Assessing and predicting granular‐materials hydrophobicity by parametrizing a general equation of state§

Abstract

AbstractWettability is related to forces or surface tensions (σ) acting at the solid–liquid–vapor interface. Given that surface tensions are difficult to assess directly, contact angles (CA) are used as indicators of the balance of forces acting in the system. In this context, the ES theory was developed as an alternative to assess the σ parameters of the Young's equation. This research evaluates the applicability of a proposed equation of state (ES), which is in theory also able to predict the CA of a sample when two σ parameters of the Young's equation and two fitting parameters (β and α) of the function Φ = (σSL) are known (Φ: molecular interaction parameter, σSL: solid–liquid surface tension). These parameters were determined by different experimental methods assessing the CA, in order to determine which method is more suitable to validate the ES theory: Capillary Rise Method (CRM) and Wilhelmy Plate Method (WPM). For WPM, two modes (advancing and equilibrium CA) were applied. Finally, results of CA between the experimental measurements obtained by WPM and those predicted by the ES were compared. Samples of coarse silt, fine sand, and nonporous glass beads with different levels of water repellency were used as test materials. In line with previous research, all methods showed lower values of σ for samples with increasing hydrophobicity. At the same time, Φ reduces consistently with higher hydrophobicity, up to ≈ 0.5–0.6 based on CRM and WPM‐ECA data, and even lower (≈ 0.1–0.2) in WPM‐ACA. α increased with higher hydrophobicity, having relatively stable values in samples having σSV > 40 mN m–1. β showed stable values for samples having σSV > 35–40 mN m–1. In hydrophobic samples (σSV < 35 mN m–1), β decreased in calculations based on WPM‐ACA data, and to a minor degree in WPM‐ECA. The agreement between the CA directly measured (WPM) and those predicted by the ES was low, although better for the WPM‐ECA, suggesting that this last approach can be considered more suitable to evaluate the ES theory.