The effects of stress on soft glasses and biological membranes

本文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质无序材料的影响。当务之急是了解治疗软眼镜的新方法。玻璃，可以看作是液体冻结成它们的一种构型，总是会老化，也就是说经过很长一段时间，它们演变成更低能量的构型，其特征是更长的松弛时间。在软胶体玻璃实验中观察到，在冷却的聚合物熔体中通过计算机模拟（我们的工作），当受到压力时，松弛时间以非平凡的方式变化，特别是可以显着增加。从目前所知的情况来看，受压的“过度老化”玻璃并不像自然老化的玻璃。它更均匀，表现出更高的有序性，但不一定是更低的能量。了解过时效的起源以及这些无序体系的微观结构和力学性能的变化，可以为玻璃的制备提供更好的方法。第二个重点是控制构成细胞壁的脂质双分子层的破裂。脂质尾部不愿与水分子接触，这就是它们的疏水性。当这个双层破裂时，细胞就失去了它的内容物。由渗透压差或电场的作用触发，这种破裂是由孔隙的形成介导的。毛孔也可以通过吸收多肽形成，细菌和细胞本身利用多肽使细胞破裂。实验的破裂不一定是不受欢迎的结果。小的单分散囊泡，例如用于包封药物，是通过纳米尺寸的孔挤压大的多层囊泡而获得的，在通过过程中它们会破裂。挤压后的碎片重新形成更小的囊泡，理想的大小是单分散的。了解破裂过程在保护细胞免受攻击和开发靶向药物递送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.