Surface Functionalization of Additively Manufactured Polypropylene and Stainless Steel 316L: Impact on Wettability and Oxygen Nucleation

Bubble nucleation, a fundamental solid–gas interaction,
plays a critical role in both natural phenomena and industrial processes.
In water electrolysis, it is relevant not only for the generation
of H2 bubbles but also for the formation of O2 as a byproduct, which significantly impacts heat and mass transfer
efficiency. While surface functionalization for bubble management
has been actively explored for electrodes, the electrolyzer periphery
has received little attention to date. Additive manufacturing enables
the design of advanced geometries to optimize the multiphase flow
in such systems. In this study, the wettability of additively manufactured
polypropylene and stainless steel 316L substrates is modified by plasma-enhanced
chemical vapor deposition based on 1H,1H,2H,2H-perfluorooctyl acrylate
(PFAC-6) and hexamethyldisiloxane (HMDSO), creating hydrophobic or
hydrophilic surfaces with final water contact angles ranging from
40 to 145°. Surface morphology and properties are assessed by
digital microscopy, water contact angle measurements, and X-ray photoelectron
spectroscopy. The coating stability and long-term wettability in contact
with air or water, as a first step toward industrial conditions, were
monitored over a period of up to 150 days. X-ray photoelectron spectroscopy
identified distinct aging mechanisms: defluorination for PFAC-6 coatings
and reorientation of polar surface groups for HMDSO. Bubble dynamics
in O2-oversaturated solutions are recorded and analyzed
using machine learning techniques, showing O2 bubble coverage
increased by up to 500% for polypropylene and 350% for stainless steel
with PFAC-6, while HMDSO coatings reduced nucleation down to 30%.
This work highlights the pivotal role of surface structure and chemical
modification in governing solid–gas interactions and offers
valuable insights for the development of optimized materials in industrial
applications such as electrolysis.