Abstract :

Dr. George Gogos finished his bachelor’s degree in Mechanical Engineering from the Massachusetts Institute of Technology, USA, in 1980. Later he did MS and PhD from the University of Pennsylvania in the years 1982 and 1986. Presently, he is a Professor of Mechanical Engineering at the University of Nebraska Lincon, USA. His research interests include: Transport phenomena in vaporizing and combusting sprays, Blast wave mitigation, Propane flaming for weed control, Thermal processes in plastics manufacturing, Thermal processes in DNA multiplication for detection of biological agents and Microfluidics. Femto second Laser Surface Processing (FLSP) is a dynamic and powerful technique for the fabrication of bio-mimetic self-organized multi scale surface structures on a variety of materials, including metals. Multi scale surfaces are characterized by micrometer and nano meter roughness and are formed via multi pulse illumination through a combination of ablation, deposition of ablated material, and fluid flow of molten material.  Precise control over the surface geometry enables the tailoring of interfacial phenomena such as wettability and wicking to achieve both super hydrophobic and super hydrophilic surfaces for heat transfer, electrolysis, and lab-on-chip systems.  Additionally, FLSP enables advanced control over the optical, chemical, and electrical surface properties. Surface features are created by ablating and reshaping the substrate material directly as opposed to additive fabrication techniques including surface coatings and lithography.  This approach has a critical advantage: permanent structures capable of withstanding high temperatures needed by many applications are readily fabricated.  An interdisciplinary team between the Departments of Electrical Engineering and Mechanical & Materials Engineering at UNL has been created to pursue transformational research on both the optimization of the FLSP process and the application of laser-produced surfaces to enhance heat transfer across a range of applications.  This effort has led to several technological advances associated with the fabrication and application of tailored multiscale surfaces with advanced heat transfer properties.  One of the primary challenges in the field is the development of a model to explain and predict the shot by shot development of self-organized surface features in response to ultra short laser illumination.  We have demonstrated that up to seven distinct classes of multi scale surface features can be fabricated solely through precise control of the laser fluence and number of pulses incident on the sample.  These classes of structures are distinguished either by their dominant surface features or the physical mechanisms that govern their production.  Through the innovative use of high-resolution stop-motion imaging techniques, we have captured the shot-by-shot development of surface structures via FLSP and have characterized the impact of fabrication parameters such as laser fluence on the physics of structure formation as well as the final structure morphology. We have successfully demonstrated many applications, such as utilizing tailored surfaces to reshape the boiling curve, which is one of the most important pieces of engineering information for the power industry.  Specifically, we have demonstrated extraordinary shifts in the Leidenfrost temperature (>175 °C), which is a critical point of the boiling curve that determines the operational range of efficient nucleate boiling; demonstrated a 7X increase in the heat transfer coefficient during nucleate boiling; and have developed a technology platform that achieves pump less control of fluids for micro-fluidic applications.