The effects of stress on soft glasses and biological membranes

采用理论建模和计算机模拟相结合的方法，研究了应力对聚合物熔体和生物膜等软无序材料的影响。当务之急是了解什么可能成为治疗软性眼镜的新方法。玻璃可以被视为冻结成它们的一种构型的液体，随着时间的延长，它们总是老化，它们演变成以更长的驰豫时间为特征的低能量构型。在软胶玻璃中的实验中观察到，通过在冷却的聚合物熔体中的计算机模拟(我们的工作)，当受到应力时，弛豫时间以不平凡的方式变化，特别是可以显著地增加。从目前所知的情况来看，压力过大的“老化”玻璃不像自然老化的玻璃。它更均匀，显示出更多的有序性，但不一定是较低的能量。如果了解过度老化的原因以及这些无序系统的微观结构和机械性能的变化，可能会带来更好的制备玻璃的方法。第二个优先事项是控制形成细胞壁的脂质双层的破裂。这种双层是通过脂尾不愿与水分子接触而结合在一起的，这被称为疏水性。当这种双层破裂时，细胞会失去其内容物。在渗透压差异或电场的触发下，这种破裂是通过形成毛孔来调节的。毛孔也可以通过吸收多肽而形成，这些多肽被细菌和细胞本身用来破坏细胞。破裂不一定是实验的不受欢迎的结果。例如，用于包裹药物的小型单分散囊泡是通过将大的多层片状囊泡挤出纳米大小的孔而获得的，这些孔在通过过程中会破裂。挤出的碎片会转化为更小的囊泡，理想情况下其大小应该是单分散的。了解破裂过程在保护细胞免受攻击和开发靶向给药方法方面具有巨大的医学潜力。

We study the effects of stress on soft disordered materials, such as polymer melts and biological membranes, using theoretical modeling and computer simulation. One priority is to understand what may turn out to be a new way to treat soft glasses. Glasses, which can viewed as liquids frozen into one of their configurations, always age, that is over long times, they evolve into lower energy configurations characterized by longer relaxation times. It has been observed experimentally in soft colloidal glasses, and through computer simulations in a cooled polymer melt (our work), that, when subjected to stress, the relaxation times change in non-trivial ways, and in particular can be increased significantly. From what is known at this point, the stressed "overaged" glass, is not like a naturally aged glass. It is more homogeneous and exhibits increased ordering but not necessarily a lower energy.  Understanding the origins of the overaging and the changes in the microstructure and mechanical properties of these disordered systems may lead to better ways to prepare glasses.   A second priority is controlling the rupture of lipid bilayers, which form the walls of cells. This double layer is held together by the reluctance of the lipid tails to be in contact with water molecules, known as their hydrophobicity.  When this bilayer ruptures the cell loses its content.  Triggered by osmotic pressure differences or the application of electrical fields, this rupture is mediated by the formation of pores.  Pores can also form through the absorption of peptides, which are used by bacteria and cells themselves to rupture cells. Rupture is not necessarily an unwelcome outcome of an experiment.  Small monodisperse vesicles used for instance for encapsulating drugs are obtained by extrusion of large multi-lamellar vesicles, through nanometer size holes, where they rupture during passage.  The extruded fragments reform into smaller vesicles, whose size ideally would be monodisperse. Understanding the rupture process offers great medical potential, in protecting cells against attack, and in developing methods for targeted drug delivery.