FOR1394

The nature of the glass transition is one of the frontier questions in Statistical and Condensed Matter Physics; slow and complex structural relaxation processes distributed over many decades in time develop in glass forming melts. While considerable progress has been made in recent decades towards understanding viscoelasticity and dynamical arrest in quiescent glass formers, the interplay of glassy dynamics with external fields reveals a wealth of novel phenomena yet to be explored. Our research unit (RU) focuses on the effects of strong external fields in glass forming systems in order to understand the complex structural and transport phenomena under far-from equilibrium conditions. Combining experimental, simulational, and theoretical efforts we measure and determine the nonlinear response of supercooled metallic, polymeric, and silica melts, of colloidal dispersions, and of ionic liquids. Applied fields include electric and mechanic fields, and forced active probing ('microrheology'), where a single probe is forced through the glass forming host, so that nonlinear stress-strain and force-velocity relations, as well as nonlinear dielectric susceptibilities can be observed. While the physical manipulation of melts and glasses is interesting in its own right, especially technologically, we suggest to use the response to strong homogeneous and inhomogeneous fields as technique to explore on the microscopic level the cooperative mechanisms in dense melts of strongly interacting constituents.

Open questions concern the (de-)coupling of different dynamical degrees of freedom in an external field, and the ensuing state diagrams. What forces are required to detach a localized probe particle from its initial environment in a supercooled liquid, in a glassy or granular system?  Do metallic and colloidal glasses yield homogeneously or by strain localization under differently applied stresses?   Which mechanisms determine field dependent susceptibilities in dielectric and ionically conducting glass formers?