Roughness-dependent wetting behavior of vapor-deposited metallic thin films

We studied the wetting behavior of silver and copper thin films versus their kinetic roughening upon deposition at room temperature on glass substrates. Time-dependent height-height correlation functions were extracted from atomic force microscopy images, and they demonstrated a nonstationary growth front of the film roughness associated with a temporal evolution of the local surface slope. As a result, we tried to correlate the roughness statistical properties such as the root-mean-square (rms) roughness $\sigma$, the correlation length $\xi$, and the local surface slope $\left(\rho \approx \sigma /\xi \right)$ with the wetting behavior of the films’ surfaces. The contact angle behavior was also studied by analyzing the variation of the energy of the system with water penetrating into the surface cavities, and the associated Laplace pressure induced by the local surface curvature. Hence, it was demonstrated that the wetting transition from a metastable Cassie-Baxter state to a Wenzel state as well as the penetration of a droplet into the surface crevices occur at the smaller local surface slopes for the higher surface energy material.

Figure 1

GIXRD spectrum of (a) S5 (Ag) and (b) C5 (Cu) samples.

Figure 2

2D AFM images of thick (a) Ag (sample S5) and (b) Cu (sample C5) surfaces. Plot (c) shows the section height profile of sample S5 as an example by a stylus profilometer tracing a distance of 3 mm along a step to determine the film thickness.

Figure 3

The time-dependent height-height correlation function for (a) Ag and (b) Cu samples.

Figure 4

The local surface slope variation versus the deposition time for the Ag and Cu samples.

Figure 5

The variation of the contact angle versus the deposition time (a) for the as-prepared Ag and Cu samples and (b) for the samples after one year.

Figure 6

The contact angle versus the rms roughness for the Ag (solid circle) and Cu (solid square) samples.

Figure 7

The contact angle versus the correlation length for the Ag (solid circle) and Cu (solid square) samples.

Figure 8

The contact angle versus the local surface slope for the Ag (solid circle) and Cu (solid square) samples. The inset illustrates the relevant roughness parameters and the local surface slope as the water penetrates the surface crevices.

Figure 9

The energy of the system per unit cell, E, and the Laplace pressure variations of the Ag (solid circle) and Cu (solid square) samples with respect to the deposition time.