Unusual Dynamic Dewetting Behavior of Smooth Perfluorinated Hybrid Films: Potential Advantages over Conventional Textured and Liquid-Infused Perfluorinated Surfaces

Abstract

From a viewpoint of reducing the burden on the environment and human health, an alternative method for preparing liquid-repellent surfaces without relying on the long perfluorocarbons (C((X-1)/2)F(X), X ≥ 17) has been strongly demanded lately. In this study, we have successfully demonstrated that dynamic dewettability toward various probe liquids (polar and nonpolar liquids with high or low surface tension) can be tuned by not only controlling surface chemistries (surface energies) but also the physical (solid-like or liquid-like) nature of the surface. We prepared smooth and transparent organic-inorganic hybrid films exhibiting unusual dynamic dewetting behavior toward various probe liquids using a simple sol-gel reaction based on the co-hydrolysis and co-condensation of a mixture including a range of perfluoroalkylsilanes (FASX, C((X-1)/2)F(X)CH2CH2Si(OR)3, where X = 3, 9, 13, and 17) and tetramethoxysilane (Si(OCH3)4, TMOS). Dynamic contact angle (CA) and substrate tilt angle (TA) measurements confirmed that our FASX-hybrid films exhibited excellent dynamic dewetting properties and were mostly independent of the length of perfluoroalkyl (Rf) groups. For example, 10 μL droplets of ultralow surface tension liquids (e.g., diethyl ether (γ = 16.26 dyn/cm) and n-pentane (γ = 15.51 dyn/cm)) could move easily on our FAS9-, FAS13-, and FAS17-hybrid film surfaces at low substrate TAs (<4°) without pinning. This is comparable or superior to the best perfluorinated textured and flat surfaces reported so far. This exceptional dynamic dewetting behavior appeared only when TMOS molecules were added to the precursor solutions; we assume this is due to co-condensed TMOS-derived silica species working as spacers between the neighboring Rf chains, enabling them to rotate freely and in doing so provide a surface with liquid-like properties. This led to the distinguished dynamic dewettability of our hybrid films, regardless of the small static CAs. Our FASX-hybrid films also displayed excellent chemical and physical durability against thermal stress (~250 °C), high-temperature (150 °C) oil vapor, and various other media (perfluoro liquid, boiling water, and weak acid) without degrading their dynamic dewettability. Such exceptional durability has been rarely seen on conventional perfluorinated surfaces reported so far.

Novel transparent, liquid-repellent smooth surfaces with mechanical durability

“…In addition, the surface energy of smooth film was calculated to be 10.5mN m -1 by using three liquids method. Packing density of the perfluoroalkyl groups on the surfaces could be gradually reduced with the increasing rubbing times; menwhile, physical mobility of the neighboring perfluoroalkyl groups might be enhanced, meaning liquid droplets in contact with them can experience very low energy barriers between metastable states, leading to formation of excellent liquid-repellent surface [25,30]. Additionally, when the surface was rubbed at overfull times, more silanol groups could expose on the surface and further weaken the liquid-repellency.…”

“…However, such liquid-infused smooth surface is easily contaminated by impurities, and unstable under rigorous conditions because of evaporation or washout of the infused liquid. Surprisingly, development of smooth hybrid film surfaces exhibiting exceptional dynamic dewetting behavior against organic liquids has offered new approaches and practical advantages for production of omniphobic coatings [25]. However, fluorinated silane was directly used as adhesive on substrates, which could limit potential applications due to poor adherence of the fluorinated compounds.…”

Turning a surface superrepellent even to completely wetting liquids

“…For low energy liquids such as oils or organic solvents, a roughness with overhanging topology was necessary to make the hydrophobic material superoleophobic (13,14) or so-called omniphobic (15) or superomniphobic (16,17). Despite the use of prefix omni- (6,(15)(16)(17)(18), however, no natural or man-made surface has been reported to repel liquids of extremely low surface tension/energy (i.e., γ < 15 mJ/m 2 ), such as fluorinated solvents, which completely wet existing materials (10,(19)(20)(21). Departing from the prevailing approach of roughening a hydrophobic material, we first propose the material's inherent wettability, depicted by the intrinsic contact angle θ Y , is irrelevant when dealing with a completely wetting liquid (θ Y = 0°).…”

3D Printing of Bioinspired Liquid Superrepellent Structures

“…To achieve such ability, combination of hierarchical micro/nanostructures and hydrocarbon/fluorocarbon compounds is an optional strategy, which usually involves etching the substrates or introducing extra coatings . Unfortunately, these artificial surfaces are unable to repel most fluorinated solvents (γ < 15 mN m −1 ), which exhibit a contact angle of 0 for all existing materials . Therefore, it is highly necessary to search for more reasonable structures to universally repel organic liquids.…”