Drop penetration dynamics of rhamnolipid biosurfactant on peat

Peat and peat-based growing media continue to serve as the major constituents in soil-less cultivation due to their favorable physical and hydraulic characteristics. However, the substrate’s inherent high organic nature, and the intensive dehydration  process employed to compact peat into bales for cost effective transportation , results in the substrate development of water repellency. Consequently, this alters the media's optimal physical and hydraulic characteristics and reducing productivity in cultivation systems. Efforts to ameliorate substrate water repellency have predominantly involved the application of wetting agents, primarily focusing on their effect on various plant growth parameters. Synthetic surfactants were proposed to treat the substrate’s water repellency, but given the environmental concerns, alternative strategies become imperative. Biosurfactants, particularly rhamnolipids, have emerged as intriguing compounds at the scientific and commercial levels. Nonetheless, comprehensive quantitative investigations on the rate and extent of wetting and spreading behaviors of aqueous biosurfactant solutions which is essential for understanding drop penetration dynamics on peat, are currently inadequate. In this regard, the main objective of this study is to quantify the drop penetration dynamics of biosurfactants on peat porous substrate under different initial moisture conditions and on peat pellet compressed at different pressures to account for different densities and surface roughness. The study involved measurements of contact angle (CA), drop height, base diameter, and volume of aqueous biosurfactant solutions on prewetted peat with water and biosurfactant and peat pellets using optical tensiometer (OCA-15, Data Physics, Germany). The results demonstrate the relative degree of water repellency with the droplet dynamics of water exhibiting a CA of ~120⁰ and relatively constant drop height and base diameter. Conversely, biosurfactant droplets reduced drop height and volume, and the CA of 120⁰-80⁰ was dependent on prewetting conditions. Furthermore, drop penetration dynamics into pellet peat highlighted the role of surface roughness with higher CA at lower compression pressure (69 bars) relative to higher one (517 bars). Furthermore, the study revealed a pronounced dependence on biosurfactant solution surface tension (ϒ_lv), with negligible CA changes within the higher ϒ_lv ( from 72 down to 41mN/m) domain and significant alterations within the lower ϒ_lv (down to 33 mN/m) domain. These outcomes underscore the effectiveness of rhamnolipid biosurfactant in aiding drop infiltration into initially hydrophobic peat, implying its role in aiding to peat wettability.

Keywords: Peat, Pellets, water repellency, contact angle, surface tension, rhamnolipid biosurfactant, droplet dynamics.