Spontaneous Formation of Structures with Micro- and Nano-Scopic Periodic Ripple Patterns

We report the first study on the formation of structures with micro- and nano-scopic periodic surface patterns created by the spontaneous flow of liquid metal over thin metallic solid films. Minute details of the flow of liquid gallium over gold are captured in situ at very high magnifications using a scanning electron microscope, and a series of experiments and microstructural characterization are performed to understand the underlying principles of the liquid flow and the pattern formation. This phenomenon is solely driven by wetting, with little influence of gravity, and is aided by a tenacious semi-solidus envelope of the intermetallic compound formed due to the reaction between the liquid metal and the metallic substrate. This complex flow creates highly periodic patterns with features ranging from hundreds of nanometers to tens of micrometers, which can be tuned a priori. We propose a model capturing the essential mechanics of the ripple formation and apply it to simulate the formation of a single ripple, along with its essential asymmetry, that forms the basis for generating the observed patterns.