Controlling the breakup of liquified metal filaments into metal nanoparticles: from edge melting to thermal scissors

We consider a metallic nanoscale filament deposited on a solid substrate and exposed to localized perturbations that modify the filament's material properties, particularly its viscosity. The considered model geometry and material parameters are motivated by a setup involving metal filaments subjected to laser heating, which liquefies them, leading to fluid flow while the temperature is above the melting point. The localized perturbations are created by adding disjoint metal pillars, which, due to the effect of 'thermal crowding' — meaning increased thermal energy absorption due to the additional deposited metal — modify the local filament properties. Depending on the positioning of the pillars, one could consider them acting either as 'thermal scissors' (splitting the filament into parts), or as the source of the filament's edge melting and consequent retraction and breakup. A precise understanding of the mechanism leading to the filament's breakup, supported by efficient simulations, allows for rationalizing the dynamics and final pattern formation, as well as controlling the size and positioning of the resulting metal particles. In particular, we numerically identify a bifurcation structure in which the positioning and number of pillars lead to a dramatic transition in the final outcome. While we focus on a rather specific setup, we expect similar mechanisms to be relevant to other systems in which material parameters could be locally modified by externally imposed perturbations.