Granular group:

Tamás Börzsönyi
Group leader

Ellák Somfai
Scientific Advisor

Sára Lévay

  Bo Fan
PhD student

Balázs Szabó  (MSc 2010, PhD 2015, PostDoc 2015-2017, TB, now at Mediso Kft)
Katalin Gillemot (PostDoc 2014-2016, TB, now MC Fellow at the University of Vienna)
Gábor Törös (MSc 2011, TB, now at GE Hungary)
  Zsolt Kovács (BSc 2011, MSc 2012, TB, now at Semilab)
  Gábor Bíró (BSc 2013, TB, now at Wigner RMI)
Béla Csengeri (BSc 2014, TB)
Bence Szabó (BSc 2017, TB)
Dávid Kálmán (MSc 2017, ES)
Viktor Nagy (BSc 2018, TB)
Dániel Nagy (BSc 2015, MSc 2017, ES)

Aims and techniques:

We study the flow properties of granular materials in various geometries taking benefit of high speed digital imaging, X-Ray Computed Tomography or MRI.

Latest results:

We analyzed the statistical properties of granular packings to test Edwards approach based on equiprobable jammed states. Identical spheres packed in a nearly two-dimensional geometrical confinement were studied in experiments and numerical simulations. When tapped, the system evolves toward a ground state, but due to incompatible domain structures it gets trapped. Analytical calculations reproduce relatively well our simulation results, which allows us to test Edwards theory on a coupled system of two subsystems with different properties. We find that the joint system can only be described by the Edwards theory if considered as a single system due to the constraints in the stresses. The results show counterintuitive effects as in the coupled system the change in the order parameter is opposite to what is expected from the change in the compactivity. Phys. Rev. E  103, 042901 (2021)     (download pdf)

We studied the outflow dynamics and clogging phenomena of mixtures of soft, elastic low-friction spherical grains and hard frictional spheres of similar size in a quasi-two-dimensional (2D) silo with narrow orifice at the bottom. We show that the addition of small amounts, even as low as 5%, of hard grains to an ensemble of soft, low-friction grains already has significant consequences. The mixtures allow a direct comparison of the probabilities of the different types of particles to clog the orifice. We analyze these probabilities for the hard, frictional and the soft, slippery grains on the basis of their participation in the blocking arches, and compare outflow velocities and durations of non-permanent clogs for different compositions of the mixtures. Experimental results are compared with numerical simulations. The latter strongly suggest a significant influence of the inter-species particle friction.  Soft Matter 17, 4282 (2021)    (download pdf)

We performed laboratory experiments and numerical simulations of silo discharge with traditional (frictional hard) granular materials and grains with reduced surface friction and hardness. We show, that particle stiffness has a strong effect on the qualitative features of flow rate. For deformable grains lowering the friction coefficient leads to a gradual change in the discharge curve: the flow rate becomes filling height dependent, it decreases during the discharge process. For hard grains the flow rate is much less sensitive to the value of the friction coefficient. New J. Phys. 23, 023001 (2021)     (download pdf)

Silo discharge was studied numerically for the case of a silo with a rotating bottom plate. We show, that in accordance with recent experimental findings, for intermediate orifice diameter the discharge rate shows a non-monotonic behaviour on the rotation frequency of the bottom plate. Phys. Rev. E  102, 042902 (2020)     (download pdf)

We studied the discharge of low friction soft (hydrogel) particles from a 2D silo with narrow orifice experimentally. The outflow velocity and the clogging probability both depend on the filling height, which is different from the case of hard grains where these quantities are fill height independent. The reason is the fill height dependence of the pressure at the orifice. Decreasing orifice size leads to increasing fluctuations of the flow. Decreasing orifice size leads to increasing intermittency of the flow.For an orifice size smaller than 2 particle diameters the outflow can stop completely, but in contrast to clogs formed by rigid particles, these congestions may dissolve spontaneously. This is connected to slow reorganizations of the grains due to the viscoelastic character of the particles. Restarting the flow leads to delayed particle movement at heigher locations above orifice (seee space-time plot).  Soft Matter 16, 8013 (2020)    (download pdf)

The rheology of frictional spherocylinders was investigated in shear flows. We explored how the effective friction changes with the inertial number and the particle aspect ratio L/D.  For frictional grains the effective friction increases with the elongation of the particles, while for low friction particles a non-monotonic behavior is observed. New J. Phys. 22, 073008 (2020)     (download pdf)

We used ultrafast X-ray tomography (1000 fps) to study discharge of a granular material from a cylindrical container. In this procedure 2 horizontal slices (separated by 11 mm) can be recorded continuously. Space time plots were used to reconstruct velocity profiles and packing structures for low friction soft grains and frictional hard particles. New J. Phys.  21, 113054 (2019)     (download pdf)

We studied the packing and flow profiles of low friction soft grains using X‐ray computed tomography during discharge of a 3D bin. For hard frictional grains (left image), flow is concentrated in the middle part of the bin (red region) with stagnant zones near the wall, for low friction soft grains we find grain motion everywhere in the bin (right image). Granular Matter 21, 56 (2019)    (download pdf)

The rheology of elongated frictional particles has been investigated in inclined plane flows. In this geometry we can easier test the systems response at higher inertial numbers (compared to simple shear). Our discrete element simulations and laboratory experiments revealed that density and friction are well-defined functions of the effective inertial number.  The effective friction clearly increases with grain elongation. Phys. Rev. Fluids  3, 074301 (2018)     (download pdf)

The stationary flow field in a quasi-two-dimensional hopper was investigated experimentally. The behavior of materials consisting of beads and elongated particles with different aspect ratio was compared. We show, that while the vertical velocity in the flowing region can be fitted with a Gaussian function for beads, in the case of elongated grains the flowing channel is narrower and is bordered with sharper velocity gradient. For this case, we quantify deviations from the Gaussian velocity profile. Relative velocity fluctuations are considerably larger and slower for elongated grains. Phys. Rev. E 96, 062903 (2018)    (download pdf)

We studied the packing of spheres experimentally and numerically in 2 + e dimensions, realized by a container which is in one dimension slightly wider than the spheres. The particles organize themselves in a triangular lattice, while touching either the front or rear side of the container. This system appears to be similar to a frustrated spin-glass, but it has a well defined ground state built up from isosceles triangles. When the system is agitated, it evolves very slowly towards the potential energy minimum through metastable states. We show that the dynamics is local and is driven by the optimization of the volumes of 7-particle configurations and by the vertical interaction between touching spheres. Soft Matter. 13, 415-420 (2018)     (download pdf)

We studied the outflow of soft, practically frictionless hydrogel spheres from a quasi-2D bin experimentally. Prominent features are intermittent clogs, peculiar flow fields in the container, and a pronounced dependence of the flow rate and clogging statistics on the container fill height. The latter is a consequence of the ineffectiveness of Janssen’s law: the pressure at the bottom of a bin containing hydrogel spheres grows linearly with the fill height. Phys. Rev. Fluids 2, 132302 (2017)2222, 123302 (2018)   (download pdf)   Phys. Rev. E 96, 062903 (2017)   

The rheology of dense granular flows for frictionless spherocylinders was investigated by means of 3D numerical simulations in simple shear. This configuration is especially suitable for testing the case of slow shearing (quasistatic limit). geometry we can. The effective friction is non-monotonic, but predominantly decreasing when the aspect ratio Q is increased: it first sharply increases, reaches a maximum around Q=1.05, and then gently decreases until Q = 3, passing its initial value at Q=2.   Phys. Rev. E 96, 062903 (2017)    (download pdf)

We report the first experimental demonstration of bulk segregation in a shear-driven dry granular mixture, where the particles only differ in their surface friction coefficients. The smoother particles tend to sink to the bottom of the shear zone, while rough particles migrate to the top of the sample. This phenomenon is similar to the well known kinetic sieving in particle mixtures with size heterogeneity. In the present case the smooth particles have a higher probability to penetrate into voids created by the shearing than the rough ones. Discrete element simulations were carried out and
reproduced the experimentally observed segregation patterns. Moreover, simulations performed in the absence of gravity revealed that rough particles tend to remain in the shear zone, while the smooth particles are being expelled from it. We propose a mechanism in which the smooth particles are driven towards regions of lower shear rate.
Soft Matter. 13, 415-420 (2017)     (download pdf)