Browsing by Subject "glass transition"

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    The Colloidal Glass Transition Under Confinement
    (2018-05) Zhang, Bo
    Understanding the nature of the glass transition is one of the most challenging problems in condensed matter physics. Although ubiquitous and technically important, glasses still elude a universally accepted theoretical description. Here, we use colloidal particles as hard-sphere models and experimentally study particle dynamics of colloidal suspensions under different confinements near the glass transition. In three dimension (3D), we design a colloidal system, where particles are confined inside spherical cavities with an amorphous layer of particles pinned at the boundary. Using this novel system, we capture the amorphous-order particle clusters proposed in the framework of the random first-order transition (RFOT) theory and demonstrate the development of a static correlation near the glass transition. Moreover, by investigating the dynamics of spherically confined samples, we reveal a profound influence of the static correlation on the relaxation of colloidal liquids. In analogy to glass-forming liquids with randomly pinned particles, we propose a simple relation for the change of configurational entropy of confined colloidal liquids, which quantitatively explains our experimental findings and illustrates a divergent static length scale during the colloidal glass transition. In two dimension (2D), we prepare quasi-2D confined colloidal liquids with optical tweezers. We confirm the existence of a divergent static length in quasi-2D liquids. We further use the confinement as a tool to probe the Mermin-Wagner long-wavelength fluctuations. We find that the fluctuations have a logarithmic dependence on the system size in quasi-2D when the system approaches to the glass transition. Ellipsoidal and rodlike particles are also used to directly compare the translational and rotational dy- namics. We show a decoupling between translational and rotational dynamics and the decoupling is not affected by the confinement. What’s more, constant values of critical volume fractions are observed regardless of types of particle aspect ratios, measurement methods, fitting functions, and values of structural factors. Lastly, we have also conduct an experimental study on the 1D dynamic self-assembly of charged colloidal particles in microfluidic flows. Using high-speed confocal microscopy, we systematically investigate the influence of flow rates, electrostatics and particle poly- dispersity on the observed string structures. By studying the detailed dynamics of stable flow-driven particle pairs, we quantitatively characterize interparticle interac- tions. Based on the results, we construct a simple model that explains the intriguing non-equilibrium self-assembly process. Our study shows that the colloidal strings arise from a delicate balance between attractive hydrodynamic coupling and repulsive electro- static interaction between particles. Finally, we demonstrate that, with the assistance of transverse electric fields, a similar mechanism also leads to the formation of 2D colloidal walls. Our study provides key experimental evidences to support the development of RFOT theory to better understand the glass transition in both 3D and 2D. The fundamental differences of particle dynamics between 3D and 2D are also studied. In addition to providing experimental results for assessing general glass transition theories and par- ticle self-assembly, our studies also provide new insights into the dynamics of confined colloidal liquids and may shed light on the behavior of atomic/molecular liquids under nano-confinements.