Characterization of the state of a droplet on a micro-textured silicon wafer using ultrasound

Abstract

In this work, we propose acoustic characterization as a new method to probe wetting states on a superhydrophobic surface. The analysis of the multiple reflections of a longitudinal acoustic wave from solid-liquid and solid-vapor interfaces enables to distinguish between the two well known Cassie-Baxter and Wenzel wetting configurations. The phenomenon is investigated experimentally on silicon micro-pillars superhydrophobic surfaces and numerically using a finite difference time domain method. Numerical calculations of reflection coefficients show a good agreement with experimental measurements, and the method appears as a promising alternative to optical measurement methods.

In-situ ATR-FTIR for dynamic analysis of superhydrophobic breakdown on nanostructured silicon surfaces

“…Contact angle measurements are the most commonly used methods in experimental studies, but they cannot provide detailed information on the progress of superhydrophobic breakdown. A few optical or acoustic based techniques have been developed to characterize wetting states more accurately at the microscale 17 , 34 , 42 – 46 . For example, the solid-liquid interface and contact line motions upon wetting/dewetting can be visualized using confocal microscopy 42 , 44 , 45 .…”

The challenges, achievements and applications of submersible superhydrophobic materials

“…A transducer is integrated into the back-side of the examined surface, producing an ultrasound wave that propagates and reflects from the above surface. 93,101,102 Similar to the principle described in QCM, the reflection coefficient (r*) decreases for waves reflected at solid-water interfaces (Wenzel), compared to those reflected at solid-air interfaces (Cassie). 93 This is explained by the decrease in the amplitude of waves reflected from solid-water interfaces.…”

“…Another form of acoustic-based methods was reported by Saad et al that employed ultrasound waves to track wetting behaviour. 102 Fig. 13b illustrates the setup used in their experiment.…”

“…101 In addition, some reports showed that measuring WCAs does not always reflect surface micro-wetting changes and that different wetting configurations can show the same WCA/hysteresis reading, which makes accurate tracking of plastron decay challenging. 93,[102][103][104] As a way to directly visualize the small-scale wetting, methods such as cryogenic focused ion beam/scanning electron microscopy (cryo-FIB-SEM) have been reported. 105 Although such techniques can give an accurate visualization of plastron degradation, they require advanced technology and difficult preparation/operation conditions, in addition to the inability of dynamic tracking of wetting transitions.…”

Brightness of Microtrench Superhydrophobic Surfaces and Visual Detection of Intermediate Wetting States

“…Rathgen et al measured the diffraction pattern of reflected light to detect the detailed meniscus shape on parallel microtrenches. Acoustic sensing, − quartz crystal microbalance, microparticle image velocimetry, and transmission small-angle X-ray scattering were utilized to detect the intrusion depth (wetting depth) of water into surface roughness. On the other hand, a variety of strategies have been used for more direct visualization of the water–air interface on an SHPo surface, including optical microscopy, ,, confocal microscopy, − interference microscopy, atomic force microscopy, − environmental scanning electron microscopy (SEM), X-ray computed tomography, , and freeze fracturing approach. , Unfortunately, however, they are not suitable or convenient for large samples or general field studies (e.g., a boat on the sea or a model in a tow tank, , where the SHPo surface of interest may travel at high speed under water and cannot accommodate installation of any bulky instrument).…”