Sistemas democra ́ ticos

Despite the immense progress that is achieved by the research community in successfully transitioning from plasma-corona-flame dependent to UV-photoactivated superhydrophilicity, truly intervention-free superhydrophilicity remains a distant goal. One primary limitation that continues to persist in the above methods lies in the need to preserve ultra-high surface energies.[238] Without self- regeneration[5,261-263] characteristics, longevity is scarcely possible. In very recent years, the appearance of a slower but equally effective state of superwetting, which is known as hemi-wicking superhydrophilicity,[124] appears to resolve these standing issues (Figure 2.29).

Figure 2.29. Hemi-wicking dynamics. a) Evolution of the facet size-ratios Ddi/Dax and velocity-ratio Udi/Uax as a function of the angle φ. The dashed line corresponds to Uax/cosφ = Udi/cos(π/4−φ). Inset: top view of an isopropanol droplet spreading on a surface roughness defining the variables Udi, Uax, Ddi, Dax, and φ.

These hemi-wicking superhydrophilic surfaces are a) permanently superhydrophilic in ambient-air without the need for re-activation and are b) capable of sustaining much larger degrees of contamination while preserving function.[30,124] On hindsight, this reflects the surface characteristics of the superhydrophilic leaves belonging toRuellia devosiana, a wild Brazilian petunia. The petunia’s

leaves are composed of a complex surface hierarchy of hairs and channel-like structures, which enables superior capillarity effects, akin to hemi-wicking superhydrophilicity.[199]

2.4.3.1. Capillarity-(Structure) Enhanced Fluid Imbibition

Wicking is a ubiquitous phenomenon which is seen in every-day mundane materials such as paper, fabrics, rocks and even kitchen sponges. However, wicking[124] presents fresh perspectives towards the development of alternative superhydrophilic interfaces with contamination-proof capabilities. While the basis of wetting in both hemi-wicking and ideal superhydrophilicity is similar in terms of contact line advancements, they remain fundamentally different. Wicking departs from the traditional understanding of superhydrophilicity, primarily due to its dominant use of structure-dependent capillary-based driving forces.[2] Contrasting the ideal state of superhydrophilicity, hemi-wicking does not require very high interfacial surface energies. Deviation from this draconian requirement comes as an advantage as it suggests that surfaces can sustain much larger degrees of atmospheric contamination before an eventual loss of functionality. The use of lower, but more robust surface energy states, coupled to capillary-wicking features, realizes the steady and gradual fluid imbibition that is observed within hemi-wicking.

However, in contrast to ideal superhydrophilicity, hemi-wicking superhydrophilicity tends to result in complete wetting (CA = 0°) owing to the constant interfacial driving forces.[130] The dynamic wetting behaviors have been classically studied using a forest of micropillars.[124] Wicking dynamics within a regularly patterned rough surface is explained through similar Washburn equations that were originally defined for the capillary effect.[2,124] Per these definitions, taller micro-structured films will result in much more rapid wicking behaviors as compared to a shorter, similarly structured surface. The increasing conduit / channel sizes culminate in improved wicking speeds. However, this effect can plateau, and wicking speeds eventually become independent of increasing channel dimensions. This occurs because the driving forces through the conduits / channels eventually balances against the viscous friction posed by movement of the fluid.[124] Despite the many advantages posed by this

system, hemi-wicking’s demonstrated rate of droplet spread is not comparable to speeds observed in ideal superhydrophilicity.[30,124] This drawback could be severe depending on its intended application. Anti-fogging for instance, requires droplet spread to occur in less than 0.5 s for function,[123] contrasting the much larger timeframe (seconds to minutes) taken by hemi-wicking superhydrophilicity. Unfortunately, owing to such slower dynamic spreading behavior, the full potential of hemi-wicking superhydrophilic coatings is still largely unexplored.

Today, research in this area continues to be directed towards the synthesis of superhydrophilic coatings that are 1) highly functional (rapid spread), 2) UV-independent, 3) long-lived and contamination-proof.[123,269]

2.4.3.2. Organically-enhanced Wettability

Thiol-Gold Functionalization

Surface assembly of organic layers on materials is a known and adequately studied phenomenon. In the early 1990s, Whitesides et al. pioneered the use of self-assembled monolayers (SAMs) for tuning the wettability of surfaces.[275-277] A combination of organothiols[275] and carboxylic acids[276,277] were used in conjunction with gold for self-assembling wetting-tunable surfaces via permanent sulphur- gold bonding[278]_{. In later years, Notsu et al. demonstrated the achievement of superhydrophilic states} by imparting multi-scale roughness to the gold substrates.[236] The use of organothiols can also be applied to other (precious) metals, such as silver, copper, platinum and palladium.[8]

Silane Functionalization

Chloro- and alkoxy- silanes have been used extensively for the functionalization of metallic oxides, ranging from SiO2,[279-282] _Al2O3,[283,284] _TiO2[285] _{amongst others. Functionalization occurs through} the nucleophilic substitution of the surface hydroxyl, resulting in the formation of a silanized graft and the side-products of HCl or alkyl-ol.

Huang et al. demonstrated the use of zwitterionic sulfobetaine silane (SBSi) for the functionalization of oxidized surfaces (wire meshes). Coatings demonstrate excellent wettability, with CAs of < 5°, coupled to excellent transmissivity, ca. 100% at 480 nm. Surfaces were also resilient to pencil scratch test and ambient environment exposure for more than a year.

Thiol-yne Click Functionalization

The thiol-yne reaction occurs between a thiol (R-S-H) and an alkyne (R≡R), first reported in 1949,[286,287] _{and later re-discovered in 2009}[288]_{. Today, it is also known as click chemistry, which} demonstrates immense potential for designer-polymers. Click-chemistry enables facile functionalization of polymers with alkyne functional groups, thus departing from traditional metal- and oxide- dependent substrates for surface functionalization (Figure 2.30).[15,45,289]

Figure 2.30. Organothiol functionalization. a) Kinetics of organothiol adsorption for wettability- modifications.[275] _{b) Tri-alkoxy silanization of SiO2 surfaces.}[279] _{c) Sequential addition and} hydrogen abstraction during thiol-ene polymerization.[288]

Amphiphiles

Amphiphilic assembly of mono- or multi-layer organics can occurviasurface adsorption from either a solution or vapor phase.[290] _{This has been traditionally demonstrated through a Langmuir-Blodgett}

(LB) film. Thin organic layers self-assemble spontaneously on surfaces, depending on the relative surface free energies, thus altering the wettability of materials. The choice of organic layers for achieving hydrophilicity can be made by selecting polar end groups. More specifically, hydrophilicity is best attained with functional groups that possess enhanced affinity with water molecules, such as hydroxyls (-OH) or carboxyls (-COOH).[276,277,291] However, as of the time of writing, despite evidently improved hydrophilicity, this method has not been able to independently achieve perfect superhydrophilicity that demonstrates WCAs of 0°.[8] Physically adsorbed monolayers are also known to possess poorer stability as compared to chemically bonded layers during fluid interaction.[290] These temporarily induced states of wettability can, however, be potentially exploited for facile, reversibly-switchable states of wetting.