vapor phase reaction of trimethylaluminium and water, which were encapsulated in a silicon oxide matrix via Atomic Layer Deposition. Subsequently, the surfaces were modified with a vapor phase deposition of tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane. Silicon wafers modified using this coating have a water contact angle of contact angle of 111°± 4°.53 In turn, the silicon nanowires (NWSHS) with diameters between 50 nm and 200 nm and height of about 2.5 µm were grown on Si(111) substrate using Chemical Vapor Deposition at 850 °C with SiCl4/H2/N2 gaseous mixture. The vapor-liquid-solid (VLS) nanowire growth was catalyzed by gold nanoparticles formed by annealing a 5 nm thick gold film. The NW surface was modified using vapor deposition of 1H,1H,2H,2H-Perfluorodecyltrichlorosilane (Alfa Aesar), which exhibits a water contact angle of 104°± 4° on a flat silicon substrate.52 Further details of the NW-SHS and NP-SHS fabrication and detailed characterization of the morphology and of the wetting properties of these surfaces during water condensation are found elsewhere.50, 54-55 The 10 µm tall square silicon micro-posts with 10 µm width and 5 µm, 10 µm, and 25 µm inter-pillar spacing were patterned using photolithographic and etched using Deep Reactive Ion Etching (DRIE). The textured substrates were cleaned using Piranha solution and were coated with octadecyltrichlorosilane (OTS from Sigma Aldrich) using solution deposition method. Flat silicon wafers modified with this coating exhibits an advancing water contact angle of 110°± 4°. The three samples with varied inter-pillar spacing were chosen such that the critical impregnation angle on each of these was greater than the contact angle of Krytox-1506 on OTS coated silicon surface (about 28°).45 Further details of the microfabrication and the coating processes as well as characterization of the wetting properties are found elsewhere.45-46 The SHS


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consisting of micro-posts are referred to as X µm-SHS, where X corresponds to inter-post spacing. All textured hydrophobic surfaces had macroscopic water contact angles above 150° and thus are referred to as superhydrophobic. However, we note that only the nanostructured SHS retained their wetting properties during water condensation.55 SEM images of all the SHS samples are shown in Figure 1.

Figure 1. SEM images of all used structured samples.

To prepare the LIS, all samples were cleaned with isopropanol, dried with nitrogen gas, and were withdrawn vertically from a perﬂuorinated oil (Krytox-1506, DuPont) bath at a constant speed using a dip-coater. To prevent formation of the excess oil layer, the dip-coater speed was adjusted to keep the capillary number below 10-5.49 Further details of the dip-coating procedures are described by Anand et al.45 The LIS consisting of Krytox infused NP-SHS, NW-SHS, and micro-post SHS are referred to as NP-LIS, NW-LIS, and X-µm-LIS, respectively (where X corresponds to inter-post spacing).


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Macroscale Condensation Frosting Experiments To evaluate the macroscopic anti-frost characteristics of the different surfaces, about 1 by 1 cm samples of NP-SHS, NW-SHS, 10 µm-SHS, NP-LIS, NW-LIS, 5 µm-LIS, 10 µm-LIS, and 25 µm-LIS were attached to a 4 cm by 4 cm Analog Technologies Thermoelectric Cooler (TEC) module mounted to a 2.5 cm x 5 cm x 5 cm aluminum heat sink placed in an ice bath. The temperature was monitored using a thermocouple firmly mounted to the TEC surface. The samples were cooled from about 20 to -10°C in room environment of 22°C and relative humidity of 25 to 30% and the condensation frosting process was imaged using a light microscope. Corresponding images were recorded every 10 s. The frosting experiments were repeated six times with varied locations of the samples on the TEC. The time required for complete frost coverage of each of the sample was measured. The reported values are averages of times from six experiments and the reported uncertainty is calculated with a coverage factor of 1.

ESEM imaging procedure The microscale dynamics of frost formation in high humidity conditions on the both SHS and LIS were imaged using FEI Quanta 200 FEG ESEM with custom built cooling stage. The samples were attached directly to the surface of a 1.2 cm by 1.2 cm Analog Technologies Thermoelectric Cooler (TEC) module using double sided copper tape. The TEC was mounted at 45° with respect to the electron beam on a water cooled copper block. The block was cooled with flow of water chilled to about 4°C. To reduce interfacial thermal resistances we applied vacuum compatible thermal grease (Apiezon-N from SPI) on all interfaces.

The surface

temperature of the TEC was measured using LM35 Precision Centigrade Temperature Sensor in


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TO92 plastic package from Texas Instruments. The plastic encapsulation of the LM35 was sanded down so that the element was of similar thickness as the sample. The TEC, the samples, and the LM35 were mechanically clamped to the cooling block (see Figure S1). After sample loading and two standard purging cycles, the pressure was allowed to settle for two to three minutes to value of 600 Pa. Frost formation was triggered by decreasing the sample temperature to -6°C at a constant chamber pressure. The drop condensation and freezing were imaged using Gaseous Secondary Electron Detector with electron beam energy and current of 10 to 15 keV and 0.16 nA, respectively. The dynamics of the condensation process were imaged with a dwell time of 1 to 3 µs per pixel and 512 pixel by 471 pixel or 1024 pixel by 943 pixel frame sizes. The corresponding images were saved every 0.2 or 1 s.

Cryo-FIB/SEM imaging procedure The cryo-FIB/SEM experiments were carried out using an FEI Nova Nanolab 600 Dual Beam equipped with a Quorum PP2000T cryo-transfer system. About 7 mm by 7 mm samples were attached to a 10 mm diameter copper stub using double sided copper tape and placed in a brass cryo-stage holder. The sample-holder assembly was cooled from room temperature to about 10°C using a 1.2 cm by 1.2 cm Analog Technologies Thermoelectric Cooler module mounted to a 2.5 cm x 5 cm x 5 cm aluminium heat sink placed in an ice bath. The relative humidity varied from 30 to 40%. The temperature of the copper stub was monitored using a thermocouple temporarily mounted within a ~1 mm diameter hole drilled into the stub. The frosting process was imaged and recorded using a light microscope (see Figure S2). Freezing of the drops was recognized through abrupt stop of their movement and localized change in sample reflectivity. Once majority of the sample was frosted over, it was submerged into liquid nitrogen slush, which


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was made by partially evacuating a chamber containing liquid nitrogen. The rest of the sample preparation and imaging procedure was identical to that described by Rykaczewski et al.50 We note that we used the cryo-FIB/SEM method to image only initial stages of frost growth. We found that even deposition of the grounding metal layer did not prevent detrimental electron beam induced charging of frost layers thicker than about 150 µm.

RESULTS AND DISCUSSION We study frost formation on nanostructured and microstructured superhydrophobic surfaces with and without the perfluorinated oil layer. The textured surfaces were modified using a low surface energy silane in order to render them superhydrophobic, while being preferentially wetted by oil. The textured samples were impregnated with oil by removing them at a controlled rate from a bath of the oil.45-46, 49 The cross sectional cryo-FIB/SEM images in Figure 2 show that by properly adjusting the dip-coating speed we avoided formation of the temporary excess oil film (see Figure S3 for cross sections of all the LIS samples).49


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Figure 2. Cross sectional images obtained through cryo-FIB/SEM technique contrasting morphology of (a) LIS with about 8 µm thick excess oil film and (b) LIS systematically prepared using dip-coating procedure which avoids formation of the temporary excess oil layer.

Frost formation can occur via desublimation or freezing of subcooled liquid condensate.18, 39, 56 Similar to most recent studies on the subject,17-18, 39, 47 we focused on studying the dynamics of the second mechanism, often referred to as condensation frosting. Specifically, we imaged frost growth on SHS and LIS samples cooled using a Thermoelectric Element from room temperature (22 °C) to -10 °C in about 25 % relative humidity (dew point of 1 °C). To provide the most conservative assessment of the anti-frosting properties of the LIS, the samples were oriented horizontally as to prevent gravity aided condensate shedding. Rapid cooling resulted in formation of droplets on all samples, however, the time required for subsequent sample frosting varied dramatically (see Figure 3). Specifically, the histogram in Figure 3 b shows that the average time required for complete frost coverage of the 10 µm-SHS was only about 9 minutes, while it was about 12 and 22 minutes for the NP-SHS and the NW-SHS, correspondingly. In turn, it took about 10, 12, and 14 minutes for the microstructured 5 µm-LIS, 10 µm-LIS, and 25 µm-LIS and about 22 minutes for the nanostructured NP-LIS and NW-LIS to be fully covered with frost.


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Figure 3. (a) Example optical images of SHS and LIS samples from the beginning of cooling until full frost coverage and (b) Histogram of average times required full sample frosting.

To provide an insight into our macroscale results, we imaged the microscale dynamics of frost formation on SHS and LIS using optimized Environmental Scanning Electron Microscopy51-52, 57-


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During imaging the microscope chamber was maintained at a constant pressure of 600 Pa,

while the sample temperature was decreased from about 8°C to about -6°C using the custom built cooling stage. The sequence of ESEM images in Figure 4 a shows that the 10 µm-SHS does not retain its superhydrophobic characteristics during condensation. As a result, droplets condensed on this surface have a large solid-liquid contact area and freeze within 1 s to 2 s. In contrast, the sequence of ESEM images in Figure 4 b shows that the NP-SHS is superhydrophobic during condensation. Specifically, this surface promotes formation of nearly spherical microdroplets with minimal solid-liquid interface, which become highly mobile during coalescence.55 As a result, direct freezing of drops on this surface is rare and frost growth proceeds primarily from the edge of the substrate or from hydrophilic defects highlighted in Figure 4 c. Water drops with large solid-liquid area quickly condense and freeze within such defective areas and serve as initiation sites for further frost growth. The presence of a number of such defective sites is responsible for significantly faster frosting of the NP-SHS as compared to NW-SHS, which also retains its superhydrophobic characteristics during condensation.50, 55 The NP-SHS was deposited on a thin copper foil and consequentially is significantly more prone to physical damage during handling (for example localized coating peeling due to underlying copper foil bending) than the NW-SHS grown on a rigid silicon wafer (if grown on the copper foil, the NW forest would also be damaged). This dramatic degradation of anti-frosting characteristics due to physical damage to NP-SHS induced by the copper foil flexibility highlights the self-healing advantages of LIS. The nanostructured LIS delayed frost growth as effectively as the NW-SHS, irrelevant of the number of defects induced by underlying substrate’s properties. The significantly faster frosting of all the microstructured LIS likely occurred because of the large direct contact areas between the micro-posts tops and water drops.


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The effective thermal resistance of the interface increases with the volume fraction of oil, explaining the slight increase in the time required for complete frosting of the microstructured LIS with decreased micro-post density. On all the SHS, drop freezing is linked with formation of pointy peaks which initiate growth of dendritic frost structures.61 In contrast, the sequences of ESEM images in Figure 4 d & e show that after freezing on the 10 µm-LIS and the NP-LIS, the drops roughen on the exterior and gradually expand without visible growth of any major dendritic structures. These morphological changes are accompanied by gradual draining of the oil from the drop’s wetting ridge and from in-between the neighboring micro-posts. The oil was nearly completely drained from the wetting ridge of the drop freezing on the NP-LIS within about 3 s, while draining of the substantially larger volume of oil near the drop freezing on the 10 µm-LIS took about 18 s (see also Movie 1 and Movie 2 in Support Information).


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Figure 4. Sequences of ESEM images of frost formation via droplet condensation and freezing on (a) 10 µm-SHS, (b & c) NP-SHS, (d) 10 µm-LIS, and (e) NP-LIS.

To determine the fate of the drained oil, we explored the three dimensional morphology of


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condensed and frozen drops on the LIS using cryogenic Focus Ion Beam and SEM (cryoFIB/SEM) procedure recently developed by Rykaczewski et al.50 Cryo-FIB/SEM images in Figure 5 show the drastic difference in the shape of condensate drops before and after freezing. Individual phases were identified using EDS elemental analysis.50 Before freezing, the drops sit on top of oil impregnated textured substrates and are surrounded by oil wetting ridges near their perimeters. We note that for the imaged drops the wetting ridges have heights of several micrometers and contain a substantial volume of oil (see cross section in Figure 5 b, c, and g). In contrast, after draining of the oil frozen drops morph into multifaceted crystals (see Figure 5 c, d, f, h, and i). The individual frozen drop cross sections (see Figure 5 d, f, and h) and three dimensional cryo-FIB/SEM destructive tomography (see Figure 5 i and Movie 3) reveal that these crystals consists of a frozen water droplet core decorated by a complex dendritic network of nano-icicles entrenched in a non-uniform oil matrix. We note that the majority of the nanoicicle surfaces are buried within the oil matrix (i.e. are not in direct contact with the external environment). For the nanostructured NP-SHS and NW-SHS, the oil depletion from the wetting ridges observed using ESEM is clearly confirmed by contrasting cross sections of unfrozen and frozen drops (Figures 5 e vs. f and g vs. h). We observed that on all of the microstructured LIS the oil drained not only from vicinity of the drop but also from underneath it (see Figure S4 b & c). In numerous investigated cases, the drained oil from underneath the drop was replaced by water. The drop transition into a Wenzel state likely occurred during droplet freezing, but could not be captured with topographic ESEM imaging. Water did not penetrate in-between nanoparticles or nanowires because surfaces consisting of these structures retained their superhydrophobic characteristics during condensation.55 In addition, we observed that some of the oil remained in-between these nanostructures; however, EDS quantification of the remaining


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amount of oil within the nanostructures was challenging.

Figure 5. Cryo-FIB/SEM images of drops before and after freezing on (a-d) 10 µm-LIS, (e & f) NW-LIS, and (g & h) NP-LIS; and (i) 3D cryo-FIB/SEM destructive tomography of a drop frozen on 10 µm-LIS; faces of three vertical progressive cuts into the structure are shown and marked as I, II, and III.

The thick oil matrix covering the frozen drops revealed using cryo-FIB/SEM confirms that majority of the oil migrates from the wetting ridge and the substrate’s texture to the frozen drop’s surface. We note that even prior to freezing the drops are partially cloaked in a thin layer of the 16 ACS Paragon Plus Environment

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oil.45-46 The presence of the thin oil film swirls is directly visible in ESEM image in Figure 6 a and indirectly confirmed with observation of several stable touching unfrozen “compound drops”62 in ESEM image in Figure 6 b (see also Movie 2 in Support Information). However, as we schematically show in Figure 6 c, substantial oil migration onto the drop surface is initiated after drop freezing and is caused by capillary forces arising from the exterior surface roughness created by nucleating nano-icicles (seen as surface roughing in ESEM images). We confirmed frost’s capillary attraction of the perfluorinated oil by observing that about 3 mm diameter drop of Krytox placed on a flat OTS coated silicon wafer (initial contact angle of about 28°) spreads completely following frosting of the sample (i.e. 0° contact angle—see Figure S5). In the case of textured substrates, nucleation of nano-icicles initiates a competition between the capillary attraction of the oil to the solid substrate’s topography and the frost layer. This competition is clearly illustrated by our observation of the amount of perfluorinated oil drained from underneath drops depends on the substrate texture’s length scale during initial stages of frost growth (i.e. oil drains easier from the microtextured than from the nanotextured substrates). However, the capillary attraction arising from frost’s texture will only increase during further frost growth. This trend is clearly demonstrated by calculating critical impregnation angles42,

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hypothetical ordered frost pillars with diameters and pitch of 500 nm. The critical impregnation angle for this frost structure increases from about 38° to 74° and 88° with pillar height increase from 100 nm to 1 µm and 10 µm. In other words, even an almost “icephobic” liquid with contact angle approaching 90° on a flat ice surface is likely to be eventually wicked into the growing frost layer. However, we note that that there is a possibility that substantial decrease of the oil draining rate, which might be achieved by proper tuning of the substrate’s texture and the lubricant properties, might make anti-frosting LIS practical.


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Figure 6. ESEM images of (a) oil swirls on top of drops and (b) compound drops condensed on NP-LIS and schematic illustrations contrasting (c) mechanism of frost growth via drop condensation and freezing on SHS and LIS (the extent of illustrated oil depletion and water penetration into the substrate are dependent on the length scale of the underlying texture).

The observed oil migration mechanism has implications on self-healing and frost repelling properties of LIS. To investigate this issue we imaged defrosting of the 10 µm-LIS and NP-LIS through two mechanisms: sublimation and melting followed by evaporation. This was accomplished in the ESEM by increasing the temperature to about 5°C with chamber pressure reduced to 300 Pa or maintained constant at 600 Pa (i.e. below and above the triple point of 18 ACS Paragon Plus Environment

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water). By performing the defrosting below the triple point, we forced the ice to sublimate. In the case of the 10 µm-LIS water sublimation allowed the drained oil to wick back into the microtexture, demonstrating self-healing abilities of LIS (see Figure 7 a). However, when defrosted above the triple point, the hybrid ice-oil frost layer changed into a thick liquid film that was partially wicked away upon contact with the clamp securing the sample and the Peltier element to the cooling block. In case of the 10 µm-LIS, significant amount of the oil was wicked away with this liquid film, causing nearly full depletion of the oil from the micro-texture (see Figure 7 b). Next, we used this defrosting via melting and evaporation procedure to test the stability of the NP-LIS. ESEM images in Figure 7 c show that the first defrosting step causes an observable decrease in the oil level, which is demonstrated by emergence of the outlines of the topographical features. In turn, after the second frosting and defrosting cycle the underlying nanostructure is clearly visible, indicating that nearly all oil is depleted. These observations show that for any LIS that are in contact with non-LIS materials (i.e. bolts, fittings, metal sheets etc.), the oil will partially deplete with continued frosting and defrosting cycles. Furthermore, the oil depletion problem will be exacerbated by any shedding of the hybrid ice-oil frost due to other physical perturbations such as vibration. This indicates that over time frost grown on any LIS will be in direct contact with the solid substrate and will adhere to it as strongly as if it was directly grown on the “dry” substrate (see schematic in Figure 7 d).


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Figure 7. ESEM images of defrosting of (a) 10 µm-LIS through desublimation at 300 Pa after which oil wicks back into the microstructure “self-healing” the LIS, (b) 10 µm-LIS through melting and evaporation at 600 Pa which results in irreversible oil depletion, and (c) NP-LIS showing progressive oil depletion due to subsequent frosting and defrosting cycles at 600 Pa, and (d) schematic of previously described (ref. 47) and actual morphology of frost on LIS after a few frosting-defrosting cycles.

CONCLUSIONS In summary, we performed a multiscale investigation of frost growth via freezing of condensed 20 ACS Paragon Plus Environment

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drops on SHS and systematically prepared LIS with nanoscale and microscale topographical features. The anti-frosting performance of horizontally oriented nanostructured LIS rivalled that of nanostructured SHS which promoted spontaneous drop motion. In addition, the self-healing characteristics of the LIS made them less prone to hydrophilic defects induced by flexible substrate handling, which was shown to significantly degrade the anti-frosting performance of the nanostructured coating deposited on a thin copper foil. However, we observed that any LIS with perfluorinated oil was susceptible to irreversible damage and loss of self-healing characteristics within a few frosting and defrosting cycles. The LIS damage was caused by oil migration from the wetting ridge and substrate’s texture onto frozen drops. Substantial oil migration was driven through capillary forces arising from nucleation of nano-icicles. The oil was observed to fully drain from the microstructured LIS within one frosting step and from the nanostructured LIS within two frosting and defrosting cycles. Thus, in any practical anti-frosting applications, the oil is likely to be depleted from any LIS consisting of the perfluorinated oil. Hence use of liquid reservoirs that allow replenishment of the lubricant would become necessary for sustained performance of LIS. Alternatively, further research is necessary to develop liquidtexture pairs, which when properly combined, will significantly decrease the rate of oil depletion into frost providing a more sustainable frost suppression method.

ACKNOWLEGEMENTS KKV gratefully acknowledges the NSF Career Award (0952564), MIT Energy Initiative, and Doherty Chair in Ocean Utilization for funding this work. KR kindly acknowledges Dr. J.H.J Scott and Dr. M.L. Walker from NIST for insightful discussions and M. Staymates and L. King from NIST for help with fabrication of the ESEM cooling stage. The authors also kindly 21 ACS Paragon Plus Environment

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acknowledge Dr. Albert Davydov and Dr. Sergiy Krylyuk from NIST for providing the VLS silicon nanowire samples and Dr. Jeff Chinn from Integrated Surface Technologies for providing the NP-SHS samples. Part of this work was performed using facilities at National Institute of Standards and Technology in Gaithersburg, MD.

Supporting Information ESEM movies of frost growth on 10 µm and NP SHS and LIS; Movie of 3D cryo-FIB/SEM ice-oil crystal reconstruction; images of the cryo-FIB/SEM and custom ESEM cooling experimental setups; cryo-FIB/SEM images of cross sections of all LIS; cryoFIB/SEM images of condensation and frost on SHS and LIS with inter-post spacing of 5 µm and 25 µm; and sequence of images showing of frost gradually wicking Krytox oil drop deposited on a hydrophobic silicon wafer. This material is available free of charge via the Internet at http://pubs.acs.org.

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SYNOPSIS TOC


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Mechanism of Frost Formation on Lubricant-Impregnated Surfaces

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