26 Octobre – Thesis defense - Muhammad Arshad

10 h Room 123 - LOMA (University of Bordeaux - campus of Talence)

Friction and elastohydrodynamic measurements using atomic force microscope.

The studies of fluid flow at nanoscale has shown significant rise due to its potential applicability in different fields including soft matters, medical sciences, mechanics and engineering. In this work, we have used the colloidal AFM in dynamic mode to study the rheological properties of thin polymeric films and the lubricated friction between polystyrene microspheres.
We have reported contactless measurement of viscoelastic rheological properties of polymer thin film. The mechanical response is measured as a function of the liquid gap thickness for different oscillation frequencies. Our measurements reveal an elastohydrodynamic coupling between the flow induced by an oscillating colloidal probe and the viscoelastic deformation of the polymer film. We have obtained loss and storage moduli of the film and the data are quantitatively described by a viscoelastic lubrication model. The frequency-dependent loss and storage moduli of soft thin PDMS film are in good agreement with the Chasset-Thirion theory. Our measurements demonstrate that contactless colloidal probe methods are powerful tools that can be used for probing the soft interfaces over a broad range of frequencies.
The second part of thesis is dedicated to the rheology of non-Brownian suspensions with a focus on the role of inter-particle friction in shear-thinning behavior. To this aim, AFM is used to measure friction coefficient between polystyrene microspheres to highlight the close link between the microscopic friction properties and the macroscopic rheological properties of the suspensions. The normal and friction forces between two approaching microspheres is measured by recording the deflection and the twist produced in the colloidal probe, respectively. The friction coefficient of polystyrene microspheres is measured for different normal loads. The inter-particle friction coefficient decreases with the increasing load, which is contrary to the behavior of macroscopic contacting bodies. The measured friction coefficient is then introduced into the model proposed by Lobry et al to predict the viscosity of suspensions made by these polystyrene particles. The good agreement between the measured viscosity using a Rhometer and Lobry’s predictions shows that the microscopic friction law governs the macroscopic viscosity of the suspensions.

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