Dynamics of High Weber Number Droplets Impacting on Hydrophobic Surfaces with Closed Micro-Cells
Authors: Zhang Rui , He Feng , Hao Pengfei 
Published: February 2020

Droplets impacting on solid surface, which consists of interesting physical phenomena and obsessed scientists all the time, is commonplace in nature and relevant for many industrial and technical applications, playing important roles in ink-jet printing, spray coating and cooling, crop dusting, vehicle soiling, forensic science, aircraft and powerline designing. Superhydrophobic surfaces that are excellently water-repellent and ice-preventive are important to modern technology. Much attention has been given to the dynamic behaviours of droplets impacting on superhydrophobic substrates with smooth surfaces, rough surfaces and surfaces structured with regular micro-posts. However, closed-cell textured surfaces with micro-cavities perform  better repellency against drop impact due to their improved mechanical and pressure stability. The one-tier hydrophobic surfaces with open and closed micro cells are designed and fabricated here. The impact dynamics and bouncing performance of high Weber number droplet on these surfaces are investigated. Central wetted rings are observed on both closed-cell and open-cell surfaces under high Weber number collisions, which are proposed to be the key element affecting the bouncing behaviours. It is found that the droplets rebound on closed-cell surfaces where the central area is in “hybrid wetting state” at high Weber number, while the droplets adhere to the open-cell surfaces where the centre region is in Wenzel state. A theoretical model is developed to explain this interesting phenomena, in which the liquid cannot reach the bottom of the closed-cell one-tier hydrophobic surfaces since the air stored in micro cavities prevents the sliding motion of the liquid film and plays as a “gas spring” lifting the liquid lamella. These findings are expected to be crucial to a fundamental understanding, as well as a remarkable strategy to guide the fabrication of novel super water-repellant and anti-icing surfaces with one-tier structures. 

Fig.1. demonstrates the microscopic structure of (a) closed-cell (micro-cavity) and (b) open-cell (micro-post) hydrophobic surfaces captured using ESEM; (c) measurement results of contact angle (CA) and contact angle hysteresis (CAH) of substrates with different depths; and (d) schematic diagram of experimental devices.

Fig.2. shows the dynamic behaviours of drops (We=980) impacting on (a) closed-cell and (b) open-cell micro-structured hydrophobic surfaces with depth of 27μm.

Fig.3. plots the comparison of wetting and bouncing properties of drops impacting on (a) closed-cell and (b) open-cell micro-structured surfaces with varied depths. The blue and yellow colour indicates the partition between rebound and adhesion. The red dots present the experiments where the centre area was partially or completely wetted while the black crosses signify that the centre area was not wetted. Hydrophobic surfaces with micro cavities are partially wetted at high Wenumbers with a central air bubble region while those with micro-posts are completely wetted with a central wetted ring in the Wenzel state.

Fig.4. presents the graphics of the centre wetted area of (a) open-cell hydrophobic, (b) closed-cell hydrophobic, (c) closed-cell hydrophilic, and (d) closed-cell superhydrophobic surfaces magnified by a drop convex lens at t=0.4 ms at We=1100. Insets are the centre wetted images at t=5 ms.

Fig.5.shows the evolution of wetting processes of drops impacting on closed-cell and open-cell surfaces with We number. At a low Wenumber, drops completely rebound with the centre of the surfaces being dry. At a high We number, drops rebound on closed-cell surfaces with the centre wetted area being in the “hybrid state”; while adhere to open-cell surfaces with the wetted area being in the Wenzel state.

Fig.6. describes (a)sketch of the “touch down” model for CWT on open-cell surfaces; (b) sketch of the “sliding” model for CWT on open-cell surfaces; and (c) schematic diagram of stressed state analysis of moving liquid lamella in closed-cell cavities.

Figure 1