The goal of this project is to understand the movement of micrometer-size colloidal particles by a process called diffusiophoresis. Small particles can move through a liquid when molecules dissolved in the liquid are distributed unevenly around the particle. The velocity of the particle is affected by chemical interactions between the solute molecules and the particle, which are described by an empirical interaction parameter. This project will use computer-based methods to predict the interaction parameter, and hence the speed of the particle, for various solutes including salt, polymers, nanoparticles and other species. The project will focus on non-electrolyte solutes, whether aqueous or organic, to broaden the range of media in which self-propelled particles can be designed to deliver cargo or enhance mixing. These media include geological reservoirs, living systems, and electronic devices. The advantage of using computer-based models to predict particle speed is that expensive experiments do not have to be carried out for each distinct system. Instead, the computer methods, which are based on fundamental descriptions of molecular level forces between the solute and the particle, can be employed in advance. Then, using results from computer calculations, the research team will synthesize particles that can create uneven distributions of solute around their surfaces to generate self-propelled motion. Results from the project will be used in online course titled, "Creativity, Innovation, and Change."
The utility of electrolyte diffusiophoresis as a propulsion mechanism is limited, especially at high-salt concentrations and in dielectric (apolar) media where many key applications lie. This project explores non-electrolyte diffusiophoresis, in which a particle generates local concentration gradients of non-electrolyte species to cause self-propulsion. The critical gap in knowledge is the connection between system chemistry and the speed of the self-propelled particle. This project will develop methods to calculate interaction energies based on rigorous descriptions of van der Waals attractions and hard-sphere repulsions between the particle and non-electrolyte solutes. Having the energies then enables the calculation of the chemical interaction parameter and the particle speed. The project will also construct self-propelled particles that incorporate appropriate ferrocene-containing polymers, gold nanoparticles, organometallic catalysts, and other species. The resulting motor chemistries will include attractive or repulsive moieties with the proper materials, sizes, and structures, to cause transport in a given set of solution conditions. From the known reaction kinetics, the concentration gradients will be calculated, and for the first time, the value of the interaction parameter will be measured. A measure of success includes observing non-electrolyte diffusiophoresis transport n three challenging test beds: aqueous with 5 M salt, physiological fluid, and hexane.
