We also evaluated the effects of varying the pillar height on the transparency of the fabricated sur- faces. With increasing pillar height, the PDMS/F-POSS particles were found to agglomerate more
Figure 6.11: Time-lapsed movie frames corresponding to flat PDMS (top), 20 µm high pillars with D∗ = 100 (middle), and 40 µm high pillars with D∗ = 100 (bottom). The frames support the proposed model, as fewer particles accumulate between the 20 µm high pillars than between the 40 µm high pillars.
quickly around the pillars because of the increase in surface area (and decreasing βcr). To better understand this phenomenon, spray coating was combined with time-lapsed optical microscopy for different pillar heights and D∗values over a spray time of 240 seconds, in intervals of 15 seconds. In Fig. 6.11, still frames are shown that correspond to flat PDMS, pillars with D∗ = 100 and H = 20 µm, and pillars with D∗ = 100 and H = 40 µm, respectively.
It should be emphasized that all surfaces were sprayed together within 5 mm of one another, so that the observed effects were purely due to geometry. It was evident that a greater pillar height lead to faster lateral growth of the PDMS/F-POSS aggregates, which in turn lowered the overall transparency. However, because of the small pillar size (2R = 15 µm) and sparse pillar density (2D = 140 µm), the optimally sprayed surfaces were visually still close to a transparency of 100%. The low ∆θ∗ and the flexibility of our fabricated surfaces are highlighted in Fig. 6.12. The fabricated surfaces with D∗ = 100 (sprayed for 120 seconds) were curved into arcs, and various droplets of liquids with high or low surface tension were deposited on them. The droplets rolled back and forth along the length of the surface until the edge was reached. The trajectories of both water and ethanol molecules are shown in the superimposed frames, spaced every 10 ms.
Figure 6.12:Movie frames taken with 10 ms intervals were superimposed for droplets of water and ethanol, respectively. The droplets rolled back and forth several times before reaching the edge of the substrate, which highlights the ultra-low ∆θ∗.
6.3.5
Conclusions
To the best of our knowledge, this is one of the first flexible and transparent surfaces that displays ultra-low contact angle hysteresis with low surface tension liquids, such as ethanol. Furthermore, the simple mold and spray fabrication method may be well-suited to scale-up to functionalize sig- nificantly larger areas. However, one shortcoming of these systems is their mechanical durability. PDMS as a base material is mechanically poor, and thus even relatively mild mechanical abrasion renders these surface wetting to both high and low surface tension liquids. In the next chapter I discuss a new method for fabricating extremely mechanically robust superhydrophobic surfaces (note, not superomniphobic).
CHAPTER 7
Designing Self-Healing Superhydrophobic Surfaces
with Exceptional Mechanical Durability
7.1
Introduction
Superhydrophobic surfaces have garnered much attention over the last few decades for their ability to be self-cleaning[32], drag-reducing[126], stain-resisting[174] and anti-fouling[127]. By trap- ping pockets of air in their porous texture, SHSs display water contact angles θ∗ > 150◦ and low roll-off angles[145]. The design and optimization of such surfaces have been well studied[32,126,
174,127,145,175,176,128,129,26,41,146,177,178,179,14,180]. However, most natural and artificial SHSs suffer from poor mechanical durability, as their fragile and porous surface texture can be easily removed even by the swipe of a finger[146]. Only a few SHSs have been reported to exhibit mechanical durability, as characterized by sand impact[181, 182, 130, 183, 184], rub- bing with a soft cloth[174,185, 186,187], tape peel tests[176,182, 188,133, 189], or sandpaper abrasion[129,146,178,179,14,182,186,190,191,192,193,194,195,141,196, 197, 198, 199,
200]. However, all such reports present single material systems. The development of design cri- teria to aid in the systematic fabrication of durable SHSs, generalizable to multiple chemistries or fillers, is expected to be extremely useful to the field. In the first part of this work, we develop such criteria.
Even the most durable SHSs will eventually become damaged by extreme or repeated me- chanical abrasion, which damages a SHS’s low surface energy and/or texture. SHSs that can regenerate both their surface texture and chemistry[201,185,11], akin to the lotus leaf’s ability to regenerate its nano-structured wax[32], would be highly desirable. Herein we also report mechan- ically durable SHSs that exhibit physical and chemical self-healing. The developed surfaces can fully recover their water-repellency even after being abraded, scratched, burned, plasma cleaned, flattened, sonicated and chemically attacked. These surfaces, and the design parameters used to develop them, may find immediate usage in a wide range of academic and industrial sectors across
7.2
Materials and methods
7.2.1
Materials and synthesis
All solvents, pre-polymers, and crosslinking agents were used as-received. Fluorinated solvents HCFC-225ca/cb (Asahiklin-225, Asahi Glass Co.) and HFC-43-10mee (Vertrel XF, DuPont) were purchased from Techspray and TMC Industries, Inc. respectively. Poly(methyl methacrylate) (PMMA), polystyrene (PS, 45 kDa or 1.2 kDa) and polyisobutylene (PIB) were purchased from Scientific Polymer. Luxecolor 4FVBA fluorinated polyol resin (FPU, 55% solids in n-butyl ac- etate) was purchased from Helicity Technologies, Inc. Desmophen 670BA polyol was provided by Bayer MaterialScience, A.G. Isocyanate crosslinkers Desmodur N3200 and Wannate HMDI (4,4-Diisocyanato-methylenedicyclohexane) were provided by Bayer MaterialScience, A.G. and Wanhua Chemical Group Co., Ltd. respectively. Crosslinker ratios were 9.7 and 3.4 wt% respec- tively with FPU, and 28.5 wt% N3200 with 670BA. Propylene glycol, a chain-extending agent that increases the modulus of the final cross-linked polyurethane network, was obtained from MP Biomedicals, LLC. A polyurethane elastomer (Vytaflex 40) was purchased from Smooth-On, Inc., and was prepared according to manufacturer directions. CNR (chlorinated polyisoprene) was pro- vided by Covestro. Polydimethylsiloxane elastomer (Dow Corning Sylgard 184) was obtained from Krayden, Inc. and a 10:1 base:crosslinker ratio was used according to manufacturer di- rections. Acrylate-terminated perfluoropolyether resin (CN4001, purchased from Sartomer USA, LLC) was mixed with 5 wt% radical photoinitiator (Irgacure 2022, provided by BASF Corporation) to yield a UV-curable fluorinated polymer matrix. Cyanoacrylate adhesive (3M Scotch-Weld SF- 100) was purchased from Pack-n-Tape, Inc. Two-part epoxy adhesive (Selleys Araldite 90 seconds) was used in an approximate 1:1 volume ratio of the components, per manufacturer instructions.
Fluorodecyl and fluorooctyl polyhedral oligomeric silsesquioxanes (F-POSS, FO-POSS) were prepared by condensing perfluorinated triethoxysilanes as previously reported[134]. Octaisobutyl polyhedral oligomeric silsesquioxane (IB-POSS) was purchased from Hybrid Plastics, Inc. Eicosane was purchased from Acros Organics.