# Direct-Write Dewetting of High Melting Temperature Metals on Flexible Substrates
## 3 Results
### Directed-Dewetting of Ni and Ag on Par-C Substrates
#### 3.1.2 Silver  
Processing Ag films in a similar fashion to Ni also leads to a melt and dewet event. While Ag has a relatively high melting point (but lower than Ni), the challenges are exacerbated by the higher inherent reflectivity of Ag vs. Ni, (> 95% vs. \(<\) 75%, respectively) at a 1060 nm wavelength. In addition, Ag exhibits the highest thermal conductivity of all the metals. The latter is detrimental as higher laser flux densities are required for the initial melting, but also helpful as the heat-induced damage of the Par-C substrate will be decreased by the rapid expulsion of heat after dewetting. To compensate for the increased reflectivity and thermal conductivity, the laser was operated in a continuous wave mode at a power of 25 W, which delivered a higher incident intensity to the target film. The intensity overcame the reflectivity and allowed the melting and subsequent dewetting process to take place. The use of high-speed scanning prevented the destruction of the underlying polymer substrate. The morphology of the film and substrate after the dewetting event is displayed in Figure 5. While the gross destruction of the polymer substrate has been avoided, there are clear ripples within the exposed polymer substrate that extend out past the ridges formed from the dewetted material. The ripples are clear deformations of the Par-C, but it is unclear whether they form within the polymer first and transfer to the deposited metal film, or if the metal film contracts and forms the ripples during the rapid cooling associated with the laser processing. As seen in Figure 2, the morphology of Ni films tends toward continuous or semi-continuous ridges along the length of the trench; however, the dewetted Ag films in Figure 5 show the formation of more distinct spheres--most likely due to a faster instability growth relative to the other films. Similar instabilities have been observed in the rims of dewetting holes in polymer films and are analogous to Plateau-Rayleigh instabilities [39; 40; 41]. As [PERSON] and [PERSON] pointed out in their investigations of such rim instabilities, their growth rate, \(q\), is described using Equation (3). \[q=\frac{\gamma_{m}}{6R_{0}\mu_{l}} \tag{3}\]  
where \(\gamma_{m}\) is the surface tension of the liquid, \(R_{0}\) is the radius of the hole formed, and \(\mu_{l}\) is the viscosity of the liquid. The authors describe a positive correlation between the dynamic contact angle of the dewetting film and the growth rate of the perturbations. The observed contact angles of the dewetted Ag ridges ranged between \(130^{\circ}\) and \(155^{\circ}\), which is in line with the high surface tension of Ag noted in Table 1. A similar analysis suggests that the growth rate of the instabilities is larger within the Ag film ridges than the rate exhibited by the Bi or Sn films when deposited upon Par-C [20]. The growth rate of the instability may be hastened by the high thermal conductivity of the Ag film. During scanning, there exists a thermal gradient both perpendicular and parallel to the direction of the scanning. Similar gradients have been studied during the high-speed welding of metal plates using a laser with beam qualities on par with the current system [42]. To prove that Ag was displaced, as opposed to being ablated, an accounting of the film volume was achieved using profilometry (Figure 6) in a similar way as shown for Ni (Figure 3). The accounting of the displaced volume (Figure 6) reveals that only 7.9% of the excess positive displaced volume is left unaccounted, proving a displacement via dewetting vs. that of an ablation process. The 7.9% discrepancy may be explained by small shadowing effects created by the aforementioned spherical accumulations along the dewetted Ag ridges, as well as the measurement variation of the profilometer and/or ablation of a small amount of Par-C.  
Figure 5: Optical microscopy of a dewetted Ag film on the Par-C polymer substrate. The laser scan of the dewetted Par-C surface appears along the dark center with ridges of dewetted metal along the edge.