Freiberger N, Medebach M, Glatter O
Institute of Chemistry, University of Graz, Heinrichstrasse 28, Graz, Austria.
J Phys Chem B. 2008 Oct 9;112(40):12635-43. doi: 10.1021/jp804027z. Epub 2008 Sep 13.
The crystal growth of dense and almost monodisperse colloids has been investigated during recent years, but less is known about the melting behavior. The current study thus focuses on this topic. Monodisperse hard spheres were found to crystallize for certain concentrations (49-58 vol %), after sufficiently long times. The characteristics of the crystal growth change when the colloidal particles are polydisperse. Finally, when the size distribution function of the particles is broad enough, the crystallization no longer took place. Dense oil-in-water emulsions with polydispersities of around 10% were successfully produced, and in a first approximation, these emulsions behaved like hard spheres. The polydispersity of the emulsions was sufficiently high to avoid crystallization in equilibrium but low enough to induce a disorder-to-order transition under shear. The formed crystals started to melt once the shear was discontinued. The melting behavior of these "oil droplet crystals" was investigated by means of time-resolved static light scattering experiments, and it was found that crystallization could be induced in a concentration regime between 46 and approximately 74 vol %. The melting behavior of these crystals depended strongly on the concentration. The typical melting times ranged from a few seconds to several hours or days when the concentration was increased. It was speculated that this phenomenon could be explained by the strong dependence of the mobility of the oil droplets on the volume fraction, as verified by dynamic light scattering experiments on oil-in-water emulsions in a similar concentration regime.
近年来，人们对致密且几乎单分散的胶体的晶体生长进行了研究，但对其熔化行为了解较少。因此，本研究聚焦于这一主题。发现单分散硬球在足够长的时间后，在特定浓度（49 - 58体积%）下会结晶。当胶体颗粒为多分散时，晶体生长的特性会发生变化。最后，当颗粒的尺寸分布函数足够宽时，结晶不再发生。成功制备了多分散性约为10%的致密水包油乳液，初步看来，这些乳液的行为类似于硬球。乳液的多分散性足够高，以避免在平衡状态下结晶，但又足够低，以在剪切作用下诱导无序到有序的转变。一旦停止剪切，形成的晶体就开始熔化。通过时间分辨静态光散射实验研究了这些“油滴晶体”的熔化行为，发现可以在46至约74体积%的浓度范围内诱导结晶。这些晶体的熔化行为强烈依赖于浓度。当浓度增加时，典型的熔化时间从几秒到几小时或几天不等。据推测，这一现象可以通过油滴迁移率对体积分数的强烈依赖性来解释，这在类似浓度范围内的水包油乳液的动态光散射实验中得到了验证。
