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Colloids
2D colloids provide a convenient model system for studying defects and order in condensed matter physics. We pioneered the method of using optical tweezers to create point defects in a 2D colloidal crystal and taking advantage of the fact that in 2D, point defects can be thermally excited into dislocation pairs, which are topological defects. We were able to observe for the first time the famous “Peierls barrier” proposed by Rudolf Peierls 70 years ago but was never observed in a real experiment. We also carried out a random pinning experiment in which we realized experimentally a “2D solid in random pinning potentials” which was extensively theorized. We showed that the quasi-long-range order (QLRO) of a 2D crystal is destroyed by random pinning, and the system behaves as a “pinned liquid”. Upon application of a driving force, instead of depin the system from the random potential, thereby causing the system to re-crystalize, we found that the system exhibits the classic thermally activated creep phenomena, i.e. a slow liquid. We are developing techniques to study open issues in 2D melting in spherical colloids and 2D glass transition in rod-shaped colloids. For a recent publication, see: https://www.pnas.org/content/