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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.

Long range mass transport driven by an electric field has many applications in the fields of nanoscience and technology. Liquid-phase mass transport ranging from the micrometer to the millimeter scale and its application to nanopatterning have been demonstrated on chromium (Cr) thin films using a DC electric field. Under the influence of an electric field, the metal seems to undergo a chemical reaction, and the resulting liquid material flows out radially in all directions. In this study, we have explored the effect of an alternating (AC) electric field on this kind of liquid-phase material transport. Within the scope of this work, mass transport has been studied on Cr films 30 nm thick using an alternating square waveform with frequencies ranging from 100 Hz to 1000 Hz in steps of 50 Hz. The dependence of the material’s formation, flow distance, and flow velocity on frequency, for a constant applied root mean square (RMS) voltage, was studied in detail. An analytical model is presented to explain the experimental results. This study, in particular the frequency parameter and the intermittent nature of the applied bias, will help us get a better control over the mass flow process, will lead to better resolutions for the electrolithography process.