Many commonplace products are composed of collections of particles suspended in liquid. Examples include paints, food products and cosmetics. An important feature of these products is that the particles remain in particular arrangements, rather than aggregating or separating from the suspending liquid. Maintaining the proper arrangement of the particles can be challenging, especially for products with long shelf lives. The key aspect of maintaining the desired arrangement of particles is to control what happens in the small gaps where the particles almost touch each other. This project takes advantage of a recent development in our ability to make well-defined examples of these small gaps. The project examines how molecules adsorb into the gaps, and how the adsorption is affected by the gap size. This molecular adsorption is critical to controlling whether the gaps close or widen with time, and thus whether consumer products maintain the most useful arrangement. The research in this project will be integrated into programs that teach undergraduate and graduate students, and will provide educational tools to inspire high school students, especially those underrepresented in STEM fields, to continue in science and engineering studies.
Adsorption of ions, surfactants, polymers, and particles is a key factor in determining the stability of colloidal dispersions in thin wetting films, and thus is central to many engineering processes. Adsorption at single interfaces has been well studied, but the directly relevant quantity for colloid stabilization is adsorption in a thin film between two interfaces. This has only rarely been measured in prior work. A new technique has been developed to measure the amount of a species in nanometer-scale films between two solids. This technique will be used to measure adsorption in critical regimes where simple modeling based on Poisson-Boltzmann theory is expected to fail, such as in concentrated solutions, multivalent ions, surfactants, and particles. The technique has three elements: (1) fabrication of a robust thin film between two solids with a continuous gradient of thicknesses in the range of 1?200 nm; (2) measurement of film thickness by interferometry; and (3) measurement of the amount of adsorbed molecules from the intensity of fluorescence emission. The expected outcomes are: (1) accurate measurements of amounts of species in thin films; (2) assessment of the validity of common models; (3) insight into the stabilization of films in various applications; and (4) some development of the method to measure the profile of fluorophore concentration in the direction normal to a planar film. We will integrate this work into the "Computers and Technology at Virginia Tech" (C-Tech2), which is an annual two-week summer program for 120 female high school to participate in hands-on activities to help develop and sustain interest in science and engineering. The work will also provide interdisciplinary training for a graduate student in the fields of optics, microfabrication, modelling, and microscopy. Undergraduates will work on the project, with the aim of inspiring them to pursue graduate study in engineering or science.
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.
