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On the cause and correction of the anomalously high contact angles measured on soils and granular materials
Geoderma ( IF 6.1 ) Pub Date : 2021-02-15 , DOI: 10.1016/j.geoderma.2021.114973
Helen M. Balshaw , Peter Douglas , Stefan H. Doerr

Anomalously high water contact angles are often measured for soils or other granular materials when using a goniometer which is designed for use with flat surfaces. For many years such high contact angles have been rationalised in terms of Cassie and Baxter, and/or Wenzel models for contact angles on non‐planar surfaces, but it is becoming increasingly apparent that the theories behind these models are fundamentally flawed. Here, we present an alternative interpretation of these anomalously high contact angles which takes into consideration how a water drop sits on the particulate surface, and propose a geometric correction factor to address this anomaly. Experimental data from studies with precisely arranged needles and spheres, and model and natural soils, were used to explore this approach and examine the validity of the method. Application of the correction factor to measurements of water drops on 1 mm diameter steel spheres hydrophobised with paraffin wax gave a reduction of the measured contact angle of 140.4(±0.6)° to a corrected contact angle of 108.2(±1.0)°, which is within 3.5° of the flat‐plane contact angle for water on the wax of 111.7(±0.6)°. For paraffin wax coated soils, measured contact angles are shown to be 15–25° higher than comparable flat-plane contact angle. This correction factor may be useful in interpreting goniometer contact angle measurements of irregular surfaces since it is the flat-plane, rather than measured, contact angle which gives a measure of the polarity/hydrophobicity of the surface.



中文翻译:

关于在土壤和颗粒物料上测量到的异常高接触角的原因和纠正方法

当使用设计用于平坦表面的测角仪时,通常会测量土壤或其他颗粒状物质的异常高的水接触角。多年来,已经根据Cassie和Baxter和/或Wenzel模型对非平面表面上的接触角进行了合理化,但是越来越明显的是,这些模型背后的理论存在根本性的缺陷。在这里,我们提出了对这些异常大的接触角的另一种解释,其中考虑到了水滴在颗粒表面上的位置,并提出了解决这一异常的几何校正因子。来自精确定位的针和球以及模型和天然土壤的研究实验数据被用于探索这种方法并检验该方法的有效性。将校正因子应用于测量用石蜡蜡疏水化的直径为1 mm的钢球上的水滴时,可将测得的接触角从140.4(±0.6)°减小到校正后的接触角108.2(±1.0)°,即在111.7(±0.6)°的蜡上,水与平面的接触角在3.5°以内。对于石蜡涂层的土壤,测得的接触角比可比的平面接触角高15–25°。该校正因子在解释不规则表面的测角仪接触角测量中可能很有用,因为它是测量表面极性/疏水性的量度的平面而非接触角。

更新日期:2021-02-15
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