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Abhinendra Singh

Micro-macro and rheology of sheared granular matter

Promotion date: May 14.

Promotor: Stefan Luding

Assistant Promotor: Dr. Vanessa Magnanimo

Soil is not always solid. Think of the leaning Tower of Pisa or a landslide, where the soil flows. Soil is a perfect example of granular material: it is made up of countless interacting grains, whose size may vary from millimeter to a fraction of a micrometer. The shape of the grains, the way they interact through contact, and the presence of humidity, are all crucial in order to predict whether the soil beneath a house can withstand the load or if it might fail. What makes the behavior of granular materials so hard to model? The difficulty comes from the way they interact.

In the past couple of decades, computer simulations, especially Discrete Element Method (DEM) simulations, have become important tools to study granular matter. In this thesis, DEM simulations are used, by focusing inside the flowing zones, induced by a special geometry called split-bottom geometry.

At first, the pair wise collisions between elasto-plastic cohesive particles were studied. A contact model, which takes all essential effects into account, was introduced. Two sticking regimes were found: one at relatively low impact velocity, the other at higher velocities.

Effects of contact properties like friction, cohesion and softness were studied. Increasing friction at contact decreases the contact number density, while it increases both the shear resistance and contact anisotropy. Next, the effects of contact cohesion were studied. To estimate the strength of cohesive forces, a non-dimensional number was introduced which very well predicts the transition from free flowing non-cohesive granular assemblies to cohesive ones.

Further, effects of particle softness and gravity on bulk behavior were studied. Conventional wisdom suggests that these properties do not affect bulk behavior of granular assemblies. This was proven to be otherwise: the shear resistance decreases as either softness or gravity increases. This anomalous behavior contributes to changes in the anisotropy of the contact network. Shear resistance accompanies the anisotropy.

Was your work fundamental or application oriented?

My thesis was part of a FOM (Fundamenteel Onderzoek der Materie) funded program on “Jamming and Rheology”. The research team consists of PhD students from various groups in the Netherlands, such as the group of Prof. Detlef Lohse and Prof. Devaraj van der Meer from the University of Twente, Prof. Martin van Hecke from Leiden, and Prof. Peter Schall and Daniel Bonn from Amsterdam.

I studied granular materials from a very fundamental point of view, starting at the basic 2-particle collisions, which form the basis of all granular assemblies where single particles can have many contacts. In the following years during my PhD, I moved along a stepwise approach studying the effects by adding one complexity at a time.

At first I added friction at contact. Having understood that, I further studied the effect of contact cohesion (attractive forces) on the bulk behavior of granular assemblies. As a next step the effects of particle softness and gravity on bulk behavior were studied.

Conventionally, the flow behavior is assumed to be independent of particle softness and gravity. However, my work showed that both these properties do affect bulk behavior. This study also led to another interesting conclusion: that changing either particle softness or gravity, has the same global effect. Interestingly we showed - performing in situ experiments under low gravity conditions, which are difficult and expensive - similar flow behavior can be observed by simply tuning the softness of the particles.

Can you recall some special moments during your PhD project period?

Understanding the effects of particle softness and gravity on bulk behavior, is one of the results that was very special to me. Intuitively one would expect both of them to have no effect at all. However, I showed that the macroscopic friction decreases with increase in either softness or gravity. This study also showed that the macroscopic friction accompanies, or is due to, the anisotropy in the contact network.

Can you mention some of your publications?

Articles are under revision in Powder Technology, and Physical Review Letters E. One article is about to be submitted soon, while a few more are in preparation.

How did you develop personally, as a researcher and scientist in these four years?

I got mature as a researcher. Working on the PhD project, I changed my way of thinking about research: going deep into literature, framing my own problem and then coming up with clever hypothesizes.

I learnt a great deal about how successful collaborations can result in outstanding scientific contributions. In choosing my direction of research, I was given a great deal of freedom, and I developed a healthy habit of questioning things if necessary. By doing so I grew more confident about the knowledge base from which I work.

I was able to improve my productivity by actively collaborating within the group. I thoroughly enjoyed and improved my research skills, including writing skills. Now that I am an experienced researcher, interestingly, I have some answers to my questions, but I have more questions than ever before.

What are your future plans?

I am first planning to take a break. As my parents are here with me at the day of my defense, I would like to take them around in the Netherlands. After that I am going to work here at a post-doc position for a few months, finishing some promising simulations and finishing some articles. Also, I am going to help the PhD student who will continue the work on the same setup. I have some ideas, which came up during my thesis writing, so I am planning to collaborate with my colleagues in the group on these a little further.

How did you experience working for Mesa+ on this project?

The working atmosphere is the most extraordinary, especially the way the group members collaborate inside among themselves and outside with external partners. I really liked the freedom given to me to perform fundamental research. My supervisors very much welcomed new ideas and perspectives of research. They provided additional views, experience and showed alternative directions. Also, regular discussions and collaboration with my daily supervisor Vanessa, helped me to understand new phenomena, like ratcheting. I liked the informal way of working, discussing with people about my research and implementing solutions. I was amazed to get easy access to the facilities the University of Twente provides to one’s employees. The harmony between work and personal life, the fun environment I felt at the UT, will surely make me wish sometimes to come back here.

On a daily basis I was not very much involved in the Mesa+ type of research, nor did I use the Mesa+ labs or other shared facilities, as I am a theorist. However, I very much liked attending the Mesa+ seminars. And I enjoyed the Mesa+ Days, in which one can get acquainted with colleagues working on various research topics.