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We consider a free-surface thin film placed on a thermally conductive substrate and exposed to an external heat source in a set-up where the heat absorption depends on the local film thickness. Our focus is on modelling film evolution while the film is molten. The evolution of the film modifies local heat flow, which in turn may influence the film surface evolution through thermal variation of the film's material properties. Thermal conductivity of the substrate plays an important role in determining the heat flow and the temperature field in the evolving film and in the substrate itself. In order to reach a tractable formulation, we use asymptotic analysis to develop a novel thermal model that is accurate, computationally efficient, and that accounts for the heat flow in both the in-plane and out-of-plane directions. We apply this model to metal films of nanoscale thickness exposed to heating and melting by laser pulses, a set-up commonly used for self and directed assembly of various metal geometries via dewetting while the films are in the liquid phase. We find that thermal effects play an important role, and in particular that the inclusion of temperature dependence in the metal viscosity modifies the time scale of the evolution significantly. On the other hand, in the considered set-up the Marangoni (thermocapillary) effect turns out to be insignificant.
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Laser heating and melting of metals on nanoscale: breakup of metal filaments
We apply a previously-developed asymptotic model to study instability and breakup of metal filaments of nanometric dimensions exposed to heating by laser pulses, and placed on thermally conductive substrates. One particular aspect of this setup is that the considered heating is volumetric, since the absorption length of the applied laser pulse is comparable to a typical filament thickness. In such a setup, absorption of thermal energy and filament evolution are coupled, and must be considered self-consistently. The asymptotic model that we use allows for significant simplification, since it reduces a complicated problem involving Navier-Stokes equations coupled with heat transport. Such simplification is crucial both for understanding the main features of the problem, and for the purpose of developing efficient simulations of the filament evolution and subsequent nanoparticle formation. The presented computational results are obtained in the GPU computing environment, which allows for fully nonlinear time-dependent simulations in large three-dimensional computational domains. We focus in particular on the influence of filament size on the evolution. It is found that filaments’ width and thickness play an important role, with thicker and/or wider filaments absorbing more energy and therefore evolving differently from thinner ones. This finding opens the door to considerations of self- and directed-assembly of metal nanoparticles via suitable choice of the initial metal geometry on the nanoscale.
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- Award ID(s):
- 1815613
- PAR ID:
- 10478639
- Publisher / Repository:
- Begell House
- Date Published:
- Journal Name:
- Proceedings of the 17th International Heat Transfer Conference
- ISSN:
- 2377-424X
- Format(s):
- Medium: X
- Location:
- Cape Town
- Sponsoring Org:
- National Science Foundation
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