Reducing droplet contact time and area by craterlike surface structure

Reduction in contact time and area between the rebounding droplet and the solid surface has attracted great attention due to the potential applications in many fields, such as self-cleaning and anti-icing. In this paper, we provide an alternative strategy for designing a functional surface, craterlike structure, with which the interaction parameter (the integration of contact area over time) can be reduced by over $75\%$. Systematic investigations on the parameters of craterlike structure, including its diameter, depth, and landing location, have been conducted by a particle-based numerical method, many-body dissipative particle dynamics. This work could be helpful for the design and preparation technology of functional surfaces.

Figure 1

Three different renders of the simulation result. (a) Particle view; (b) Mesh view; (c) Ray-tracing render view. The DPD simulations directly provide liquid particles position information (a); then, we reconstruct the surface mesh by identifying different clusters and the particles near the surface (b); last, we render the mesh by adding light, material, etc. (c).

Figure 2

The comparison of experimental images and DPD simulation results. (a)–(d) Experimental snapshots of a droplet impacting a superhydrophobic surface decorated with a ring structure. The droplet stretches into the air and forming a three-dimensional water bowl. Finally, the droplet retracts and liftoff. (e)–(h) DPD simulation snapshots. The DPD results generally capture the shape evolution in different stages.

Figure 3

(a) Illustration of the craterlike surface. (b) Comparison of contact area and contact time on a plane surface and craterlike surface. The contact area and time are both normalized. (c),(d) Droplet spreads to lamella along the plane surface but spread into the air on the craterlike surface. (e) Cross-section images of a droplet impacting on plane and craterlike substrate. Half of the crater and half of the droplet are hidden to observe the liftoff clearly. On the craterlike surface, the wetting area is much smaller, and the liftoff is earlier.

Figure 4

The effect of crater radius ($a$ in inset plot) on interaction parameter ($I$) and contact time ($T$). The $a$ ranges from 0 (plane) to $5.5R$ (larger than the droplet). (a) Four representative liquid shapes. If the crater is too small (case A), the liquid overrun and spread along the substrate; once the size is in an optimized range (case B), the droplet would be redirected to spread in the air; if the crater radius is bigger than the size of maximum spreading lamella (case C and D), the droplet hardly spread in the air. (b) The measured $I$ and $T$ versus crater radius. All values are normalized by $I_0, T_0$, and $R$, respectively. There exists a minimum $I/I_0$ at $a=1.22R$, and the $T/T_0$ share the similar trend. The results indicate to achieve the best performance of reducing the contact, the crater radius should be slightly bigger than the droplet radius (around $1.2R$).

Figure 5

Different crater radius leads to overrun or ejection phenomenon. (a) Overrun when $a/R=1.0$; (b) Ejection when $a/R=1.4$. The yellow line indicates the slope of the main body of liquid, showing where the surface's downward momentum hits.

Figure 6

The effect of crater depth ($h$ in inset plot) on $I$ and contact time $T$. The $h$ ranges from 0 (plane) to $1.0R$. (a) Four representative liquid shapes. Hitting a plane surface (case A), the liquid will spread along the substrate; Even on a very shallow crater (case B), the droplet can gain noticeable upward momentum to detach when spreading; however, some overruns appear. Deepen the crater a little (case C), then none of the surface outside the crater is wetted. Once the spreading is entirely in the air, a deeper crater (case D) hardly improves the performance. (b) The measured $I$ and $T$ versus crater depth. All values are normalized by $I_0, T_0$, and $R$, respectively. The results indicate crater depth should be above a threshold to achieve the best performance of reducing the contact.

Figure 7

Different crater depths lead to overrun and ejection phenomena. (a) Overrun as $h/R=0.16$; (b) Ejection as $h/R=0.6$. The yellow line indicates the slope of the main body of liquid, showing where the surface's downward momentum hits.

Figure 8

The effect of landing location (off-center distance $s$) on $I$ and $T$. The landing location ranges from crater center ($s=0$) to far from crater ($s=2.33a$). (a) The measured $I$ and $T$ versus $s$. The values are normalized by $I_0, T_0$, and $a$, respectively. The $I/I_0$ increase from around $0.2$ (at the center) to $1.0$ (far from the crater) monotonous as the effect of redirecting the droplet is diminished. That indicates to achieve the best performance, landing far off-center should be eliminated. (b) Three representative snapshots time series showing different liquid shape evolution.

Figure 9

A new design of substrate. The surface is trimmed by many spheres and forming a crater array. The craters are compactly packed to ensure no plain between craters. We test the $I/I_0$ and $T/T_0$ on different landing locations. The selected locations are on the path from one crater center A to its neighbor center B or C.

Figure 10

The measured $I/I_0$ and $T/T_0$ versus landing location, (a) from A to B, (b) from A to C. The landing location is described by distance s normalized by distance AB or AC. Landing at the center still has the best performance; however, other landing locations all show reduced $T$ and $I$.

Figure 11

Snapshots of six separate cases. The labels correspond to those in Fig. 10. The three cases in the first row are landing locations on path A-B, the three cases in the second row are on path AC. On path AB, the droplet may be broken by the peak trimmed by four craters; on path AC, the droplet may be divided into two parts by the ridge between two craters. The complexity of the shape evolution contributes to the asynchronous contact time and interaction parameter. However, all landing locations are effective in reducing contact area and time.