Abstracts

Large Amplitude Oscillatory Shear (LAOS) is a recently evolving rheological technique offering rich insights for material characterization. In this talk, I trace the history of LAOS and indicate different non-linear material responses specific to bitumen when subjected to LAOS. I also discuss specific applications for such a technique for bitumen rheology.

In this talk, I will describe two particulate/polymer systems for which non-standard rheological protocols were used. Thixotropy is generally observed using steady state or as transitions between two steady states. In our work, we used oscillatory shear to characterize thixotropic response of fumed silica - polyisobutelyene - paraffin oil system. Another example that will be discussed pertains to thin film coatings. In this case, hollow glass sphere dispersions in polyurethane matrix were studied for their rheological response at gaps comparable to the particle size.

Many dense particulate suspensions show a stress-induced transformation from a liquid-like state to a solid-like shear jammed (SJ) state. However, the underlying particle-scale dynamics leading to such a striking, reversible transition of the bulk remains unknown. Here, we study the transient stress relaxation behaviour of SJ states formed by a well-characterized dense suspension under a step strain perturbation. We observe a strongly non-exponential relaxation that develops a sharp discontinuous stress drop at a short time for high enough peak-stress values. High-resolution boundary imaging and normal stress measurements confirm that such stress discontinuity originates from the localized plastic events, whereas, the system spanning dilation controls the slower relaxation process. We also find an intriguing correlation between the nature of transient relaxation and the steady state shear jamming phase diagram obtained from the Wyart-Cates Model.

Rheological studies of cement-based materials are now gaining more importance, especially with the advent of new construction technologies like concrete 3D printing. Compared to conventional workability tests, rheological studies can give a fundamental description of flow behaviour. In this talk, the different rheological models and the commonly used rheological test methods for cementitious materials will be discussed. To avoid errors during rheological measurement, many challenges, such as sample drying, wall slip, the occurrence of plug flow, and sample settling, need to be tackled. Finally, the talk will also include concrete pumping. The formation of a lubrication layer at the pipe wall interface by shear-induced particle migration and analytical models to predict pumping pressure will be briefly discussed.

Visco-elastic materials can behave as elastic solid-like and viscous-liquid like depending upon the time scale of the applied shear stress or shear strain. In addition, they possess complex chemical and molecular structures which make their manufacturing and post-processing diverse. This, along with the wide range of possibility of modifying these materials open a whole new window of mechanical and chemical characterization of such materials at its molecular level. The current study is a comprehensive review of test and analysis methodologies that help in determining various aspects of thermal, mechanical and molecular characterization of such materials and their derivatives, in general. In a nutshell, the current study aims at giving a review of rheometry techniques to understand the flow behavior of complex materials (Rheology) and also the behavior of the interacting surfaces in relative motion in a system as a whole (Tribology).