steven higgins, ph.d. - research
Welcome to the Higgins Research Group
Focus: Research in Solid-Liquid Interface Dynamics and Chemistry
Crystal Growth and Dissolution: A major project in my group will focus on understanding inorganic and bio-mineralization processes that influence the chemistry of fragile ecosystems such as those found in marine environments and will also have a direction toward understanding crystallization in solid-phase carbon management strategies and long-term radioactive waste immobilization. My approach to a better understanding of these problems will involve detailed in-situ Atomic Force Microscopy (AFM), and hydrodynamic (e.g., channel-flow cell, and wall-jet flow cell) studies of heterogeneous kinetics under well-defined chemical and transport conditions. Since the quality and properties of many materials, and the heterogeneous kinetics of surface reactions depend on defect and impurity content, my research will provide a new level of understanding of the interaction and influence of inorganic as well as organic contaminants on processes ranging from surface dynamics of non-linear optical materials such as Potassium Dihydrogen Phosphate (KDP) to growth and dissolution kinetics of scaling minerals such as alkaline earth sulfates, phosphates, and carbonates and molecular-scale step dynamics and chemistry of solid surfaces.
Adsorption at Solid-Liquid Interfaces: Many problems in aqueous geochemistry, corrosion and scaling inhibition involve adsorptive interaction between organic molecules and solid surfaces. By choosing appropriate non-linearly active molecules, it is possible to determine adsorption free energies, and in some cases, information on the molecular orientation at the surface may be determined through optical second harmonic generation. It is my intent to utilize this surface sensitive approach to determine relationships between the concentration of adsorbed molecules and the kinetics of a particular process (i.e., dissolution, precipitation). This approach will incorporate a new hydrodynamic experimental design to ensure well-defined and controllable mass transport in these dynamic systems. With the fluid chemistry and transport conditions under experimental control and means for simultaneously characterizing the surface chemistry and reaction flux, this very powerful methodology can be utilized in the interrogation of specific heterogeneous reaction mechanisms.
Instrumentation: The observation of molecular scale reactions with scanning probe microscopy represents one of my key research areas. I have developed a Hydrothermal AFM (HAFM) that operates in highly corrosive fluids and significantly extends the pressure and temperature range of AFM technologies. There are two major reasons why this development is important to materials-related and environmental problems. First, in situ observations are usually required to describe the underlying mechanism in a reaction of interest. Second, the AFM is a valuable structural characterization tool, but has poor detection limits when used to observe dynamics. It is difficult to discuss mechanisms without kinetic experimental data to interrogate. To surmount this shortcoming of the AFM, elevated temperatures assist by accelerating heterogeneous reaction rates. My research will continue to push the limits of temperature and pressure in the AFM that will expand our research base to include studies under supercritical CO2, which is important in many materials formation processes and is gaining increased attention as a replacement for some organic solvents. With the capability to study surfaces with the AFM under supercritical fluids, we may begin to observe polymerization reactions in real time at the macromolecular level. These studies will be the first of their kind and may lead to a better understanding, and ultimately, a better control over the formation of new polymeric materials.