Spreading and absorption of silicone oil droplets on silicone elastomer films

Abstract

When a liquid droplet completely wets a hard substrate, its spreading dynamics follow Tanner's law, with the droplet radius growing as the one-tenth power of time. Here, we investigate how these dynamics change when silicone oil droplets spread on soft silicone elastomer and gel films supported by a rigid silicon substrate. While the droplets fully wet the elastomer surface, they also simultaneously swell the elastomer film. By varying the film thickness, we observe deviations from the classical power-law scaling, which we interpret in terms of changes to the effective stiffness and the absorption potential of the system. We describe the spreading behavior using a phenomenological model that accounts for both absorption and mechanical contributions.

Figures (7)

• Figure 1: Film thickness measured before and after the washing procedure. The as-prepared film thickness is $h_{\text{prep}}$, while $h_{\text{elast}}$ is the thickness of the elastomer film following washing. The data is fit to a straight line with a best-fit slope of 0.66. Error bars represent typical 5% uncertainty.
• Figure 2: Schematic of the sample geometry. Silicone oil droplet spreads on a film with thickness $h$, supported on a silicon substrate. The film consists of either a gel or an elastomer (see text). The contact radius of the droplet, $R$, is measured as a function of time.
• Figure 3: Microscopy images of silicone oil droplets wetting different surfaces at $t=0$ and $t\approx 20~\mathrm{h}$: (a) bare silicon, (b) a $232~\mathrm{nm}$ elastomer film supported on silicon, (c) a $1575~\mathrm{nm}$ elastomer film supported on silicon, (d) a $312~\mathrm{nm}$ gel film supported on silicon, and (e) a $2078~\mathrm{nm}$ gel film supported on silicon. Vertical dashed lines are to facilitate comparing droplet radii between samples. The contact line of each droplet is highlighted with a red line in the third column.
• Figure 4: Contact radius evolution for droplets of droplet of silicone oil spreading on two different V35 elastomer film thicknesses, and on a Si substrate (see Fig. \ref{['fig:schematic']}). On the Si substrate, the droplet follows Tanner’s law, with the solid black line indicating a $t^{1/10}$ power law. In contrast, droplets on elastomer films exhibit modified dynamics that depend on the film thickness, reflecting the influence of substrate compliance on the spreading dynamics.
• Figure 5: Plot of the prefactor, $C$, as a function of the film thicknesses normalized by the size of the droplet $\Omega^{1/3}$ for elastomer (circles) and gel (triangles) films. The black line shows the fit of the model described by Equation (\ref{['eqn:prefactor']}) to the data. Uncertainties were estimated from the scatter in the data.