X-ray imaging of wetting ridge on a soft solid*

Softness of solids affects a microscopic deformation, called a `wetting ridge', at a three-phase contact line. We present a direct visualization of wetting ridges by high-resolution x-ray imaging, which shows a spatial transition between elastic and fluidic wetting behaviors on a soft solid. The fluidic behavior that corresponds to Neumann's triangle occurs at the vicinity of the triple point while the elastic deformation at \textbar x\textbar \textless le (the elasto-capillary length). Real-time x-ray imaging clearly shows temporal variation of wetting ridge.