Characterization of Underwater Stability of Superhydrophobic Surfaces Using Quartz Crystal Microresonators

Abstract

We synthesized porous aluminum oxide nanostructures directly on a quartz crystal microresonator and investigated the properties of superhydrophobic surfaces, including the surface wettability, water permeation, and underwater superhydrophobic stability. After increasing the pore diameter to 80 nm (AAO80), a gold film was deposited onto the AAO80 membrane, and the pore entrance size was reduced to 30 nm (AAO30). The surfaces of the AAO80 and AAO30 were made to be hydrophobic through chemical modification by incubation with octadecanethiol (ODT) or octadecyltrichlorosilane (OTS), which produced three different types of superhydrophobic surfaces on quartz microresonators: OTS-modified AAO80 (OTS-AAO80), ODT-modified AAO30 (ODT-AAO30), and ODT-OTS-modified AAO30 (TS-AAO30). The loading of a water droplet onto a microresonator or the immersion of a resonator into water induced changes in the resonance frequency that corresponded to the water permeation into the nanopores. TS-AAO30 exhibited the best performance, with a low degree of water permeation, and a high stability. These features were attributed to the presence of sealed air pockets and the narrow pore entrance diameter.

Study of Transitions between Wetting States on Microcavity Arrays by Optical Transmission Microscopy

“…21−23,25,30,38−41 Among these studies, some either tracked the wetting transitions macroscopically using experimental techniques with less control 21,23,25,40,41 or used complex experimental setups and numerical calculations. 26,38,39,42,43 Salvadori et al reported on the wetting properties of microcavity arrays but did not consider the diffusion of gas molecules in water. 44 Lv et al reported a quantitative study of the air/water interface in single microcavities submerged in water.…”

Highly stable superhydrophobic surfaces under flow conditions

“…16,20 The pore diameter, pore-to-pore distance, and height of the resulting AAO nanostructure were 35 nm, 100 nm, and 1.2 lm, respectively. Pore widening was conducted for various durations and AAO nanostructures with various pore diameters were produced, as shown in the scanning electron microscopy (SEM) images in increases in the acoustic impedance (i.e., the increases in the void fraction) of the AAO layers.…”

“…Most studies, however, have focused on the fabrication of superhydrophobic surfaces and only a few have reported their stability in water. [13][14][15][16] Lee and Yong used a video camera to investigate the influence of hydrostatic pressure on the stability of W 18 O 49 nanowire-grown superhydrophobic surfaces with various surface chemistries. 15 The total reflection of white light between the water layer and the air pockets produced images of silvery surfaces, and the numbers of white and black pixels was converted to the surface areas of the nonwetted and wetted regions of the surface, respectively.…”

Communication: Anti-icing characteristics of superhydrophobic surfaces investigated by quartz crystal microresonators

“…The resonance frequency of a quartz crystal is sensitive to changes in mass and viscous damping only in the vicinity of its surface (∼1 µm) because the acoustic wave decays rapidly with the distance from the surface, 27,28 which enables the sensitive evaluation of anti-icing performance. We synthesized anodic aluminum oxide (AAO) or ZnO nanorods directly onto gold-coated quartz crystal substrates and rendered their surfaces hydrophobic or superhydrophobic via chemical modifications with octyltrichlorosilane (OTS), octadecyltrichlorosilane (ODS), or octadecanethiol (ODT).…”

“…27 The contact radius of the water droplets on the quartz crystals increases in the following order (which is the order of decreasing contact angle): C18-ZnO (0.33±0.02 mm) < C18-AAO (0.35 ± 0.02 mm) < C8-AAO (0.45 ± 0.05 mm) < C18-Au (1.31 ± 0.08 mm) < UV-Au (5.52 ± 0.57 mm). As a result, the largest decrease in resonance frequency upon the placement of a 2 µl water droplet was observed for the hydrophilic UV-Au due to its largest contact radius.…”