Browsing by Subject "confinement"

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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.
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    Novel Properties and Emergent Collective Phenomena of Active Fluids
    (2021-01) Liu, Zhengyang
    An active fluid denotes a suspension of particles, cells and macromolecules that are capable of transducing free energy into systematic motions. Converting energy at individual constituent scales, these systems are constantly driven out of equilibrium and display unusual phenomena, including a transition to a zero viscosity superfluid-like state and a transition to a collective moving turbulent state. These curious transitions are consequences of the self-propulsion of active particles, and are absent in classical complex fluids without spontaneous motions. In this thesis, we present an experimental investigation on the rheology of active fluids under confinement. Specifically, we find the viscosity of bacterial suspensions is significantly reduced by confining walls. We show that this effect results from upstream swimming bacteria near the confining walls, which collectively exert stress on the fluids and push the fluids to flow. A phenomenological model is proposed which qualitatively captures the confinement effect on the viscosity of bacterial suspensions. The collective motions in dense bacterial suspensions are investigated. In particular, we measure the critical conditions of the transition from disordered state to turbulent state in bacterial suspensions. We present the experimental results in a phase diagram, serving as a benchmark for existing and future theories. We put forward a heuristic model based on two-body hydrodynamic interactions, hoping to understand the transition in a more intuitive way and to stimulate theoretical advancement. In addition, we present the first experimental study on the giant number fluctuation - a landmark of collectively moving active particles - in 3-dimensional bacterial suspensions. Our measurements are free from effect of frictional walls and thus allow quantitative comparison with previous theoretical and computational works. We also present a detailed analysis on the flow fields generated by the swimming bacteria, and reveal a strong coupling between flow strength and giant number fluctuations spanning all length scales. By elucidating the causes and consequences these phenomena, we not only expand the knowledge of active fluids, but also provide deeper understandings on the biological and ecological significance of living organism behavior. Our experiments deepen understanding of the self-organization processes in active fluids and lay the foundation of engineering machines composed of active constituents which mimics the properties of real living matter.