Theoretically modeling the high thermal emission/formation dynamics of femtosecond laser functionalized surfaces to optimize surfaces

Theoretically modeling the high thermal emission/formation dynamics offemtosecond laser functionalized surfaces with the goal to optimize their performance. The efficient control and enhancement of metallic surfaces with tuned reflectivity/emissivity without the use of coatings is difficult to be achieved without the use of a rapidly developing technology known as femtosecond laser surface processing (FLSP). The efforts of the currently proposed Office of Naval Research (ONR) Young Investigator Program (YIP) research project will be focused on developing a complete multiphysics modeling technique, combiningelectromagnetic, thermal, and fluid dynamic simulations, with the goal to theoretically understand and predict the micron and nanoscale surface formations along FLSP surfaces and their absorption, emission, and thermal transfer response. The FLSP technique can be used to functionalize, or tailor, the surface properties of metals by creating finely tuned, self-organized micron and nanoscale surface features combined with laser induced chemistry changes. This approach has several distinct advantages compared to traditional functionalization techniques using either lithography or coatings: surface features are generated directly in the metallic surface through a combination of self-organization processes including preferential laser ablation, fluid flow, andredeposition of ablated material in the form of nanoparticles. FLSP generates a durable permanent surface suitable for use in harsh environments including marine and high-temperature applications.However, the fundamental understanding of the theoretical mechanisms behind the formation of FLSP surfaces and the linkage to the broadband absorption/ tunable emission response has not been developed. The goal of the proposed YIP project is to provide for the first time a complete theoretical model with experimental validation to accurately explain why FLSP functionalized surfaces demonstrate nearly perfect and controllable absorption and emission over a very broad frequency range. The two main objectives of the proposed project are: a) Provide a complete theoretical foundation to accurately model the emissivity/absorption processes obtained by FLSP functionalized surfaces. b) Understand how the proposed surfaces are formed and optimize theirdesign to efficiently tune the emissivity and absorption. The first objective will provide fundamental scientific answers to why FLSP surfaces are black over such a large range of wavelengths. The second objective will provide directions on how to optimize the optical properties of surfaces for specific applications and will also support the ongoing experimental work at UNL, where FLSP has been used in minichannels (two black FLSP surfaces are spaced 500 ~m apart) to enable two-phase flow to reach heat transfer coefficients of 540,000 W/m2~K,with applications of cooling high-powered laser diodes. In addition, the proposed YIP project directly relates to other Navy operations such as stealth technology for Navy unmanned aerial vehicles (UAV~s), to avoid their detection from Light Detection And Ranging (LIDAR) systems, anti-icing surfaces (passive heating) without using coatings utilized in helicopter blades or Navy UAVs during winter missions or in ship cables sailing in harsh cold environments, radiativecooling to passively cool high temperature gas turbines of Navy aircrafts, black surfaces to reduce stray light in sensors or high powered laser systems, and design of efficient new thermophotovoltaic devices to produce electricity for ships and Navy UAV~s.