ARR - Annual Report
EXECUTIVE SUMMARY
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 thousands of Ångstroms. 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 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.
Following on our new sensing strategies to enable on-line measurements in concentrated and, using different approaches, for dilute flowing mixtures reported in the second year of our work, we have since concentrated on the dilute flowing system using the Particle Gymnasium approach.
Over the last year extensive work has been conducted on design, fabrication and testing of a particle flow system, a tube sensor and its on-line data acquisition system. Significant results have been obtained in terms of the relationship between the sensor signal and particle size and shape. The benefits of the revised approach using a 'sensor tube' rather than an orifice (Coulter) have been confirmed.
An on-line data acquisition system was designed and assembled, including means of reducing signal noise without affecting the signal response, as we intended to use the response time for particle length estimation. An experimental apparatus for particle flow was designed and built for examining the effect of particle flow behaviour on the measurement signals. Most importantly, a new Pt wire sensor of a ring shape was designed and fabricated, which can be easily fitted via flanges into the pipeline for measurements. Special designs for adding particles into either a horizontal or vertical pipe flow system were also made to examine the effect on signal response.
Using the newly designed experimental apparatus, it is possible to carry out both dynamic tests (adding particles into to the flow system) and static tests (particles fixed on a string to achieve a specific particle radial position and orientation) tests. Results revealed that the change of the particle radial position, up to 83% of the tube diameter, did not affect the peak value of the signal, while particle orientation had a great effect on the peak, giving a 63% difference for a cylindrical particle of 2.2x5.35mm (diameter x length). Particles moving close to the electrodes produced some more complex voltage signals. Particles that rotate during measurements also yielded more complex voltage perturbations. Therefore it is possible to use the orientation effect for particle shape estimation. Experimental and modelling results have also shown that for cylindrical particles of constant orientation both the particle diameter and length could be estimated using the Particle Gymnasium sensor.
Further work is needed to confirm the above finding using particles of arbitrary shape and size. It is also necessary to examine whether we can recognise the particle shape, other than only non- spheroid, from the signal profiles. Other data analysis methods, such as neural networks, may be appropriate especially when the relationship between the profile and size and shape becomes too complicated to be described by simple equations and when particulates can be classified into different user-defined categories.
Work on concentrated systems as well as multiphase flows has also continued, as summarised at the Annual Meeting and in this report.
A revised forward programme has been discussed to focus upon microstructure sensing of concentrated flowing mixtures.
Executive Summary
Uniform anatase-type TiOl nanoparticles were prepared by Gel-Sol method, in which a condensed aqueous solution of Ti-triethanolamine (TEOA) complex is first aged at 100 “C for 24 h for the hydrolysis of the Ti-TEOA complex to Ti(OH)4 gel network, and then aged at 140 “C for 72 h for the nucleation and growth of the final product by gradual dissolution of the Ti(OH)4 gel. The Ti-TEOA complex was previously prepared by mixing titanium isopropoxide (TIPO) directly with TEOA in a dry box at a molar ratio 1:2. Typically, uniform TiOz particles of ca. 21 nm in mean diameter were obtained by aging an aqueous solution of 0.25 mol dm” TIP0 stabilized with 0.50 mol dmT3 TEOA (initial pH = 9.5) at 100 “C for 24 h, followed by aging at 140 “C for 72 h. Effect of pH on the particle size was remarkable, since the mean diameter of cuboidal particles increased from ca. 5 to 30 nm with increasing pH from 1 to 11.5. However, no TiO2 particles were obtained over pH 12 even after the 2nd aging for 72 h. If TIP0 is not stabilized by TEOA in advance, Ti(OH)4 flock is formed instantly on mixing TIP0 with water at room temperature, and it is completely converted to ill-defined polydispersed TiOl particle of ca. 9 nm in mean diameter after the 1 st aging at 100 “C for 24 h. From the reduction of the reaction rate with increasing pH, irrespective of the presence or absence of TEOA, the increasing particle size with pH is basically elucidated in terms of the reduction of the nucleation rate by the lowered concentration of precursor complexes to TiOZ particles with increasing pH. In addition, this pH effect on the final particle size of TiOz was pronounced by the presence of TEOA, since TEOA liberated after the 1st aging significantly lowered the solubility of the produced Ti(OH)4 gel by adsorption, and since this etTect of TEOA was 111 enhanced with increasing pH. The particle size was also varied systematically by adding different amounts of seeds. Dramatic increase in the formation of TiOz with the increasing amount of seeds provided us with information that the dissolution of the Ti(OH)4 gel is not the rate-determining step of the particle growth of TiOz, and that the particles are grown by deposition of monomeric solute and not by aggregative deposition of particulate matters such as hypothetical primary particles of TiO2. It was also found that the particle shape was changed from cuboidal form to rod-like one when the pH was increased to 11 S. This result was explained in terms of the adsorption of TEOA to the crystal planes parallel to the c-axis of the anatase crystals.
As an application of thus-prepared well defined TiOz nanoparticles, their catalytic performance as a photocatalyst for water photolysis was studied. For this purpose, a special reactor was designed for precise measurement of the quantum efficiency of the photo-excited electrons and holes. Using this reactor with Pt/TiOz catalysts prepared by the selective deposition of Pt onto TiOz particles precisely controlled in size and shape, we performed preliminary experiments for the effects of the particle size and shape of TiOz, Pt loading, pH, atmospheric pressure, and concentration of electrolyte on the quantum efficiency. As a consequence, we found significant effects of these factors. However, we also found that the most imminent issue to be resolved is the surface modification of the Ti02 particles to introduce separate electron and hole trapping centers for the prevention of the drastic recombination of photoelectrons and holes.
Present Annual Report - 2000
The present annual report-2000 described results on the third years’ task in the theme of “Mechanochemistry of Materials” approved by the IFPRI organization. The task covers development of novel material processes by means of mechanochemical treatment and its relation with a ball mill simulation work. The simulation work plays a significant role to elucidate mechanochemical phenomena of materials. The present report contains considerable findings on the relation of mechanochemical phenomena and information obtained from the simulation.
Report Composition
The report composes of three parts:
- Mechanochemical treatment of EP dust, forming soluble vanadium (V) compound in water.
- Mechanochemical treatment of fluorescent powder, accelerating its structure change.
- Dechlorination of PVC by its mechanochemical treatment with inorganic material such as CaO.
Findings
Regarding the first example, the yield of vanadium extracted by water leaching is well correlated with impact energy of balls in the mill calculated from the result obtained by the Particle Element Method (PEM). As for the second example, the dry grinding the EP dust enables us to form a water soluble vanadium compound. The well correlation between the V-yield and the impact energy of balls is obtained, suggesting that the ball impact energy plays a significant role to control the formation of vanadium compound.
The third one is dechlorination of polymers such as PVC (poly-vinyl chloride), PVDF (poly-vinylidene fluoride) and PTFE (poly-tetra fluoro ethane) by their mechanochemical treatment with inorganic material such as CaO. This work has been presented at the IFPRI AGM 2000 held at Scheveningen, Netherlands. The present report described only the dechlorination of PVC and its correlation with the impact energy of balls in the mill calculated from the result simulated by the PEM.
All the approximately same, the impact energy of balls in a mill is a significant key to control mechanochemical effect and reaction. In such sense, the computer simulation based on the PEM regarding the ball motion during milling is a quite useful tool for determining the optimum operational parameters, mill design with scaling-up.
This report introduces a quasicontinuum formulation for heterogeneous granular systems. The description assumes that the relative displacements between particles are described by a constrained field while the interparticle forces are resolved locally. Equilibrium is enforced weakly by virtue of the principle of virtual displacement. The methodology accounts for particles of variable size and different species. Interaction forces between particles in different cells are computed using a rule which allows for local operations and renders symmetric tangent operators. Energy relaxation from static condensation of an internal node is proposed as an indicator for adaptive meshing. Simulations of uniaxial densification show the robustness and versatility of the formulation for heterogeneous granular beds. The numerical tests also recover the general trends observed in compaction experiments.
Executive Summary
The major objective of our IFPRI project, which started in 1998, is to develop a comprehensive mathematical model of the extrusion process for “concentrated” dense suspensions using the Finite Element Method. The results of the numerical analysis need to be verified through experimental studies carried-out on industrial-scale and well-instrumented extruders. Other objectives include the development of various methods and apparati to probe the rheological behavior and microstructural characteristics of concentrated suspensions (“dense suspensions”) especially in the confines of the extrusion process. Single and twin screw extrusion processes (including co-rotating and counter-rotating twin screw extruders) and flow through dies are included in the scope of the project. The ability to determine the distributions of the pressure, stress, velocity and temperature which develop during the course of the extrusion process is the minimum information wherewithal necessary to understand the development of the structure and the ultimate properties of the processed suspensions. The project is carried out with massive support from other sources also including te US Army, Naval Surface Warfare Center and various corporations.
The major tasks of Year #2 were the modification of our FEM based source codes to accommodate the wall slip behavior of dense suspensions in 3-D, the initiation of the experimental studies of dense suspensions using a well-instrumented and industrial-size co-rotating twin screw extruder and the comparisons of the experimental and simulation results on various isolated sections of the twin screw extruder. The coupled flow occurring at a slit die and the exit section of the twin screw extruder was focused upon using a model suspension, which was well characterized. The experimental results (pressure and temperature distributions) agreed well with the results of our numerical simulation using 3-D FEM. During the next year we intend to expand our mathematical analysis to multiple sections of the extruder simultaneously on the single screw extruder, carry-out detailed experimental studies with realistic suspensions using again a well-instrumented single screw extruder with a complex shaping die and compare the experimental results with the numerical results.
EXECUTIVE SUMMARY
Our basic or beginning 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 or contact on a single particle. To realize the concept we proposed and performed, in our previous works, so called “impact charging experiments” in which, however, samples were spherical particles mainly with 3 mm in diameter. The size was too much bigger comparing with normal powder size. The major subject of this project of IFPRI was, therefore, to perform the single particle study in the region of the normal particle size from tens to hundreds micro-meter. We proposed two approaches to realize the project:
Approach 1:
Improvement of the Original Method: To apply the former version “impact charging experiments” to the smaller particle size, there were two major subjects: improvement of the sensitivity of the charge measurement, and trajectory control of the accelerated particles.
- In the first year: the trajectory problem was solved with adopting “aerosol beam generator,” also the sensitivity of the charge measurement was improved successfully. These resulted in that the particles with hundreds micro-meter became available.
- In the second year: Actual “impact charging experiments” were performed successfully with using PMMA particles mainly of 200 pm in diameter. The experimental results showed good agreements with “charge relaxation model” which we have proposed as a scheme determining impact charges.
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 particle due to a contact from a measurement of adhesive force curve at approaching and separating particle against a metal target.
- In the first year: A concept of the mechanism was studied and clarified, and a strict theory was developed to calculate the force curve of a particle charged partially only on a supposed contact area.
- In the second year: As a preparation to realize an actual equipment, performances of some essential parts were tested. After the results of the first and second year activities, the project work objects in the third year are:
In approach 1:
- Actual and further data accumulation will be continued.
- The model called “charge relaxation model” may be verified its applicability for an actual powder region of the particle size, the hundreds micro-meter particle size.
- A further improvement of the sensitivity of the charge measurement to make possible to apply a smaller particle size, from tens under to some micrometer particle. Especially, to realize the measurement of particle size around ten micro-meter may be important because the size is of toner particles in electrophotograpy process.
In approach 2:
In the first and second year, the concept of the new experimental apparatus was established, and some performances of essential parts of it were tested. In the third year, a prototype will be established and its performance will be tested.
The increasing particle-particle-interactions with increasing fineness are an essential problem during wet comminution in stirred media mills. These interactions have an influence on the stability of the product suspension towards agglomeration and on the rheology. Experimental results show that during comminution the measured particle size increases again after reaching a product fineness of approx. 500 nm, although a further increase of the specific surface can be determined (BET-method). The reasons for this phenomenon are the increasing particle-particle-interactions and spontaneous agglomeration.
Possibilities of the stabilization of the product suspension in the stirred media mill are therefore mainly discussed in this report. With respect to this, first results of sample preparation are mentioned. It is shown that during the comminution changes of the pH value, the conductivity and thus the ionic strength as well as the zeta potential occur in dcpendencc of the materials of the grinding chamber lining and the stirrer discs as well as the grinding media material and the product material.
In further investigations the product suspension shall be stabilized electrostatically during the grinding process. The comminution progress as well as the electrochemical properties of the product suspension shall be characterized by online measurements. For this measurements a possible experimental set-up is introduced and discussed. Only after this steps the systematic investigations concerning the influence of the grinding media size are reasonable.
This year research on the rheological behavior of concentrated suspensions has proceeded along three parallel efforts as described below. The goal is to provide a quantitative microstructurally-based description of suspension rheology for a range of concentrations and particle-level interactions. The problem is addressed by both analytical and computational (simulation) approaches. Not all of the work described here has been supported by IFPRI; for example, the work on an accelerated version of Stokesian Dynamics has not. The report is intended to give an integrated view of our current efforts.
The process of disintegration of liquid/solid suspension sheets and jets is analysed in a fundamental manner and visualized by suitable measurement method 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 based on water and water/glycerol mixture with various suspended particles will be atomized by means of conventional and specifically designed atomizers.
The second year activities which are reported here include:
- Experimental investigations of suspension rheology
- Stability analysis of twin fluid suspension atomization
- Experimental investigations of suspension atomization in twin-fluid atomizer
Model suspensions based on distilled water with neutrally buoyant polymer particles have been studied.