2018

Hydrodynamic theory of wet particle systems

In this thesis wet granular materials are studied. ‘They are ubiquitous in geology and in many real-world applications,’ says Sudeshna Roy.

On the basis of literature research and advanced simulations, at the Multi Scale Mechanics (MSM) Group, Sudeshna aimed at better fundamental understanding a range of phenomena. ‘I started from models and flow rules from the rheology of dry granular materials,’ she says. ‘Then I added micro and macro properties of (unsaturated) wet granulars. These flow rules differ in complexity and in the number of parameters, which are combined into equations.’

The main focus areas in this PhD project were:  (i) the formulation of suitable constitutive equations for the hydrodynamic density-stress-strain relations; (ii) the deduction of the constitutive equations from discrete element simulations; (iii) the validation of the micro-macro transition with numerical,   theoretical and experimental results.

‘It is great to observe novel liquid migration phenomena and work on understanding the physics underlying these,’ says Sudeshna. ‘We distinguished homogeneous liquid migration from inhomogeneous complex geometries, where shear is more localized.’

Because her supervisor, Professor Stefan Luding, has long-term working relations with ETH Zürich, Sudeshna had access to highly qualified models which served as an inspiration, to extend the work on liquid migration and to look more into the theoretical perspective. Also, collaborations were valuable with Freiberg University of Mining and Technology. ‘We were able to further verify some of their continuity flow dynamics models,’ Sudeshna says.

Sudeshna observed that in partially saturated systems, the formation of liquid bridges between particle pairs, leads to the development of microscopic tensile forces, resulting in cohesion at macroscopic scale. ‘Conservation laws are obtained by time averaging and spatial coarse graining of the discrete constituents,’ she explains.

A special phenomenon is the macroscopic torque measured at the walls, which is an experimentally accessible parameter. This is predicted from simulation results and from the model in dependence on the steady state cohesion.

Experimental

The final chapter of Sudeshna’s PhD thesis concerns experimental work, exploring surface flow profiles for different dry and wet granular materials. The novel experimental technique used here, is a combination of Particle Tracking Velocimetry (PTV) and Coarse Graining (CG), in order to obtain continuum velocity fields of granular flow.  

‘Performing experiments within my PhD project, led to enriched results and insights,’ Sudeshna says. ‘Also it added to my personal development as a researcher, giving more depth to my overall knowledge and to my simulation skills. My supervisor welcomed my initiative to widen my horizon, and my colleagues were very keen helping me to set up and perform the experiments.’

Future plans

Sudeshna was also attracted to the various seminars as scheduled within Mesa+. ‘I learned about the content of work and experimental techniques my colleagues use, sometimes in quite other fields of research,’ she says. ‘I love to learn what is happening outside my research area and widen my circle.’

It fits Sudeshna’s ambition to proceed her career in academics. ‘After my Defense I will go and work as a post-doc, at Pennsylvania State University. The topic is more biophysical oriented. Simulation techniques on bacterial suspensions are central here. I expect the bulk behavior will be quite different. I hope my mathematical modelling skills may contribute to multi-scale modeling of soft matters.’