You are invited to attend Youssra Rahham's thesis defense.
Thesis Title: Transport and Irreversible Retention of Hydrophobic Nanoparticles by Fluid-Fluid and Fluid-Solid Interfaces in Porous Media
Abstract:
Hydrophobic nanoparticle (NP) transport in porous media has implications for aquifer transport and retention of a wide range of contaminants that infiltrate water resources and threaten human health as well as aquatic environments. Comprehension of NP transport and interactions with hydrophobic surfaces and interfaces -given their ubiquity in porous aquifers- is essential for groundwater remediation from organic contaminants, toxic engineered NPs, and nanoplastics.
This research investigates the transport and attachment of hydrophobic NPs under varying physicochemical conditions in saturated and unsaturated porous media by integrating experimental observations across multiple scales, theoretical extended-DLVO predictions, and numerical modeling. A non-toxic, negatively-charged, hydrophobic model NP system synthesized from ethyl cellulose (EC), and exhaustively characterized for colloidal stability and interfacial interactions, was employed to systematically explore NP interactions with fluid-fluid and solid-fluid interfaces.
The upscaling capability of an advection-dispersion-retention continuum model was compared vis-à-vis a pore network model of irreversible NP attachment onto fluid interfaces in 3D columns packed with spherical glass beads, showing that the latter captures key pore-scale dynamics such as bypassed interfaces, slow-moving corner flows, and diffusion-dominated retention.
Transport experiments in 2D microfluidic pore networks confirm that the dynamics of NP retention in unsaturated porous media depend not only on the saturation of the non-wetting phase, but also on its connectivity and the accessibility of immobile fluid-fluid interfaces.
Experimental evidence demonstrates that ethyl cellulose nanoparticles
(EC-NPs) irreversibly attach onto immobile fluid-fluid interfaces and experience delay in slow moving zones owing to geometric effects. Similarly, hydrophobic solid-fluid interfaces represent permanent sinks for EC-NPs. The attraction between a hydrophobic particle and a hydrophobic solid surface may be strong enough for irreversible attachment to take place, even under conditions of strong electrostatic repulsion. The strength of this hydrophobic interaction between an EC-NP and a hydrophobic collector surface is demonstrated using octadecyltrichlorosilane-treated glass and quantified via systematic contact angle measurements. Under destabilizing ionic conditions, irreversible EC-NP aggregation results in the formation of a secondary porous structure within hydrophilic porous media, altering permeability and retention patterns. Both phenomena are inadequately captured by macroscopic breakthrough curve (BTC) analyses alone. For example, attachment onto fluid-fluid and fluid-solid interfaces manifests itself on BTCs at low injection concentrations, whereas the opposite effect emerges in the presence of salt.
This research advances the field by conducting transport experiments under carefully controlled conditions. The findings, supported by theoretical analysis and supporting experimental evidence, highlight key limitations in current modeling approaches and provide foundational experimental data that should advance the development and validation of numerical models of nano-colloid transport in porous media. Besides enhancing predictive capabilities for the fate of hydrophobic nanomaterials in the subsurface, this research informs risk assessment and the design of groundwater remediation strategies, ex-situ (i.e., NP filtration media) and in-situ (e.g., permeable adsorptive barriers for fluorinated contaminant capture and oil spill cleanup).