Article pubs.acs.org/Langmuir
Fog-Harvesting Potential of Lubricant-Impregnated Electrospun Nanomats Boor Singh Lalia,† Sushant Anand,‡ Kripa K. Varanasi,‡ and Raed Hashaikeh*,† †
Institute Center for Water and Environment (iWATER), Masdar Institute of Science and Technology, P.O. Box 54224, Abu Dhabi, United Arab Emirates ‡ Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States S Supporting Information *
ABSTRACT: Hydrophobic PVDF-HFP nanowebs were fabricated by a facile electrospinning method and proposed for harvesting fog from the atmosphere. A strong adhesive force between the surface and a water droplet has been observed, which resists the water being shed from the surface. The water droplets on the inhomogeneous nanomats showed high contact angle hysteresis. The impregnation of nanomats with lubricants (total quartz oil and Krytox 1506) decreased the contact angle hysteresis and hence improved the roll off of water droplets on the nanomat surface. It was found that water droplets of 5 μL size (diameter = 2.1 mm) and larger roll down on an oil-impregnated surface, held vertically, compared to 38 μL (diameter = 4.2 mm) on a plain nanoweb. The contact angle hysteresis decreased from ∼95 to ∼23° with the Krytox 1506 impregnation.
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INTRODUCTION A grim reality of our present time is the lack of access to clean water among large sections of the population in many parts of the world.1 The problem is acute, especially in areas that experience insufficient precipitation and/or may have become barren because of unsustainable deforestation activities.2 Reclaiming water from available sources in a cleaner and efficient way and developing alternate sources of water thus have become important tools in managing the water scarcity crisis.3 The most common technique for reclaiming fresh water is water desalination and is currently used in many parts of Europe and in most arid parts of the Middle East.2 Harvesting water directly from the atmosphere can be a potential resource for fresh water. Even in arid environments, there may be sufficient water vapor present in the atmosphere that can potentially be harvested in the form of dew or fog. Indeed there are numerous plants4,5 and animals6,7 that survive under harsh conditions because they have evolved to harvest water from the atmosphere. Water harvesting for human consumption through dew collection has been utilized for centuries.8 Typical dew condensers use large plates that need to be subcooled below the saturation temperature in the atmosphere to condense water from water vapor.8−12 In the absence of natural radiative cooling, the plate needs to be subcooled by artificial means requiring the expenditure of electrical energy for the operation. However, at many locations conditions exist wherein water can be harvested from the ambient atmosphere even without the need for condensers.13 For example, typical summer climatic conditions such as those in the United Arab Emirates (UAE) © 2013 American Chemical Society
coastal regions comprise high humidity and heat resulting in the formation of large quantities of “fog” that makes it a potential renewable source of fresh water.8 In essence, fog consists of condensed microdroplets suspended in air with diameter sizes ranging from 1 to 40 μm with fall velocities of 90° or vice versa. In contrast to the Wenzel model, the Cassie model allows θ* > 90° even if θ < 90°. However, more importantly, the droplets in the Wenzel state show a large amount of hysteresis even when θ* > 90° as compared to droplets in the Cassie state.52 Intermediate between the two states is the so-called Cassie-impregnating state where a droplet with a very high contact angle (>150°) on a surface also displays very high adhesion.53,54 The Cassieimpregnating state may arise on surfaces with hierarchical surfaces where droplets are pinned on the microscale asperities while the air remains entrapped in nanoscale asperities. Knowing the contact angle hysteresis and the geometry of the surface, we can define the state of the droplet on the surface. On the electrospun nanomat, the advancing (θa) and receding (θr) contact angles were measured to be 154 ± 4° and 59 ± 4°, respectively, indicating very high hysteresis (∼95 ± 4°) and thus very high adhesion of water droplets on the surface (Figure 3a). To estimate the strength of pinning forces,
Figure 3. Optical images of differently sized water drops on the vertical surface of (a) a nanomat and (c) a nanomat impregnated with total quartz oil. Schematic of water drop sliding (b) on the nanomat and (d) on the nanomat impregnated with total quartz oil.
the mobility of a water droplet on the nanomat surface was determined in terms of the minimum size of the droplet that starts moving on the vertical surface. A drop of 1 μL was placed on a vertically held surface, and the volume of the droplet was subsequently increased steadily by 1 μL until the droplet motion was initiated (Figure 3a,b). Increasing the volume resulted in a large asymmetrical distortion of the droplet related 13084
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Figure 4. Optical images of fog-collection−coalescence behavior of water droplets on (a) a plain unimpregnated nanomat, (b) a nanomat impregnated with total quartz oil, and (c) a nanomat impregnated with Krytox-1506. The scale bar corresponds to 10 mm.
the water collection rate remained nearly constant during the entire time of the experiment (Supporting Information, Figure S3), thus signifying that the adhesion of droplets to the surface remained unchanged during this time. Water collected from the sample (Mcollect) and retained (Mresidual) on the sample was subsequently measured. Figure 5 shows the fog collection versus time behavior of the nanomats
As can be seen from Figure 4a−c, there is a distinct difference in the behavior of droplet growth on unimpregnated and impregnated nanomats. Fog droplets impacting the surface grow by coalescence and subsequently shed.57 The drop impaction on unimpregnated nanomat leads to droplets growing within the sample as Wenzel droplets. After a period of 10 min, because of the high hysteresis a thick film is formed on the surface in between other droplets. However, on the impregnated nanomat samples, impacted water droplets retain the droplet shape and increase in size. It should be noted that although both of the lubricants cloak the water droplets, cloaking effects are less obvious in this case and the coalescence of droplets is observed, leading to increases in the size of droplets on the surface. As mentioned previously, the lack of observable wetting ridge formation is associated with strong capillary forces within the texture that help to retain the lubricant on the surface. Because of interfacial forces, the lubricants have a tendency to spread on water droplets; however, the thickness of the oil film around the water droplets may be significantly less as a result of strong capillary forces of the porous nanomat structure. Furthermore, the impacting droplets may carry sufficient energy to dewet the existing thin films on the water droplet surface. On the lubricantimpregnated nanomat surfaces, the droplets grow in size, and after reaching the minimum threshold volume (i.e., 5 μL), as discussed in Figure 3c, they start to roll off the vertical surface and are subsequently collected at the bottom. It should be noted that the progressive loss of lubricant from the surface is associated with increased contact angle hysteresis.30 However,
Figure 5. Fog-collection behavior with time for nanomats with and without lubricant impregnation. 13085
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water. As a comparison, the TOC analysis from pure DI water was found to be 1 ppm. From these measurements, it is clear that there is considerably less leeching of the oil from the samples during the experiment.
(with and without oil). From the unimpregnated nanomat sample, the water collected from the nanomat without oil was found to be 77 ± 4 mg/cm2/h whereas 11 ± 0.55 mg/cm2/h water was found to be retained on the sample. The total amount of water harvested (Mtotal = Mcollect + Mresidual) per unit area per hour on the nanomat samples impregnated with total quartz oil and Krytox-1506 oil was measured to be 100 ± 5 and 118 ± 6 mg/cm2/h, respectively. Videos showings the water collection on nanomats (with and without oil) is given in the Supporting Information. The net increase in water collection due to the impregnation of the sample is associated with the low hysteresis of the impregnated samples that results in faster drainage from these surfaces as compared to that of unimpregnated nanomats. Because the viscosity of total quartz oil is larger than the viscosity of Krytox-1506, the droplet mobility is higher on the sample impregnated with the later lubricant37 and thus results in higher collection rates. This indicates improvements of 14 and 35% using total quartz oil and Krytox-impregnated surfaces over the unimpregnated nanomat sample, respectively. In a recent study, fog-collection measurements on the desert beetle and desert grass were made by Nørgaard et al.58 In their studies, the desert beetle and desert grass were found to collect ∼12.4 and ∼22.2 mg/cm2/h of water respectively, whereas ∼6 and ∼9 mg/cm2/h of water were collected in the sample tube from the two surfaces, respectively. Compared to these values, the fog-collection ability of the nanomat without impregnation is 2.8 times whereas the Krytox-impregnated surface has a collection rate of 3.8 times that of the total collection by desert grass. In dew-collection studies by Lee et al., 0.25 g/h water over a 9 cm2 area was collected from the vertically aligned hydrophilic silicon wafer under controlled conditions of 25 ± 1 °C and 90−95% RH.12 Thickett et al. used a micropatterned dewetted surface kept at 0 °C (surrounding temperature 20 °C) and exposed to two different fog flow rates of 1.4 L/min (86% RH) and 9.8 L/min (79% RH).11 The fog-collection rate was found to be ∼750 mL/m2/h with a 1.4 L/min flow rate and ∼3250 mL/m2/h with a 9.8 L/min flow rate. Although a direct comparison cannot be made because of dissimilarities in the experimental conditions used in this study and reported results, the water collection rates observed in our experiments is over 57% of the values reported by Thickett et al. for a low flow rate of 1.4 L/min and 325% of the reported values of Lee et al. For fog-harvesting applications, it is important to estimate the contamination in collected water.59 Contaminants such as bacteria, dust, and organic aerosols are found to occur in water collected from dew or fog.60 Lubricant surfaces have been shown to inhibit the growth of bacteria on surfaces.40 For fogharvesting applications, this feature may be useful. However, the contamination is still likely to occur because of the drainage of oil from the surface. Because the lubricants used in the current work are expected to cloak the water droplets, there may be more oil drainage. To estimate the oil drained from the sample along with water droplets, the total organic carbon (TOC) analysis of the water collected from the impregnated surfaces was used to estimate the oil leaching from the impregnated nanomats. The water collected from total quartz oil and Krytox-1506-impregnated nanomats contains 28.24 ppm (equivalent to 5.54 μL of oil per liter of water) and 16.18 ppm (equivalent to 3.03 μL of oil per liter of water) total organic content, respectively. The conversion of TOC (ppm) to μL/L was achieved by measuring the TOC of known amounts of total quartz oil and Krytox-1506 in the distilled
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CONCLUSIONS In the current work, we investigated the potential of lubricantimpregnated nanomats for fog harvesting. PVDF-HFP polymer was used to prepare electrospun nanomats because of its inherent hydrophobicity. Because they have strong adhesive forces with water, the prepared nanomats exhibit a great potential to prepare stable lubricant-impregnated surfaces as a result of their high porosity by virtue of which they wick a large amount of lubricant proportional to their weight. The impregnation of lubricant in nanopores/micropores of nanomats dramatically lowers the retentive force between water droplets and the nanomat surface indicated by the low-contactangle hysteresis and promotes the roll-off of small droplets. The fog-collection studies show that collection rates on the lowhysteresis impregnated sample is higher than the water collection rates on the high-hysteresis unimpregnated sample. Furthermore, we show that nanomats impregnated with the lubricant show significantly less drainage of oil from the surface along with shedding water.
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ASSOCIATED CONTENT
S Supporting Information *
Morphology analysis. Schematic of fog-collection holder. Videos showing fog collection on a PVDF-HFP electrospun mat and on a PVDF-HFP mat impregnated with Krytox-1506. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Author Contributions
B.S.L. and S.A. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Masdar Institute-MITEI. This project was supported by the National Research Foundation (NRF) of the United Arab Emirates (UAE) through the Research and Scholarship Award program (RSA-1108-00282). S.A. acknowledges logistical support by Shruti Sachdeva and Thomas Braun during this work.
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