ARR - Annual Report

Our basic concept in the project was as follows: for a fundamental discussion on the phenomena of electrostatic charging of powder, it is essential to study the charge generation due to a single impact/contact on a single particle. To realize the concept we proposed two directions:

Approach 1: Impact Charging Experiment:

To apply the previous version of “impact charging experiment,” in which rather bigger particles were used as samples, to smaller particle sizes, the sensitivity of the charge measurement and trajectory control of the accelerated particles were required to be improved. In the first year project, we carried our the improvements, and new rig, which is available for hundreds micro- meter particles, was manufactured successfully. In the second year, actual experiments were performed, but we encountered with data scattering, while the order of magnitude of the data agreed well with that predicted by “charge relaxation model” which we have proposed as a scheme determining the amount of the impact charge.

In the third year, we tried to improve the rig with some points which could cause the scattering. In a point, particle trajectory control, was improved successfully, and the data accumulation was improved. However, the data scattering did not disappear: a question to be answered here was if there was a reasonable cause of this data scattering. Some discussions were done, and as its result, a localization of the initial charge was supposed. A top and a rear side localization against the contact point, gave two limitations. The scattering data were indeed held within the range. As a conclusion, the charge relaxation model works in the range of the particle size from 100 to 300 micro-meter. For the future study, a successive impact experiment for electrostatic charging of particles would be recommended.

Approach 2: Development of the Method of Electrostatic Adhesive Force Measurement:

This is the completely new approach to determine the amount of charge transferred onto a par- ticle due to a contact from a measurement of adhesive force curve at approaching and separating particle against a metal target.

In the third year project, we have actually launched AFM study. Now the experiments are still pilot status, but a force curve due to electrostatic charge was measured successfully for a 35 micro-meter particle. The amount of the electrostatic charge was estimated in the order of 0.1fC, and at this moment the sensitivity is enough for the order of magnitude of the amount of charge, which corresponds 1000 electrons (elementary charges). In the next year project, actual and detailed discussion will be available.

This report summarises progress in IFPRI project 37 in 2001. This is the first year of the continuation project. We have established new areas of investigation with new objectives. Thus, we report here a summary of work in progress at a fairly early stage.

Iveson and Page at The University of Newcastle

have continued to investigate the mechanics of partially saturated powders. For a range of powders, both spherical and non spherical, dimensionless strength is independent of strain rate for Capilliary No (Ca) less than 10-4 and strongly dependent on Ca above this value. Particle shape has a strong effect on yield stress due to its effect on particle packing, friction and particle interlocking. The failure mode also changes with strain rate. At low strain rate, brittle fracture is observed, while at high strain rate pellets deform plastically.

Forrest and Bridgewater at Cambridge University

have established a methodology for characterizing powder flow during granulation using Positron Emission Particle Tracking (PEPT). The technique is being used to study powder flow in a plough share mixer granulator throughout a batch granulation. Powder flow characteristics change significantly as liquid is added to the granulation. Changes in the granule velocity and local bed density are linked to regions of granule motion that are characterised by inter-granule contact time and force. Further experiments are required to determine values for the relaxation ratio and develop the relationship between granule motion and granule properties. Extension of this analysis to a greater range of experimental conditions will develop the relationship between granule motion and granule properties for a ploughshare mixer.

Wet Granule Breakage Research

A brief review of the status of wet granule breakage research is presented. Wet granule breakage is a relatively poorly studied phenomenon. Experimental evidence shows that it may play an important role in controlling granule size distribution in mixer granulators and can contribute significantly to binder distribution under some conditions. However, little attempt has been made to link breakage rates quantitatively to the mechanical properties of the granules (or even to characterise them). The theory of granule breakage and the mechanical characterisation of granules under dynamic conditions need further development in order to produce quantitative predictions of conditions for granule breakage. The work of Tardos and coworkers based on a Stokes deformation number analysis combined with the mechanical characterisation approach of Iveson and Page provide a good starting point for further development.

The objective of this work is systematic understanding of particle-particle nanorheology based on the single particle-particle contact of two atomically-smooth solid surfaces in molecularly-thin proximity. The main relevance is to understand the origins of suspension rheology, especially the origins of rheological anomalies that arise when interfacial films between two solid bodies are so thin that the intuition of what to expect based on bulk rheology no longer applies. Based on this understanding, we are seeking to develop new methods to control and manipulate the properties of their interfacial films.

Specifically, the dynamic mechanical properties of the resulting interfacial film are being studied directly with special emphasis on how they depend on both vibration frequency and strain rate. A homebuilt apparatus is employed to this purpose with the following unique properties:

• Surface-surface spacing are variable from thousands of Ångstroms to molecular contact. The surface force needed to produce this separation is measured while at the same time measuring shear nanorheology. The tip is imaged in situ directly during each experiment, therefore force can be normalized by area to produce stress.
• Oscillatory deformations with variable frequency in the range 0.01 to 103 rad-sec-1 can be applied, with deformations either in the shear direction or in the normal (pumping) direction.
• The amplitude of deformation can be varied from sub-Ångstrom to microns. The reason to use very small deformations is to produce a linear viscoelastic response (representative of the rest state, because this can be studied by methods of equilibrium statistical thermodynamics). The reason to use very large deformations is to produce strongly nonlinear deformations characteristic of very high shear rates.

To the best of our knowledge, no other instrument with these properties exists in any other laboratory in the world. We would like to take this opportunity to encourage IFPRI members to suggest new systems that would be interesting to study using these unique methods. The main finding during Year I was to develop criteria with predictive power to understand whether opposed particles will move past one another with intermittent stick-slip motion or with smooth sliding. We found that stick-slip motion occurred only when 2 thin films were deformed faster than their intrinsic relaxation time. The observation offered a new strategy to look for methods to avoid stick-slip motion by engineering the relaxation time of a confined film. The main findings during Years II and III concerned methods to control suspension rheology with surface coatings. First, we found that because the interparticle potential is an equilibrium quantity, whereas nanorheological responses depend on rate-dependent processes, the interparticle potential failed to correlate directly with shear nanorheology when dealing with films that showed a viscoelastic response. We initiated studies concerning forces in a “tapping” mode – to hydrodynamics when particle surfaces come together and are pulled apart with fluid media in between. These studies focused on nonaqueous systems and some preliminary studies were undertaken in aqueous media. During Year IV, studies were initiated concerning the boundary condition, ‘no-slip’ or ‘partial slip’ of continuum hydrodynamics. Preliminary findings indicated massive breakdown of the conventional ‘no-slip’ assumption, but those conclusions were tempered by the fact that the experiments at that point had concerned only surfaces that were atomically smooth.

This year research on the rheological behavior of concentrated suspensions has focused on the connection between viscous suspensions and granular media – from wet to dry. The goal has been to understand the similarities and differences between these two classes of particulate materials. How the microstructure changes as one moves from viscous to rapid granular flow and how this microstructural evolution manifests itself in macroscopic properties such as stress. The effort has been numerical, as the ASD technique has been extended to incorporate particle inertia, which is necessary to model granular systems. Please note that not all of the work described here has been supported by IFPRI; for example, the development work for ASD has been primarily supported by NASA.

The major conclusions of this study are that there basically exist two distinct states as the effect of particle inertia is varied. At low particle inertia (or low Stokes number) – wet suspensions – viscous forces dominate, stresses scale linearly with the shear rate and the fluctuational motion of the particles as characterized by the suspension or granular temperature is small. This is the so-called ‘quenched’ state first predicted by Koch and co-workers (Koch 1990, Tsao & Koch 1995). At high particle inertia (high Stokes number) the fluctuational motion is large, corresponding to a large suspension temperature – the so-called ‘ignited’ state (Sangani et al 1996). At high Stokes number – dry suspensions – stresses scale inertially and are proportional to the shear rate squared. The transition from the quenched to the ignited state occurs at a Stokes number of roughly 10 over the wide range of volume fractions studied here, 0.01 ≤ φ ≤ 0.45.

Present annual report-2002 described results on the second year’s work after renewal of the first period (1998-2000) in the theme of “Mechanochemistry of Materials” approved by the IFPRI organization. The work focuses on structural change of cellulose by grinding and its dissolution in aqueous NaOH solution. Under this concept, the contractor (F. Saito) and his group have extensively investigated the mechanochemical (MC) work from both viewpoints of experimental and computer chemistry. Several characterizations such X-ray diffraction (XRD), TG-DTA and FT-IR analyses have been conducted for the ground sample, and regarding the XRD analysis, the grinding enables us to transform the structure of cellulose into disordered system like an amorphous state in the prolonged grinding. The TG-DTA and FT-IR spectra are almost the same as those in the initial stage, the dissolution of cellulose in the aqueous NaOH solution is however improved. Cellulose has donor and accepter composing hydrogen bonds in the molecule, and the bond is strong relatively in comparison with other similar structured materials such as glucose and lactose. In spite of this, improvement in the dissolution of cellulose in the solution may be due to the disturbance of internal molecules of hydrogen bonds and their rupturing. This is followed by the investigation on the computer chemistry work, which has shown the weakest part of the chain structure of cellulose in the all, so that the rupture may be initiated to break at C-O bonds in the molecule. Of course, other parts of the structure are also subjected to damage by the grinding, however, the unit cell like cellubiose may remain unaltered in the prolonged grinding. This may be confirmed from the results of TG-DTA and FT-IR analyses. The substance re-crystallized from the solution may be cellulose, of which structure may have slightly different from the initial one. Grinding the cellulose results in the agglomeration of particles, of which sizes are not significantly changed. These facts suggest the suitable application of the ground cellulose to food and pharmaceutical fields.

This work has been presented at the IFPRI AGM 2002 held at Sendai, Japan in 15-18, July 2002. At this moment, it is still unknown, but “mechanochemistry of cellulose” has a potential to extend to food and pharmaceutical fields, because the ground sample of cellulose is, in fact, still maintaining the same unit cell even in the prolonged grinding stage, changing its part of the structure, correspondingly its physico-chemical properties such as solubility. During the course of this investigation, the evidence may be initiation to start an application of mechanochemistry to food and drugs field by modification of morphology and structure of such products. This is also a great expectation of this series of work to come.

According to the project work objectives there were the following four aims to be achieved in the second year of the project extension:

1. Carrying out validation of drying tests in the counter-current spray drying tower
2. Carrying out experimental small-scale drying kinetics tests on selected products
3. Comprehensive validation of the CFD model for scaling-up spray drying process in co- and counter-current system
4. Carrying out experimental investigations to identify the effect of initial process parameters (feed properties, feed rate, air flow rate, drying temperature, atomization parameters) on the final product properties (porosity, bulk density, solubility, etc.)

This project has the goal of providing experimental evidence for the influence of interparticle surface forces and hydrodynamic forces on the moderate to high shear rheological properties and shear stability of wet dispersions that span the colloidal to particulate range. The current emphasis is on particle shape and its effect on the shear thickening transition, as well as the dynamics of the shear thickening transition. Ongoing research into the influence of polymer stabilization, and in particular on the role of adsorbed polymer in modifying the hydrodynamic and interparticle forces acting between particles under flow will not be summarized here, but in the next year’s report.

PART I

In PART I of this report, the transient shear rheology (i.e. frequency and strain dependence) is compared to the steady rheology for a model colloidal dispersion through the shear thickening transition. Reversible shear thickening is observed and the transition stress compares well to theoretical predictions. Steady and transient shear thickening are observed to occur at the same value of the average stress. The critical strain for shear thickening is found to depend inversely on the frequency at fixed applied stress for low frequencies (high strains), but limits to an apparent minimum critical strain at higher frequencies. This minimum critical strain is shown to be an artifact of slip. Lissajous plots illustrate the transition in material properties through the shear thickening transition, and the energy dissipated by a shear thickening suspension is analyzed as a function of strain amplitude. This work provides crucial evidence for the dynamics of the shear thickening transition, and also suggests that dynamic oscillatory measurements can be used to study shear thickening in addition to steady shear.

PART II

PART II provides experimental evidence for the effects of varying particle shape on the shear rheology and shear thickening in concentrated colloidal dispersions. A series of 2:1, 5:1 and 8:1 aspect CaCO3 dispersions were prepared, characterized, and examined rheologically. Prelimary SANS measurements corroborates the supposition that alignment can reduce the low shear viscosity and severity of shear thinning at higher particle loadings. In comparison to previous work on charged silica dispersions, these dispersions shear thicken at lower stresses, but still shear thicken at higher stress than would be expected for hard sphere dispersions at the same loading and average particle size. Ongoing experimentation and modeling is designed to fully elucidate the influence of particle shape on the low and high shear suspension rheology.

This report summarises progress in IFPRI project 37 in 2001/2. The breakage of wet granules in a granulator environment is being studied by Rachel Smith (UQ PhD student). Preliminary results from the dynamic compression of single wet pellets using an Instron Dynamite load frame are reported. The methodology developed by Iveson and Page is used with sone improvements. Fast frame video is used to capture in detail the deformation of the pellets.

The deformation behaviour of the pellets varied widely from very plastic behaviour to failure by brittle crack propagation. Brittle or semi-brittle behaviour was more likely at high strain rate and with high viscosity binders. Pellets made from non-spherical, broad size distribution lactose powder were also more likely to fail in a brittle mode than closely sized glass ballotini pellets. Preliminary results suggest the failure mode can be related to a critical capillary number. The fast frame video analysis of the compression is a very useful tool for understanding the failure behaviour of these partially saturated materials. A hypothesis for wet granule breakage in mixer granulators is suggested and a plan for testing the hypothesis using a specially designed breakage only granulator is presented.

A summary of the PhD studies of Hans Wildeboer on regime separated granulation is given. This work proposes that to optimise granulator design, the key rate processes for granulation should be separated. A conceptual design for a two stage granulator – nucleation only, followed by consolidation and layered growth - is presented. Mathematical models for both stages are developed. The models are compared to results from a partly regime separated continuous drum granulator. The models are very promising in predicting and understanding the granulation behaviour.

A new granulator design is developed which completely separates the nucleation regime from consolidation and growth. A novel nucleation device is tested which gives near mono-sized nuclei at a required size. Further details of Wildeboer’s work are given in his PhD thesis, which will be sent to IFPRI members in early 2003.

The process of disintegration of liquid/solid suspension jets and sheets by atomization is analysed in a fundamental manner and visualized by suitable measurement methods, which allow qualitative and quantitative evaluation of the process. Supporting numerical analysis and theoretical derivations will contribute to basic understanding and control of the suspension atomization process. Model suspensions will be atomized by means of conventional and specifically designed atomizers. The fourth year activities that are reported here include:

• Experimental investigations of suspension atomization in twin-fluid atomizer
• Experimental investigations of suspension atomization in rotary-atomizer

Model suspensions based on water with industrial relevant suspended particles (China Clay) have been atomized by means of twin-fluid atomizer. Model suspensions based on water and water/CMC-(carboxymetylcellulose) mixture with suspended glass particles have been atomized by means of a rotary atomizer.