A novel femtosecond laser surface processing system for functionalization of the inside of tubes with applications in enhanced thermal-fluid performance

Roughening the inner surfaces of tubes is a widely used manufacturing approach to enhance heat transfer capabilities in both single-phase and two-phase flow regimes by increasing the effective surface area, increasing nucleation sites, improving wicking, and promoting turbulence and mixing. However, conventional fabrication techniques involve multistep, complex processes that are time-consuming, geometry limited, not easily scalable, and result in non-uniform structures. Here, we present a novel functional prototype, a single-step method for processing interior tube surfaces using self-organized laser functionalization (SOLF) with a high-power 300-watt femtosecond laser system (Amplitude Tangor 300). SOLF is a cutting-edge technique that produces self-organized permanent hierarchical micro- and nano-scale structures proven to significantly improve thermal-fluid performance. While past research with SOLF has focused predominantly on flat surfaces, many practical energy systems rely on tubular components, where SOLF implementation has been limited and understudied. Previous attempts to apply laser-based surface texturing inside tubes have required cutting or disassembling the tube, undermining structural integrity, practicality, and scalability. Our approach allows for direct, nondestructive internal application of SOLF to intact tubes. We demonstrate the formation of tunable and uniform self-organized micro- and nano-scale structures throughout the interior of stainless-steel tubes. The resulting functionalized surfaces were characterized using scanning electron microscopy (SEM) and benchmarked against SOLF structures created on flat samples that were similar to SOLF structures that had previously shown enhanced wicking and heat transfer capabilities. We also discuss the fundamental optical and mechanical limitations of applying SOLF inside tubes and propose practical solutions to overcome them. This method offers a scalable path forward for the fabrication of advanced heat exchanger and solar absorber components, pushing the boundaries of SOLF technology towards confined three-dimensional geometries. This work supports the development of more efficient, compact, and sustainable energy systems in response to the growing energy demands.