Nishant Kumar

Micro-Macro and jamming transition in granular materials

Promotion date: March 14.

Promotor: Prof.dr.rer.-nat. Stefan Luding

Assistant Promotor: Dr. Vanessa Magnanimo

Granular and other soft materials are used ubiquitously everywhere in nature and daily life and pose many scientific challenges. They are widely used as raw materials in various industries, and their processing, handling and storage in the form of particle systems is widespread in most industrial sectors, beyond the scope of our imagination.

The bulk behavior of particulate materials and interesting phenomena like the jamming transition (where the transition from fluid-like to solid-like occurs) depend on the behavior of their constituent particles. They, from nano- to micro- to macro-size interact through contact forces – and the challenge is the micro-macro transition towards understanding their bulk micro-based macro-behavior. To gain more insight into the micro-structure of granular materials, numerical studies and simulations were performed on various deformation experiments, namely purely isotropic and deviatoric (volume conserving) and a mixed, uniaxial deformation mode, using the Discrete Element Method (DEM).

Initially, a guideline for calibrating simplified theoretical models with DEM simulations was formulated, emphasizing the effect of polydispersity, and then to predict another element test using this calibrated model. Next, the goal was to relate the elastic moduli (small strain stiffness) with the state variables of the polydisperse anisotropic material, in order to predict the constitutive behavior along a generic deformation path. Small perturbations to various static equilibrium states that previously experienced different history were applied, i.e. different finite/large compression and pure shear strains and investigated the effect of volume fraction, stress state and microstructure (fabric) on the elastic bulk response of the material. The calibrated anisotropic constitutive model is then able to predict quantitatively the evolution of pressure, shear stress and deviatoric fabric for an independent cyclic pure shear test. This approach was also successfully applied to other (reasonable) particle size distributions like, lognormal and Weibull distributions.

Finally, based on the study of soft, frictionless, polydisperse spheres, a quantitative model is proposed for how the jamming density changes, with history, representing a memory state-variable of the system, using simple isotropic and shear deformation tests. One can explain both: how the packing efficiency increases logarithmically slow under gentle ‘tapping’ or repeated compression, and for shear deformations, in contrast, how it rapidly decreases. Finally, by modifying the anisotropy continuum model – adding the memory (history) dependent jamming point – its predictive power was shown in order to quantitatively explain many real-world observations.

Was your work fundamental or application oriented?

The research was fundamental in nature, and the development/calibration of the theoretical models for granular materials was the main goal. The research carried out at the Multi Scale Mechanics group, MSM, ( was funded by a Marie Curie FP7 ITN, named PARDEM ( and was driven by industrial applications with the goal of developing such a theoretical model that can be applied at large/industrial scales; we also had inputs from our industrial partners (Nestle, NASA, BASF, Zeppelin, P&G to name a few). My goal was to develop optimal practises that can help to explain the macroscopic observations based on information obtained at the micro level. We used DEM as tool, for learning from simplified model systems to develop this. In this sense, the approach was theoretical. However, the involvement of our industrial partners to this approach was application orientated, where they were providing us the input on our approach from an applied industrial perspective. Hence, our fundamental research can have practical implications, for example understanding the jamming phenomena in a silo, that has huge impacts on its design, and can provide insights on the procedures of avoiding them.

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

There are two things that I find the most remarkable during this period, first, how the deformation mode and the sample history effect the jamming transition in granular systems. The second one was related to the measurement of the ‘classical’ material stiffness and their novel link with stress and microstructure. The unification of these results for polydisperse materials, that was done in my final year, was really interesting.

Can you mention some of your publications?

Journal articles appeared and are about to appear in Particuology, KONA and Acta Mechanica, a few more are to be submitted.

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

The scholarship is highly esteemed for its international character and the involvement of industrial partners. The focus was multidisciplinary and international since researchers from many different countries were involved. Also the special mix of fundamental and application oriented approaches of the research possesses high added value for me and my future. At MSM, I had a great deal of freedom, in choosing my own direction to achieve the research objectives, making the experience more efficient and productive for me. I developed a good, healthy habit of questioning things if necessary. By doing so I have been more confident about the knowledge base I worked with. I was able to improve my productivity by actively collaborating within the research group as well as with international partners, which also led to several internationally recognized publications. I thoroughly enjoyed and improved my research skills, including writing skills.

What are your future plans?

After the defense, I am first planning a travel period in the Balkans, as I am very much interested in the culture over there. Then, I am starting a job at Nspyre as a model driven engineer.

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

I liked the curiosity driven way of scientific research. The academic freedom felt, when working within Mesa+, is great. The supervisors and colleagues are equally open and willing to listen to my ideas and sharing knowledge at all levels. Also the seminars (e.g. the M&M Computational Science and Lecture series) were very interesting, and I learned a lot from these.