(Nanowerk News) “The long divided, must unite; long united, must divide. Thus it has ever been.”
The opening lines of the great Chinese historical novel Romance of the Three Kingdoms condense its complex and spectacular stories into a coherent pattern, that is, power blocs divide and unite cyclically in turbulent battle years.
A good philosophy or theorem has general implications. Now, published in the journal Nature Synthesis (“Oscillatory bifurcation patterns initiated by seeded surface solidification of liquid metals”), scientists from Australia, New Zealand, and the US reported a new type of solidification patterns that resembles the plots in the Chinese classic, but this time appearing on the surface of solidifying liquid metals.
Figure 1. a, Schematic drawing of the seeded surface solidification and pattern formation process. b, Oscillatory bifurcation patterns form on the surface of solidified Ag-Ga alloy (scanning electron microscopy image).
The team dissolved a small amount of metals such as silver (Ag) in low-melting-point solvent metals such as gallium (Ga), and investigated how the metallic components interact and separate to form patterns when the metallic liquid mixtures (alloys) solidify.
The researchers found that a single silver–gallium system can produce distinct patterns such as particles or bundle-like structures of a Ag2Ga compound.
The individual Ag2Ga structures that build the patterns are small, with micrometre or nanometre thicknesses, tens or hundreds of times less than a human hair.
Figure 2. a, Time-lapse of the seeded surface solidification process, with arrows indicating the propagating direction of the surface solidification front. b, Scanning electron microscopy image reveals multiple surface subdomains with different patterns. c, Atomic force microscopy image of the surface patterns.
Most surprisingly, the researchers observed that the patterns divide and unite in a repeated manner. “The first time I saw such cyclic divergent-convergent patterns, it immediately reminded me of the famous opening lines of the Romance of the Three Kingdoms,” said Dr Jianbo Tang from University of New South Wales (UNSW, Australia), who is the first author of the study.
Pattern formation is a fundamental yet ubiquitous phenomenon which has interested and inspired scientists for a long time. Some pattern types are more common than others.
Observation of surface patterns formation during the solidification of a Ag-Ga alloy under an optical microscope.
Among all the diverse patterning behaviours, divergent pattern formation, or bifurcation, is frequently seen in nature because this particular arrangement generally favours energy conversion or distribution. “I.e. it’s the ‘easiest’ path,” explains Dr. Tang. River networks, tree branches, lightning pathways, and vascular systems are all examples of bifurcation.
In comparison, convergent pattern growth, or inverse bifurcation, is encountered less frequently as it is contrary to the energetically favourable bifurcation.
Figure 3. Examples of oscillatory bifurcation patterns observed in (a-c) Ag-Bi alloys and (d,e) Bi-Ga alloys. Scanning electron microscopy images (a,b,d) and energy dispersive spectroscopy images (c,e).
The strange cyclic divergent and convergent growth, called oscillatory bifurcation, is rare and has not been observed in solidification structures prior to the new published work.
Despite this, the researchers observed oscillatory bifurcation patterns on the surface of several liquid alloys after solidification, which suggests that this counter-intuitive behaviour may be quite general for solidification patterns forming on the surface of liquid metals.
Analogous to the dramatized novel where the turbulent forces between and within a large number of power blocs drive those groups to divide and unite, the team found that it is also the instability of the liquid metal surface that underlies the emergence of the exotic oscillatory bifurcation patterns.
Figure 4. Initial and final (50 picoseconds) atomic configurations of the Ag atoms (pink) and Ga atoms (grey) seen in one of the molecular dynamics simulations.
“Surface pattern formation of liquid metal alloys is a new but exciting topic. The surface or interfacial nature of the process enables us to better understand and control fundamental phase transition and pattern formation.” Dr. Tang added, “We will continue our work on designing crystalline surface patterns and structures using liquid metals to enable cutting-edge applications such as plasmonic sensing, high-efficiency electronics and optics, and high-precision spectroscopic.”
The experimentation of the study was conducted at the Centre for Advanced Solid and Liquid based Electronics and Optics (CASLEO) based at UNSW, led by CASLEO Director Prof Kourosh Kalantar-Zadeh. Two molecular dynamics simulation groups, one led by Prof Salvy Russo (RMIT University, Australia) and the other by Prof Nicola Gaston (MacDiarmid Institute for Advanced Materials and Nanotechnology and University of Auckland, New Zealand), carried out supercomputer simulations to provide atomistic insights to the phenomenon.
