Solid structure formation during the liquid/solid phase transition

This review examines six different mechanisms of forming solid structures by freezing: solidification driven extrusion (SDE), hydrodynamic sputtering, laser-induced periodic surface structures (LIPSS), capillary waves, the Mullins–Sekerka instability, and laser zone texturing. Particular emphasis is placed on how these mechanisms relate to structures formed after melting of surfaces with laser irradiation, even though several of these mechanisms operate also for more conventional melts. The Bally–Dorsey model of SDE explains a mechanism for making spikes of materials that expand upon melting and that scales from the centimeter regime down the nanoscale. Capillary waves are often said to “cause” or be “responsible for” the formation of structures with periodicities Λ ranging anywhere from the wavelength of the incident light to over 10× the wavelength. Here it is shown that while capillary waves formed in conjunction with femtosecond to nanosecond pulsed laser irradiation above the melting threshold support structures with 150 nm ⩽ Λ ⩽ 5 μm, they do not cause the structures to form and some other stimulus is required to select the dominant capillary wave. Capillarity actually inhibits the formation of the smallest structures. Features with larger periodicities can be formed during laser irradiation but they require mass transport that is achieved by, e.g., thermocapillarity or the pressure of the laser ablation plume.