Reports: DNI557019-DNI5: Fundamental Investigation of Scale Formation and Corrosion on Hydrophobic and Oleophobic Coatings, and Their Role on Coating Degradation
Nenad Miljkovic, PhD, University of Illinois, Urbana-Champaign
The main goal of this project is to create multifunctional nanoengineered surfaces to manipulate fluidic and heat transport processes for robust and long-lasting high performance oil and gas solutions which utilize both high (water) or low (refrigerant) surface tension fluids. Heat transfer is a critical bottleneck for the advancement of a variety of important petroleum systems. Phase-change based systems promise compact solutions with high-heat removal capability. However, challenges in implementation lead to poor heat transfer performance. More recently, multifunctional nanoengineered surfaces have been developed that can significantly enhance the stability and performance of these systems using thin film evaporation, dropwise and jumping-droplet condensation. Although proven in lab scale environments, the wide spread utilization of these surfaces has not been successful due to their poor durability. To successfully implement these phase-change based approaches, the first critical step is obtaining the fundamental understanding of the complex degradation mechanisms on such surfaces with a range of working fluids having high and low surface tensions. Year 1 of the project focused on three main functional surfaces and their degradation mechanisms. First, we observed the formation of high surface energy agglomerates on hydrophobic coatings after condensation/evaporation cycles in ambient conditions. To investigate the deposition dynamics, we studied the agglomerates on fluorine based hydrophobic coatings as a function of condensation/evaporation cycles via optical and scanning electron microscopy, microgoniometric contact angle measurements, nucleation statistics, and energy dispersive X-ray spectroscopy (EDS). While the contact angle did not change, EDS analysis showed the agglomerates to be comprised on mainly carbon, oxygen, and sulfur. The SEM and EDS results indicated that the agglomerates stem from adoption of volatile organic compounds such as methanethiol (CH3SH), dimethyl disulfide (CH3SSCH), and dimethyl trisulfide (CH3SSSCH3) on the liquid-vapor interface during water vapor condensation, which act as preferential sites for heterogeneous nucleation after evaporation. The insights gained from this first study elucidate fundamental aspects governing the behavior of both short and long term heterogeneous nucleation on hydrophobic surfaces, suggest previously unexplored microfabrication and air purification techniques, and present insights into the challenges facing the development of durable dropwise condensing surfaces. In a parallel study, we investigated the fundamental degradation mechanisms of Lubricant-Infused Surfaces (LIS) or Slippery Liquid-Infused Porous Surfaces (SLIPS), which have recently been developed where the defect free slippery surface leads to lesser pinning of water droplets, resulting in their easy removal. Such surfaces enhance rate of condensation of steam, are self-cleaning and self-healing. The remarkable results of LIS with water droplets gives hope of their viability with low surface tension fluids. However the presence of additional liquid in the form of lubricant for LIS brings other issues to consider. We investigated miscibility of such low surface tension fluids with widely used lubricants for designing LIS. We considered a wide range of low surface tension fluids (12 to 48 mN/m) and different categories of lubricants with varied viscosities, namely fluorinated Krytox oils, silicone oils, mineral oil and ionic liquid. We also calculated the cloaking behavior of the lubricants for immiscible pairs of lubricant and the working fluids. We conclude there are very few choices for lubricant selection in designing stable LIS for the low surface tension fluids. Lastly, using steady state condensation experiments, we show that polymeric functional coatings do not fail due to abrasion or mechanical wear during phase change. The main mechanism of failure is water permeation through the polymeric coating to the high surface energy substrate and mechanical delamination of the coating. This finding will be key to future design considerations of functional coatings for durable and high-performance phase-change heat transfer applications. The project funded multiple full-time graduate students (not simultaneously). Due to the heavy experimental nature of the project, all students have revived training in surface science characterization techniques at the Material Research Laboratory and Micro-Nano Manufacturing Systems Lab. Examples include: Scanning Probe Microscopy, Scanning Electron Microscopy, Atomic Force Microscopy, Ellipsometry, Time Domain Thermoreflectance, and microfabrication. Furthermore, the students have developed professionally though the attendance of conferences and professional meetings to present their work stemming from this project. The main thrusts of year 2 will be the implementation of scanning probe microscopy techniques. Year 1 focused on atmospheric condensation conditions as well as optical microscopy results mainly due to the long lead time for vacuum chamber construction. The three chambers developed are coming online during the summer of 2017 and will be utilized heavily during Year 2 of the project. Although SPM techniques such as AFM spatial and phase mapping have been used for the studies conducted in Year 1, we would like to transition to CRVM and AFM-IR techniques to gain a better understanding of what it happening to coatings at the nanoscale level. In addition to more chamber experiments and SPM characterization, the miscibility and cloaking results from Year 1 of the project have redirected our attention away from LIS and SLIPS surfaces and more towards covalently attached liquids which should dramatically reduce condensate drainage. In parallel, the water permeation results have demonstrated that decreasing the water diffusion coefficient through functional surfaces, instead of increasing abrasion resistance, will be the way to achieve long lasting and robust functional surfaces. As such, we will be continuing our collaborations with PPG, Chemours, and Dr. Gozde Ince on developing and testing cross linked iCVD based polymeric coatings with unprecedented water barrier function.