Contact angle as a non-destructive method to determine wettability changes induced by sub-aerial biofilms on built heritage porous substrates

Wettability and water transport mechanisms play a relevant role in the weathering of built heritage since liquid water is directly involved in many deterioration mechanisms. Moreover, water availability can promote the colonization of microorganisms on surfaces of built heritage. While microbial colonization of stone monuments was previously considered as a significant biodeteriorative threat in heritage studies and conservation practices, recent investigations have approached it from a different perspective, shedding new light on its actual impact. Recent studies have high- lighted that microbial communities, known as sub-aerial biofilms (SABs), can have a neutral or even protective role in certain situations. In the present work, a benchtop contact angle instrument was used for studying the surface wettability induced by sub-aerial biofilms (SABs) on laboratory limestone samples. Complementary information on the water interaction with the substrate in the presence of a SAB was acquired via capillary absorption tests. Field measurements of wettability and water absorption properties of biocolonized plastered wall of a historic case study, Casa a Ponente, of Palazzo Rocca in Chiavari (Genova, Italy) were also performed. Results confirmed the potential of contact angle measurements as a non-destructive monitoring tool for the wettabil- ity of biocolonized stone substrates. The presence of SABs is associated with measurable changes in the surface wetta- bility, resulting in near-hydrophobic conditions observed in both the lab-scale colonized samples and the case study.