Crystal-Liquid Transitions Studied With Colloids in an Electric Bottle

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Crystal-Liquid Transitions Studied With Colloids in an Electric Bottle

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Title: Crystal-Liquid Transitions Studied With Colloids in an Electric Bottle
Author: Hwang, Hyerim ORCID  0000-0001-8098-6030
Citation: Hwang, Hyerim. 2016. Crystal-Liquid Transitions Studied With Colloids in an Electric Bottle. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
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Abstract: In this thesis, we have presented the experimental investigations on the crystal-liquid transitions in a colloidal system. Colloids behave as big atoms, thus they are good model systems to study the dynamics of condensed matter. Their phases are determined by the particle concentration which can be controlled by external forces. We studied the transitions such as crystallization and melting in a controlled way. With a confocal microscopy, we were able to obsserve the transitions at single particle level in three-dimension.

In Chapters 2-3, we introduced the electric bottle setup which played a significant role to induce the transitions in this thesis. The electric bottle was designed to generate inhomogeneous electric fields, and we were able to employ dielectrophoresis to manipulate the particle concentration using this setup. The colloidal suspension we used here is composed of PMMA particles (Ɛp=2.3), the mixture of cis-decalin and tetrachloroethylene (Ɛm = 2.6$), and surfactant AOT molecules to give repulsive interaction between the particles. We also introduced analysis methods to obtain particle location information from the raw confocal images and to distinguish between the crystal and liquid phases by using their different structures.

In Chapters 4-6, we investigated the crystal-liquid transitions and a crystal-crystal transition. We studied the growth kinetics in crystallization and melting in a system which is covalent to the collision-limited growth of pure metals. We measured the attachment and detachment rates, which can be denoted as jump rates. It was found that the process is governed by the Brownian motion of the particles which is dealing with the random walk. The free energy difference between the two phases gives bias to the random walk, thus we insist that the growth process is a biased random walk. We also studied the equilibrated interfaces in a BCC crystal-liquid system. We measured the equilibrium fluctuations of the interface, which gives an interfacial stiffness of the interface. Although the orientation of the interface plane doesn't have high-rotational symmetry, the stiffness was observed to be isotropic in a long wavelength limit. The last transition we observed is the one between crystals, BCC and FCC crystals.

We explored the crystal-liquid transitions at single particle level using the combination of the electric bottle and colloids. Instead having multiple samples to study the phase behaviors as a function of volume fraction, we were able to obtain a concentration-dependent phase diagram in a single electric bottle sample. The design of the sample cell can be further developed to induce the various kinds of density gradient. Also, many other phase behaviors resulted from different type of interactions can be studied.
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