PhD Thesis Defence by Animesh Nepal

The predoctoral researcher Animesh Nepal, from the Groundwater and Hydrogeochemistry group, will defend his thesis on 17th July at 10:30h in Escola Tècnica Superior d’Enginyeria de Camins, Canals i Ports de Barcelona (UPC, Campus Nord Building B1. Classroom: 003 C/Jordi Girona, 1-3 08034 Barcelona)

Directors: Marco Dentz and Juan J. Hidalgo

Thesis Committee: Luis Cueto-Felgueroso, Ramón Planet and Maarten Saaltink

Abstract:

Immiscible fluid displacements in porous media is governed by the interplay of capillary forces, pore geometry, wettability, and driving conditions. Under quasistatic conditions, this interplay gives rise to metastable interfacial configurations, abrupt interface rearrangements (Haines jumps), interface pinning, and macroscopic pressure–saturation hysteresis. This thesis develops a pore-scale framework to explain how these phenomena emerge and how they can be predicted. The work combines laboratory experiments, numerical simulations in two and three spatial dimensions, and analytical models derived from hydrostatic–capillary pressure balance and interfacial energy arguments. The investigation progresses from simple geometries representing a single pore (ink-bottle and wavy capillaries) to connected porous media (homogeneous and heterogeneous porous media), allowing direct identification of fundamental mechanisms and their collective effects.

In single-pore ink-bottle geometries, the results show that interfacial behavior is controlled by a critical geometric condition: below this threshold, interfaces evolve smoothly, whereas above it, interfaces become unstable and jump, resulting in hysteresis during imbibition and drainage. The occurrence and magnitude of jumps depend systematically on the contact angle, surface tension, and constriction angle, and are accurately captured by theory. The thesis then demonstrates that driving mode is also a primary control parameter by analyzing two different capillaries (ink-bottle and wavy capillary tube). Under pressure-driven cyclic displacement, jumps and hysteresis arise from the loss of interfacial stability. Under volume-driven displacement, however, imposed volume constraints suppress jump transitions, eliminate capillary hysteresis, and force the interface to retrace identical imbibition and drainage paths, while interfacial pinning can still occur. In the volume-driven regime, the fluid can traverse tensile and otherwise inaccessible interfacial configurations that are inaccessible under pressure-driven fluid displacement, confirming that the choice of driving mode can decouple pinning and hysteresis. In wavy capillaries, smooth curvature change prevents interface pinning, but pressure-driven displacement can still trigger interfacial jumps. An energy-based analysis shows that pressure changes drive saddle-node transitions between local energy minima, producing jumps and hysteresis. This yields a simple jump criterion that links contact angle to wavy-capillary geometry. Extending these insights to porous media, the thesis shows that homogeneous pore channels can be represented with wavy-capillaries that reproduce key pressure–saturation signatures, including jumps and hysteresis. In heterogeneous porous media, stronger geometric variability generates intermittent collective rearrangements, also known as Haines jump, hysteresis, and return-point memory during internal cycles, evidencing a rugged energy landscape with history-dependent pathways. Together, these results establish a coherent mechanistic link between pore-scale metastability and macroscopic hysteresis. The framework provides predictive tools for controlling displacement efficiency and energy dissipation in applications such as CO2 sequestration, enhanced oil recovery, microfluidics, and capillary transport in natural and engineered porous media.