****Technical Abstract**** The structures and transitions of colloidal clusters may reveal local mechanisms governing bulk phenomena such as nucleation and structural arrest. Such clusters are also useful model systems for understanding self-assembly pathways. This experimental program will investigate three-dimensional colloidal clusters composed of 6-30 particles held together by weak interactions such as depletion or DNA-mediated binding. An advanced imaging method, digital holographic microscopy, will be used to measure the positions and motions of every particle in a cluster with sub-millisecond temporal resolution and 10 nm precision in all three dimensions. This technique enables detailed, quantitative studies of the ground states, equilibrium structural transitions, and topological transitions of the clusters. Results will be compared and used as input to energy landscape calculations, with the broad goal of understanding how phase transitions and nonequilibrium phenomena emerge as the size of the system increases. The program will train a graduate student and undergraduate in soft matter physics and applied optics. An outreach and education program will bring 20-40 high-school physics students from Cambridge and Boston-area public schools to Harvard for a series of discovery-based labs on waves, optics, and holography.
This award supports experimental research on unusual materials called clusters, sometimes called an "intermediate state of matter." Compared to the usual states of matter we encounter in our daily lives -- gases, liquids, and solids -- clusters consist of a far smaller number of atoms or particles. Because they are less complex than ordinary matter, they are in principle easier to understand. Studying how these clusters change their shapes over time and in the presence of external forces could help solve longstanding challenges in materials science, such as how crystals form and why they sometimes form disordered structures called glasses, both of which are important classes of materials for industry. However, clusters remain difficult to prepare and study experimentally. This project will take advantage of a new way to make clusters using microscopic particles called colloids, which can be seen with an optical microscope. The motion of particles in the clusters will be observed using an advanced imaging method involving holograms. The project will train a Ph.D. student and undergraduate in physics, materials science, and holography, an emerging high-technology field. The project will also educate 20-40 public high school students in physics and optics through a series of labs designed to be fun, engaging, and aimed toward motivating them to continue studying science.
