Cohesion in Particulate media

Project Information

CohPa

Grant agreement ID: 101097842

DOI

10.3030/101097842

EC signature date 28 June 2023

Start date 1 November 2023

End date 31 October 2028

Funded under

• European Research Council (ERC)

Total cost

€ 2 469 604,00

EU contribution

€ 2 469 604,00

Coordinated by

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE CNRS
France

Periodic Reporting for period 1 - CohPa (Cohesion in Particulate media)

Reporting period: 2023-11-01 to 2026-04-30

Summary of the context and overall objectives of the project

How grains flow ? Can we predict the behavior of sand, snow, powders and more generally of a collection of particles? In many situations both in nature or industry, this question is of importance to predict avalanches, landslides, or designing silos and industrial processes. While for simple fluids the measure of the viscosity is sufficient to compute and predict the flow, for granular materials the question remains open. The challenge of understanding the physics of granular flows has motivated many researches and constitutive laws have been developed for grains interacting by friction and collision. However, in many applications, grains present cohesion and predicting their behavior is much more challenging. Sticky or humid grains, powders, snow, fibers are examples of granular media having difficulties to flow, with clogging, agglomeration, intermittency. The CohPa project aims at a fundamental understanding of the behavior of cohesive granular materials, by combining experimental, numerical and theoretical approaches, with the ultimate goal of developping constitutive laws able to predict their behavior in any configuration. The two work packages correspond to two types of cohesive materials. In WP1, I will study cohesion resulting from the presence of adhesive forces between particles and I will focus on the development of model materials made of sticky grains with tailored surface properties as well as on real powders. By developing innovative rheological technics coupled with numerical simulations we hope to get a new understanding of the rheology. In WP2, I will study the cohesion arising from entanglement between frictional particles, as observed in an assembly of fibers (as in a bird nest or a cotton ball), or when mixing fibers and grains (typically roots in soils). They constitute a new class of disordered materials, in which the tensile strength results from a subtle coupling between geometry, elasticity and friction, which is not yet understood.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

In WP1, dedicated to the study of cohesive granular materials, we have developed several activities and obtained the following key results:

-Design of model cohesive granular materials: Different model systems have been designed, including coated particles, polymer particles, and wet particles.
-Development of an original experimental setup: A cantilever-based setup has been developed to precisely analyze the interaction properties between particles, focusing on detachment force and detachment energy (a measurement never previously discussed in the literature).
-Rheology of coarse coated particles: The rheology of these particles has been studied in detail, revealing for the first time a lubrication effect induced by the coating. We have successfully linked microscopic tribological measurements to macroscopic flow laws through experiments, mean-field modeling, and DEM simulations. This work provides the first framework for describing dry granular systems with complex, non-Coulombic interactions, offering insights into the design of granular materials with tunable flow properties, directly relevant to industrial applications. Two papers have been published on this topic (Dumont et al., PRF 2025; Rocha et al., Soft Matter 2026).
-Mean-field theory of the Stribeck effect: Related to the lubrication effect, we have developed a mean-field theory of the Stribeck effect (V. Bertin, O. Pouliquen, submitted to PRF). Observing lubrication transitions between coated particles, we revisited the theoretical description of the Stribeck effect, which characterizes the influence of normal force and sliding velocity on lubrication at the interface of two spheres. Our analysis reveals that, contrary to conventional understanding, this transition is governed by at least three dimensionless numbers. We explore the different limits, from solid contact to fully lubricated regimes, providing a more comprehensive theoretical framework.
-Novel shear cell design: A major technical achievement was the design of a novel shear cell, addressing one of the project’s most challenging goals: measuring the inertial rheology of fine cohesive particles under low confining stress. This innovative design has enabled us to obtain the first measurements in this regime for small particles, revealing striking new features in the shape of the rheological curves. Shear-weakening flow curves have been identified, and new scaling laws with new dimensionless numbers have been proposed, opening new avenues for describing cohesive granular media.

In WP2, concerning the packing of entangled fibers, the following activities have been developed:

-Compaction of fiber packings: An extensive study has been carried out to analyze the compaction of fiber packings. Results show that the standard description (Van Wyck theory) does not apply, and we have proposed new scaling laws. Thanks to DEM simulations, we have demonstrated the importance of sliding friction during compaction and are working toward proposing a new scenario to understand these scaling laws.
-Tensile strength measurements: An experimental setup has been developed to pull on a fiber packing and measure its tensile strength. New scaling behaviors have been observed, which we are currently investigating through micro-CT imaging to understand the underlying microstructure.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

-The observation of a lubrication transition in coated particles, along with its comprehensive analysis—coupling bulk-scale rheological measurements, particle-scale tribology, DEM simulations, and theoretical modeling—represents a major, unanticipated achievement. Initially, our goal was to investigate the influence of adhesion between particles. However, we uncovered the presence of lubrication, a phenomenon that had not been previously explored in this context. To our knowledge, this study is the first to systematically examine the role of coatings in granular media, a technique widely employed in industry to enhance flowability. This work not only advances fundamental understanding but also holds significant potential for industrial applications.

Another groundbreaking development is the possibility of measuring the inertial rheology of powders under low confinement using our novel shear cell, a capability that has never been achieved before. This achievement marks a critical first step toward our understanding of cohesive material flow, pushing the boundaries beyond the current state of the art. By enabling the exploration of rheology in previously inaccessible regimes, this work may unlock the physics underlying the concept of powder flowability, a priority for the remainder of the project.