Tamás Börzsönyi Group leader 
Ellák Somfai Scientific Advisor 
Dániel Nagy PhD student (supervisor: E. Somfai) 
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) 
We study the flow properties of granular
materials in various geometries taking benefit of high speed
digital imaging, XRay Computed Tomography or MRI.
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 Janssen’s law: the pressure at
the bottom of
a bin
containing
hydrogel
spheres grows
linearly with
the fill
height. Phys. Rev. Fluids 2, 123302 (2017) (download pdf)

The rheology of dense granular flows for frictionless spherocylinders was investigated by means of 3D numerical simulations. 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 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 (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) 
We studied the
outflow
and clogging
of
shapeanisotropic
grains in
3D hoppers
with small
apertures.
We
show that an increasing aspect ratio Q of the grains leads to
lower flow
rates and
higher
clogging
probabilities compared
to spherical
grains.
On the other
hand, the number of grains forming the clog is larger for elongated
grains
of comparable
volumes, and
the long axis
of these
blocking
grains is
preferentially aligned
towards the
center of the
orifice. Soft
Matter 13,
402412 (2017)
(download pdf) 
When a granular material composed of shapeanisotropic grains is sheared in a cylindrical split bottom container, a secondary flow is generated that leads to the formation of a considerable heap of material near the rotation center. We demonstrate that this effect can be found not only with prolate grains, as shown in a previous study, but also for oblate particle shapes. Numerical (DEM) simulations reproduce this secondary flow effect. New J. Phys. 18, 113006 (2016) (download pdf) 
We investigated the packing fraction, grain alignment, orientational order parameter, and flow field in a 3D hopper based on Xray CT measurements. We analyzed subsequent clogged states for 6 materials including elongated particles (pegs), lentils, and nearly spherical grains (peas). We have shown that for elongated particles the grains get ordered in the flowing parts of the silo. Similarly to the case of simple shear flows the average orientation of the rods is not parallel to the streamlines but encloses a small angle with it. The order parameter increases as the grains travel downwards the silo and the local shear deformation grows. In most parts of the hopper the orientational distribution of the grains did not reach the stationary orientational distribution observed for simple shear. New J. Phys. 18, 093017 (2016) (download pdf) 
Axial segregation of bidisperse granular mixtures of glass beads was investigated in a spherical container, rotating about its horizontal axis. Depending on the filling fraction of the mixer and on the composition of the mixture, qualitatively different spontaneously formed patterns are observed. For technical applications, the welllocalized segregated bands allow a convenient separation of individual components of the mixtures. It is particularly surprising that the initial compositions of the granular mixtures have a fundamental influence on the location of the segregated bands. This evidences a collective pattern forming mechanism. The spontaneous formation of these bands cannot simply be traced back to individual particle dynamics. [Phys. Rev. E 93, 032903 (2016)] (download pdf) 
Secondary flow and heaping has been observed in sheared granular rods in a cylindrical split bottom geometry. Flow reversal transiently reverses the secondary flow, leading to a quick collapse and slower regeneration of the heap. We present a symmetry argument and experimental data that show that the generation of the secondary flow is driven by a misalignment of the mean particle orientation with the streamlines of the flow. This general mechanism is expected to be important in all flows of sufficiently anisometric grains. [Soft Matter 11, 2570 (2015)] (download pdf) 
The evolution of wide shear zones was investigated experimentally and numerically for quasistatic granular flows in split bottom shear cells. Shearing an initially random sample, the zone width (w) was found to significantly decrease in the first stage of the process. The characteristic shear strain associated with this decrease is about unity and it is systematically increasing with shape anisotropy, i.e. when the grain shape changes from spherical to irregular (e.g. sand) and becomes elongated (pegs). The strongly decreasing tendency of the zone width is followed by a slight increase which is more pronounced for rod like particles than for grains with smaller anisotropy (beads or irregular particles). [Phys. Rev. E 90, 032205 (2014)] (download pdf) 
The packing fraction of a sheared granular material has been studied by Xray Computed Tomography. We quantified the shear induced (Reynolds) dilation of an initially random sample. We also show, that for elongated grains the dilation is partially compensated by a compaction due to the shear alignment. The deformation scale corresponding to the dilation is considerably smaller than that of the alignment process. Shearing identical spheres results in a strong positional ordering of the grains. [Soft Matter 10, 5157 (2014)] (download pdf) 
Granular physics has made considerable progress during the past decades in the understanding of static and dynamic properties of large ensembles of interacting macroscopic particles, including the modeling of phenomena like jamming, segregation and pattern formation, the development of related industrial applications or traffic flow control. The specific properties of systems composed of shapeanisotropic (elongated or flattened) particles have attracted increasing interest in recent years. Orientational order and selforganization are among the characteristic phenomena that add to the special features of granular matter of spherical or irregularly shaped particles. An overview of this research field is given. [Soft Matter 9, 7401 (2013)] (Review paper) (download pdf) 
We report shear experiments with macroscopic shapeanisotropic particles and discuss induced orientational order and alignment. Optical observations of the top layer are accompanied by Xray computed tomography, where positions and orientations of each individual grain in the bulk can be resolved. The induced orientational order influences local packing and other macroscopic properties like the shear resistance. A comparison is drawn with molecular liquid crystals (LC). Many observations are qualitatively and even quantitatively comparable to the wellunderstood nematic phase of rodlike molecules, even though the types of interactions are completely different. [Powders and Grains, AIP Conf. Proc. 1542, pp. 7477 (2013)] (download pdf) 
Shear
induced alignment of elongated particles
was studied experimentally and numerically.
We show that shear alignment of ensembles of
macroscopic particles is comparable even on
a quantitative level to simple molecular
systems, despite the completely different
types of particle interactions. We
demonstrate that for dry elongated grains the
preferred orientation forms a small angle with
the streamlines (see
example image for rice),
independent of shear rate across
three decades. For a given particle shape, this angle
decreases with increasing aspect ratio of
the particles. The shearinduced
alignment results in a considerable reduction of
the effective friction of the granular
material. [Phys. Rev.
Lett. 108,
228302 (2012)] (download
pdf) 
We used Xray computed tomography (CT) to obtain threedimensional images of the particle orientations in sheared systems (reconstructed CT image shown here). All individual particle positions and orientations were extracted, the orientational distribution functions and the complete order tensor were determined. The evolution of these quantities was monitored as the shear induced alignment developed starting from an initially random configuration. [Soft Matter 8, 10950 (2012)] (download pdf) 
The alignment, ordering, and rotation of elongated granular particles was studied in shear flow. The time evolution of the orientation of a large number of particles was monitored in laboratory experiments by particle tracking using optical imaging and Xray computed tomography. At the grain level the steady state is characterized by a net rotation of the particles, as dictated by the shear flow. The distribution of particle rotational velocities was measured both in the steady state and also during the initial transients. The average rotation speed as a function of particle orientation is seen on the image. The rotation speed for particles with their long axis perpendicular to the shear alignment angle is larger, while shear aligned particles rotate slower. The ratio of this fast/slow rotation increases with particle aspect ratio. During the initial transient starting from an unaligned initial condition, particles having an orientation just beyond the shear alignment angle rotate opposite to the direction dictated by the shear flow. [Phys. Rev. E 86, 051304 (2012)] (download pdf) 
The
geometry of shear zones was
investigated in layered granular
materials. The presence of the
material interface can lead to a special
type of “total internal reflection”
of the shear zone. In a wide
range of configurations the
reflection is characterized by a
fixed angle which is analogous to
the critical angle of refraction in
optics. The zone leaves and reenters
the high friction region at this
critical angle and in between
it stays near the interface in the low friction
region. [Soft
Matter
7,
8330 (2011)] (download
pdf) 
The
nature of the grain motion was
investigated during resonant
silo discharge (called silo music). The
grains do not oscillate in phase at
neighboring vertical locations (see
Fig.a), but information propagates
upward in this system in the form of
sound waves. We show that the
wave velocity U is not constant throughout the
silo (see Fig.b), but considerably
increases toward the lower end of the
system, suggesting increased pressure in
this region, where the flow changes from
cylindrical to converging flow. In the
upper part of the silo the wave
velocity matches the sound velocity
measured in the same material when
standing (in the absence of flow).
Grain oscillations show a
stickslip character only in the
upper part of the silo.
[Phys.
Rev. E 83,
032301 (2011)] (download
pdf) For details and movies click here. 
Refraction
and deflection of shear zones in layered
granular materials was studied
experimentally and numerically. We show,
that (i) according to a recent theoretical
prediction [T.
Unger, Phys. Rev. Lett. 98, 018301
(2007)] shear zones refract in
layered systems in analogy with light
refraction, (ii) zone refraction obeys
Snell's law known from geometric
optics and (iii) under natural pressure
conditions (i.e. in the presence of
gravity) the zone can also be deflected
by the interface so that the deformation
of the high friction material is avoided.
[Phys.
Rev. E 80,
060302(R) (2009)] (download
pdf) For details and movies click here. 
We present experimental and
numerical results that show the
formation of longitudinal stripes that
arise from instability of the uniform
flowing state of granular media on a
rough inclined plane. For reltively
dense flows we find a robust form of
stripes that consists of fast sliding
plug like regions (stripes) on top of
highly agitated boiling
material (see
image a)  a configuration reminiscent of the Leidenfrost
effect when a droplet of liquid lifted
by its vapor is hovering above a hot
surface. We
determine the effective friction as
function of the inertial number I
and
find, that the increasing trend known
for dense flows breaks down at about I=0.7
and further increasing the inertial
number leads to decreasing effective
friction (see
image b). [Phys.
Rev. Lett. 103, 178302
(2009)] (download
pdf) For details and movies click here. 
We show that the properties of
avalanches in a gravitationallyforced
granular layer on a rough inclined plane
 a model system for rock avalanches on
a hillside  depend dramatically but in
a predictable manner on the shape
(angularity) of the grains. Measuring major
characteristics of avalanches
as the the typical height, the ratio of
the particle and front velocities and
the growth rate of avalanche speed with
increasing avalanche size we find that
they correlate well with the most
basic property of the material  the
angle of repose. For rough
nonspherical grains (i.e. materials
with a high angle of repose), avalanches
are faster, bigger and overturning in
the sense that individual particles have
downslope speeds that exceed the front
speed as compared with avalanches of
rather spherical particles that are
quantitatively slower, smaller and where
particles always travel slower than the
front speed. [Phys.Rev.E.
78,
011306 (2008)] (download
pdf) For details and movies click here. 
The flow
rule of dense flows on
an incline was tested for 14 samples
of granular materials. We find, that the
Pouliquen flow rule (PFR) provides
reasonable but not perfect collapse of
the u(h)
curves measured for various plane
inclinations and mean particle diameter
d.
Improved
collapse is obtained for sand
and glass beads by using a
recently proposed scaling referred to as
PouliquenJenkins
flow rule (PJFR). Measuring the
slope \beta of the PJFR for ten
different sizes of sand and glass beads,
we find a systematic, strong increase of
\beta
with the divergence angle \theta_1
of h_s.
The copper materials with different
shapes are not well described by either
flow rule. [Phys.Rev.E.
76, 031301 (2007)]
(download
pdf) For details click here. 
The overall phase diagram of granular flows
on an incline with emphasis on high inclination
angles was determined. A new
method was developed for the measurement of the
density of the flow for a wide
range of the plane inclination. For low
volume flow rates, a transition was
detected between dense and very dilute
(gas) flow regimes. We show using a
vacuum flow channel that air did not
effect the flow properties except for
small changes in the very dilute
gaslike phase. [Phys.Rev.E.
74, 061301 (2006)] (download pdf) For details click here. 
The dynamical
properties of avalanches
depended strongly on the shape anisotropy of
the particles used. For rough
nonspherical grains, avalanches are
faster, bigger and overturning.
Individual grains have downslope speeds
that exceed the front speed as compared
with avalanches of spherical glass beads
that are quantitatively slower, smaller
and where particles always travel slower
than the front speed. [Phys.Rev.Lett.
94, 208001 (2005)]
(download
pdf) For details and movies click here. 