Contact angle hysteresis: A new paradigm?

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 mea-surements 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 mea-surements. Observations reveal that on average, contact angle patterns depend strongly on the type of non-wetting fluid: dodecane and decane, for instance, show large contact angle hysteresis between drainage (drying) and imbibition (wetting) cycles, while air or Soltrol (R) 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.

Automated summary

This study investigates the contact angle variations of fluids at different stages of flow using direct pore-scale measurements. The results show that the contact angle patterns depend strongly on the type of non-wetting fluid, and the interplay between pore geometry and the forces deforming the fluid interface has a significant impact on the contact angle.