spreading of granular pastes: from the particle to end use properties – PASTFLOW

a- Influence of vibrations on the rheology, optimization of transport (macroscopic scale)
Our goal is here to extract macroscopic constitutive laws for vibrated granular pastes. We will compare laws extracted for confined flows with the ones obtained for gravitational free surface flows during pastes spreading. We will quantify the influence of both vibrations and intergrain interaction potentials, described on the meso- and microscales, on the macroscopic flow. These results will be used for identifying relevant parameters to optimize the transport of model pastes of given composition (particles of various size, softness, wettability, interstitial fluid viscosity and particle concentration).
b- Evolution of the porosity for flows of granular pastes (mesoscopic scale)
By performing local measurements, we will quantify for the first time the influence of both flow shearing and vibrations on the porosity. We will relate the porosity variations to the macroscopic constitutive laws. We will determine how the porosity adapts to the applied shear rate/stress and vibrations intensity (A, f) by a direct measurement of the dilatancy/compaction law. We will also identify the influence of vibrations on the shear localization usually observed in yield stress fluid. Results obtained for confined flows will be compared with those obtained for spreading dynamics.
c- Linking the porosity with particles motion (microscopic scale)
The evolution of the porosity induces a change in the mean free volume available for particles to rearrange and modify the apparent viscosity of the paste. Using CFD-DEM simulation we will describe the particle motion and interaction dynamics, extract the typical reorganization times at the particle scale and relate these typical times to both the porosity and the effective viscosity. We will obtain information on structural relaxation properties for various vibration intensities, shear stresses and dispersions varieties for both confined and spreading dynamics.
The “Pastflow” project is divided in two main tasks devoted to the study of the influence of vibrations on the rheological behavior of granular pastes.

During this first period, we study the rheological behavior of vibrated granular dispersions in stationary conditions using an innovative powder rheometer linked with a vibration cell. We have experimentaly shown that applying well-controlled mechanical vibrations to granular dispersions made of glass beads give remarkable properties to these systems, such as controlling their viscosity. Our recent experiments show that fluid saturated or dry granular dispersions have similar behaviour and that vibrations mainly modify the dynamics of the contact network and the apparent friction. A coupled CFD-DEM model of the paste was established by TU Kaiserslautern without vibrations which predicts the experimentally obtained flow behavior of the suspension. In the next period, the influence of vibration will be investigated. We have developed an experimental device consisting of a vibrating plate and an adapted optical metrology in order to follow the spreading and the evolution of the free surface of a granular dispersion subjected to vibrations. The nature and size of the particles were modified as well as the roughness of the plate by the manufacture of a 3D printed plate with controlled structure. We have shown that the vibrations allow to control the spreading dynamics by showing a two-step kinetics: a first step of compaction of the pile and a second step of erosion of the surface induced by a decrease of the apparent friction in connection with the results of task 1. Numerical simulations performed by the university of Kaiserslautern using discrete elements methods are in agreement with experimental results and will allow us to study the particles dynamics in each regime. The next step will be both to conduct experiments with liquid-saturated dispersions following the dynamics of the gains by refractive index matching methods and to finalise the development of CFD-DEM tools for the simulation of the spreading in that case. We will then extend our measurements to dispersions of different natures (dry, fluid saturated, partially saturated dispersions) .

The fact that we wish to study both the effects of vibrations and flow shearing on the spreading of granular pastes is both relevant and innovative. It will lead to a better description of granular pastes flows and will allow to apply these results to complex transport processes of industrial interest. Our work will lead to scientific progress on the rheology of granular pastes. We will identify parameters and vibration conditions to control their flows. In this framework, we will be able to propose a rheological law depending on the local properties of the dispersions. This law will be implemented in an original numerical code and this will be a first stage towards the development of a predictive, modular and robust code to simulate vibrated granular dispersions flows in complex configurations of industrial interest. Our project deals with fundamental research but we will take advantage of this collaboration for defining the potential industrial applications of our work and estimating technology maturity of the project. In that sense, this collaboration will favor to increase the technology maturity of the project. We expect a transition from a TRL 1 to a TRL 3 by the five next years. This project is also an opportunity for our team to participate to an up-front work with potential industrial practical applications in the long term. We hope that this collaboration will help us to identify and facilitate the potential industrial transfer to the industry and comes to industrials contracts in the next ten years.

1. Krull, F., Mathy, J., Breuninger, P., Antonyuk, S.: Influence of the surface roughness on the collision behavior of fine particles in ambient fluids, Powder Technology 392 (2021), 58-68 (2021)
2. Hesse, R., Krull, F., Antonyuk, S.: Prediction of random packing density and flowability for non-spherical particles by deep convolutional neural networks and Discrete Element Method simulations, Powder Technology 393 (2021), 559-581
3. Goldnik, D., Lösch, P., Ripperger, S., Antonyuk, S.: Diafiltration of highly concentrated suspensions with fine particles by dynamic disc filtration, Chemical Engineering & Technology (2021), accepted, doi.org/10.1002/ceat.202100194
4. 1. Communication orale, Powders and grains conferences (07/2021), EPJ Web of Conferences 249, 03008.

Granular pastes as highly concentrated particle suspensions, such as gypsum pastes and fresh concretes, play an important role in the manufacturing of different products in the construction, chemical and food industries. These materials are non-Newtonian fluids whose complex rheological properties (yield stress, thixotropy) need to be understood and described with physical based models to optimize the end-use properties of these products in connection with their formulation. One possibility to optimize the flow behavior of the pastes is to apply mechanical vibration for targeted control of their viscosity. In this cooperation project, we will combine the experimental and numerical tools to investigate the flow behaviour of granular pastes under influence of vibration. On the microscale the characteristic parameters of the micro processes and the interactions between phases (particle-particle, particle-fluid, particle-wall and fluid-wall) will be obtained. The particles will be simulated with the help of the Discrete Element Method (DEM). To consider the micro interactions between the liquid and particles the DEM will be coupled with a CFD approach. These models will be developed at the Institute of Particle Process Engineering (University of Kaiserslautern, Germany). They will be validated with the rheological experiments performed at LEMTA laboratory (University of Lorraine, Nancy, France). Based on the experimental and numerical results we will be able to identify the mechanisms of the shear flow, particle transport and energy dissipation to provide the parameters that can be used in physical models on the macroscale. With the help of a developed multiscale model, it will be possible to obtain detailed information about the influence of the microprocesses on the macroscopic flow behavior of the granular pastes during vibration. This can be used for the optimization of the spreading properties of granular paste with controlled formulation.