Following the Wetting of One-Dimensional Photoactive Surfaces

Abstract

This article aims toward a full description of the wetting conversion from superhydrophobicity to superhydrophilicity under illumination with UV light of high-density ZnO nanorods surfaces by (i) following the evolution of the clusters and superstructures formed by the nanocarpet effect as a function of the water contact angle (WCA); (ii) characterization of the superhydrophobic and superhydrophilic states with an environmental scanning electron microscope (ESEM); and (iii) using the nanocarpet effect as a footprint of both local and apparent water contact angles. Thus, the main objective of the article is to provide a general vision of the wettability of 1D photoactive surfaces. In parallel, the nanocarpet (NC) formation by clustering of vertically aligned ZnO nanorods (NR) when water is dripped on their surface and then dried is studied for the first time by taking advantage of the possibility of tuning the surface water contact angle of the ZnO NR structure under UV preillumination. As a result, we demonstrate the feasibility of controlling the size and other morphological characteristics of the NCs. Moreover, a strong anisotropic wetting behavior, characterized by a Δθ = θ∥ – θ⊥ = 30°, is shown on an asymmetrically aligned NC surface resulting from arrays of tilted NRs. The study of the condensation/evaporation of water on/from an as-prepared (superhydrophobic) or a preilluminated (superhydrophilic) NR surface examined by an environmental scanning electron microscope has evidenced the formation of supported water droplets with polygonal shapes in the first case and the complete filling of the inter-NR space in the latter. The long-term stability of the NC clusters has been utilized as a footprint to track the penetration depth of water within the inter-NR space in the three borderline regions of water droplets. This analysis has shown that for moderately hydrophobic surfaces (i.e., water contact angles lower than 130°) water droplets do not present a well-defined borderline trace but a spreading region where water penetrates differently with the NR interspace. The transition from a Cassie–Baxter to a modified Cassie–Baxter to finish in a Wenzel wetting state is found on these surfaces depending on the UV preillumination time and is explained with a model where water interaction with the NR units is the critical factor determining the macroscopic wetting behavior of these surfaces.

Fabricating Superhydrophobic Polymeric Materials for Biomedical Applications

“…This permanent or semi-permanent water repellency is a useful property that is employed to prepare self-cleaning surfaces 7 , microfluidic devices 8 , anti-fouling cell/protein surfaces 9,10 , drag-reducing surfaces 11 , and drug delivery devices [12][13][14][15] . Recently, stimuli-responsive superhydrophobic materials are described where the nonwetted to wetted state is triggered by chemical, physical, or environmental cues (e.g., light, pH, temperature, ultrasound, and applied electrical potential/current) 14,[16][17][18][19][20] , and these materials are finding use for additional applications [21][22][23][24][25] .…”

“…Wettability is also controlled by the choice of copolymer dopant species. In this case, 6.5-7.5-µm fiber PLGA meshes doped with 30% PLA-PGC 18 (90:10) or 30% PLA-PGC 18 (60:40) exhibit apparent contact angles of 133° or 154°, respectively ( Figure 3C). Altering (i.e., reducing) the fiber size also enhances hydrophobicity independent of dopant selection and/or concentration.…”

“…For example, thermal analysis using differential scanning calorimetry (DSC) reveals that PLA-PGC 18 polymers containing 10, 20, 30, or 40 mol% PGC 18 monomer gradually become more crystalline with increased PGC mol%. The thermal properties of PCL-PGC 18 and PLA-PGC 18 copolymers are summarized in Table 2.…”

“…While PCL possesses an advancing water contact angle of 84°, the advancing contact angle for PCL-PGC 18 (80:20) is ~120°. Likewise, PLGA possesses an advancing contact angle of 71°, while PLA-PGC 18 (90:10) and PLA-PGC 18 (60:40) exhibit advancing contact angles of 99° and 105°, respectively.…”

“…Figure 3A illustrates how doping in 30% PCL-PGC 18 or PLA-PGC 18 transitions meshes from hydrophobic to superhydrophobic. Superhydrophobicity is defined as an apparent water contact angle ≥ 150° with a low contact angle hysteresis-defined as the difference between advancing and receding water contact angle measurements.…”