Solid-supported thin elastomer films deformed by microdrops

Abstract

A sessile droplet can deform the surface of a soft solid not only with its weight. The surface tension pulls up a ridge at the perimeter of the drop, and the capillary pressure embosses a quasi-spherical dimple underneath the drop. This holds for the case of a bulk solid. However, if the solid forms a film with thickness comparable to the deformation scale the shape and the depth of the dimple are strongly distorted. We investigated dimples on elastomer films with a Young's modulus of 25 kPa and thickness in the range 4-104 mm embossed by sessile ionic liquid droplets. The films are supported by an undeformable glass slide. Below a certain critical film thickness, the dimple is shallower and the ridge at the drop rim is less elevated than for the bulk elastomer. The deviations are more pronounced for thinner films. Further, troughs form at the two sides of the ridge. Their distance from the rim is equivalent to the layer thickness. The measurements are qualitatively reproduced by an analytical model and quantitatively by numerical simulations. A consistent physical picture of the deformation on the bulk elastomer and of the distortions on the thin films is given.

Deformation of an Elastic Substrate by a Three-Phase Contact Line

“…al. [8] recently measured the topography at the free surface of a PDMS substrate due to a drop of ionic liquid. Their data showed agreement with Long's theory in the long and short wavevector limits, but large discrepancies were found at distances from the contact line comparable to the substrate thickness.…”

Physics of solid–liquid interfaces: From the Young equation to the superhydrophobicity (Review Article)

“…[34][35][36]. Anyway, this distortion is not negligible for soft materials such as elastomers and it contributes essentially to the contact angle hysteresis.…”

“…12) must be equilibrated, and this leads necessarily to some distortion of the substrate near the triple line, called the "wetting ridge" [34,35]. This distortion is negligible for rigid substrates such as glass or steel, but it should be considered for soft substrates such as rubbers (elastomers) [36]. This wetting ridge (depicted in Fig.…”

“…where µ is the elastic (shear) modulus of the solid, and d is the depth (thickness of the substrate) [34][35][36]. For distances much larger than the thickness d the vertical displacement ζ (see Fig.…”

“…Equation 37 is true for x > Є, where Є is a cutoff length, below which the solid no longer behaves in a linearly elastic manner (typically on the order of a few nanometers for an elastomer) [34][35][36]. At short distances ( ) x d < δ the vertical displacement ζ is estimated as:…”