Wetting hysteresis of nanodrops on nanorough surfaces

Nanodrops on smooth or patterned rough surfaces are explored by many-body dissipative particle dynamics to demonstrate the influence of surface roughness on droplet wetting. On a smooth surface, nanodrops exhibit the random motion and contact angle hysteresis is absent. The diffusivity decays as the intrinsic contact angle $\left({\theta }_Y\right)$ decreases. On a rough surface, the contact line is pinned and the most stable contact angle $\left({\theta }_Y^{\prime }\right)$ is acquired. The extent of contact angle hysteresis $\left(\mathrm{\Delta }\theta \right)$ is determined by two approaches, which resemble the inflation-deflation method and inclined plane method for experiments. The hysteresis loop is acquired and both approaches yield consistent results. The influences of wettability and surface roughness on ${\theta }_Y^{\prime }$ and $\mathrm{\Delta }\theta$ are examined. ${\theta }_Y^{\prime }$ deviates from that estimated by the Wenzel or Cassie-Baxter models. This consequence can be explained by the extent of impregnation, which varies with the groove position and wettability. Moreover, contact angle hysteresis depends more on the groove width than the depth.

Figure 1

(a) The variation of surface tension, CA evaluated by Young's equation, and drop shape with solid-liquid attractive parameter. (b) The comparison between CAs determined by Young's equation $\left({\theta }_Y\right)$ and obtained by simulations $\left({\theta }_m\right)$.

Figure 2

The mean-squared displacement $〈\mathrm{\Delta }{r}^2\left(t\right)〉$ is plotted against the time $t$ for nanodrops with different ${\theta }_Y$. The inset shows the variation of the diffusion coefficient $D$ with the contact area $A$.

Figure 3

The structure of the patterned rough surface with cuboidal cavities. $L,W$, and $H$ represent the length, width, and depth of the groove, respectively. $G$ is the distance between two adjacent cavities.

Figure 4

(a) The variation of the apparent CA $\left(\theta \right)$ (top loop) and base diameter of the drop (BD) (bottom loop) with the drop volume (V). (b) Some snapshots associated with the hysteresis loop.

Figure 5

(a) The variation of the drop shape with the applied force $\left(f\right)$. (b) The plot of the front CA $\left({\theta }_f\right)$, rear CA $\left({\theta }_b\right)$, and major length $\left(l_m\right)$ against $f$.

Figure 6

(a) The side view slice of the sessile nanodrop on the patterned rough surfaces with various ${\theta }_Y^{\prime }$. (b) The extent of impregnation is exhibited by the contour plot of liquid beads in grooves with various ${\theta }_Y^{\prime }$.

Figure 7

The contour plot of the liquid density in the groove compared to that in the bulk for various groove depths $H$.

Figure 8

The contour plot of the liquid density in the groove compared to that in the bulk for various groove widths $W$. As $W$ is increased, the contact line gradually becomes noncircular.