A study of phase transitions in two-dimensional colloidal particle systems at an oil/water interface
Date Issued
2007
Date
2007
Author(s)
Lin, Bo-Jiun
DOI
zh-TW
Abstract
Melting phenomenon in two dimensions exhibits a different scenario from that in three dimensions. According to KTHNY theory there exists an intermediate phase called “hexatic phase” between solid phase and liquid phase and that makes the melting process into two stages. First stage is from solid phase melting into hexatic phase and followed by hexatic phase melting into liquid phase. Due to the long range repulsive force, the colloidal particles adsorbed at an oil/water interface could form crystal structure that makes it suitable for observation of melting phenomena in two dimensions. We observed the melting phenomena in two dimensions induced by concentration change, temperature and addition of small particles by trapping colloidal particles at an oil/water interface. The potential function between the colloidal particles was also experimentally determined.
Digital video microscopy method was used to calculate two dimensional structure factor and transverse line shape of first peak. In addition we also evaluated the radial distribution function and bond orientation correlation function of the system which are used in KTHNY theory to distinguish the phase behavior. Further defect analysis was accomplished by the Voronoi diagram to analyze the percentage of different coordination number of particles.
The sequence of phase transitions of charged sulfate polystyrene particle at oil/water interface, a pure solid phase pure hexatic phase liquid-hexatic coexisting pure liquid phase, was observed at three different temperatures. In addition, the boundary line of each phase shifts to a higher density with raising temperature.
The pair interaction potential of the two-dimensional colloidal particles was found to be a very long-ranged repulsion. The potential function decays with distance to the power of –3 and exhibits the same feature at three different temperatures (20℃、30℃ and 40℃). When the surface charge of particles is decreased, the potential function is no longer decay with distance to the power of -3. In contrast, the potential function decays exponentially.
Sequence of phase transition induced by addition of small particles in binary mixture was found to be exactly the same as the sequence of phase transition observed in the pure single component system. The boundary line of liquid phase was found to shift to a higher density with increasing the number of small particles. The depletion potential of the colloidal particles at oil/water interfaces exhibits a repulsive force when the number of small particles gets concentrated, consistent with A-O model.
Subjects
膠體粒子
二維相變
界面
位能模型
colloidal particles
two-dimensional melting
interface
potential function
Type
thesis
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