Abstract Biochar hydrophobicity is crucial for understanding its interaction with environmental substances (e. g. , soil, water, pollutants). Contact angle (CA) and water droplet penetration time (WDPT) are commonly used methods for assessing biochar hydrophobicity. However, occasional inconsistencies between CA and WDPT measurements introduce uncertainties, emphasizing the need for more accurate evaluation. This study addressed these temporal inconsistencies by proposing a new method using the dynamic contact angle (DCA) to evaluate the hydrophobicity of 17 standard materials and 18 types of biochars. The DCA method, which considers droplet diffusion recorded CA changes over 90 s and compared the significance (p 90°) to hydrophilic (CA < 90°) within this period, revealing their hydrophilic nature. The combination of CA₀ and the rate of CA change over time (| k |) was considered as a new criterion for hydrophobicity evaluation. Through a 90-day incubation experiment of biochar and soil, most biochars significantly increased the water repellency of the biochar-amended soil, as evidenced by increases in both CA₀ and CA₉0. Our DCA method, along with the definition ‘pseudo'-hydrophobicity, resolves contradictions between CA and WDPT measurements for both soil and biochar, enhancing the accuracy of hydrophobicity assessments. Graphical Abstract
Jing et al. (Sun,) studied this question.