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.