Imagine a liquid drop resting on the surface of a solid material. The classical picture, due to Young and Laplace, assumes that the drop rests on the substrate without deforming it. However, if the solid is sufficiently thin, the capillary forces affect its shape dramatically, yielding a pattern of wrinkles that emanate from the liquid-solid contact line (see attached figure). Furthermore, the contact angle deviates from the classical Young’s law, which only accounts for surface energies and ignores the solid’s elasticity. In a paper, published in Physical Review Letters (R.D. Schroll et al. , 2013) we analyzed this effect and found that it is attributed to the high bendability of thin solid objects. This insight suggests new regimes of “elasto-capillary” phenomena, governed by surface energy, elasticity and geometry. The characterization of the wrinkle pattern enabled by this theory opens the way to new quantitative techniques for the metrology of nano-metrically thin solid sheets.
Figure: Image (interferometry) of an ultrathin film near a solid-liquid-vapor contact line.