Chemical Reactions and Chemically-driven Transport in Channels and Porous Media

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The principles of equilibrium physical chemistry are well known when modeling environmental and transport processes at interfaces that occur at the boundary between a liquid and a solid, between two liquids, or between a liquid and a gas. Interfacial transport processes are important for handling and studying colloids in a wide range of industrial or environmental systems, but they are not understood and may not even be appreciated for systems that are out of equilibrium, such as when there are chemical gradients that introduce pH and surface charge effects. In this project, configurations that mimic important features common to transport of colloids in porous materials will be studied with experiments, mathematical modeling, and numerical simulations. The project will examine the integrated effects of pH-regulated surface charge, flows in micro- and nano-scale systems, material type, and mode of transport (e.g., accumulation or dispersion), yielding a coherent framework linking physical chemistry to transport, from nano- to macro-scales. The broader impacts of the project include training graduate students and undergraduates to study these timely problems with experimental and simulation tools of fluid dynamics, physical chemistry, and transport phenomena so as to develop quantitative understanding of these problems. The outreach efforts included characterize the PI’s approaches to engaging with, teaching, and mentoring future research scientists, including continuing an annual “holiday” lecture that has been delivered since 2002. The goal of this project is to couple knowledge of physicochemical processes at and near solid-liquid interfaces to understand, and control, particle transport and fluid flow in a myriad of small-scale and porous materials. The focus is on non-equilibrium phenomena driven by pH-induced changes in surface charge, which is coupled to macroscopic gradients in chemical concentrations, to produce phoretic and osmotic flows. These ideas will be tackled using experiments, simulations, and theory based on the principles of electrokinetics, including recent extensions to the modified Poisson-Boltzmann description that accounts for molecular-level features important to transport phenomena. The research has the following three systematic aims: (1) The study of surface reactions and diffusiophoretic particle motions in the presence of pH and electrolyte gradients, for different pH, ionic strength, and particle type; (2) The study of surface reactions and diffusioosmotic channel flows by using microfluidic methods to control the spatial heterogeneity of the surface charge of the channel walls and so understand how the induced flows affect particle transport as a function of pH, ionic strength, and particle type; and (3) the flow of suspensions in porous materials, where qualitative and quantitative conditions (chemical, particle type, geometry, etc.) can lead to particle accumulation or dispersion that until now is poorly understood and poorly characterized. The research themes will advance the understanding of surface-dominated transport processes, which occur in a wide range of environmental and industrial problems and processes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.