Functional and versatile superhydrophobic coatings via stoichiometric silanization

Superhydrophobic coatings have tremendous potential for applications in different fields and have been achieved commonly by increasing nanoscale roughness and lowering surface tension. Limited by the availability of either ideal nano-structural templates or simple fabrication procedures, the search of superhydrophobic coatings that are easy to manufacture and are robust in real-life applications remains challenging for both academia and industry. Herein, we report an unconventional protocol based on a single-step, stoichiometrically controlled reaction of long-chain organosilanes with water, which creates micro- to nano-scale hierarchical siloxane aggregates dispersible in industrial solvents (as the coating mixture). Excellent superhydrophobicity (ultrahigh water contact angle >170° and ultralow sliding angle <1°) has been attained on solid materials of various compositions and dimensions, by simply dipping into or spraying with the coating mixture. It has been demonstrated that these complete waterproof coatings hold excellent properties in terms of cost, scalability, robustness, and particularly the capability of encapsulating other functional materials (e.g. luminescent dyes).

forming head-to-head linear fibers subsequently.

Supplementary Fig. 5 Progression of OTS/water reaction.
For better visualization, a 3 mL reaction mixture prepared according to the procedure described in the Method section was transferred to a plastic cuvette. The reaction was initially fast, particularly for the first hour, with a noticeable amount of gas generated. It slowed down after 2 h. At about 6 h, the reaction was completed, and the product turned into a gel that was no longer dispersible in a solvent. Side view of a FIB-cut particle on the treated glass surface. The EDX data b shows the elemental composition of red square in a. As gold salt is soluble in water and insoluble in organic solvents (hexane and OTS), the presence of Au in every nanofiber, indicates that these fibers are formed from the sols. c SEM image of the surface in between microparticles (inset is the overlay of gold mapping with surface morphology). d Elemental composition with the gold peak clearly shown.  Fig. 9 Abrasion test of treated filter paper. A small piece of the treated paper (1 × 2 cm 2 ) was affixed at the bottom of a weight (50 g), which was then dragged on the surface of sandpaper (Grit No. 400). From a to c shows one cycle of abrasion (50 cm); this method was adapted from Ref. 9. Water contact angles were measured before (inset in a) and after (inset in c) the test.   Micro/nano morphological structures were created on silicon wafer with an aluminum-induced crystallization technique. Then OTS SAM was deposited on the modified silicon surface by immersing in an OTS/toluene solution.

155
OTS self-assembly and crosslink on naturally hydrophilic micro/nanostructured surfaces to render superhydrophobicity.

Chem. Eng. J.
Fiberglass cloth fiber was pre-"epoxidated" with an epoxy resin, followed with the deposition of aminofunctionalized silica particles. It was then immersed into an OTS ethanol solution (60 C for 2 h).
151 -154 Generate sufficient surface roughness by depositing amino-functionalized silica particle; and achieve low surface energy through reacting OTS with amine groups on amino-silica surface and with epoxy groups on epoxy resin.
Pre-cleaned silicon nanowires/nanowalls are first modified with OTS, followed by another coating of n-octadecane onto silicon wafer substrates.
165° ± 5 The nanowire/nanowall structure contributes to the surface morphology, while the OTS modification lowers the surface energy. SiO 2 nanoparticles are modified with OTS first, followed by mixing with epoxy resin and polydimethylsiloxane in ethyl acetate solution. The suspension is then sprayed on the substrate.

159.5
SiO 2 nanoparticles provides a "skeleton" that increases the surface roughness. OTS modification generates a low surface tension coating.

RSC Adv.
Cotton substrate is coated with a layer of SiO 2 particles by immersing into a TEOS solution, followed by dipping in an OTS/ethanol solution for 2 h.
151° ± 1.2 SiO 2 nanoparticle coating introduces surface roughness, while the surface tension of cotton fibers is reduced via modification with TEOS and OTS. * Considering marginal profits, only material cost is calculated. The price of mineral spirits and OTS are1000 US$/ton and 1100 US$/ton, respectively (on average industrial price). Hence, the total material cost is 0.1 US$. We assume that the total cost is three times of the material expense.
S-16 Table 3. Comparison of representative superhydrophobic coatings with the present coating protocol.

Reference Fabrication WCA Robustness
This work Small amount of water first mixed with bulk amount of OTS based on stoichiometry. The mixture is later diluted and applied onto substrates by dipping or spraying.

Ref. 6 Deng et al. Science
Substrate is initially covered with candle soot, followed by coating with tetraethoxysilane. The carbon soot core is later "burnt" through calcination, followed by a second coating of a semifluorinated silane via vapor deposition.

Nat. Mater.
Mixing polytetrafluoroethylene nanoparticles, perfluoropolyether, epoxy suspension, and fluorinated amine in acetone. The coating solution can be applied to substrates by spraying, brushing, or roll-coating.

~158°
The modified substrate is resistant to 30 cycles of tape peel test, 100 cycles of the standard Taber abrasion with loads of 150 g and 200 g, harsh chemical corrosion, and water jet at >35 m/s.

Ref. 9 Lu et al. Science
Mixing two types of TiO 2 nanoparticles in an ethanol solution of perfluorooctyltriethoxysilane to make a "paint" before applying onto substrates along with adhesives (double-sided tape/spray adhesive) via spraying, dipcoating, or syringe extruding.

>150°
Superhydrophobic Pretreated-copper foam is dipped into a mixture of tetramethylethylenediamine, pyridine, and acetone, followed by adding hexakisbenzene dropwise. The resulted porous foam is coated with PDMS through vapor deposition.

157.6°
The contact angle dropped to 147.7° after 5 cycles of sandpaper abrasion (with 100 g weight on top, 20 cm abrasion distance each cycle).