Droplet impact on a prewetted mesh

The impact of droplets on dry and prewetted meshes was investigated using high-speed photography. The prewetting resulted from previous impacts. It was observed that an increase of the Weber number We resulted in a transition from no penetration of the mesh through an incomplete penetration to a complete penetration. It was found that both penetrations could be entirely suppressed by liquid trapped in the mesh (prewetted mesh), with the height $h$ of the sessile droplet being a key factor. The phase diagram in the $(\mathrm{We}, h)$ plane determining the expected type of impact has been determined. The transition Weber numbers for both types of penetration increased monotonically with $h$. A model for predicting the transition thresholds is proposed and compared with experimental results. It is suggested that the impact type is determined by the momentum exchange between the impacting droplet and the liquid trapped in the mesh.

Figure 1

Sketch of the experimental setup. The inset illustrates the mesh structure with L and D denoting the pore size and the wire diameter, respectively.

Figure 2

Effect of the number of preceding droplets' impacts on the next impact on a mesh for We = 69.6. (a)–(e) correspond to the impact number $N_i$ of 1–5, respectively. The mesh is dry for the first impact and is wetted for the remaining impacts. The scale bar corresponds to 2.0 mm. (a) Complete penetration. (b), (c) Incomplete penetration. (d), (e) No penetration.

Figure 3

Variation of $h=H/R$ as a function of We and $N_i$. Symbols identify the experimental points. Error bars indicate standard deviation.

Figure 4

Time evolution of a droplet impacting a dry mesh (blue background, top) and a mesh wetted by one previous impact at the same We (no background, bottom). Each column represents a different time t. The scale bar is 2.0 mm. (a) We = 7.4, top and bottom: no penetration; (b) We = 16.4, top: incomplete penetration, bottom: no penetration; (c) We = 69.6, top: complete penetration, bottom: incomplete penetration; (d) We = 78.5, top and bottom: complete penetration.

Figure 5

Phase diagram illustrating the outcome of droplets' impacts on meshes with varying We and $h$. $h=0$ represents impacts on dry meshes. Red circles—complete penetration; blue circles—incomplete penetration; green circles—no penetration. The dashed and solid lines represent the theoreticall -predicted $\left({\mathrm{We}}_{p1}\right)$ and $\left({\mathrm{We}}_{p2}\right)$, respectively.

Figure 6

(a) Sketch of the flow configuration before the impacts and (b) after coalescence of the impacting droplet with the liquid trapped in the mesh. $V_d$, $V_{\mathrm{under}}$, and $V_{\mathrm{total}}$ are the volumes of the droplet, the liquid mass below the droplet, and the coalesced liquid, respectively. $U_1$ is the velocity of the coalesced liquid resulting from the momentum exchange.