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Nanoengineered Surfaces for Enhanced Thermal-Fluid Transport Properties : Studies on Dynamic Wetting and Phase Change Phenomena
Thermal-fluid-surface interactions are ubiquitous in multiple industries including Energy, Water, Agriculture, Transportation, Electronics Cooling, Buildings, etc. Over the years, these systems have been designed for increasingly higher efficiency using incremental engineering approaches that utilize system-level design trade-offs. These system-level approaches are, however, bound by the fundamental constraint of the nature of the thermal-fluid-surface interactions, where the largest inefficiencies occur. In this talk, we show how surface/interface morphology and chemistry can be engineered to fundamentally alter these interactions in a wide range of processes involving fluid, heat and mass transport processes including, droplet impact, condensation, boiling, and freezing. We study the wetting energetics and wetting hysteresis of droplets in an Environmental SEM (ESEM) as a function of surface texture and surface energy and establish various wetting regimes and conditions for wetting transitions. We extend these concepts to dynamic wetting and establish optimal design space for droplet shedding and impact resistance. We show for droplets bouncing off of non-wetting superhydrophobic surfaces that their contact time can be reduced below the inertial-capillary time scale by introducing macroscale features that counter-intuitively enhance rather than attenuate surface interactions. We then present the behavior of surfaces under phase change, such as condensation, and freezing at both macroscale and microscale (using ESEM) and find their non-wetting properties can be compromised due to nucleation of water or frost within texture features. Based on these insights we introduce lubricant-impregnated surfaces that can result in two to three orders of magnitude reductions in ice adhesion and promote dropwise condensation. We discuss the unconventional contact line morphology, thermodynamics and dynamics of droplet shedding on these surfaces. Manufacturing approaches, robust materials, and applications of nanoengineered surfaces in various energy, water, and transportation systems including oil & gas (flow assurance and energy efficiency), turbines, power and desalination plants, and electronics cooling will be highlighted.