Dirk Kadau | Others - Academia.edu (original) (raw)

Papers by Dirk Kadau

Journal of Colloid and Interface Science, 2010

Soft-agglomerate restructuring, break-up (or fragmentation) and relaxation are studied in a simpl... more Soft-agglomerate restructuring, break-up (or fragmentation) and relaxation are studied in a simple shear flow by a discrete element method (DEM). The agglomerates, held together by van der Waals forces, rotate in the shear flow and are stretched into nearly linear structures (fractal dimension approaches unity) until they fracture at their weakest point resulting in lognormally-shaped fragment size distributions asymptotically. Individual fragments relax in the flow towards more compact agglomerates than the parent ones. The evolution of the average number of particles per fragment is described by generalized scaling laws between shear rate, onset (time-lag) of fragmentation, asymptotic fragment mass and size consistent with experimental and theoretical studies in the literature. The initial effective fractal dimension of the agglomerates influences the final one of the fragments.

(will be inserted by the editor) The effect of contact torques on porosity of cohesive powders

The discrete element method constitutes a general class of modeling techniques to simulate the mi... more The discrete element method constitutes a general class of modeling techniques to simulate the microscopic behavior (i.e., at the particle scale) of granular/soil materials. We present a variant of the contact dynamics method, originally developed to model compact and dry systems with lasting contacts. This variant accounts for the cohesive nature of fine powders and soils. The attractive force plays an important role in the stabilization of large voids, leading to highly porous systems as e.g. in fine cohesive powders. Using this model we can explain how large pores are stabilized, and which contact laws are crucial. The model is applied for compaction of fine powders and compared to experiments, both showing a power law behavior in the high stress regime. We use a modification of the model to investigate the ``quicksand'' behavior of a collapsing soil material. Our contact dynamics model as a microscopic description is adjusted to capture the essential physical processes underlying the dynamics of generation and collapse of the system. Our physical model is validated with real data obtained from in situ measurements performed with a specific type of natural quicksand at the shore of drying lagoons. Cyanobacteria form an impermeable crust, giving the impression of stable ground. After breaking the crust a person rapidly sinks to the bottom of the field. We measured the shear strength of the material before and after perturbation and found a drastic change. We show that the shear strength behavior of our collapsing soil model is consistent with the behavior of this quicksand, for both the unperturbed and the collapsed phases of the material. We also investigate how deep the object can be pushed in and how well the intruder is captured by the material after it collapsed above the intruder. During the penetration process we measured the relation between the driving force and the resulting velocity of the intruder. We also investigated the influence of different strength of viscous drag acting on the grains.

Various industrial processes involve materials consisting of grains. The spectrum ranges from coa... more Various industrial processes involve materials consisting of grains. The spectrum ranges from coarse grained matter and fine powders to nano-powders. As grain size decreases, the influence of cohesion increases leading to an enormous pore volume, which sometimes can exceed 90%. One main goal of this thesis is to understand this macroscopic behavior on a microscopic level and to quantify it. By using the contact dynamics method we simulate the evolution of an ensemble of grains in two dimensions. An extension dealing with cohesion and rolling friction turns out to be crucial for explaning the pore stabilization mechanism. Furthermore, our model shows that strong cohesion can stabilize even low coordinated packings such as particle chains. After compaction the porosity depends on the ratio of applied pressure to the cohesive force (scaled by the particle radius). For high pressures this dependence can be described by a power law. Consolidation experiments by the group of Prof. Schwede...

Physical Review Letters, 2013

We propose a simple model, supported by contact-dynamics simulations as well as rheology and fric... more We propose a simple model, supported by contact-dynamics simulations as well as rheology and friction measurements, that links the transition from continuous to discontinuous shear thickening in dense granular pastes to distinct lubrication regimes in the particle contacts. We identify a local Sommerfeld number that determines the transition from Newtonian to shear-thickening flows, and then show that the suspension's volume fraction and the boundary lubrication friction coefficient control the nature of the shear-thickening transition, both in simulations and experiments.

Granular Matter, 2006

Two dimensional simulations of non-cohesive granular matter in a biaxial shear tester are discuss... more Two dimensional simulations of non-cohesive granular matter in a biaxial shear tester are discussed. The effect of particle elasticity on the mechanical behavior is investigated using two complementary distinct element methods (DEM): Soft particle molecular dynamics simulations (Particle Flow Code, PFC) for elastic particles and contact dynamics simulations (CD) for the limit of perfectly rigid particles. As soon as the system dilates to form shear bands, it relaxes the elastic strains so that one finds the same stresses for rigid respectively elastic particles in steady state flow. The principal stresses in steady state flow are determined. They are proportional to each other, giving rise to an effective macroscopic friction coefficient which is about 10 % smaller than the microscopic friction coefficient between the grains.

A novel method to investigate the compaction behaviour of cohesive powders is presented. As a sam... more A novel method to investigate the compaction behaviour of cohesive powders is presented. As a sample, a highly porous agglomerate formed by random ballistic deposition (RBD) of micron sized spherical particles is used. A nanomanipulator deforms this small structure under scanning electron microscope observation, allowing for the tracking of individual particle motion. Defined forces are applied and the resulting deformations are measured. The hereby obtained results are compared to results from threedimensional discrete element simulations as well as macroscopic compaction experiments. Relevant simulation parameters are determined by colloidal probe measurements.

As an example for history dependent mechanical behaviour of cohesive powders experiments and comp... more As an example for history dependent mechanical behaviour of cohesive powders experiments and computer simulations of uniaxial consolidation are compared. Some samples were precompacted transversally to the consolidation direction and hence had a different history. The experiments were done with two carbonyl iron powders, for which the average particle diameters differed by a factor of ca. 2. Whereas the particle diameter was the only characteristic length in the simulations, the evaluation of the experimental data indicates that at least a second characteristic length must be present.

Granular Matter, 2008

Steady state flow of powders is a crucial concept to investigate the powders’ strength and flow p... more Steady state flow of powders is a crucial concept to investigate the powders’ strength and flow properties. This state is defined as a continuous deformation of the material without volume change while the stresses at the specimen’s boundaries remain constant. Recent investigations have shown that this state, especially for cohesive powders, is not always as constant as it should be by definition. This paper presents experimental and numerical work, both for cohesive and non-cohesive powders. The experimental part focuses on the use of different control strategies, leading to steady state flow of different quality. Differences between steady state flow for cohesive and non-cohesive powders are shown.

Journal of Nanoscience and Nanotechnology, 2012

As an example for history dependent mechanical behaviour of cohesive powders experiments and comp... more As an example for history dependent mechanical behaviour of cohesive powders experiments and computer simulations of uniaxial consolidation are compared. Some samples were precompacted transversally to the consolidation direction and hence had a different history. The experiments were done with two carbonyl iron powders, for which the average particle diameters differed by a factor of ca. 2. Whereas the particle diameter was the only characteristic length in the simulations, the evaluation of the experimental data indicates that at least a second characteristic length must be present.

It is shown that computer simulations can qualitatively reproduce experiments, where a powder of ... more It is shown that computer simulations can qualitatively reproduce experiments, where a powder of cohesive, round, hard particles is periodically deformed at constant volume. Two types of initial configurations are considered: Uniaxially precompacted ballistic deposits and biaxially precompacted DLA-clusters. Both initial configurations had the same volume fraction, but due to the different precompaction procedure completely different principal stresses. After a transient which lasts only less than a period, the stresses follow the same periodic function, i.e. the powder forgot its history.

Particulate Science and Technology, 2007

ABSTRACT Steady-state flow of powders is defined as a continuous deformation of the material with... more ABSTRACT Steady-state flow of powders is defined as a continuous deformation of the material without volume change while the stresses at the specimen's boundaries remain constant. Recent investigations have shown that this state, especially for cohesive powders, is not always as steady as it should be by definition. In this article a recent extension of the true biaxial shear tester is introduced that allows a view of the shear motion of the brick-shaped powder specimen inside the tester. By applying a dark-colored powder pattern onto a light powder sample, the movement of the powder can be captured using a CCD camera. Development of shear bands and inhomogeneities of the sample can be visualized. Experiments with a cohesive powder with purely strain-controlled, volume-preserving shear cycles, as well as mixed stress-strain controlled experiments, are presented. The recorded images as well as stress-strain data from discrete elements simulations are compared to the experimental results.

We report numerical results of effective attractive forces on the packing properties of two-dimen... more We report numerical results of effective attractive forces on the packing properties of two-dimensional elongated grains. In deposits of non-cohesive rods in 2D, the topology of the packing is mainly dominated by the formation of ordered structures of aligned rods. Elongated particles tend to align horizontally and the stress is mainly transmitted from top to bottom, revealing an asymmetric distribution

We combine a DEM simulation with a stochastic process to model the movement of spherical particle... more We combine a DEM simulation with a stochastic process to model the movement of spherical particles in a turbulent channel flow. With this model we investigate the mixing properties of two species of particles flowing through the channel. We find a linear increase of the mixing zone with the length of the pipe. Flows at different Reynolds number are studied.

The porosity of uniaxially compacted cohesive powders depends on the applied stress (including gr... more The porosity of uniaxially compacted cohesive powders depends on the applied stress (including gravity). The case, where these stresses are weak, is considered. The compaction results in a porosity which is a function of sliding, rolling and torsion friction. By contact dynamics simulations it is shown that the influences of contact torques (static rolling and torsion friction) on the porosity

It is shown that computer simulations can qualitatively reproduce experiments, where a powder of ... more It is shown that computer simulations can qualitatively reproduce experiments, where a powder of cohesive, round, hard particles is periodically deformed at constant volume. Two types of initial configurations are considered: Uniaxially precompacted ballistic deposits and biaxially precompacted DLA-clusters. Both initial configurations had the same volume fraction, but due to the different precompaction procedure completely different principal stresses. After a

As an example for history dependent mechanical behaviour of cohesive powders experiments and comp... more As an example for history dependent mechanical behaviour of cohesive powders experiments and computer simulations of uniaxial consolidation are compared. Some samples were precompacted transversally to the consolidation direction and hence had a different history. The experiments were done with two carbonyl iron powders, for which the average particle diameters differed by a factor of ca. 2. Whereas the particle

Physical review letters, Jan 26, 2008

A new contribution to friction is predicted to occur in systems with magnetic correlations: Tange... more A new contribution to friction is predicted to occur in systems with magnetic correlations: Tangential relative motion of two Ising spin systems pumps energy into the magnetic degrees of freedom. This leads to a friction force proportional to the area of contact. The velocity and temperature dependence of this force are investigated. Magnetic friction is strongest near the critical temperature, below which the spin systems order spontaneously. Antiferromagnetic coupling leads to stronger friction than ferromagnetic coupling with the same exchange constant. The basic dissipation mechanism is explained. A surprising effect is observed in the ferromagnetically ordered phase: The relative motion can act like a heat pump cooling the spins in the vicinity of the friction surface.

The discrete element method constitutes a general class of modeling techniques to simulate the mi... more The discrete element method constitutes a general class of modeling techniques to simulate the microscopic behavior (i.e. at the particle scale) of granular/soil materials. We present a contact dynamics method, accounting for the cohesive nature of fine powders and soils. A modification of the model adjusted to capture the essential physical processes underlying the dynamics of generation and collapse of loose systems is able to simulate "quicksand" behavior of a collapsing soil material, in particular of a specific type, which we call "living quicksand". We investigate the penetration behavior of an object for varying density of the material. We also investigate the dynamics of the penetration process, by measuring the relation between the driving force and the resulting velocity of the intruder, leading to a "power law" behavior with exponent 1/2, i.e. a quadratic velocity dependence of the drag force on the intruder.

Journal of Colloid and Interface Science, 2010

Soft-agglomerate restructuring, break-up (or fragmentation) and relaxation are studied in a simpl... more Soft-agglomerate restructuring, break-up (or fragmentation) and relaxation are studied in a simple shear flow by a discrete element method (DEM). The agglomerates, held together by van der Waals forces, rotate in the shear flow and are stretched into nearly linear structures (fractal dimension approaches unity) until they fracture at their weakest point resulting in lognormally-shaped fragment size distributions asymptotically. Individual fragments relax in the flow towards more compact agglomerates than the parent ones. The evolution of the average number of particles per fragment is described by generalized scaling laws between shear rate, onset (time-lag) of fragmentation, asymptotic fragment mass and size consistent with experimental and theoretical studies in the literature. The initial effective fractal dimension of the agglomerates influences the final one of the fragments.

(will be inserted by the editor) The effect of contact torques on porosity of cohesive powders

The discrete element method constitutes a general class of modeling techniques to simulate the mi... more The discrete element method constitutes a general class of modeling techniques to simulate the microscopic behavior (i.e., at the particle scale) of granular/soil materials. We present a variant of the contact dynamics method, originally developed to model compact and dry systems with lasting contacts. This variant accounts for the cohesive nature of fine powders and soils. The attractive force plays an important role in the stabilization of large voids, leading to highly porous systems as e.g. in fine cohesive powders. Using this model we can explain how large pores are stabilized, and which contact laws are crucial. The model is applied for compaction of fine powders and compared to experiments, both showing a power law behavior in the high stress regime. We use a modification of the model to investigate the ``quicksand'' behavior of a collapsing soil material. Our contact dynamics model as a microscopic description is adjusted to capture the essential physical processes underlying the dynamics of generation and collapse of the system. Our physical model is validated with real data obtained from in situ measurements performed with a specific type of natural quicksand at the shore of drying lagoons. Cyanobacteria form an impermeable crust, giving the impression of stable ground. After breaking the crust a person rapidly sinks to the bottom of the field. We measured the shear strength of the material before and after perturbation and found a drastic change. We show that the shear strength behavior of our collapsing soil model is consistent with the behavior of this quicksand, for both the unperturbed and the collapsed phases of the material. We also investigate how deep the object can be pushed in and how well the intruder is captured by the material after it collapsed above the intruder. During the penetration process we measured the relation between the driving force and the resulting velocity of the intruder. We also investigated the influence of different strength of viscous drag acting on the grains.

Various industrial processes involve materials consisting of grains. The spectrum ranges from coa... more Various industrial processes involve materials consisting of grains. The spectrum ranges from coarse grained matter and fine powders to nano-powders. As grain size decreases, the influence of cohesion increases leading to an enormous pore volume, which sometimes can exceed 90%. One main goal of this thesis is to understand this macroscopic behavior on a microscopic level and to quantify it. By using the contact dynamics method we simulate the evolution of an ensemble of grains in two dimensions. An extension dealing with cohesion and rolling friction turns out to be crucial for explaning the pore stabilization mechanism. Furthermore, our model shows that strong cohesion can stabilize even low coordinated packings such as particle chains. After compaction the porosity depends on the ratio of applied pressure to the cohesive force (scaled by the particle radius). For high pressures this dependence can be described by a power law. Consolidation experiments by the group of Prof. Schwede...

Physical Review Letters, 2013

We propose a simple model, supported by contact-dynamics simulations as well as rheology and fric... more We propose a simple model, supported by contact-dynamics simulations as well as rheology and friction measurements, that links the transition from continuous to discontinuous shear thickening in dense granular pastes to distinct lubrication regimes in the particle contacts. We identify a local Sommerfeld number that determines the transition from Newtonian to shear-thickening flows, and then show that the suspension's volume fraction and the boundary lubrication friction coefficient control the nature of the shear-thickening transition, both in simulations and experiments.

Granular Matter, 2006

Two dimensional simulations of non-cohesive granular matter in a biaxial shear tester are discuss... more Two dimensional simulations of non-cohesive granular matter in a biaxial shear tester are discussed. The effect of particle elasticity on the mechanical behavior is investigated using two complementary distinct element methods (DEM): Soft particle molecular dynamics simulations (Particle Flow Code, PFC) for elastic particles and contact dynamics simulations (CD) for the limit of perfectly rigid particles. As soon as the system dilates to form shear bands, it relaxes the elastic strains so that one finds the same stresses for rigid respectively elastic particles in steady state flow. The principal stresses in steady state flow are determined. They are proportional to each other, giving rise to an effective macroscopic friction coefficient which is about 10 % smaller than the microscopic friction coefficient between the grains.

A novel method to investigate the compaction behaviour of cohesive powders is presented. As a sam... more A novel method to investigate the compaction behaviour of cohesive powders is presented. As a sample, a highly porous agglomerate formed by random ballistic deposition (RBD) of micron sized spherical particles is used. A nanomanipulator deforms this small structure under scanning electron microscope observation, allowing for the tracking of individual particle motion. Defined forces are applied and the resulting deformations are measured. The hereby obtained results are compared to results from threedimensional discrete element simulations as well as macroscopic compaction experiments. Relevant simulation parameters are determined by colloidal probe measurements.

As an example for history dependent mechanical behaviour of cohesive powders experiments and comp... more As an example for history dependent mechanical behaviour of cohesive powders experiments and computer simulations of uniaxial consolidation are compared. Some samples were precompacted transversally to the consolidation direction and hence had a different history. The experiments were done with two carbonyl iron powders, for which the average particle diameters differed by a factor of ca. 2. Whereas the particle diameter was the only characteristic length in the simulations, the evaluation of the experimental data indicates that at least a second characteristic length must be present.

Granular Matter, 2008

Steady state flow of powders is a crucial concept to investigate the powders’ strength and flow p... more Steady state flow of powders is a crucial concept to investigate the powders’ strength and flow properties. This state is defined as a continuous deformation of the material without volume change while the stresses at the specimen’s boundaries remain constant. Recent investigations have shown that this state, especially for cohesive powders, is not always as constant as it should be by definition. This paper presents experimental and numerical work, both for cohesive and non-cohesive powders. The experimental part focuses on the use of different control strategies, leading to steady state flow of different quality. Differences between steady state flow for cohesive and non-cohesive powders are shown.

Journal of Nanoscience and Nanotechnology, 2012

As an example for history dependent mechanical behaviour of cohesive powders experiments and comp... more As an example for history dependent mechanical behaviour of cohesive powders experiments and computer simulations of uniaxial consolidation are compared. Some samples were precompacted transversally to the consolidation direction and hence had a different history. The experiments were done with two carbonyl iron powders, for which the average particle diameters differed by a factor of ca. 2. Whereas the particle diameter was the only characteristic length in the simulations, the evaluation of the experimental data indicates that at least a second characteristic length must be present.

It is shown that computer simulations can qualitatively reproduce experiments, where a powder of ... more It is shown that computer simulations can qualitatively reproduce experiments, where a powder of cohesive, round, hard particles is periodically deformed at constant volume. Two types of initial configurations are considered: Uniaxially precompacted ballistic deposits and biaxially precompacted DLA-clusters. Both initial configurations had the same volume fraction, but due to the different precompaction procedure completely different principal stresses. After a transient which lasts only less than a period, the stresses follow the same periodic function, i.e. the powder forgot its history.

Particulate Science and Technology, 2007

ABSTRACT Steady-state flow of powders is defined as a continuous deformation of the material with... more ABSTRACT Steady-state flow of powders is defined as a continuous deformation of the material without volume change while the stresses at the specimen's boundaries remain constant. Recent investigations have shown that this state, especially for cohesive powders, is not always as steady as it should be by definition. In this article a recent extension of the true biaxial shear tester is introduced that allows a view of the shear motion of the brick-shaped powder specimen inside the tester. By applying a dark-colored powder pattern onto a light powder sample, the movement of the powder can be captured using a CCD camera. Development of shear bands and inhomogeneities of the sample can be visualized. Experiments with a cohesive powder with purely strain-controlled, volume-preserving shear cycles, as well as mixed stress-strain controlled experiments, are presented. The recorded images as well as stress-strain data from discrete elements simulations are compared to the experimental results.

We report numerical results of effective attractive forces on the packing properties of two-dimen... more We report numerical results of effective attractive forces on the packing properties of two-dimensional elongated grains. In deposits of non-cohesive rods in 2D, the topology of the packing is mainly dominated by the formation of ordered structures of aligned rods. Elongated particles tend to align horizontally and the stress is mainly transmitted from top to bottom, revealing an asymmetric distribution

We combine a DEM simulation with a stochastic process to model the movement of spherical particle... more We combine a DEM simulation with a stochastic process to model the movement of spherical particles in a turbulent channel flow. With this model we investigate the mixing properties of two species of particles flowing through the channel. We find a linear increase of the mixing zone with the length of the pipe. Flows at different Reynolds number are studied.

The porosity of uniaxially compacted cohesive powders depends on the applied stress (including gr... more The porosity of uniaxially compacted cohesive powders depends on the applied stress (including gravity). The case, where these stresses are weak, is considered. The compaction results in a porosity which is a function of sliding, rolling and torsion friction. By contact dynamics simulations it is shown that the influences of contact torques (static rolling and torsion friction) on the porosity

It is shown that computer simulations can qualitatively reproduce experiments, where a powder of ... more It is shown that computer simulations can qualitatively reproduce experiments, where a powder of cohesive, round, hard particles is periodically deformed at constant volume. Two types of initial configurations are considered: Uniaxially precompacted ballistic deposits and biaxially precompacted DLA-clusters. Both initial configurations had the same volume fraction, but due to the different precompaction procedure completely different principal stresses. After a

As an example for history dependent mechanical behaviour of cohesive powders experiments and comp... more As an example for history dependent mechanical behaviour of cohesive powders experiments and computer simulations of uniaxial consolidation are compared. Some samples were precompacted transversally to the consolidation direction and hence had a different history. The experiments were done with two carbonyl iron powders, for which the average particle diameters differed by a factor of ca. 2. Whereas the particle

Physical review letters, Jan 26, 2008

A new contribution to friction is predicted to occur in systems with magnetic correlations: Tange... more A new contribution to friction is predicted to occur in systems with magnetic correlations: Tangential relative motion of two Ising spin systems pumps energy into the magnetic degrees of freedom. This leads to a friction force proportional to the area of contact. The velocity and temperature dependence of this force are investigated. Magnetic friction is strongest near the critical temperature, below which the spin systems order spontaneously. Antiferromagnetic coupling leads to stronger friction than ferromagnetic coupling with the same exchange constant. The basic dissipation mechanism is explained. A surprising effect is observed in the ferromagnetically ordered phase: The relative motion can act like a heat pump cooling the spins in the vicinity of the friction surface.

The discrete element method constitutes a general class of modeling techniques to simulate the mi... more The discrete element method constitutes a general class of modeling techniques to simulate the microscopic behavior (i.e. at the particle scale) of granular/soil materials. We present a contact dynamics method, accounting for the cohesive nature of fine powders and soils. A modification of the model adjusted to capture the essential physical processes underlying the dynamics of generation and collapse of loose systems is able to simulate "quicksand" behavior of a collapsing soil material, in particular of a specific type, which we call "living quicksand". We investigate the penetration behavior of an object for varying density of the material. We also investigate the dynamics of the penetration process, by measuring the relation between the driving force and the resulting velocity of the intruder, leading to a "power law" behavior with exponent 1/2, i.e. a quadratic velocity dependence of the drag force on the intruder.