Spatially resolved rheology modeling of complex fluids

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Mikael Mohtaschemi Name of the doctoral dissertation Spatially resolved rheology modeling of complex fluids Publisher School of Science Unit Department of Applied Physics Series Aalto University publication series DOCTORAL DISSERTATIONS 152/2015 Field of research Theoretical and Computational Physics Manuscript submitted 15 June 2015 Date of the defence 2 November 2015 Permission to publish granted (date) 26 August 2015 Language English Monograph Article dissertation (summary + original articles) Abstract Many complex fluids show yield stress behavior. However, the term yield stress has been subject of much controversy. The separation of yield stress fluids into thixotropic and simple ones resolves many of these issues. This division is mainly driven by experimental results and is suspect to active theoretical development.Many complex fluids show yield stress behavior. However, the term yield stress has been subject of much controversy. The separation of yield stress fluids into thixotropic and simple ones resolves many of these issues. This division is mainly driven by experimental results and is suspect to active theoretical development. This thesis addresses yield stress fluids and associated phenomena through continuum modeling for fluids with time dependent structure evolution. In addition to homogeneous laminar shear modeling, the emergence of spatial effects in viscometric flow situations is addressed. Therefore the models are coupled to the creeping flow solution (1-D Stokes equation) of a concentric cylinder geometry, which enables comparisons with experimental observations. Further, the results from thixotropic yield stress fluids are applied to the analysis of rheology measurements of nanocellulose suspensions, which have peculiar rheological properties. In particular, shear rate sweeps are simulated utilizing a structural model for thixotropic yield stress fluids. The results indicate that spatial flow heterogeneities have to be taken into account. Additionally wall slip, which is known to play an important role in the flow of complex fluids is addressed through a simple model. The results in this thesis add to the understanding of nanocellulose suspensions and complex fluids in general.