Time-Evolved Characterization and Performance of Slips (Slippery Liquid-Infused Porous Surface) Tubing With Potential Drag Reduction

The development of energy-efficient, low-drag fluid transport systems would have a substantial impact on many promising industrial and commercial applications including fire-fighting operations, irrigation systems, pipeline transport, district heating systems, marine vessels, turbo machines, sports gear, and next-generation biomedical apparatus. The potential for traditional superhydrophobic surfaces to address this need is severely limited by high-pressure instability, a weakness that is exacerbated when subjected to low surface tension flow medium. Wong et al. have recently introduced Slippery Liquid-Infused Porous Surfaces (SLIPS), a pressure-stable and truly omniphobic approach that promises to overcome these issues. Herein we rationally evaluate the viability of SLIPS as a means of drag reduction for fluid transport through metal pipes. We applied this functionalization, Slippery Liquid-Infused Porous Inner Pipeline Extruded Surfaces (SLIPIPES), to 3 mm ID aluminum pipes, characterized the system for longevity and, and observed effects on pressure drop. We used water as a flow medium to assess longevity and compare friction factor of untreated aluminum (untreated-Al), boehmitized-fluorinated aluminum (BF-Al), and SLIPS aluminum tubing (SLIPS-Al) as a function time. First, to gauge the longevity of various lubricants, we studied the SLIPIPES as a function of time and Reynolds number, with the lubricant retention and resultant slip characteristics monitored by weight changes and sliding angle of a water droplet, respectively. The lubricant layer depletes over time at a rate proportional to the thickness and viscosity of the lubricant layer and the shear rate of the flowing medium. We observe a transition in thinning rate from the excess layer to the bound, ‘critical’ layer, and the lubricant layer is depleted after 30 minutes for nominal flow rates. Lastly, we observe no significant drag reduction in turbulent regime as the increasing drag caused by lubricant shedding outweighs the inner surface roughness improvement by SLIPS.