Free-Standing, Flexible, Superomniphobic Films - ACS Applied

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Free-standing, Flexible Superomniphobic Films Hamed Vahabi, Wei Wang, Sanli Movafaghi, and Arun K. Kota ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06333 • Publication Date (Web): 19 Aug 2016 Downloaded from http://pubs.acs.org on August 21, 2016

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ACS Applied Materials & Interfaces

Free-standing, Flexible Superomniphobic Films Hamed Vahabi1‡, Wei Wang1‡, Sanli Movafaghi1, Arun K. Kota1,2* 1

Department of Mechanical Engineering, Colorado State University, Fort Collins, CO, USA;

2

School of Biomedical Engineering, Colorado State University, Fort Collins, CO, USA.

‡ These authors contributed equally to this work. Keywords: superomniphobic, free-standing, flexible, enhanced chemical resistance, enhanced weight bearing capacity Abstract

Fabrication of most superomniphobic surfaces requires complex process conditions or specialized and expensive equipment or skilled personnel. In order to circumvent these issues and make them end-user friendly, we developed the free-standing, flexible superomniphobic films. These films can be stored and delivered to the end-users, who can readily attach them to virtually any surface (even irregular shapes) and impart superomniphobicity. The hierarchical structure, the re-entrant texture and the low solid surface energy render our films superomniphobic for a wide variety of liquids. We demonstrate that our free-standing, flexible, superomniphobic films have applications in enhanced chemical resistance and enhanced weight bearing.

ACS Paragon Plus Environment

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Superomniphobic surfaces (i.e., surfaces extremely repellent to virtually all liquids) are of great interest because of their applications in anti-fouling, self-cleaning, corrosion resistance, micro-robots, membrane separation, and drag reduction.1-6 Recent years have seen rapid and noticeable advances in the design and fabrication of superomniphobic surfaces.2-4,7,8 However, fabrication of most superomniphobic surfaces requires complex process conditions or specialized and expensive equipment or skilled personnel. Further, some fabrication techniques are not easily scalable. In order to circumvent these issues and make them end-user friendly, in this work, we developed free-standing, flexible superomniphobic films. Prior work has primarily focused on the fabrication of rigid superomniphobic surfaces or free-standing, flexible omniphobic films, but not on free-standing, flexible superomniphobic films.9,10 Our freestanding, flexible superomniphobic films can be stored and delivered to the end-users, who can readily attach them to virtually any surface (including irregular shapes) and impart superomniphobicity. The importance of free-standing, flexible superomniphobic films is more pronounced when there is lack of skilled personnel or lack of necessary fabrication equipment. We envision that our free-standing, flexible, superomniphobic films will have numerous applications including tapes with enhanced chemical and biofouling resistance, and devices (e.g., water drones and microrobots) with enhanced weight-bearing capacity etc. The primary measure of wetting of a liquid on a non-textured (i.e., smooth) solid surface is the equilibrium (or Young’s) contact angle θ.11 When the liquid droplet contacts a textured (i.e., rough) solid surface, it displays an apparent contact angle θ*, and it can adopt one of the following two configurations to minimize its overall free energy – the fully wetted Wenzel12 state or the Cassie-Baxter13 state. The Cassie-Baxter state is preferred for designing super-repellent * * surfaces14-18 because it leads to high θ* and low contact angle hysteresis Δθ * = θ adv , where − θ rec


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* * and θ rec are the apparent advancing and apparent receding contact angles, respectively. Low θ adv

Δθ* leads to low roll off angle ω (i.e., the minimum angle by which the surface must be tilted relative to the horizontal for the droplet to roll off).19 A surface is considered superhydrophobic if it displays θ* > 150° and Δθ* < 10° (or ω < 10°) with water16,20,21, and superoleophobic if it displays θ* > 150° and Δθ* < 10° (or ω < 10°) with low surface tension (typically γlv < 30 mN m1

) liquids2,3,22,23. Superomniphobic surfaces are both superhydrophobic and superoleophobic.2,3

Superomniphobic surfaces can be fabricated by combining a surface chemistry possessing low solid surface energy (typically γsv < 15 mN m-1) with re-entrant texture (i.e., multivalued surface topography).7,8,22,24 In this work, we fabricated superomniphobic surfaces by coating a substrate (e.g., glass slide) with a water-soluble sacrificial layer of poly(sodium 4-styrenesulfonate) (PSS), followed by a polyurethane (PU) layer and then a layer of fluorinated silica (F-SiO2) particles (Figure 1a; see supporting information (SI), section 1). PU serves as an adhesive to hold the layer of F-SiO2 particles together. The re-entrant texture and the low solid surface energy (γsv ~ 10 mN m-1; see SI, section 1) of the F-SiO2 particle layer results in superomniphobicity. Subsequently, the PSS layer is dissolved in water to release the layered PU–F-SiO2 free-standing, flexible superomniphobic film from the substrate. We chose a water-soluble sacrificial layer because water does not adversely affect the surface chemistry or the morphology of the layered PU–FSiO2 superomniphobic film. The flexibility of the free-standing superomniphobic film arises from the flexibility of the PU layer.


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Figure 1. (a) Schematic illustrating the fabrication of a free-standing, flexible superomniphobic film. (b) The influence of silanization time (τs) on apparent contact angles and roll off angles of dodecane. (c) Influence of surface density (ρA) on apparent contact angles and roll off angles of dodecane. The left and right insets show SEM images of films with ρA = 0.1 mg cm-2 and ρA = 1.6 mg cm-2, respectively.


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The key parameters influencing the surface chemistry and the surface texture (and hence superomniphobicity) are the silanization time (τs) (i.e., time required to functionalize SiO2 particles with the fluorinated silane and impart low γsv) and the surface density (ρA) (i.e., mass per unit area), respectively, of F-SiO2 particles. In order to systematically investigate the influence of

τs on superomniphobicity, we fabricated superomniphobic films using F-SiO2 particles with ρA = 1.6 mg cm-2 (corresponding to complete surface coverage upon visual inspection) and different

τs. Our results (Figure 1b) indicate that the surfaces are not superomniphobic for τs < 3 days, perhaps due to insufficient silane graft density. For τs = 3 days, the surfaces are both * * superhydrophobic ( θ adv = 171 ° , θ rec = 162 ° , ω = 4° for water with γlv = 72.1 mN m-1) and * * superoleophobic ( θ adv = 158 ° , θ rec = 149 ° , ω = 8° for dodecane with γlv = 25.3 mN m-1), i.e.,

superomniphobic, indicating a sufficiently high silane graft density. Further increase in τs did not improve the superomniphobicity indicating that there is no further increase in silane graft density. In order to systematically investigate the influence of ρA on superomniphobicity, we fabricated superomniphobic films using F-SiO2 particles with τs = 3 days and different ρA. Our results (Figure 1c) indicate that at low ρA, the surface coverage is insufficient to render the surface superomniphobic. At ρA = 1.0 mg cm-2, the surfaces are both superhydrophobic and superoleophobic i.e., superomniphobic. Further increase in ρA did not improve the superomniphobicity indicating that there is no further increase in roughness or re-entrant texture. Based on these results, we fabricated our free-standing, flexible, superomniphobic films with τs = 3 days and ρA = 1.0 mg cm-2 for the results discussed below. We characterized the morphology of our free-standing, flexible superomniphobic films using a scanning electron microscope (SEM). The morphology indicates a hierarchical structure (i.e., a


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finer texture superimposed on a coarser texture; Figure 2a). The coarser texture consists of ~1050 μm aggregates of the F-SiO2 particles. The finer texture consists of ~10 nm individual F-SiO2 particles. The hierarchical structure, the re-entrant texture and the low γsv result in a robust Cassie-Baxter state with air trapped at both the coarser and the finer length scales (see SI, section 2). This renders our films superomniphobic for a wide variety of liquids with γlv ≥ 25.3 mN m-1 (Figure 2b). The high liquid-air area fraction results in high θ* and the droplets bead up on the film (Figure 2c). Also, the low Δθ* resulting from the low solid-liquid area fraction allows the droplets to roll off and bounce (Figure 2d also see Movie S1) easily. Our experimentally measured roll off angles match reasonably well with the predictions based on the work by Furmidge19 (see SI, section 3). The robust Cassie-Baxter state is further evident from the stable plastron (i.e., layer of trapped air) over 24 h when our superomniphobic films are immersed in dodecane (Figure 2e).


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Figure 2. (a) SEM image showing the hierarchical structure of the free-standing, flexible superomniphobic film. Inset shows the finer texture. (b) Advancing and receding contact angles as well as roll off angles of liquids with a wide range of surface tension values on the freestanding, flexible superomniphobic film. (c) Droplets of dodecane (γlv = 25.3 mN m-1), hexadecane (γlv = 27.5 mN m-1), rapeseed oil (γlv = 35.7 mN m-1), and water (γlv = 72.1 mN m-1) beading up on the free-standing, flexible superomniphobic film. (d) A series of snapshots showing a droplet of dodecane bouncing on the free-standing, flexible superomniphobic film tilted by 5° relative to horizontal (also see Movie S1). (e) A stable plastron on the superomniphobic film immersed in a liquid bath of dodecane.


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Decoupling the fabrication of the superomniphobic film from the target application offers operation freedom for both the film manufacturer and the end user. It allows the manufacturer to fabricate a wide variety of superomniphobic films by changing the material processing conditions as well as fabricate superomniphobic films in bulk with reproducible quality. It also allows the end users to utilize the superomniphobic films on a wide variety of surfaces for different applications. Flexibility of the PU–F-SiO2 free-standing superomniphobic film allows it to be easily attached to virtually any surface using ordinary glue on the PU side of the film. Due to their flexibility, the free-standing superomniphobic films can be wrapped around spools of different sizes and delivered to the end-user (Figure 3a). This flexibility is further evident from the highly curved shape of the film held with tweezers (Figure 3a, inset). Our free-standing, flexible superomniphobic films are well-suited for chemical resistance applications due to the excellent chemical stability of the F-SiO2 particles. To demonstrate the chemical resistance, we immersed two aluminum samples – one wrapped with our flexible superomniphobic film and another wrapped with just the PU film (not superomniphobic) – in concentrated 98% sulfuric acid (Figure 3b). The untreated PU film darkened shortly after contacting sulfuric acid and eventually degraded. In contrast, the superomniphobic film remained unaffected (see Movie S2 and Figure 3c). In a similar manner, our superomniphobic films remained unaffected upon contacting a wide variety of aqueous and organic acids and bases indicating good chemical resistance. Further, our films retained their superomniphobicity even after exposing them to different corrosive environments (i.e., liquids with pH values ranging from 1 to 13) for 24 h (see SI, section 4).


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Figure 3. (a) Free-standing, flexible superomniphobic film wrapped around a spool and ready to be applied on any surface (even irregular shapes). Inset shows a highly curved shape to emphasize the flexibility of the superomniphobic film. (b) and (c) Two aluminum samples wrapped in – a PU film (left) and in the PU–F-SiO2 superomniphobic film (right) – before and after immersion, respectively, in concentrated 98% sulfuric acid (also see Movie S2). (d) An image depicting the weight-bearing capacity measurement of a film floating on water (dyed blue) using a force probe. Inset shows the schematic of the measurement and emphasizes the flexibility of the film. (e) Influence of the flexibility of the film on the weight-bearing capacity. The upper left and right insets depict the deformation of the film (dotted red curve), deformation of the liquid-vapor interface (dotted blue curve) and the undeformed free surface of the liquid (dotted yellow line) for a barely deformable configuration and a highly deformable configuration, respectively. The lower schematics show circular force probes with different cross sectional areas that allow precise control over the degree of flexibility (or deformability) of the film.


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Recent work25-28 has indicated that free-standing, flexible films floating on a liquid have higher weight bearing capacity and better floating stability compared to rigid sheets of similar thickness and lateral dimensions. Here, the weight-bearing capacity is the maximum weight (Wmax) that the film can support before sinking. In order to systematically investigate the influence of flexibility on Wmax, we fabricated free-standing, flexible superomniphobic circular films that are coated with F-SiO2 on all sides of the film. These films were then placed on water and subjected to gradually increasing force using circular force probes until the films were completely submerged in water (Figure 3d). Force was measured with a force gauge (±0.5 mN resolution) and Wmax was determined just before the film began to submerge in water. We used circular force probes with different cross sectional areas (see inset in Figure 3e) to precisely control the degree of flexibility (or deformability) of the film. When the cross sectional area of the force probe is equal to the cross sectional area of the film, the deformable area of the film is zero. Consequently, the film behaves as a rigid sheet under applied force. As the cross sectional area of the force probe decreases relative to the cross sectional area of the film, the deformable area of the film increases, i.e., the behavior increasingly resembles that of a flexible film. Here, we define the fraction of deformable area (Adef) as:

Adef = 1 −

2 D probe

D 2film

(1)

Here, Dprobe is the diameter of the force probe and Dfilm is the diameter of the flexible superomniphobic film. We measured Wmax on our flexible superomniphobic films for different Adef. Our results (Figure 3e) indicate that Wmax increases with increasing Adef, i.e., increasing flexibility of the film. However, at very high Adef (> 0.93), the film witnesses an onset of wrinkles or folds along the perimeter29,30 that leads to a hydrodynamic instability and expedited sinking. Consequently, very high Adef results in low Wmax. These results indicate that our free-standing,


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flexible, superomniphobic films can have ~20% higher Wmax on water compared to a rigid superomniphobic sheet. In a similar manner, the free-standing, flexible superomniphobic films can be used to enhance the weight-bearing capacity on a wide variety of liquids. In summary, we fabricated the free-standing, flexible superomniphobic films by coating a substrate with a water soluble sacrificial layer, followed by a polyurethane layer and a layer of fluorinated silica particles, and subsequently dissolving the sacrificial layer in water.

The

hierarchical structure, re-entrant texture and the low solid surface energy of the fluorinated silica particle layer results in superomniphobicity with a wide variety of liquids of γlv ≥ 25.3 mN m-1. Our free-standing, flexible superomniphobic films can be stored and delivered to the end-users, who can readily attach them to virtually any surface (even irregular shapes) and impart superomniphobicity. Further, our free-standing, flexible superomniphobic films can be used for applications that require enhanced chemical resistance and enhanced weight-bearing capacity. While our free-standing, flexible superomniphobic films display reasonable mechanical durability against liquids flowing past the surface, they do not have sufficient mechanical durability to withstand harsh and abrasive environments (see SI, section 5). Efforts are currently underway to improve the mechanical durability of our free-standing, flexible superomniphobic films.

Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Section 1 – Experimental details. Section 2 – Influence of hierarchical structure on superomniphobicity. Section 3 – Roll off angles. Section 4 – Chemical resistance. Section 5 – Mechanical durability. Movie S1 – Dodecane droplet bouncing on a free-standing, flexible,


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superomniphobic film. Movie S2 – Chemical resistance of the free-standing, flexible, superomniphobic film. Corresponding Author * E-mail: [email protected] Author Contributions HV and WW contributed equally to this work. HV, WW and SM conducted the experiments. HV and WW conducted the analyses. HV, WW and AKK wrote the manuscript. Acknowledgement We thank Colorado Office of Economic Development and International Trade for financial support under award EDA 14-246. References (1) (2) (3) (4) (5) (6) (7) (8) (9)

Chu, Z.; Seeger, S. Superamphiphobic Surfaces. Chem. Soc. Rev. 2014, 43, 2784-2798. Kota, A. K.; Choi, W.; Tuteja, A. Superomniphobic Surfaces: Design and Durability. MRS Bull. 2013, 38, 383-390. Kota, A. K.; Kwon, G.; Tuteja, A. The Design and Applications of Superomniphobic Surfaces. NPG Asia Mater. 2014, 6, e109. Kota, A. K.; Mabry, J. M.; Tuteja, A. Superoleophobic Surfaces: Design Criteria and Recent Studies. Surf. Innovations 2013, 1, 71-83. Liu, K.; Yao, X.; Jiang, L. Recent Developments in Bio-Inspired Special Wettability. Chem. Soc. Rev. 2010, 39, 3240-3255. Nishimoto, S.; Bhushan, B. Bioinspired Self-Cleaning Surfaces with Superhydrophobicity, Superoleophobicity, and Superhydrophilicity. RSC Adv. 2013, 3, 671-690. Tuteja, A.; Choi, W.; Ma, M.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Designing Superoleophobic Surfaces. Science 2007, 318, 1618-1622. Tuteja, A.; Choi, W.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E. Robust Omniphobic Surfaces. Proc. Natl. Acad. Sci. 2008, 105, 18200-18205. Kim, J. H.; Shim, T. S.; Kim, S. H. Lithographic Design of Overhanging Microdisk Arrays toward Omniphobic Surfaces. Adv. Mater. 2016, 28, 291-298.


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(10) Campos, R.; Guenthner, A. J.; Meuler, A. J.; Tuteja, A.; Cohen, R. E.; McKinley, G. H.; Haddad, T. S.; Mabry, J. M. Superoleophobic Surfaces through Control of Sprayed-on Stochastic Topography. Langmuir 2012, 28, 9834-9841. (11) Young, T. An Essay on the Cohesion of Fluids. Philos. Trans. R. Soc. London 1805, 95, 65-87. (12) Wenzel, R. N. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem. 1936, 28, 988-994. (13) Cassie, A.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday Soc. 1944, 40, 546551. (14) Johnson Jr. R. E.; Dettre, R. H. Contact Angle Hysteresis. III. Study of an Idealized Heterogeneous Surface. J. Phys. Chem. 1964, 68, 1744-1750. (15) Marmur, A. Wetting on Hydrophobic Rough Surfaces: to be Heterogeneous or not to be? Langmuir 2003, 19, 8343-8348. (16) Nosonovsky, M.; Bhushan, B. Superhydrophobic Surfaces and Emerging Applications: Non-adhesion, Energy, Green Engineering. Curr. Opin. Colloid Interface Sci. 2009, 14, 270-280. (17) Öner, D.; McCarthy, T. J. Ultrahydrophobic Surfaces. Effects of Topography Length Scales on Wettability. Langmuir 2000, 16, 7777-7782. (18) Patankar, N. A. On the Modeling of Hydrophobic Contact Angles on Rough Surfaces. Langmuir 2003, 19, 1249-1253. (19) Furmidge, C. Studies at Phase Interfaces. I. The Sliding of Liquid Drops on Solid Surfaces and a Theory for Spray Retention. J. Colloid Sci. 1962, 17, 309-324. (20) Lafuma, A.; Quéré, D. Superhydrophobic States. Nat. Mater. 2003, 2, 457-460. (21) Mittal, N.; Deva, D.; Kumar, R.; Sharma, A. Exceptionally Robust and Conductive Superhydrophobic Free-Standing Films of Mesoporous Carbon Nanocapsule/Polymer Composite for Multifunctional Applications. Carbon 2015, 93, 492-501. (22) Kobaku, S. P.; Kota, A. K.; Lee, D. H.; Mabry, J. M.; Tuteja, A. Patterned Superomniphobic–Superomniphilic Surfaces: Templates for Site-‐Selective Self-‐Assembly. Angew. Chem., Int. Ed. 2012, 51, 10109-10113. (23) Brown, P. S.; Bhushan, B. Mechanically Durable, Superoleophobic Coatings Prepared by Layer-by-Layer Technique for Anti-Smudge and Oil-Water Separation. Sci. Rep. 2015, 5, 8701. (24) Ahuja, A.; Taylor, J.; Lifton, V.; Sidorenko, A.; Salamon, T.; Lobaton, E.; Kolodner, P.; Krupenkin, T. Nanonails: A Simple Geometrical Approach to Electrically Tunable Superlyophobic Surfaces. Langmuir 2008, 24, 9-14. (25) Burton, L. J.; Bush, J. W. Can Flexibility Help You Float? Phys. Fluids 2012, 24, 101701. (26) Ji, X.-Y.; Wang, J.-W.; Feng, X.-Q. Role of Flexibility in the Water Repellency of Water Strider Legs: Theory and Experiment. Phys. Rev. E 2012, 85, 021607. (27) Tennenbaum, M.; Liu, Z.; Hu, D.; Fernandez-Nieves, A. Mechanics of Fire Ant Aggregations. Nat. Mater. 2015, 15, 54-59. (28) Andreotti, B.; Marchand, A.; Das, S.; Snoeijer, J. H. Elastocapillary Instability under Partial Wetting Conditions: Bending Versus Buckling. Phys. Rev. E 2011, 84, 061601. (29) Holmes, D. P.; Crosby, A. J. Draping Films: A Wrinkle to Fold Transition. Phys. Rev. Lett. 2010, 105, 038303. (30) Pocivavsek, L.; Dellsy, R.; Kern, A.; Johnson, S.; Lin, B.; Lee, K. Y. C.; Cerda, E. Stress and Fold Localization in Thin Elastic Membranes. Science 2008, 320, 912-916.


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a

70

150°

60 50 40 Advancing Receding Roll off 10°

1

2

30 20 10

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5

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180

90

160

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140

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150°

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Advancing Receding Roll off

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10°

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50 µm

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0.6

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Su rface Density, ρ A (mg cm-2)

Silanization Time, τ s (day)


1.4

1.6

0

Roll off Angle, ω (°)

80

Apparent Contact Angle, θ (°)

*

c

90

Apparent Contact Angle, θ* (°)

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27180 28160 29 30140 31 120 32 33100 34 35 80 36 60 37 38 40 39 20 40 41 0 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58


b

*

Apparent Contact Angle, θ (°)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 a"15 c"16 17 18 19 20 21 22Dodecane 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

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160

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10 0

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Surface Tension, γlv (mN m-1 )

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2 mm e

d" Hexadecane

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Advancing Receding Roll off

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Rapeseed Oil

Water

d"


5 mm

70

75


a

aPage 17 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

b

0 sec

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10 mm

10 mm 80

e Weight-Bearing Capacity, W m ax (mN)

d"

5 mm

70 60

62 mN (Rigid Sample)

50 40

10 mm

30

F

F

F

F

Adef ~0

Adef =0.25

Adef =0.50

Adef =0.75

20 10 0

0.1

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0.4

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Fractio n o f Deformable Area, Adef


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Free-Standing, Flexible, Superomniphobic Films - ACS Applied

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