Improving the Mechanical Durability of
Superhydrophobic Coating by Deposition on to a
Mesh Structure
Wei-Hua Hu1, De-Quan Yang1,*, and Edward Sacher2
.
1) Solmont Technology Wuxi Co., Ltd. 228 Linghu Blvd. Tianan Tech Park, A1-602, Xinwu
District, Wuxi, Jiangsu 214135, China
2) Regroupement Québécois de Matériaux de Pointe, Department of Engineering Physics,
ÉcolePolytechnique de Montréal, Case Postale 6079, succursale Centre-Ville, Montréal, Québec
H3C 3A7, Canada
.
KEYWORDS: mechanical durability, superhydrophobic coating, Ultra-ever Dry
*Corresponding author: Dr. De-Quan Yang, Email. [email protected] ; Tel. +86-510-
8538-6636; Fax. +86-510-8538-4339
ABSTRACT
Superhydrophobic surfaces (SHSs) require a combination of a rough nano- or microscale
structured surface topography and a low surface energy. However, its superydrophobicity is
easily lost, even under relatively mild mechanical abrasion, when the surface is mechanically
weak. Here, we develop a method that significantly increases the mechanical durability of a
superhydrophobic surface, by introducing a mesh layer beneath the superhydrophobic layer. The
hardness, abrasion distance, flexibility and water-jet impact resistance all increase for the
commercially available Ultra-ever Dry superhydrophobic coating. This is attributed to the
increased mechanical durability offered by the mesh, whose construction not only increases the
porosity of the SHS coating but acts as a third, larger structure, so that the superhydrophobic
layer is now composed of a three-level hierarchical structure: the mesh, micropillars and
nanoparticles.
Graphical abstract
1. INTRODUCTION
In recent years, a growing number of researchers have been studying how to fabricate
superhydrophobic surfaces (SHSs), which displayhigh static contact angles (SCA) and low
sliding angles (SA) for a water drop1. SHSs have wide application in daily life, in situations that
require self-cleaning2, anti-freezing3, non-sticking of snow4, anti-corrosion5, anti-biofouling6, etc.
In general, the superhydrophobicity of a SHS is dependent on both the chemical composition and
the topography of the surface7. Many fabrication processes have been reported, including a
hydrothermal method8, electrodeposition9, etching processes10, spray coating11, atomic layer
deposition12 and sol−gel processing13.
However, applications are often limited by the poor mechanical stabilities of SHSs because
their nanoscale structures, with high aspect ratios, are intrinsically mechanically fragile and can
be destroyed by rather low mechanical stresses14,15. Various methods have been proposed to
improve the mechanical durabilities of SHSs. While, for example, Liu et al. used composite
ceramic coating to improve mechanical durability16, some soft materials are also good choices
for anti-abrasion SHSs17.
Hierarchal roughnesses are widely used to improve the hydrophobicity and durability of
SHS14.For example, Groten et al.10manufactured three types of photolithographically created
structural model surfaces (microscale structures, nanoscale structures and composite structures)
that were ion-couple plasma-etched to produce SHSs with improved mechanical durability.
Based ontheir results, they hypothesized that the composite structures had increased mechanical
stability.
Ou et al.15 reported similar composite structures on copper substrates, fabricated by chemical
etching. Then, treatment of the etched surface with 1H,1H,2H,2H-perfluorodecanethiol produced
a superhydrophobic surface. However, it had poor wear resistance, due to the loss of low surface
energy molecules and a change in surface morphology. Huovinen et al.18 introduced additional,
larger microscale features, and found improved mechanical wear properties.
Using a low surface energy material, such as PTFE or PVDF, which will protect the SHS from
abrasion19,20,21,22, our group introduced nano- and microscale porous structures in PTFE
superhydrophobic coatings, improving their anti-abrasion properties23.
In this paper, we present a simple method for enhancing the mechanical durability of the
commercially available Ultra-ever Dry superhydrophobic coating product. Its mechanical
durability, including hardness, and water-jet impact and abrasion resistances,are greatly
improved by introducing a larger scale nylon mesh as a substratefor the superhydrophobic
coating. We believe that the cause of the durability enhancement is directly attributable tothe
integrated 3D network introduced by the mesh structure.
2. EXPERIMENTAL SECTION
Figure1. Schematic of preparation of the superhydrophobic coating on nylon mesh
2.1. Materials
Ultra-Ever Dry coatings 4001 and 4002 were purchased from Ultra Tech International, Inc.
Low carbon steelsubstrates (30 x 25x 0.28mm), were obtained from Wuxi Guangyuan
Auspicious Metal Materials Co., Ltd. (China). The nylon mesh, available in different mesh
grades, was purchased from Shanghai Xingan Woven Fabric Co., Ltd.(China).The double-side
tape was purchased from 3M Co., Ltd. (China).
2.2. Preparation of the SHS coating
The meshes were rinsed sequentially with deionized water and ethanol, and then dried at room
temperature. The SHSs were fabricated in two steps, presented schematically in Figure 1. First,
the nylon mesh and substrate were joined by double-sided tape, forming a sandwich. The
superhydrophobic coating was sprayed on to the sandwich and then dried at room temperature.
The spraying pressure of the spray gun was 0.8MPa, and the amount of solution sprayed was
0.3mL/cm2, at a spraying distance of 15 cm; all these processes were carried according to the
manufacturer’s instructions.
2.3. Characterizations
The surface structures were examined by field emission scanning electron microscopy
(FESEM, Nova Nano-SEM, FEI, USA). The static contact angle (SCA) and sliding angle (SA)
were measured with a Kruss DSA20 apparatus at ambient temperature. The volume of the
individual water droplets were 7μL. The average SCA and SA values were obtained by
measuring at least five different positions per sample. The stability, including wear/water-
impact/hardness resistance, of the as-fabricated sample was also evaluated, and the conditions for
testing, listed in Figure 3, are those previously recommended24.
3. RESULTS AND DISCUSSION
3.1. Surface structures
The FESEM images of a typical nylon mesh (grade 400) (Figure 2a and 2b) demonstrate that
the mesh fibers are smooth; their diameters are ~ 50 ± 5μm, with the distance between fibers
being ~25 to ~ 100μm. FESEM images of the superhydrophobic Ultra-Ever Dry coating (Figure
2c and 2d) and the coating on a mesh structure (Figure 2e and 2f) reveal that pillars form on the
original surfaces. For the SHS coating, surface morphology can be composed by microstructure
pillars and nanoparticles24, the pillar diameters range from~ 20 to ~ 40μm, and the distance
between the pillars is approximately ~ 25 to ~50μm, which is consistent with a previous study24.
For the coating on the mesh structure, the pillar diameters are similar, while the distance between
the pillars is~40 to ~80μm, and they appear taller than for the coating without mesh. All the
pillars, for samples with and without mesh, are covered with nanoparticles (Figure 2). The pore
structures of the coatings on the mesh are much taller, as seen in the SEM photo micrographs in
Figure 2c, 2d, 2e and 2f.
Figure 2. FESEM images of (a) and (b) nylon mesh, (c) and (d) Ultra-Ever Dry coating, and (e) and (f) Ultra-
Ever Dry coating on nylon mesh.
3.2. Mechanical durability
Mechanical durability or robustness is one of the most important aspects necessary for the
industrial application of SH coating. It is typically evaluated by sandpaper abrasion test14, 25,26,27.
The abrasion resistance of our films was evaluated by the sandpaper abrasion test (Figure 3a).
The water droplet dependences of both the static contact angle (SCA) and slide angle (SA), as a
function of abrasion cycle or abrasion distance, have been used to characterize the mechanical
durability of a SHS. The variation in our SA values on the abrasion distance (1 cycle=2x15cm),
using different mesh grades, can be found in Figures 3 and 4. The abrasion resistances of all the
mesh-containing samples are greatly improved (Figure 3a, 3b) when using mesh. The change of
SA of the SHS both with and without mesh, as a function of abrasion distance, can be divided
into two parts: the slope first increases slowly over an abrasion distance of less than 500cm,
before becoming more rapid, reaching ~ 90°, with the water droplet pinned to the surface. The
tendency of the SA to increase with abrasion distance was slower with than without mesh,
especiallyfor the grade 400 mesh, which has the best abrasion resistance. It is interesting to note
that the SA of the SHS without mesh can be rapidly pinned after ~600cm abrasion distance (~
20 abrasion cycles). The wear-resistance, or anti-abrasion property, of the SHS on mesh depends
on the mesh grade.
Figure 3. (a) Schematic of the abrasion test employed to evaluate the mechanical durability of
superhydrophobic coatings, (b) sliding angle of SHS, as a function of abrasion distance, with a load of ∼ 200 g,
and (c) an enlargement of the first 1000 cm of abrasion distance in Figure 3b.
A typical change of SCAwith slide distance can be found in Figure 4. One notes that the SCA
slowly decreases with abrasion distance, from ~ 152º to ~142º, while SA increases from ~ 3º to
~ 90º. The scatter of the SCA data increases with abrasion distance, as seen in Figure 4. All these
results indicate that the dependence of the SCA on abrasion distance is relatively small, although
the SA variation on the abrasion distance is great, suggesting more nanostructures were lost from
the SHS during the abrasion process (the outermost nanostructures are associated with the SA2).
This is consistent with the surface morphology change seen by SEM, and discussed below. The
similarity of SA values both with and without mesh suggests both SHSs have similar abrasion
mechanisms.
Figure 4 .Typical SA and SCA as a function of sliding distance on the SH coating containing a grade 400 mesh.
Figure 5 shows photomicrographs of the coatings following abrasion testing. The outer layers
of the coatings, both with and without mesh structures, are seen to have been abraded. However,
there are many more pores in the sample with the mesh, compared to that without the mesh. The
meshes remain coated, although the nanostructures on the outer surface appear to have been
removed by the sandpaper.
Figure 5. A comparison of (a,b,e,f) SEM photomicrographs of SH coatings before, and (c,d,g,h)after the
abrasion test. (a)-(d) are of a SH coating without mesh and (e)-(h) are with mesh.
3.3. Water-jet impact testing
Figure 6 shows the mechanical durabilities of our SHSs on water-jet impact testing. Table 1
lists the hardness values of the coatings. The results show that the SA can be improved by using
mesh structures; both 300 and 400 grades are the best, on water-impact jet testing. Using 300 and
400 grade meshes, it takes 40min of water-jet impact testing to pin water droplets, while this
time is 30min without mesh structures. SCA values decrease from > 150º to ~ 135º for the mesh
structures although the value drops to <130º after 20min, in the absence of mesh. These result
sindicate that mechanical stability is greatly improved for soft impact (water-jet) on using mesh
structures.
Figure 6. (a) A water-jet impact setup and (b) SA and SCA as a function of water-jet impact time for the SH
coating on different meshes.
The hardness of the coating was evaluated by pencil scratching28. As showing in Table 1, the
hardness increases on using mesh, and it reaches a maximum for grade 400, consistent in with
our abrasion and water-jet impact testing. Figure 7 shows SEM photomicrographs of the SH
coating before (a, b, e, f) and after (c, d, g, h) water-jet impact testing. It is seen that the
nanoparticles on the outer surface have been worn away, with or without mesh. The difference
between the two SH coatings is then limited to the pores seen in the photomicrographs. This
suggests that the durability improvement of the mesh-containing SH coating is due to the
presence of larger pores for the mesh-containing coating.
Figure 7. SEM photomicrographs of the SH coating (a,b,e,f) before, and (c,d,g,h)after water-jet impact
testing.Here, (a)- (f) are of a SH coating without mesh, and (e)-(h) are of a coating with mesh.
The SA dependence on abrasion distance, as seen in Figures 3b and 3c, suggests that there is a
difference for different mesh grades. The best abrasion resistance was found for 400 grade mesh,
having an abrasion resistance distance (the distance over which water droplet remains pinned) of
almost 5000cm, compared to 600cm without the mesh. The change of SA slope with abrasion
distance, as shown Figure 4, from 0.0299 for the SHS without mesh to 0.013 for the SHS with
mesh, is more than halved. The reduced slope indicates the removal or wear of surface
nanoparticles is retarded. The same tendency for the SA as a function of abrasion distance, both
with and without mesh, is seen in Figure 4. This suggest that both have same wear mechanism,
and can be attributed to two abrasion processes, the first being the loss of nanoparticles from the
upper surface, and the second, the wear of the micropillars. The nylon mesh can be considered a
larger microstructure, with the micropillars smaller microstructures; the larger microstructures
enhance mechanical stability, as suggested in reference18. This can be understand as (1) the
increase of SHS roughness on using mesh structures, because the mesh structure is larger than
that of the original microstructure, and (2) the increased pore size and number of pores of the
SHS coating, which assists in protecting nanoparticles from loss during the abrasion process,
since the nanoparticles are better protected in the pores.
Table 1. Hardness of the SH coating as a function of different grades of mesh
Although the exact mechanism for the improved wear resistance of the SH coating, on using
the mesh structures, is unclear, it may be that the larger microscale structure assists in protecting
the micropillars as well as the fragile, fine-scale nanostructures. Because the mesh is integrated
in to the whole layer, its presence enhances both the hardness and the water-jet resistance of the
coating. Because the mesh structure is inexpensive compared with previously reported
processes18, it is easily commercialized and engineered.
4. CONCLUSIONS
The present work reveals a simple, low cost method that lends itself to the mass production of
mechanically robust SHSs. The improved mechanical durability can be attributed the large
microstructured mesh, and the increased porosity of the SHS coating that protects the smaller,
weaker micro- and nanostructures. The method can be used with any superhydrophobic coatings.
ACKNOWLEDGEMENTS
The work is supported by Haining Technology Innovation Founding program, Haining City
government.
References
1. Lafuma, A.; Quere, D., Superhydrophobic states. Nature materials 2003, 2 (7), 457-60.
2. Jung, Y. C.; Bhushan, B. Mechanically durable carbon nanotube− composite hierarchical
structures with superhydrophobicity, self-cleaning, and low-drag. ACS nano 2009, 3(12), 4155-
4163.
3. Ou, J.; Shi, Q.; Wang, Z.; Wang, F.; Xue, M.; Li, W.; Yan, G., Sessile droplet freezing
and ice adhesion on aluminum with different surface wettability and surface temperature.
Science China Physics, Mechanics & Astronomy 2015, 58 (7), 1-8.
4. Kako, T.; Nakajima, A.; Irie, H.; Kato, Z.; Uematsu, K.; Watanabe, T.; Hashimoto, K.
Adhesion and sliding of wet snow on a super-hydrophobic surface with hydrophilic channels.
Journal of Materials Science 2004, 39(2), 547-555.
5. Su, F.; Yao, K., Facile fabrication of superhydrophobic surface with excellent mechanical
abrasion and corrosion resistance on copper substrate by a novel method. ACS applied materials
& interfaces 2014, 6 (11), 8762-70.
6. Wang, F.; Lei, S.; Xue, M.; Ou, J.; Li, C.; Li, W., Superhydrophobic and Superoleophilic
Miniature Device for the Collection of Oils from Water Surfaces. The Journal of Physical
Chemistry C 2014, 118 (12), 6344-6351.
7. Yokoi, N.; Manabe, K.; Tenjimbayashi, M.; Shiratori, S., Optically transparent
superhydrophobic surfaces with enhanced mechanical abrasion resistance enabled by mesh
structure. ACS applied materials & interfaces 2015, 7 (8), 4809-4816.
8. Ou, J.; Hu, W.; Xue, M.; Wang, F.; Li, W., Superhydrophobic surfaces on light alloy
substrates fabricated by a versatile process and their corrosion protection. ACS applied materials
& interfaces 2013, 5 (8), 3101-3107.
9. Wang, Z.; Zhu, L.; Li, W.; Liu, H., Rapid reversible superhydrophobicity-to-
superhydrophilicity transition on alternating current etched brass. ACS applied materials &
interfaces 2013, 5 (11), 4808-4814.
10. Groten, J.; Ruhe, J., Surfaces with combined microscale and nanoscale structures: a route
to mechanically stable superhydrophobic surfaces? Langmuir : the ACS journal of surfaces and
colloids 2013, 29 (11), 3765-3772.
11. Wong, W. S.; Stachurski, Z. H.; Nisbet, D. R.; Tricoli, A., Ultra-Durable and Transparent
Self-Cleaning Surfaces by Large-Scale Self-Assembly of Hierarchical Interpenetrated Polymer
Networks. ACS applied materials & interfaces 2016, 8 (21), 13615-13623.
12. Hoshian, S.; Jokinen, V.; Somerkivi, V.; Lokanathan, A. R.; Franssila, S., Robust
superhydrophobic silicon without a low surface-energy hydrophobic coating. ACS applied
materials & interfaces 2015, 7 (1), 941-949.
13. Ge, D.; Yang, L.; Zhang, Y.; Rahmawan, Y.; Yang, S., Transparent and
Superamphiphobic Surfaces from One-Step Spray Coating of Stringed Silica Nanoparticle/Sol
Solutions. Particle & Particle Systems Characterization 2014, 31 (7), 763-770.
14. Verho, T.; Bower, C.; Andrew, P.; Franssila, S.; Ikkala, O.; Ras, R. H., Mechanically
durable superhydrophobic surfaces. Advanced materials 2011, 23 (5), 673-678.
15. Ou, J.; Hu, W.; Liu, S.; Xue, M.; Wang, F.; Li, W., Superoleophobic textured copper
surfaces fabricated by chemical etching/oxidation and surface fluorination. ACS applied
materials & interfaces 2013, 5 (20), 10035-10041.
16. Liu, Z.; Wang, H.; Zhang, X.; Lv, C.; Wang, C.; Zhu, Y., Robust and Chemically Stable
Superhydrophobic Composite Ceramic Coating Repellent Even to Hot Water. Advanced
Materials Interfaces 2017, 4 (7).
17. Zhou, H.; Wang, H.; Niu, H.; Gestos, A.; Wang, X.; Lin, T., Fluoroalkyl silane modified
silicone rubber/nanoparticle composite: a super durable, robust superhydrophobic fabric coating.
Advanced materials 2012, 24 (18), 2409-2412.
18. Huovinen, E.; Takkunen, L.; Korpela, T.; Suvanto, M.; Pakkanen, T. T.; Pakkanen, T. A.,
Mechanically robust superhydrophobic polymer surfaces based on protective micropillars.
Langmuir : the ACS journal of surfaces and colloids 2014, 30 (5), 1435-1443.
19. Song, H.-J.; Zhang, Z.-Z.; Men, X.-H., Superhydrophobic PEEK/PTFE composite
coating. Applied Physics A 2007, 91 (1), 73-76.
20. Xue, C.-H.; Ma, J.-Z., Long-lived superhydrophobic surfaces. Journal of Materials
Chemistry A 2013, 1 (13), 4146-4161.
21. Xu, Q. F.; Mondal, B.; Lyons, A. M. Fabricating superhydrophobic polymer surfaces
with excellent abrasion resistance by a simple lamination templating method. ACS applied
materials & interfaces 2011, 3(9), 3508-3514.
22. Li, J.; Wan, H.; Ye, Y.; Zhou, H.; Chen, J., One-step process for the fabrication of
superhydrophobic surfaces with easy repairability. Applied Surface Science 2012, 258 (7), 3115-
3118.
23. Zhang, Y.-Y.; Ge, Q.; Yang, L.-L.; Shi, X.-J.; Li, J.-J.; Yang, D.-Q.; Sacher, E., Durable
superhydrophobic PTFE films through the introduction of micro- and nanostructured pores.
Applied Surface Science 2015, 339, 151-157.
24. Wang, L.; Yang, J.; Zhu, Y.; Li, Z.; Sheng, T.; Hu, Y. M.; Yang, D.-Q., A study of the
mechanical and chemical durability of Ultra-Ever Dry Superhydrophobic coating on low carbon
steel surface. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2016, 497, 16-
27.
25. Zhu, X.; Zhang, Z.; Men, X.; Yang, J.; Wang, K.; Xu, X.; Zhou, X.; Xue, Q., Robust
superhydrophobic surfaces with mechanical durability and easy repairability. Journal of
Materials Chemistry 2011, 21 (39), 15793-15797.
26. Xiu, Y.; Liu, Y.; Hess, D. W.; Wong, C. P., Mechanically robust superhydrophobicity on
hierarchically structured Si surfaces. Nanotechnology 2010, 21 (15), 155705.
27. Wan, F.; Yang, D. Q.; Sacher, E. Repelling hot water from superhydrophobic surfaces
based on carbon nanotubes. Journal of Materials Chemistry A 2015, 3(33), 16953-16960.
28. Paints and varnishes-determination of film hardness by pencil test, ISO 15184:2012.