Rotational and translational diffusion near the colloidal glass transition

在这个由化学系实验物理化学项目资助的奖项中，来自埃默里大学的威克斯教授和他的学生将研究胶体玻璃转变附近的扩散。在常规液体中，粒子和分子经历布朗运动，使它们既平移又旋转。对于给定的颗粒(或分子)大小和形状，这些运动的速率具有恒定的比率。例如，将液体的粘度加倍将以相同的系数减慢这两种类型的运动。然而，在接近玻璃化转变的过冷分子液体中，情况并非如此。间接测量表明，在这些样本中，分子的平移速度快于它们的旋转速度。这表明，过冷液体不仅是具有大粘度的简单液体，而且扩散是通过根本改变的机制发生的。威克斯教授将使用激光扫描共聚焦显微镜同时直接观察胶体“过冷流体”中棒状粒子的旋转和平移动力学。胶体悬浮液是由微米级的固体颗粒在液体中组成的，当颗粒浓度超过临界值时，胶体悬浮液表现出玻璃化转变。因为它们可以直接观察，所以这些系统是研究这种过渡机制的理想系统。共焦显微镜将被用来观察棒状颗粒和周围胶体颗粒的运动。因此，平移和旋转运动都可以同时和直接地测量，从而了解它们是如何相关的。科学结果可能有助于设计新型玻璃材料，这可能会影响光纤等技术。威克斯教授每年至少为一群小学生到他的实验室进行一次实地考察。这些学生将研究“粘性”材料的特性，例如本项目中使用的胶体糊状物。

In this award, funded by the Experimental Physical Chemistry Program of the Chemistry Division, Professor Weeks and his students from Emory University will study diffusion near the colloidal glass transition. In regular liquids, particles and molecules undergo Brownian motion, causing them to both translate and rotate. For a given particle (or molecule) size and shape, the rates of these motions have a constant ratio. For example, doubling the viscosity of the liquid will slow both types of motion by the same factor. However, in supercooled molecular liquids close to the glass transition, this is not the case. Indirect measurements suggest that molecules translate faster than they rotate in these samples. This then suggests that supercooled liquids are not merely simple liquids with large viscosities, but that diffusion takes place by fundamentally changed mechanisms.Professor Weeks will use a laser-scanning confocal microscope to simultaneously and directly observe the rotational and translational dynamics of rod-like particles in a colloidal "supercooled fluid". Colloidal suspensions are composed of micron-sized solid particles in a liquid, and manifest a glass transition as the particle concentration is increased beyond a critical value. Because they can be viewed directly, these are ideal systems for studying the mechanisms of this transition. Confocal microscopy will be used to observe the motion of both the rod-like particles and the surrounding colloidal particles. Thus, both translational and rotational motion can be measured simultaneously and directly, leading to an understanding of how they are related.The scientific results may help in the design new types of glassy materials, which could impact technologies such as fiber optics. Professor Weeks will conduct at least one field trip each year to his laboratory for groups of primary school students. These students will investigate the properties of "squishy" materials such as the colloidal pastes used in this project.