Contact angle hysteresis: A new paradigm?

• Published in: Advances in water resources 2022-03, Vol.161, p.104138, Article 104138
• Main Authors: Behnoudfar, Diba, Dragila, Maria I., Meisenheimer, Douglas, Wildenschild, Dorthe
• Format: Article
• Language: English
• Subjects: Contact angle; Hysteresis; Multi-phase flow; Thermodynamics; Wettability; Youngs equation
• ISSN: 0309-1708; 1872-9657
• DOI: 10.1016/j.advwatres.2022.104138

•X-ray tomography was used to image pore-scale drainage and imbibition displacements using a variety of fluid pairs in packed glass bead samples. Image analysis was used to measure curvatures, wetted pore radius, contact angles, and convergence angles, to demonstrate that traditional understanding of contact angles, and contact angle hysteresis, is flawed – the simple force balance underlying the Young's contact angle criterion, cannot always describe the complicated situations where pinning forces deform interfaces, and system geometry influences the contact angle. We observed that contact angle hysteresis is pronounced for some fluid-fluid-pairs, regardless of whether the fluids exist in a two-phase or three-phase fluid configuration. The wetting of a surface by a liquid is a crucial part of many natural and industrial processes. Despite numerous existing studies, some elements of wetting-drying such as contact angle variation during flow, are still poorly understood, yet it is a crucial piece of information needed for modeling fluid displacements in capillary-dominated flows. In the context of multi-phase flow in porous media, scarcity of direct contact angle measurements inside pores (in-situ) has added to the ambiguity. This work presents an unprecedented study of contact angle variations at different stages of flow, for a variety of fluid systems, using direct pore-scale measurements. Observations reveal that on average, contact angle patterns depend strongly on the type of nonwetting fluid: dodecane and decane, for instance, show large contact angle hysteresis between drainage (drying) and imbibition (wetting) cycles, while air or Soltrol® 220 oil exhibits little hysteresis. To further explore this behavior, we evaluate contact angle from the perspective of geometry, and show that the combined effects of fluid-fluid curvature and wetted-pore-radius variations can explain the observed trends. We hypothesize that, for certain nonwetting fluids, fluid-solid chemical interactions lead to pinning of fluid interfaces and the fairly enhanced hysteresis observed for some nonwetting fluids. Overall, this study suggests that contact angle is a result of the interplay between pore geometry and the forces deforming the fluid interface (including surface chemical interactions), and thus, - despite traditional practice -, contact angle alone is not always an accurate metric for wettability.