The effects of stress on soft glasses and biological membranes

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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.