EAGER: Collaborative Research: Dynamics of Nanoparticles in Light-Excited Supercavitation

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Nanoparticles that can be propelled through liquids at high speed (called "nanoswimmers") can play important roles in applications such as targeted-drug delivery, in-situ diagnostics and nanofabrication. For such applications, controlling the direction of high-speed nanoswimmers is critical. However, high-speed nanoswimmers are mostly propelled by forces with random directions, and guided nanoswimmers are currently limited to slow speeds. Designing fully controllable yet fast-moving nanoswimmers thus has significant technological potential. Recent experiments have observed that extremely fast ( 100,000 micron/s) gold nanoparticle swimmers can be directed by light as an external energy source. However, the underlying mechanism is yet to be fully understood. This EAGER project will take the first step toward understanding this phenomenon by studying the force and energy balance of a nanoparticle driven by light. A combination of experiments and numerical simulations of the motion of the nanoswimmers will provide a basic understanding the dynamics of nanoswimmers and resolve questions about the underlying mechanisms of their motion. This, in turn, will provide new information that can lead to a wide range of advanced nanoengineering applications, such as selectively printing nanostructures at a surface for sensing applications or delivering drug-carrying nanoswimmers to biological cells under the skin using skin-penetrable near infrared light sources.The observed ultra-fast nanoswimmer motion has never been reported and could not be explained by Stokes law. This EAGER project will investigate a hypothesis that when the nanoparticle is excited by the light at the surface plasmon resonance (SPR) peak, a nanoscale bubble forms surrounding the particle (i.e., super-cavitation). This provides a near frictionless environment that allows it move at high speed, provided the bubble can remain intact. The objective of the project is to test this hypothesis by analyzing the forces (optical force and fluidic force) the nanoparticle experiences when moving inside the supercavitation bubble using multiscale modeling and experimental techniques. This project consists of two tasks. First, multi-scale modeling will be conducted to understand the nanoparticle dynamics in supercavitation. Second, experiments will be conducted to observe the nanoswimmer dynamics and validate the computation results. This project will unravel fundamental physics involving coupled effects of nanophotonic optical forces, optothermal super-cavitation, and nanoscale thermo-fluids. An essential aspect of this work will be the integration of research with education and training of the next generation of scientists and engineers in multi-disciplinary fields, which are crucial for the technology-intensive U.S. industries. We will educate graduate students at Notre Dame and undergraduate researchers at the Colorado Mesa University (CM), a Primarily Undergraduate Institution serving nearly 10,000 students including many rural, first-generation, non-traditional (single parents, veterans and returning students), Hispanic, and other student groups underrepresented in STEM disciplines.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.