Interfacial Colloid-Clusters

• Main Authors: Wu-Liou Tsai, 蔡五柳
• Other Authors: Shang-Kiang Lai
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/cd7gue

碩士 === 國立中央大學 === 物理研究所 === 94 === Recent experimental efforts on charged colloids trapped at the fluid/water interface have witnessed the formation of colloid-clusters. It was observed in these studies that the average inter-colloidal distance is surprisingly large on the order of approximately and often greater than 3 μm much farther than that in bulk colloidal dispersion. The mechanism giving rise to these mesoscopic structures remains an unclear puzzle unquestionably due to some kind of a long-range attractive force which is certainly not of van der Waals origin. In this work, we analyze theoretically the three main contributions, namely, the electrostatic (screened Yukawa and dipolar), van der Waals and capillary potentials, to the total energy of a two dimensional (2D) charged colloids spread on the fluid/water interface. Among them, we pay due attention to the capillary potential and consider it as a dominant source causing the long-range attraction. Realistically, we choose to study charged colloids possessing the same radius equal approximately to 10 μm and consult recent theoretical and experimental works for a reasonable estimation of other interfacial related quantities such as the charge of a surface colloid , Debye screening length,..etc which are indispensable in a colloid-cluster calculation. By appealing to two state-of-the-art optimization algorithms, we calculate the 2D colloid-clusters by searching their lowest energies. Our results show that the optimized total energies yield mesoscopic structures in close resemblance to surface colloid-clusters trappped at the fluid/water interface. Prevalently, we see primary clusetrs that include singlet, doublet, triplet and quadruplet occupying deeply in the cluster core center. For the range of cluster size studied here, we find, in particular, regularity in the growth pattern in three qualitative repeated sequences. As the number of colloids increases, we notice furthermore that the triangular and centered hexagonal clusters and their respective sequence are two common core clusters. Also, our predicted large clusters show tendency towards circular geometries similar to those observed experimentally.