Drop impact onto semi-infinite solid surfaces with different wettabilities

The process of liquid drop impact onto a solid surface can be widely seen in both nature and industrial applications. Depending on the distance between the impact point and the surface edge ($W$), the impact can be categorized into three groups: (a) drop impact onto an infinite surface (surface is large, $W$ is larger than the maximum value of radius of the lamella in all directions); (b) finite surface (surface is small, $W$ is smaller than the maximum value of radius of the lamella in all directions); and (c) semi-infinite solid surface ($W$ is smaller than the maximum radius of the lamella in some directions, but not all). Different from most of the literature which typically focused on the drop impact onto an infinite and/or finite solid surface, an experimental study was performed to investigate the liquid drop behavior during an impact onto a semi-infinite solid surface. In this case, part of the lamella spreads out of the surface (free lamella), while the other part of the liquid remains on the solid surface. Depending on the normalized distance ($\overline{W}=W/D$; $D$ is the drop diameter), the free lamella can either recede back onto the surface ($\overline{W}>0.61$) or completely break off at the surface edge ($\overline{W}<0.61$). The distinctive behavior can be explained by the vertical velocity component of the free lamella and the Laplace pressure difference across the interface at the edge. The surface wettability and Weber numbers cannot determine whether the free lamella can break or recede back, but can significantly affect the breaking morphology of the free lamella. The liquid lamella on the surface behaves similarly to the drop impact on an infinite surface in the spreading phase. The value of the maximal spreading radius $\left(r_{\max }\right)$ on the surface is not affected by $\overline{W}$ when $\overline{W}>0.3$; $R_{\max }$ only starts to decrease when $\overline{W}$ is smaller than 0.3. In the receding phase, the liquid lamella on the solid surface evolves into a semicircle and a sausage shape before eventually evolving to a rounded shape.

Figure 1

Schematic of drop impact on (a) infinite solid surface; (b) finite solid surface; (c) semi-infinite solid surface.

Figure 2

Lamella evolution (from both top and side views) for a water drop impact on a PEMA surface with $W=2.4\mathrm{mm}$ and $\mathrm{We}=189$ ($D=3.1\mathrm{mm}, v=2.1$ m/s). The angle $\mathit{\phi }$ is 9°. The scales for the top and side views are different and are shown.

Figure 3

Lamella evolution (from both top and side views) for a water drop impact on a PEMA surface with $W=1.3\mathrm{mm}$ and $\mathrm{We}=189$ ($D=3.1\mathrm{mm}, v=2.1\mathrm{m}/\mathrm{s}$ ). The scales for the top and side views are different and are shown in the figure.

Figure 4

The free lamella angle ($\mathit{\phi }$) as a function of the impact distance ($W$); $\mathrm{We}=189$. The hollow data points represent the cases which are in regime 2, and the solid data points represent the cases for regime 1. The maximal contact radius of the lamella ($R_{max}$) during the drop impact onto a corresponding infinite solid surface is 6.07 mm.

Figure 5

Vertical velocity distribution during a 3.1-mm water drop impact on an infinite PEMA surface with $\mathrm{We}=189$; impact velocity is 2.1 m/s. Experimental images on the left column, and the numerical simulation on the right.

Figure 6

Free lamella angle ($\mathit{\phi }$) as a function of the normalized impact distance $\overline{W}(=W/D)$. Black data points: $\mathrm{We}=416$; green data points: $\mathrm{We}=189$; red data points: $\mathrm{We}=413$; yellow data points: $\mathrm{We}=116$. The hollow data points represent the cases which are in regime 2, and the solid data points represent the cases which are in regime 1.

Figure 7

Snapshots of the 2.6-mm water drop impact on Teflon AF surface with $\mathrm{We}=416$: (a) $\overline{W}=0.29$; (b) $\overline{W}=0.58$.

Figure 8

Schematic for the liquid lamella near the edge. The nonflat region in the zoomed inset is due to the free lamella bending.

Figure 9

Snapshots of the 3.1-mm water drop impact on glass surface with $\mathrm{We}=189$ ($D=3.1\mathrm{mm}$ and $v=2.1\mathrm{m}/\mathrm{s}$): (a)$\overline{W}=0.9$; (b) $\overline{W}=0.52$; (c) $\overline{W}=0.31$.

Figure 10

Snapshots of the 3.1-mm water drop impact on Teflon AF surface with $\mathrm{We}=189$ ($D=3.1\mathrm{mm}$ and $v=2.1\mathrm{m}/\mathrm{s}$): (a) $\overline{W}=0.74$; (b) $\overline{W}=0.58$; (c) $\overline{W}=0.29$.

Figure 11

The evolution of $r$ for the case of a 3.1-mm water drop impact on PEMA surface with $W=2.4\mathrm{mm}$ at $v=2.1\mathrm{m}/\mathrm{s}$.

Figure 12

The evolution of $r$ for the case of a 3.1-mm water drop impact on PEMA surface with $W=1.7\mathrm{mm}$ at $v=2.1\mathrm{m}/\mathrm{s}$.

Figure 13

Evolution of $r$ during the impact process (PEMA surface, $v=2.1\mathrm{m}/\mathrm{s}$) for the case of $W=3.4$, 2.4, 1.7, 0.6, and 0.1 mm.

Figure 14

Value of ${\overline{r}}_{\max }$ as a function of $\overline{W}$ for all systems studied. Black data points: $\mathrm{We}=417$; green data points: $\mathrm{We}=189$; red data points: $\mathrm{We}=414$; yellow data points: $\mathrm{We}=116$.

Figure 15

Definition of the lamella geometry aspect ratio (a) $\mathrm{s}\text{emi-infinte}$ impact; (b) corresponding infinite impact.

Figure 16

Evolution of lamella geometry aspect ratio on solid surface for water impact onto PEMA surface with $W=3.4\mathrm{mm}$ and its corresponding infinite impact.

Figure 17

$\overline{S}$ as function of $\overline{W}$ for PEMA surface. Black data points: $\mathrm{We}=417$; green data points: $\mathrm{We}=189$; red data points: $\mathrm{We}=414$; yellow data points: $\mathrm{We}=116$.