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About this book

This book contains the latest scientific findings in the area of granular materials, their physical fundamentals and applications in particle technology focused on the description of interactions of fine adhesive particles.In collaboration between physicists, chemists, mathematicians and mechanics and process engineers from 24 universities, new theories and methods for multiscale modeling and reliable measurement of particles are developed, with a focus on:• Basic physical-chemical processes in the contact zone: particle-particle and particle-wall contacts,• Particle collisions and their dynamics• Constitutive material laws for particle systems on the macro level.

Table of Contents

Frontmatter

Analysis of Adsorbates and Interfacial Forces at Metal Oxide Interfaces at Defined Environmental Conditions

Abstract
This work addresses the investigation of single particle contact forces and molecular forces on the basis of a detailed understanding of the adsorbate-particle interfacial structure. Oxide surfaces are typically hydroxylated under ambient conditions and exhibit adsorbate layers consisting of mono- and multilayers of water. Moreover, most often organic molecules spontaneously adsorb onto these high-energy surfaces leading thus to a reduction in the surface energy. Such organic adsorbates play a major role for the contact forces at small distances. TiO2 and Al2O3 oxide surfaces served as reference oxide materials for the fundamental studies. AFM-based nanoshaving allowed for the analysis of such omnipresent molecular layers. Complementary to this approach, well-defined surfaces with controlled adsorbate chemistry, such as those obtained upon ultrahigh vacuum conditions, provided the basis for a detailed understanding of the measured contact forces. Thereby, the enlightenment of the role played by the molecular surface chemistry on contact forces between particles was carried out by means of the spectroscopic and microscopic analysis of controlled model adsorbates. Such monomolecular adsorbates were formed onto single-crystalline oxide surfaces under conditions of ultra-high vacuum and defined atmospheres. The spectroscopic surface analysis of the adsorbate structure was combined with AFM-based contact force-distance curve measurements to achieve a reliable correlation between measured forces and the given interface chemistry. In addition, single molecule force studies and PM-IRRAS spectroscopy in the presence of high water activities promote the understanding of molecular adsorbates on oxide surfaces under ambient conditions.
A. G. Orive, C. Kunze, B. Torun, T. de los Arcos, G. Grundmeier

Understanding and Manipulation of Nanoparticle Contact Forces by Capillary Bridges

Abstract
Since, in the presence of humidity the inter-particle processes are dominated by capillary forces, a fundamental understanding of the water adsorption and the capillary bridge formation is very important. However, the adsorbed water structure and thus the capillary bridge formation is influenced by various parameters like the particle morphology (e.g. particle size, roughness) as well as the surface chemistry (surface energy, adsorbate structure) and therefore needs to be analyzed on a submicroscopic or even molecular basis. A multi-scale approach ranging from experiments on an individual particle level (AFM and liquid bridge simulation) and investigations on small particle ensembles (combined QCM-D/FTIR) up to macroscopic description of bulk behavior is presented in this chapter. In this context, the combined in situ QCM-D/FTIR experiments are bridging the gap between experiments on an individual particle level and macroscopic bulk behavior. Variation of surface chemistry by means of adsorption of functional organic molecules allows for the correlation of macroscopic particle behavior to nanoscopic effects like the presence and structure of adsorbate layers as well as the formation of capillary bridges while keeping the disperse properties constant. Besides extensive experimental work, simulations of capillary bridges formed by condensation from humid air are presented. It is clearly shown that well known approximations which have been introduced for micron-sized particles are not valid any more for nano-scaled particles. The forces between nanoparticles by static liquid bridges and their dependency on particle size, contact angle, humidity and interparticle distance are discussed in detail. Furthermore, capillary forces during separation of particles are studied thoroughly and a constitutive law based on a contact stiffness allows the transfer to DEM simulations of wet powders. Finally, it is demonstrated by comparison to Molecular Dynamics simulations, that the used continuum approach to simulate capillary bridges might even be used down to particle sizes of a few nanometers, if some additional effects are considered correctly.
Hans-Joachim Schmid, Guido Grundmeier, Michael Dörmann, Alejandro González Orive, Teresa de los Arcos, Boray Torun

Microwave Emission During the Impact Compaction of Particle Bed

Abstract
Cost, quality and productivity in comminution processes depend on the portion of consumed energy directly applied to the particle size. Especially particle bed grinding is an inefficient process and has great potential to improve its efficiency. The modelling of the stress and breakage behavior provides complex solutions that are very difficult to apply in terms of real grinding problems. An experimental technique to detect the crack formation in particle may be helpful to obtain the real parameters for modelling. Low sized, not expansive and wireless sensors, based on microwave emission, are promising for this application. In the first step, the microwave emission from Lead zirconate titanate Pb[ZrxTi1−x]O3 (PZT) induced by mechanical stressing was investigated. The mechanical stressing occurs by impact of a sharp tungsten indenter on the upper surface of PZT-ceramics. In the second step, the microwave emission from PZT-ceramics was used for investigations of stressing behavior of particle beds. The PZT-particles were incorporated in beds of glass particles. Variable impact conditions were used to obtain the microwave response of PZT-particles located in different layers of particle bed. An acoustic sensor was used to obtain the mechanical force acting on particles bed during the impact. Based on this microwave response the stressing and breakage behavior of particle beds was characterized. The proposed method allows a direct optimization of particle size reduction in particle beds.
Sergej Aman, Alexander Aman, Werner Hintz

Formation, Deformation, Rolling and Sliding of Particles and Particle Aggregates: Mechanisms and Applications

Abstract
Particulate systems under external forces, like mechanical load, exhibit reorganization processes on various length and times scales. Here we review our investigation of the characterization of the mechanical properties of nanoporous colloidal networks and micrometer sized granular particles. To quantify the mechanical properties of nanoporous colloidal networks, we used soot templated surfaces as model system. These surfaces have the advantage that the hardness and the wetting properties of the network can be easily tuned. Bending of particle chains and breaking of single contact points were resolved by AFM. The elastic and plastic modulus of the network was monitored using the colloidal probe technique or for harder networks by nanoindentation. To gain insight into the adhesion force of hydrophobic porous networks, microspheres coated with a fluorinated soot-templated layer were investigated. In contrast to smooth surfaces, the roughness gives rise to an adhesion force which depends on the load. In the second part, we discuss particle agglomerates which are only physically linked. To relate macroscopic processes to the motion of single particles a combination of confocal microscopy and high resolution mechanical testing was used. We have developed measurement and image analysis techniques that allows an automatic tracking of the translation and rotation of the particles under mechanical load (e.g. shear). 3D imaging of granular systems under mechanical deformation allows following the trajectories of the granular particles. Here, we also describe methods to detect the rotation of spherical particles. First steps towards using simulations for a refinement of experimental data, e.g., the estimation of friction parameters is shown.
Laurent Gilson, Jennifer Wenzl, Maxime Paven, Michael Kappl, Hans-Jürgen Butt, Doris Vollmer, Günter K. Auernhammer

Contact Models and DEM Simulation of Micrometer-Sized Particles and Agglomerates at Static Loading Based on Experimental Characterization

Abstract
Caused by particle-particle and particle-apparatus interactions during various manufacturing processes and transportation steps, bulk solids are exposed to repeated mechanical stressing. The single particle interactions determine the behaviour of bulk solids and their mechanical stability, which is decisive for the product quality. The behaviour of the bulk solids can be studied with numerical simulations. Due to the rapid improvement of computational engineering, the modelling of bulk solids using the physical based Discrete Element Method (DEM) continues to gain importance. This method allows to study the microprocesses in bulk solids and to analyse the different material and adhesion effects, such as the influence of capillary and solid bridges, irregular particle shape and solidification or softening of the material. To perform such a simulation the deformation behaviour of the single particles in contact with other particles and walls must be described with an appropriate contact model. For the fast estimation of model parameters and validation of contact models, precise and robust measurement techniques are needed. In this work, a novel setup for the measurement of the contact forces and deformations during slow loading of single micrometre-sized particles in normal and shear direction under climatic conditions is developed and described. For the compression behaviour, also the influence of hardening effects related to cyclic loading was investigated. As model material micrometre-sized, irregular shaped titanium dioxide agglomerates and amorphous maltodextrin particles were used. Although bulk solids, processed in industry, mostly consist of irregular shaped particles, they are usually assumed to be spheres in DEM simulations which leads to many uncertainties. Therefore in this study, different contact models for spherical particles and two approaches for the irregular shaped particles (multi-sphere and bonded particle models) are compared regarding the prediction of the deformation behaviour found in the experiments. The shape and position of the particles related to the loading direction were obtained by X-ray computer tomography and implemented in the simulations. Moreover the results are compared with simulations using the Finite Element Method (FEM). The best agreement with experimental data from compression tests with titanium dioxide agglomerates was obtained by the simulation using the bonded-particle model based on the Maxwell viscoelastic model.
Philipp Grohn, Dominik Weis, Ulrich Bröckel, Stefan Heinrich, Sergiy Antonyuk

DEM Analysis of Breakage Behavior of Bicomponent Agglomerates

Abstract
The discrete element method (DEM) is an effective approach for the numerical investigation of micromechanical behavior of granules and agglomerates. In this chapter, we apply an extension of the DEM, the so-called bonded particle model (BPM), for modeling the breakage behavior of cylindrical and spherical multicomponent agglomerates during quasi-static compression. The considered agglomerates consist of ideally spherical primary particles connected by cylindrical solid bonds. In several previous case studies, we investigated the behavior of agglomerates consisting of two different types of particles and solid bonds, where the simulations were performed on virtual microstructures generated by a stochastic agglomerate model. For this, we varied the volumetric ratio of both types of particles as well as their material properties. The results obtained show complex non-linear dependencies of the mechanical characteristics of agglomerates on their microstructure and composition.
Maksym Dosta, Matthias Weber, Volker Schmidt, Sergiy Antonyuk

Contact Mechanisms in Ultrasound-Agitated Particulate Systems

Abstract
Proper modelling of the particle-particle contact and adhesion mechanisms of particulate systems is essential for the simulation of particle processes, e.g. for the development of plant components and adjustment of process conditions during production, handling and conditioning of finest solid powders. The subject of the project is the derivation of the dynamic particle-particle interactions with time-dependent, vibration-induced particle contacts in acoustic, ultrasound-excited particulate systems. The mechanism-based representation of the particle-particle-interactions in (ultra-)sound-excited (i.e. vibration-induced on the gas side) dispersed systems and the modelling and simulation of processes with dispersed phases is performed. The particulate contact and binding mechanisms and their dynamic behavior are derived for different primary particles and aggregate structures at defined process conditions. In addition to applying suitable acoustic parameters (frequency, intensity, …), in particular the ambient conditions (air humidity) need to be specifically adjusted in order to describe the contact behavior and conditions parametrically. Experimental investigations on acoustically-excited particle systems have been carried out in comparison to direct numerical simulations of the fluid and particle behavior in agitated flow fields and are evaluated quantitatively.
Claas Knoop, Tobias Wollborn, Udo Fritsching

Capillary Interaction in Wet Granular Assemblies: Part 1

Abstract
Liquid transport and capillary cohesion in partially saturated particulate materials are relevant to many areas of geosciences and process engineering. This chapter demonstrates that computations of capillary surfaces by numerical energy minimizations (NEM) can be utilized to study the formation and spreading of wetting films on rough surfaces, interfacial shapes and capillary forces of funicular liquid clusters, and the effect of contact angle hysteresis on liquid transport and capillary forces between spherical particles.
Stephan Herminghaus, Ciro Semprebon, Martin Brinkmann

Capillary Interaction in Wet Granular Assemblies: Part 2

Abstract
This chapter focuses on Discrete Element models using Contact Dynamics, that capture the behavior of granular assemblies with fluid on the pore scale. The effect of the timescales, introduced by the fluid transport on relaxation, fluid migration, and failure are identified and studied in particle simulations and accompanying experiments. To extend the simulation capabilities to larger fluid contents, a discretization scheme for arbitrary liquid bodies with morphological evolution laws is employed, that combines experimental observations and results from small-scale Numerical Energy Minimization (NEM) simulations (part 1). We demonstrate that this way, fluid saturation in random packings can be simulated with arbitrary liquid contents ranging from dry to full saturation with good accuracy.
Falk K. Wittel, Roman Mani, Konstantin Melnikov, Filippo Bianchi, Hans J. Herrmann

Sintering—Pressure- and Temperature-Dependent Contact Models

Abstract
Sintering granular materials involves the application of pressure and temperature to make the particulate material a permanent solid. In order to better understand this complex process, the pressure-, temperature-, and time-dependent contact behaviour of micron-sized particles has been studied in close collaboration by the groups of Luding, Staedler and Kappl within the DFG SPP PiKo. This chapter summarises the modelling advances made during the project, with direct links given to the experimental results. Two aspects have been studied: (a) the dependence of the elastic as well as frictional contact forces and torques on an applied normal pressure; and (b) the formation and evolution of adhesive bonds between particles during heat-sintering. Both contact models have been experimentally calibrated and validated, using advanced techniques such as nanoindentation and AFM. As materials, borosilicate particles were used to study the pressure-dependency, while polystyrene particles were chosen due to their low glass transition temperature to study the temperature-dependency near the transition. Combining both aspects provides a multi-purpose contact model that allows the simulations of a wide range of sinter and agglomeration processes for a variety of practically relevant materials.
T. Weinhart, R. Fuchs, T. Staedler, M. Kappl, S. Luding

A Contact Model for the Discrete Element Simulations of Aggregated Nanoparticle Films

Abstract
This chapter presents the development of a Discrete Element Method (DEM) contact model for aggregated nanoparticles. Particle synthesis from the gas phase often results in nanoparticles with a primary particle diameter smaller than 20 nm. These particles interact via sinter bridges (aggregates) or weaker adhesion forces such as capillary and solvation forces (agglomerates). Here, we present a set of five DEM contact model components to compute non-covalent adhesion forces (capillary and solvation forces), normal and tangential contact, rolling torque and stiff sinter bridges between nanoparticles with a diameter smaller than 20 nm. This model can represent nanoparticle films comprised of hundreds of thousands of primary particles under mechanical load. Validation against atomic force microscopy (AFM) force distance curves and mechanical compaction up to 3.4 MPa reproduced experiments with striking agreement. The DEM simulations allow us to gain insight into the structure and the restructuring of nanoparticle films that is impossible to obtain from experiments. This can help tailor particle films and coatings in a wide range of applications including catalysis, gas sensing and energy storage.
Valentin Baric, Jens Laube, Samir Salameh, Lucio Colombi Ciacchi, Lutz Mädler

Determination of the Adhesion Forces of Magnetic Composite Particles

Abstract
The separation of specific proteins from a fermentation broth is a challenge in biotechnology. Problematic impurities are mainly foreign proteins that are present in the fermentation broth and have similar properties to the product. One approach for the specific separation of the target protein is the so-called high gradient magnetic separation (HGMS) with functionalized magnetic particles. The functionalization is specifically adapted to the product, so that ideally only the product binds to the particles. The particles can thus be added directly into the fermentation broth with the product where the product binds to the particles. The particles loaded with the product are then separated from the rest of the fermentation broth in a magnetic separator. A particle fixed bed builds up in the magnetic separator, which must be resuspended after separation from the fermentation broth in order to be able to reuse the particles. For complete resuspension, the separating forces must exceed the adhesive forces between the particles and between the particles and the magnetic matrix. In previous studies of HGMS processes it is assumed that the magnetic force is the dominant one and thus the other forces, e.g. the van der Waals or electrostatic interactions, are negligible. In the investigations carried out so far, the focus has been on the separation of particles in the magnetic filter. However, the resuspension of the particles found little attention. Building on this, the individual interactions between the particles and between the particles and surfaces move into the focus of this work. On the basis of a detailed examination of the interactions at the particulate level, the individual forces were to be quantified and the influencing factors on the forces to be determined.
Johannes Knoll, Frank Rhein, Hermann Nirschl

Deformation and Friction at the Microscale—From Model Experiments to Process Characterization

Abstract
One of the grand challenges in Particle Technology is how to characterize the contact mechanisms and contact forces between particles. The mechanics of particles across all length scales down to the nanoscale are highly relevant for many applications including adhesion, friction, powder flow, comminution and tribology. Particular interesting new fields are additive manufacturing and 3D printing which “promise” a revolution in industrial manufacturing based on powder technologies. Mechanical particle properties are also the key parameters in discrete element models (DEM) which have been developed during the last decades and gradually reach “predictive power”. In order to make DEM truly predictive, intrinsic material properties such as Young’s modulus, hardness, plastic deformation or fracture toughness of the particles must be known quantitatively. However, measured mechanical particle properties (and their distributions) are only known in a few rare cases.
Wolfgang Peukert, Stefan Romeis

Measurement of the Adhesion Moment of a Particle on a Wall in a Gaseous Environment and Comparison to Simulated Data

Abstract
If a particle adheres to a wall, it is flattened. If such a particle is stressed sideways by an external force, it might start to roll. To send the particle into a rolling motion, a resistive moment must be overcome: The “adhesion moment”. In the first part the calculation of the adhesion force based on an atomic model is described. The results of the simulation are compared to experimental results. The calculation of the adhesion moment for flattened smooth particles with and without roughness is in the focus of the second part. Finally an experimental setup to measure the force that must be applied to an adhered particle to trigger a rolling motion in the specimen chamber of an environmental scanning electron microscope (ESEM) is presented. From this force, the adhesion moment can be calculated. A detailed explanation of the experiments is given, and the results of the experiments are discussed.
Alexander Haarmann, Eberhard Schmidt

Nanoindentation Based Colloid Probe Technique: A Unique Opportunity to Study the Mechanical Contact of Individual Micron Sized Particles

Abstract
A direct access to the mechanics of the contact of individual particles with other particles and walls allows for improved simulations and for a deeper understanding of the (process) dynamics of particle ensembles. In the work presented here, the concept of the colloid probe technique, which is well established in the atomic force microscopy community, is transferred to a nanoindenter setup. The potential of the concept is shown by studying adhesion, sliding, rolling and torsional friction. The majority of the work is based on the contact scenarios of Borosilicate microspheres featuring radii of about 10 µm with flat silicon substrates of different roughnesses for normal loading (adhesion tests), pure sliding and rolling and on silicon based rail systems for mixed rolling and torsion. The experimental results are discussed and compared to various analytical predictions and contact models, allowing for an interpretation of the effects of surface roughness, plasticity and adhesion. This finally enables us to determine rolling and torsion friction coefficients together with their associated length scales. All results indicate, that the nanoindentation based approach can be treated as valuable tool in studying quantitative contact parameters of individual micron-sized particles, dramatically extending the load and size regime typically accessibly to an AFM based colloidal probe technique.
Thorsten Staedler, Katharina Diehl, Regina Fuchs, Jan Meyer, Aditya Kumar, Xin Jiang

The Importance of Interactions Between Carrier and Drug Particles for the Application in Dry Powder Inhalers

Abstract
The design of formulations intended for inhalation is a major challenge since many different factors need consideration in order to guarantee major drug delivery. This becomes especially important in dry powder inhalation. Balanced inter-particle interactions between carrier and drug particles are key factors for an optimal aerodynamic performance. This work combines an experimental approach utilising spherical glass beads as model carrier and simulations of device properties as well as particle-particle interactions to gain deeper understanding of processes during inhalation and their effect on aerodynamic performance. Surface roughness modification of the carrier proved to influence the effective drug loading of distinct drug particles. Moreover, surface topography had a major impact on the aerodynamic performance as micron-sized indentations drastically reduced the fine particle fraction (FPF). This could be linked to their ability of sheltering drug particles from the airstream during inhalation. Nano-scale roughness on the other hand led to a significant increase of the FPF. The impact of the inhalation device itself was also taken into account. The conducted numerical calculations (CFD) have provided more insight into carrier particle transport and drug detachment in the case of carrier-based formulations. A multi-scale approach was adopted to numerically analyse the performance of inhaler devices. First, the motion of carrier particles through a swirl-type inhaler was simulated to provide information on flow stresses acting on carrier and on wall collision statistics. Fully resolved simulations of carrier covered by hundreds of drug particles in connection with measured adhesion properties showed that flow-induced drug detachment will only be possible if the van der Waals force between carrier and drug is very weak (i.e. requiring surface modification). However, in the considered swirl-type inhaler the wall collision number of carriers is quite high, being the reason for an efficient drug detachment through inertia. A developed wall collision detachment model revealed that almost 100% of the drug powder is detached within the inhaler, which does not correspond to experimental observations. Consequently, also drug powder deposition on the inhaler walls needs to be accounted for, in order to allow correct predictions of the emitted dose.
Sarah Zellnitz, Niklas Renner, Yan Cui, Regina Scherließ, Martin Sommerfeld, Hartwig Steckel, Nora Urbanetz

Rapid Impact of Nanoparticles on Surfaces

Abstract
The collision of gas-borne particles with surfaces plays an important role in many processes of particle technology such as particle separation, dry dispersion of powders and particle measuring techniques. While for coarse particles comprehensive investigations have been performed regarding sticking and bouncing behavior, in the range of nanoparticles new issues arise e.g. the influence of adhesive forces and of restructuring during plastic deformation on the impact process. In this contribution the different interactions (elastic and plastic deformation, friction, adhesion, charge transfer) between single particles as well as agglomerates impacting on solid substrates are elucidated by a combination of simulations and experiments. It was found, that size-dependent material parameters can be used to describe the collision of nanoparticles with solid substrates using continuum approaches. The effect of the impaction on the restructuring and fragmentation was investigated leading towards a dry dispersion method for nanoparticle agglomerates at ambient pressure.
Alfred Weber, Christian Schöner, Manuel Gensch, Alexander Werner, Thorsten Pöschel

Stochastic Nature of Particle Collisions and its Impact on Granular Material Properties

Abstract
The dynamics of rapid granular flows is governed by dissipative interactions of particles with each other and with the system walls. To adequately describe these interactions, roughness and particle shape must be taken into account. The coefficient of restitution for arbitrary particles thus depends not only on material properties and impact velocity but also on the angular orientation at the instant of the collision. By measurements of the coefficient of restitution from the sound signal emitted by a sphere bouncing repeatedly off the ground, it was found that small deviations from the perfect shape of the sphere lead to large measurement errors. Using stochastic methods, the effective coefficient of restitution for the collision of a rough sphere with a plane was described as a fluctuating quantity, characterized by a rather uncommon probability density function. It was shown that modelling the coefficient of restitution as a stochastic variable significantly affects the dynamics of particles under rapid granular flows. The decay of temperature of rapid granular flows in the homogeneous cooling state deviates from Haff’s law for gases of particles interacting via a constant coefficient of restitution also from the scaling law for gases of viscoelastic particles.
Nina Gunkelmann, Dan Serero, Aldo Glielmo, Marina Montaine, Michael Heckel, Thorsten Pöschel

Non-ohmic Properties of Particle Layers in Electrostatic Precipitators

Abstract
The so-called back-corona hampers the separation of highly resistive particles in electrostatic precipitators. It is linked to the resistivity of the particle layers depending on the particle material, the particle size distribution but also the temperature and humidity of the surrounding gas. In this work, however, it was shown that electret properties are imposed on the particle layers deposited in an electrostatic precipitator. These, in turn, not only explain their non-ohmic properties in electrostatic precipitators but also allows measures to be taken to avoid back-corona.
Damian Pieloth, Helmut Wiggers, Peter Walzel

Structure of Sheared Cohesive Granular Bulk

Abstract
The particle-particle interactions on micro scale determine the macroscopic flow behaviour of bulk solids as in shear testers and in industrial facilities. However, although the flow behaviour can be measured on macro scale and bulk solid facilities as silos can be designed based on reliable engineering knowledge, the microscopic physics causing the wide fluctuation in flow properties of the different bulk solids is still not deeply understood. Therefore, the motion of individual particles in shear testers was determined experimentally as well as by discrete element method (DEM) simulations. The experimental detection of the particle motion was achieved by an own-built micro torsional shear tester which can be placed into a X-ray tomography device (µCT) and a customized statistical analysis method to extract the individual trajectories of almost all particles even at large angle increments of up to 5° between the single tomographic measurements. The two bulk solids, borosilicate glass beads and potassium chloride, with particle sizes in the range of 10–100 µm show very different contact behaviour, on one side viscoelastic with constant adhesion force and on the other side elastoplastic with time dependent adhesion. By a careful calibration of the DEM contact model parameters using among others shear and nanoindentation tests the microscopic behaviour of the two different model materials could be simulated successfully to predict the shear bands and to determine the macroscopic flow properties. Moreover, a theory for the rate dependent rheology of granular materials showing time consolidation has been developed.
Lothar Brendel, Alexander Weuster, Dietrich E. Wolf, Harald Zetzener, Stephan Strege, Lutz Torbahn, Arno Kwade, Lisa Handl, Volker Schmidt

Improved Flowability of Ultrafine, Cohesive Glass Particles by Surface Modification Using Hydrophobic Silanes

Abstract
The focus of the paper is on the investigation of the surface hydrophobization to reduce the interparticle interactions and to improve the flowability of ultrafine cohesive particles. As a model for cohesive materials, microscopic glass particles were selected for experiments. The particle surface was hydrophobized by organosilanes with different length and chemical constitution of the hydrophobic chain. In order to achieve different thicknesses of the deposited coating the silane concentration was varied. The powder flow behavior of the surface modified particles was measured using a ring shear tester. Results showed that the surface modification with all silanes led to an increased powder flowability, however the flow enhancing effect of these silanes depends on the thickness of the coating layer. The contact model ‘stiff particles with soft contacts’ was used to determine micromechanical adhesion and contact properties of the modified particles to contribute to a better understanding of their flow properties on macroscopic level.
Zinaida Todorova, Steffi Wünsche, Werner Hintz
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