Tamás Börzsönyi Group leader 
Ellák Somfai Scientific Advisor 
Sára Lévay PostDoc 
Bo Fan PhD student 
Alumni: Balázs Szabó (MSc 2010, PhD 2015, PostDoc 20152017, TB, now at Mediso Kft) Katalin Gillemot (PostDoc 20142016, 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) 
We study the flow properties of granular
materials in various geometries taking benefit of high speed
digital imaging, XRay Computed Tomography or MRI.
We analyzed
the
statistical
properties of
granular
packings to
test Edwards
approach based
on
equiprobable jammed
states.
Identical
spheres packed
in a nearly
twodimensional
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 lowfriction spherical grains and hard frictional spheres of similar size in a quasitwodimensional (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, lowfriction 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 nonpermanent clogs for different compositions of the mixtures. Experimental results are compared with numerical simulations. The latter strongly suggest a significant influence of the interspecies 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
nonmonotonic
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 spacetime 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
nonmonotonic
behavior is
observed. New
J. Phys. 22,
073008 (2020)
(download pdf) 
We used ultrafast Xray 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 welldefined 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 quasitwodimensional 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 spinglass, 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 7particle configurations and by the vertical interaction between touching spheres. Soft Matter. 13, 415420 (2018) (download pdf) 
We
studied the outflow of soft, practically frictionless
hydrogel
spheres from a
quasi2D 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 Janssens 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 nonmonotonic, 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 sheardriven
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, 415420 (2017) (download pdf) 