Nanoscale objects, such as metal nanoparticles or carbon nanotubes, are prone to highly efficient absorption of electromagnetic radiation. When high intensity electromagnetic radiation is delivered to such objects, such as by a focused laser beam, the absorbed energy can lead to extreme local heating of and very large temperature increases in both the nanoscale objects and the surrounding medium. These highly localized thermal excursions correspond to heat fluxes that can be orders of magnitude larger than those sustained at the macroscale.
Intellectual Merit: This research builds upon advances in (a) laser-based ultrafast optical techniques capable of capturing relevant phenomena at pico- to nanosecond time scales and (b) computational power and modeling techniques allowing simultaneous examination of such systems experimentally and theoretically at the same temporal and special scales. In particular, molecular dynamics simulations and time resolved pump-probe experiments will advance the understanding of heat transfer, phase transformation, and phase equilibria arising at the interface between the nanoscale solids and a surrounding liquid.
Broader Impacts: The research focuses on the exchange of thermal energy between an intensely heated solid nanoparticles and a surrounding liquid. This has important implications for biomedical applications such as highly selective thermal therapy for cancer treatment. Graduate students engaged in the research will make contributions in heat transfer, materials science, soft-hard matter interactions, and phase equilibria. Undergraduate students will also be engaged in the research. Pertinent visual-learning and web-based tools will be developed to integrate the research and education activities.
In last decade there were numerous efforts in using remotely (e.g., by a laser) heated electrically conductive nanoparticles to develop better cancer thermal therapies. Such approach, in principle, allows to selectively hyperthermia cancerous cells without inducing damage to the adjacent healthy cells. While the local heat delivery is indeed possible, achieving substantial local to the nanoparticle temperature elevations requires large heating powers, that have to be delivered in short pulses to avoid macroscopic (large volume) heating. In our research we focused on the theoretical determination of the criterion for the vapor formation around intensely heated nanoparticles immersed in the liquid. To avoid making assumptions associated with modeling of the phenomena via continuum equations we simulated the system at atomic level. We determined that pulse heating leads to the vapor formation only when the liquid ~ 1 nm away from the solid nanoparticle reaches so-called spinodal decomposition temperature, i.e., when the barrier for the vapor nucleation is reduced to zero. The liquid immediately adjacent the nanoparticle surface can be even warmer due to the presence of the wetting forces. Moreover, due to a big property mismatch between hard solid and soft liquid materials there is a significant temperature drop between the heated nanoparticle and the liquid, associated with the interfacial thermal resistance. Such resistance is analogous to the electrical contact resistance. Consequently, for small nanoparticles the solid can melt and even "evaporate" directly to the liquid without liquid vaporizing. These findings present a wealth of possibilities available for thermal therapies, including controlled heating, explosive vapor nanobubble formation and collapse with tunable maximum bubble radius and atomization/ionization of the solid nanoparticle without vapor formation. We also conducted a multi-year effort developing and using visual molecular dynamics software for educational purposes. We adapted and developed features of the MDApplet freeshare Java web base software, with a target of audience of Introduction to Materials Science for Engineers course. This is a core engineering freshman/sophomore studio type course delivered annually to over 500 RPI students. The original software is free to copy and share and was developed by Dan Schroeder, Physics Department, Weber State University, Ogden, UT 84408-2508. http://physics.weber.edu/schroeder/. The initial feature development was done by an undergraduate student, an African American from the Computer Science Department at RPI, who provided protocols for the introduction of new initial atomic structures, and types of bonding to illustrate concepts we teach, such as thermal expansion of the bond length, vacancy diffusion, phase change and dislocations. Next the PI teamed up with Prof. Dan Lewis, a course coordinator and with assistance of a graduate student, they implement and integrated the software into the regular teaching delivery of the course. The software is available for the instructors and students via RPI Blackboard. The course instructors use the software during lectures and in-class activities, but, perhaps most importantly, it is used by the students to perform numerical experiments and "play with it" allowing them connect the concepts of atoms, structure and bonding with phase behavior, diffusion and other materials properties. We received significant feedback over the several years of the implementation and use of the software where students were better able to understand concepts such as: vacancy diffusion, polymer conformations, phase transitions and vibrational modes of nanoparticles. Students can, if they want, write their own initial conditions to simulate other phenomena in materials. The external visitors can access the software on the Prof. Lewis' RPI web-space at: http://homepages.rpi.edu/~lewisd2/MD/MDApplet.html.
