PhD Proposal: Brian Uthe
TITLE: Unraveling the Complex Fluid Dynamics of Simple Liquids
ABSTRACT: Over the past several decades excitement has grown over using nanoparticles as devices which maximize their unique optical and mechanical properties. Ultrasensitive mass sensors have been realized using nanostructures to provide atomic-scale mass resolution. Relying on the known mechanical vibrations of these structures, shifts in frequency are detected once a molecule adsorbs to the surface increasing the overall mass of the structure. Enhanced resolution and accurate interpretations of the frequency shifts will require a complete characterization of the vibrational damping experienced by these nanostructures. The goal is to incorporate these sensors into fluidic environments for real-time mass sensing; this will require a quantitative understanding of the fluid-structure interactions. Simple liquids, such as water, are typically treated using Newtonian fluid mechanics where the stress applied to the fluid is proportional to the rate of strain in the fluid. This treatment holds so long as the flow generated by object’s motion within the fluid is long compared to the stress relaxation time of the fluid. This assumption begins to break down for nanoscale structures and is the subject of this work.
In this work, transient absorption measurements will be employed to excite and probe the acoustic vibrations of metal nanoparticles to measure the mechanical damping by simple liquids. We will present our recent work using Au bipyramids to probe the shear viscoelastic response of water-glycerol mixtures over a range of temperatures. The measured quality factors and resonance frequencies show discrepancies between the simulation and experimental results when treating the liquid as a Newtonian fluid. This is resolved when modeling the liquid as a viscoelastic fluid; this is evidence that typical formulaic treatments of simple liquids break down with the motion of nanometer scaled objects. The aim of our work will be to determine a complete phenomenological and quantitative description for viscoelastic effects in simple liquids utilizing bipyramidal and spherical nanoparticles to probe the shear and bulk viscoelastic effects respectively.