Ice accumulation is known to have significant negative impacts on roads, aircraft, cable cars etc., that could result in economic losses. At present, passive deicing technologies are commonly used to mitigate ice threats. Although these technologies are generally effective, they rely on energy-intensive methods that make them unsuitable for future applications. Therefore, developing more effective anti-icing technology is highly desirable as one way of saving energy and improving the safety of different equipment and infrastructure. Superhydrophobic materials are promising for anti-icing applications owing to their excellent water repellence property.
The application of superhydrophobic materials as anti-icing agents has been extensively studied. Superhydrophobic surfaces have two main droplet wetting regimes: Cassie and Wenzel. Cassie regimes are characterized by air trapped in the surface cavities and droplets with large contact angles. Wenzel regime experienced an increase in the adhesive force as most cavities are filled with liquid. The Cassie wetting regime is the most suitable for preventing ice accumulation. Nevertheless, the wetting state is influenced by numerous factors like the size of the droplet, and the wetting regime may change to Wenzel due to the impact of the droplet on the superhydrophobic surface.
Due to the significant impact of the wetting regime on the icing process, several studies have studied the wetting behavior of the impact of microdroplets on superhydrophobic surfaces. Unfortunately, most studies have focused on Cassie regime and low velocities due to the complexity of capturing micro-droplets at high speed. This provides inadequate understanding of the wetting mechanism and does not reflect the practical condition of icing processes.
On this account, Professor Anjie Hu and Professor Dong Liu from Southwest University of Science and Technology numerically investigated the influence of micro droplet on structured superhydrophobic surfaces using a three-dimensional (3D)-based volume-of-fluid (VOF) model. The droplet had a 20 µm impact diameter on the superhydrophobic surface with a wide range of velocities in the range 1 – 32 m/s was considered. The influences of the pillar height and intrinsic contact angle on the droplet behavior, as well as the impact of the droplet processes on the Wenzel and Cassie regimes, were explored. The work is currently published in the International Journal of Multiphase Flow.
The authors showed an increase in the droplet wetting pressure and a corresponding decrease in the impact velocity for the impalement transition due to the larger Laplace pressure of the micro-sized droplets than that of the macro droplets. As such, it was important not to neglect the Laplace pressure effect on the droplet impinging as it facilitated the transition to Cassie impact regime. A large intrinsic angle not only reduced the penetration depth of the Cassie impact but also remarkably suppressed the adhesion force of the Wenzel impact regime.
At a sufficiently large contact angle, the droplet could rebound in Wenzel regime, though with much lower restitution coefficient than that of Cassie regime. The bouncing ability was also significantly influenced by the pillar height. Although high pillars were generally beneficial in facilitating the Cassie regime, they also enhanced surface adhesion for impacts in the Wenzel regime. In contrast, shorter pillars were suitable for facilitating droplet bouncing in Wenzel regime. It is thus important to select the most appropriate pillar height and intrinsic contact angle based on the impact velocity of the droplet.
In summary, the research team studied the impact of micro water droplets on structured rough surfaces with large impact velocity ranges. The dynamic impact of the supercool droplets was identified as one of the potential factors influencing the anti-icing property of the superhydrophobic surface. The study strengthened the compression of the mechanism underlying the impacts of supercool droplets on high-speed anti-icing superhydrophobic surfaces. In a statement to Advances in Engineering, Professor Dong Liu stated that their findings would guide the design of high-performing anti0incing technologies. the authors focused on the reliability and durability problem of superhydrophobic surface in the anti-icing application by simulating a droplet impacting on the hydrophobic surface under different conditions that the micromechanism of the wet state transition during the droplet impact on the super-hydrophobic surface under different conditions. Our results reveal the micromechanism of the wet state transition during the impact, and provide a reference for superhydrophobic surface technology development.
Hu, A., & Liu, D. (2022). 3D simulation of micro droplet impact on the structured superhydrophobic surface. International Journal of Multiphase Flow, 147, 103887.