Controlling break-up of a liquid filament: from edge melting to thermal scissors

We consider a fluid filament on a solid substrate, exposed to localised perturbations that modify its material properties, particularly its viscosity. The considered model geometry and material parameters are motivated by an experimental set-up involving metal filaments subjected to laser heating, which liquefies them, leading to fluid flow while the temperature is above the melting point. The localised perturbations are created by adding disjoint metal pillars, which, due to the effect of ‘thermal crowding’ – meaning increased energy absorption due to the additional deposited metal – modify the local filament properties. Depending on the pillars’ positioning, one could consider them either as ‘thermal scissors’ (splitting the filament at the pillar location into segments) or as the source of the filament’s edge melting, leading to retraction and break-up. A precise understanding of the mechanism underlying the filament’s break-up, supported by efficient simulations, enables rationalising the dynamics and final pattern formation, as well as controlling the size and positioning of the resulting metal particles. In particular, we identify numerically 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 set-up, we expect similar mechanisms to be relevant to other systems in which material parameters could be locally modified by externally imposed perturbations.