Toward Surfaces with Droplet Impact Robustness and Low Contact Angle Hysteresis

Abstract

the cavity bottom or reaches the local advancing contact angle. [19][20][21] To increase this critical velocity, the surface structure has to be designed such that it imposes a strong resistance to liquid impalement. Without changing the structure scale, this target can be realized by using a dense array of structures [19][20][21][22][23][24] or structures with special top to pin the liquid. For instance, sharp edge structures [25] can resist impalement better due to the additional pinning effect. On the other hand, a dense array or a special pinning top leads to a higher contact angle hysteresis due to the extra pinning, which goes against the desired properties of superhydrophobic surfaces.The whole bouncing process includes different phases: the drop falling down to first contact with the surface, spreading to a maximum contact diameter, receding back and moving backward into the air. It has been shown that the dynamic contact angle can affect the drop impact behavior, [26][27][28][29] especially during the receding stage, which also affects the energy dissipation. Different criteria for the occurrence of droplet rebound have been reported in the literature, based on droplet impacting energy, [30] receding contact angle and contact angle hysteresis [26,27] . In order to have a rebound, the energy dissipation (i.e., viscous dissipation [30,31] and contact angle hysteresis [30,31] ) has to be minimized. Viscous dissipation can be reduced by minimizing the solid-liquid contact area. To further reduce energy dissipation and to increase the number of bouncing, the contact angle hysteresis needs to be reduced as well, which requires a surface design with less dense and lower pinning structures. However, a less dense surface structure with less pinning on the structures top will enhance liquid penetration and result in higher adhesion forces, limiting the surface robustness toward droplet bouncing.In this sense, optimization of anti-impalement ability and contact angle hysteresis is conflicting. While dense structures or pinning top can have better anti-impalement ability, the contact angle hysteresis in these cases is increased, thus increasing the energy dissipation and reducing the chances of bouncing. The main way to increase the anti-impalement ability while maintaining a low contact angle hysteresis has been achieved either by reducing the structures scale, namely the use of nanostructures, or by increasing the hierarchical level. [22,32] We present another solution to decouple the combined effect of anti-impalement ability and contact angle hysteresis. We Superhydrophobic surfaces show excellent water repellent performance. However, liquid impalement occurs when a droplet impacts on the surface at high velocity. To achieve higher resistance to liquid impalement at the same scale, a dense array of structures or structures with pinning at the structures top are typically used, but they also lead to increased contact angle hysteresis and higher energy dissipation. Changing the structure side wall shape can help t...

A Modified Weber Number Capturing the Bouncing–Wetting Transition of Droplet Impact on Rough Surfaces

“…[19] In order to achieve high surface water-repellency, hydrophobic coating and roughness design are often applied. [12,20] However, surface water-repellency could be undermined when the Weber number of an impacting droplet exceeds a threshold, where the impact outcome transits from bouncing to wetting due to the Cassie-Wenzel transition. [21,22] The Cassie state refers to the scenario that air is trapped in the micro-structure beneath the droplet.…”

“…[6][7][8] Experimental investigations of droplet impact on solid substrate revealed six identical modes, including deposition, partial bouncing, complete bouncing, receding breakup, prompt splash, and corona splash. [9] The outcome of droplet collision depends on various factors, such as droplet properties, [10] surface conditions, [11,12] ambient pressure, [13] and impacting angle and velocity. [14] Dimensionless numbers are used to characterize the droplet impact dynamics,…”

Air Cushion Storing Energy Promoting Droplets Retraction and Flow on Engineering Porous Bionic Lotus Surfaces

“…[39][40][41] Ding et al suggested that compared with cylindrical pillar surfaces, appropriate design of the cone surfaces can resist liquid impalement without the increase the contact angle hysteresis. [42] However, these etched surfaces with ordered structure cannot be prepared in large area and have poor mechanical properties, it is difficult for them to meet the needs of engineering applications. [43,44] By contrast, disordered porous superhydrophobic surfaces prepared by one-step method show superior application prospect.…”