(2ib) Rheological (Structural) and Interfacial Properties of Emulsions and Foams for Environmental Applications

My research interests include the rheology of colloidal systems, such as concentrated emulsions, nanoemulsions, and Pickering foams, structure-property relationship and characterization of porous polymers and hydrogels, physicochemical properties of interfaces such as interfacial rheology of surfactants (per-and polyfluoroalkyl substances, PFAS) and nanoparticles, replacement of per-and polyfluoroalkyl substances (PFAS).

High internal phase emulsions (HIPEs) and foams are colloidal systems, which can be exploited in several environmental applications. For example, HIPE templating is used to produce porous materials, known as polyHIPEs, by crosslinking the continuous monomer phase followed by the removal of the dispersed droplet phase. The polyHIPEs can be used as water treatment membranes and adsorbents for contaminant removal. The morphology of polyHIPEs affects their specific applications, for instance, an open-cell structure for a membrane application and a closed-cell structure for an application of energy storage. The polyHIPE morphology is termed as voids (pores) and windows (the pore throats) on the polymer wall, the former appeared due to the removal of the dispersed phase, making the polyHIPEs porous, and the latter makes the pores interconnected. The formation of the windows is a complex process, and our study indicates that window formation is related to interdroplet interactions and interfacial rheology. Therefore, what we are interested in is tailoring or manipulating the morphology of polyHIPEs to apply them to environmental applications through systematically studying the interfacial and rheological (structural) properties of HIPEs and evaluating the relationships between the colloidal properties of HIPEs and the resulting porous structures.

As another example, foam flotation/fractionation can be applied as a promising remediation method for removing per- and polyfluoroalkyl substances (PFAS). The PFAS are a group of partially or fully fluorinated manmade chemicals, they have been widely used for many applications, such as aqueous film-forming foams (AFFFs), and non-stick cookware, due to their excellent chemical and thermal stability. However, PFAS are toxic and accumulative in organisms and the environment. Nonetheless, the main problem is that the relationships between the foaming process of PFAS solutions and the interfacial properties of PFAS have not been fundamentally studied. It is significant for improving the effectiveness of foam flotation, especially for removing the short-chain PFAS. Hence, we are interested in an investigation of the relationships between the interfacial properties of PFAS foams, for example, obtaining the adsorption coefficient from measuring the dynamic surface tension, and solving for the interfacial moduli and viscosity from performing the interfacial rheology, and their bulk properties, such as rheology and stability termed as drainage and coarsening.

Additionally, we synthesize the nanoparticles, such as Fe₃O₄ and graphite oxide, characterize the nanoparticles (i.e., morphology and wettability), and use them to create the Pickering emulsions and Pickering foams. We are interested in investigating the effect of nanoparticle structure on the properties of Pickering emulsions/foams, for example, the foaming and deforming properties of PFAS solutions for improved PFAS removal efficiency. The nanoparticle-adsorbed interfaces can be characterized by measuring the dynamic surface tension and performing dilatational and shear interfacial rheology. Also, the rheology of bulk samples is investigated to examine their structure, for example, the effect of intermolecular interactions and droplet/bubble size.