Dynamic contact angle as a new metric for the water repellency evaluation of biochar-amended soil

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<0.05) between initial CA (CA_0) and CA after 90 s (CA_90). Based on this, a new classification of hydrophobicity was established, encompassing super-hydrophobic, strongly hydrophobic, ‘pseudo'-hydrophobic and hydrophilic categories. ‘Pseudo'-hydrophobic materials exhibited a significant decrease in CA within 90 s, where the CA transitioned from hydrophobic (CA>90°) to hydrophilic (CA<90°) within this period, revealing their hydrophilic nature. The combination of CA_0 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 CA_0 and CA_90. 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.

Keywords

‘Pseudo’ hydrophobicity / Contact angle / Soil water repellency / Water diffusion / Biochar amendment

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Funding

Tianshan Talent Training Program(2023TSYCCX0080)

Key Research and Development Program of Xinjiang Uygur Autonomous Region(2023B02002)

Natural Science Foundation of Xinjiang Uygur Autonomous Region(2021D01C048)

RIGHTS & PERMISSIONS

The Author(s)