Fluorinated Nanocomposite Coatings for Confinement and Pumpless Transport of Low‐Surface‐Tension Liquids

Abstract

pollution. Repellent surfaces hinder liquid spreading and for that purpose, are frequently used for fluid management on open surfaces. In some cases, both repellent and adhesive domains are required for efficient fluid management. [1] Wettability engineering, i.e., the spatial modification of surface energy and morphology, is a suitable option for fluid management and has been explored extensively to deliver innovative advances in materials chemistry, fabrication techniques, and a fundamental understanding of tunable liquid properties from extreme repellency to extreme wetting. In particular, superliquid-repellent surfaces have shown great promise for self-cleaning, [2] anti-fogging, [3] anti-icing, [4] drag reduction, [5] and corrosion resistance [6] applications, among others. However, the majority of prior research has focused on applications employing high-surface-tension (γ) liquids (e.g., water, γ water ≈ 72 mN m −1 ). Difficulties arise when designing surfaces that are super-repellent to low-surface-tension liquids (20-40 mN m −1 ), where wetting interactions are more sensitive to surface chemistry and surface roughness. The methodology to making a surface repellent to these liquids is essentially the same, with surface energy and surface roughness being key factors. [7] As wetting behavior is governed by competing surface forces at the contact line, [8] liquid affinity (assuming chemical and physical attributes remain constant) is mainly dependent on the probe liquid's surface tension. Ideally, high-performing superoleophobic (extreme repellency to oils and other hydrocarbons) surfaces will exhibit Cassie-Baxter [9] type wetting across a wide range of surface-tension values. However, there may be instances where Wenzel [10] type (or complete) wetting is observed with liquids of low surface tension.In order to make surfaces with specially designed reentrant structures that promote repellency to low γ liquids, [11] researchers have typically relied on costly and complex microfabrication techniques. However, scalable, large-area, sprayable superoleophobic coatings have been explored by only a few groups. [12] In addition, an ultra-omniphobic (repellent to many liquids) material (FD-POSS; (1H,1H,2H,2H-heptadec afluorodecyl) 8 Si 8 O 12 ) has been synthesized by the Air Force Research Laboratory, and is currently considered the lowest surface-energy crystalline solid material with a solid-air surface energy of 10 mN m −1 . [13] This compound is functionalized Preparing surfaces that repel low-surface-tension liquids, such as oils and hydrocarbon fuels with surface tensions below 30 mN m −1 , poses more challenges than attaining water repellency. Oleophobic surfaces are needed when organic fluids must be contained to avoid pollutant spreading. A composite material system is presented comprised of fluorinated silica (filler), a perfluoroalkyl methacrylate copolymer (binder), and fluorinated polyhedral oligomeric silsesquioxane (additive; considered the lowest surface-energy material to date), which can be ...

Liquid Wells as Self‐Healing, Functional Analogues to Solid Vessels

“…To demonstrate, that the variety and complexity of shapes obtainable with this core–shell approach is comparable to LSTLs confined by omniphobic–omniphilic surfaces, we created several patterns with varying interior angles (60°, 90°, 120°) and a maze‐like structure (Figure 1C). [ 21,22,24,26 ] Compartments with an organic layer feature size down to 1 mm were fabricated.…”

“…[ 18–20 ] Furthermore, these methods often solely constrain the area wetted by the LSTL and there are only a few demonstrations of patterned LSTLs. [ 21–26 ] Jokinen et al. demonstrated the use of superhydrophobic–hydrophilic patterns to create designer multiphase droplets to confine an organic liquid droplet inside an aqueous droplet or for miniaturized liquid–liquid–liquid extraction application.…”

Patterning Wettability for Open-Surface Fluidic Manipulation: Fundamentals and Applications

“…Another area for continued research and investigation is the application of wettability gradients to low-surface-tension liquids, where omnidirectional spreading behavior is more commonly seen. Clearly, a different approach is needed here to attain rapid, directional transport …”

“…Regime I resembled the early stage spreading of unconfined viscous droplets through hemiwicking, yielding Washburn spreading characteristics (i.e., X ∼ T 0.5 ); the second regime displayed a linear trend (i.e., X ∼ T), ensuing from a balance between the driving capillary force and the viscous resistive force; the third spreading regime followed a density-augmented Tanner spreading trend (i.e., X ∼ T 0.1 ), which arises when the spreading film thickness becomes thin enough so that the dissipation at the contact line dictates the spreading behavior. Such mode of capillary-driven transport on wedge-shaped wettability confined tracks was not restricted only to water droplets; the principle was also harnessed to transport low surface-tension liquids with γ lg as low as 23.8 mN/ m. 163 For a surface immersed in water, a similar design with opposite contrast of wettability (i.e., a superhydrophobic wedgetrack laid on a superhydrophilic background) was shown to induce capillary-driven transport of air bubbles dispensed underneath the wettability patterned surface. 164 Unlike the spreading of liquid droplets on a wedge track, propagation of the air-bubble front, however, showed a linear temporal variation; a scaling argument revealed that the spreading velocity for the bubble on the superhydrophobic wedge track arises from an inertia−capillary force balance and varies as the inverse of the square root of the track width.…”

“…Liquid manipulation studies have primarily focused on surfaces where the wettability modification has been brought about through chemical functionalization, micro-and nanoscale roughness features, or a combination of both. 33,45,72,73,77,78,159,162,163 Roughness induced on such surfaces were random, which originated from mechanical or chemical etching and/or depositions of nanocomposites. A detailed account of liquid transport on such surfaces has already been provided in section 3.1. pattern i), whereas, the topographic gradients demonstrated by Shastry et al 102 consisted of square micropillars with silane and Teflon coatings; Figure 10, pattern ii.…”