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Title: Ageing, driving and effective temperatures : from 'soft rheology' to glassy dynamics
Author: Fielding, Suzanne
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2000
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This thesis studies non-equilibrium dynamics in disordered "glassy" systems, focusing particularly on the response to such systems to external driving and loading. Its primary motivation is a body of experimental data suggesting that glassy dynamics underlie the mechanical properties (rheology) of a wide variety of disordered soft materials (e.g. foams, dense emulsions and dense colloidal suspensions): typically, such materials show pronounced non-linearity in their stress response to slow steady shear (often with a yield stress in the limit of zero shear) and a loss modulus which curves upwards slightly to low frequencies (in apparent violation of linear response theory). In what follows, can, when rheologically driven, broadly capture this behaviour. We also show that they predict ageing at small loads, in qualitative agreement with the results of recent ageing experiments. Beyond this rheological motivation, we also use the models to study more general concepts of glassy dynamics, such as the use of fluctuation-dissipation theorems (FDTs) for defining effective non-equilibrium temperatures. As a preliminary step, we extend the existing rheological formalism to include ageing materials in which time-translational invariance (TTI) is lost. Within this generalized framework, we then analyze the rheologically driven trap model - the "soft glassy rheology" (SGR) model - which considers an ensemble of elastic elements undergoing activated local yielding dynamics, with distributed yield thresholds, governed by a noise temperature x. (Between yields, each element follows affinely the applied shear). We review the model's exact constitutive solution and discuss its mapping, in the undriven limit, to Bouchaud's trap model. We exploit this mapping to demonstrate the existence (in linear rheology at least) of an ageing glass phase (x < 1), in which the relaxation time is always of order the sample age.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available