Hydrodynamic stability window and antifouling performance of Xanthan Gum-based hydrogel coatings resolved by in situ OCT, CFD, and field exposure

Hydrogel-based antifouling coatings containing biodegradable Xanthan Gum (XG) offer a potential route to reduce biocidal Cu₂O content, but their practical implementation requires evaluation of hydrogel stability under hydrodynamic loading. This study aimed to develop a quantitative methodology linking laboratory rotor testing of XG-based hydrogel coatings to hydrodynamic conditions relevant for ship operation, while assessing their antifouling performance in the field. Hydrogel stability in self-polishing acrylic/rosin coatings was investigated using a laboratory rotor setup combined with in situ Optical Coherence Tomography (OCT) to quantify changes in hydrogel thickness, coverage, and surface morphology. OCT-derived surface profiles were incorporated into Computational Fluid Dynamics (CFD) simulations to determine roughness-dependent wall shear stresses, which were translated into equivalent ship speeds. Baltic Sea field exposure was used to evaluate the influence of XG particle size and loading on antifouling performance and Cu₂O reduction. The hydrogels remained present after 5 d under all tested conditions corresponding to tangential speeds of 0.6–5.2 knots, without evidence of bulk mechanical failure. Hydrogel thickness remained largely time-independent up to 3.7 knots tangential speed for 2 days but progressively decreased at the highest speed corresponding to an estimated equivalent ship speed of approximately 7 knots. Field exposure showed that XG <32 μm enabled reduction of Cu₂O from 10 to 6 wt% while maintaining comparable antifouling performance. The combined rotor-OCT-CFD methodology therefore provides a quantitative framework for evaluating hydrogel stability under practically relevant marine hydrodynamic conditions.