Volume 2 Number 1 Dennis R. Dinger 1 November 2003
An Update
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A year ago, when I started this E-zine, I wrote an initial, brief article about dilatancy. I want to spend time in this issue discussing the causes of dilatancy and especially dilatant blockages.
Physical Causes of Dilatancy and Dilatant Blockages
Dilatancy -- A Brief Review
To insure that we're all on the same page, we need to briefly review the rheological phenomenon known as dilatancy. Then, we will discuss its causes. When the causes are understood, it should be relatively easy for ceramic and materials engineers (and anyone else working with particle/fluid suspensions) to logically approach problems caused by dilatancy and to make adjustments to move process conditions out of the dilatancy zone.
Dilatancy is the name for the rheology of a suspension characterized by rising viscosities with rising shear rates. When a suspension's viscosity increases as the speeds of pumping, stirring, forming, flowing, or shearing increase, that suspension is dilatant. Most ceramic suspensions are shear-thinning at low shear rates. That is, as they are worked, their viscosities decrease. Dilatant behavior, however, is just the opposite. The faster one works a dilatant suspension or body, the more viscous it will be. Most ceramic suspensions and forming bodies are shear-thinning at low shear rates; some are dilatant at low shear rates; but all ceramic suspensions and forming bodies are dilatant at high shear rates.
The article entitled "Dilatancy" in Volume 1 Number 1 of this Ceramic Processing E-zine shows a typical rheogram of a dilatant ceramic process suspension. Viscosity typically decreases initially at low shear rates, but as shear rates increase, the viscosity goes through a minimum and then it increases. At the extreme (at high shear rates), viscosities of dilatant suspensions can be so high that all flow stops. When all flow stops, a dilatant blockage has been produced. When this happens during forming, disastrous results can be expected.
Many more details of all of these rheological phenomena are available in the author's book "Rheology for Ceramists."
The effects of dilatancy are clear: Viscosity increases as shear rate increases. But why? The "Why?" is the main subject of this article.
The Primary Cause -- Particle/Particle Collisions
The primary cause of dilatancy and dilatant blockages is particle-particle collisions. When particles in suspension can flow past one another without interference, dilatant properties will not appear. In suspensions of relatively low solids content, particles can easily flow through the suspending liquid without major interference from one another. But in most ceramic suspensions and forming bodies, solids contents are so high that all particles are crowded in and around one another. It is nigh unto impossible for any flow to occur in such systems without particles bumping into one another. And when particles collide, viscosities rise.
Particle/particle collisions are not only related to the solids contents of suspensions but to the particle packing potentials of the particle size distributions of the powder fraction in each suspension. If a suspension is to be formed at 50vol% solids (which many would consider to be a relatively normal solids content for a ceramic suspension), the viscosity of that suspension will be a function of the packing potential of its particles. If the particles alone, when packed as densely as possible, define a compact with 50% internal porosity, the addition of 50% water to suspend the particles will produce no fluidity at all because all the water will be necessary to fill the pores with no excess. Particles which are in contact with one another in the dry compact will remain in contact with one another after the fluid has been added. If a mixture of 50% particles and 50% water defines a suspension, bulldozers can be driven over this suspension.
But if a dry compact of the particles packs more densely and defines porosities of only 20-30vol%, then the addition of 50vol% water will be sufficient to fill the pores and separate the particles. When particles in suspension are separated and no longer touching, fluidity results.
Even though all particles are separated by fluid, when suspensions are sheared, particles move around and once again have opportunities to collide with one another. Collisions during suspension flow cause suspension viscosities to increase. At very low shear rates, few collisions will occur, and the contribution to viscosity from collisions will be minimal. Slow moving particles can usually move around other slow moving particles to pass without colliding. Imagine yourself trying to walk against the flow on a crowded sidewalk. It is difficult, but it can be done without colliding with anyone.
As shear rates and flow velocities increase, collisions will intensify and viscosities will increase noticeably. Now imagine trying to run in the same direction everyone else is walking on a crowded sidewalk. It will be very difficult to do this without colliding with others AND without pushing many of these people into other people in the process.
At high shear rates, particle structures can form which are characterized by high internal porosities. When these structures contain more porosity than the volume of fluid present, regions within the suspension begin to act like solids, unlike fluid suspensions. Such regions can then crack and break. Cracking and breaking are phenomena that are NOT characteristic of suspension behavior. But cracking and breaking occur due to the dilatant character of such suspensions when particle/particle structures form and there is insufficient fluid present to maintain any separation between particles.
Our typical demonstration of dilatancy is to mix fine nepheline syenite with water in a bowl. When it is at particularly high solids contents, simply dragging the stirring spoon across the surface of the suspension will cause particle/particle structures to form and the 'suspension' will crack and break under the stress of the spoon. Frequently the cracks will extend through the suspension to expose the bottom of the container to your view. This demonstrates that the structures formed when particles collide extend throughout large regions of the sample.
In extreme cases like this during pipe flow or in extruder dies, when shear rates are so high and collisions are so intense that whole regions of the suspensions act like solids, dilatant blockages can occur. When a blockage has formed, the whole cross-sectional area of the flow channel will be blocked, and all flow stops.
Consider, for example, a suspension flowing in a pipe. At low flow rates, the suspension may have a relatively high viscosity. If it is a typical shear-thinning suspension, as flow rates increase, the viscosity will decrease somewhat. As flow rates continue to increase, the intensities of particles collisions will increase, the viscosity of the suspension will reach a minimum, and then the viscosity will begin to rise. At high flow rates, the severity of particle collisions will intensify until they become so severe that crowded particles jam together, bridge across the whole cross-sectional area of the pipe, and block further flow. A pile-up like this that blocks all flow in a channel, is called a dilatant blockage. Let me repeat this: When dilatant blockages form, flow ceases.
In an extrusion die, the cross-sectional areas of the flow channels may be quite small, even though the extruder diameter is quite large. If a dilatant blockage forms in part of a complex extrusion die, a hole parallel to the extrusion axis may form in the extruded column. If a dilatant blockage forms across the whole die opening, the die will be completely blocked. Depending upon the hardness of the die material, the die may be chewed out and ruined through abrasion as high extruder pressures force blockages down the die channels; the die may be bent (bowed out) and ruined; or the bolts mounting the die on the extruder can pop and fly across the plant floor as lethal projectiles. The problem with extruders is that the pressure capabilities of most extruders can overpower most blockages. In many extruders, bodies will be forced forward under extreme pressures, and dies and/or parts of the machinery will be ruined in the process.
A More Common Example of A Dilatant Blockage
The easiest examples to demonstrate dilatancy and dilatant blockages use people in the place of particles. In a fire drill where students are exiting from a lecture hall, collisions between students trying to squeeze through the exit doors represent the dilatant particles. If the students manage to exit through the doorways in single file, there will be no dilatant problems or effects. But when several students try to enter a doorway together, if they bump into one another in the process, the whole exit flow rate (including everyone behind them) will be reduced. If they all try to exit the room at the same time, bunch together, and get stuck in the doorway, the doorway can be completely blocked. This is an example of a dilatant blockage. Once such a blockage has formed, more and more people will arrive at the back of the pack, and they may begin to push forward. This will increase the physical pressure on those blocking the exit, but it usually will not help speed anybody's exit from the room. This kind of pile-up of people produces what has been described as the press. When this occurs, people within the blockage, and especially those at the front, can be severely injured from the pressure.
Blockages in ceramic systems work similarly. A blockage in a pipe or extrusion die may be pushed forward by the pressure of the pump or the extruder, respectively. Pressure at the back of the blockage will build up to the maximum possible by the pump or the extruder, and usually when maximum pressures are reached, something will break. But the maximum extrusion or pump pressure has been reached, or until a break occurs, the blockage will be compressed, densified, and strengthened.
Whether or not something breaks, blockages compressed into pipes, extrusion dies, and flow channels are very difficult to remove. Generally, such blockages are impossible to remove. In many systems, the easiest solution to a blockage is to throw the blocked section of pipe away and replace it with a new clean flow channel.
When a blockage has occurred, the problem is not just the blocked channel. Replacing a blocked channel with a clean channel will not solve the problem. The source of the problem lies elsewhere. Conditions which produce dilatant blockages are affected by packing potentials, solids contents, interparticle chemistries, high shear rates, and/or all of the above. These fundamental causes will be considered next.
Fundamental Causes of Increased Particle Collisions Which Produce Dilatancy
When a blockage occurs, of course it must be removed. But without making any other changes to a suspension or process following a blockage, a new blockage may reform in the same location. Blockages are symptoms of a problem, but they are not the fundamental problem. Several different phenomena can be adjusted to reduce the occurrence of dilatancy and dilatant blockages.
Particle Packing Potential
Particle size distributions with poor packing potentials are prone to producing dilatancy. The better a system of particles can pack, the less likely a suspension of those particles will produce dilatancy. Poorest packing occurs in monodisperse systems of particles. All particles in a monodisperse system of particles are exactly the same size. All monodisperse systems pack poorly, but packing potentials become even worse as the particle size of a monodisperse system decreases. The worst case, for example, would be a sub-micron monodispersion. Suspensions of such particles can be extremely dilatant unless they remain at extremely low solids contents.
Particle size distributions that contain wide ranges of particle sizes can pack much more densely. When a system of particles can pack densely, only a little fluid is required to fill the pores, and the remainder of the fluid can then separate particles and impart fluidity. The greater the distance between particles in suspensions, the less dilatant character will be exhibited during flow. When particles pack poorly, a large fraction of fluid is required to fill pores and little if any may be available to separate particles and impart fluidity.
One adjustment to minimize dilatancy and dilatant blockages is to modify the fundamental particle size distribution of the body to produce denser packing potentials.
High Solids Contents
The higher the solids content of a suspension, the more crowded the particles. Many engineers believe that the higher the solids content, the better the process system. That is not necessarily true. But that is also another subject. Whatever the reasons used to justify raising solids contents, as solids contents are increased, particles move closer to one another, and particle collisions and dilatant blockages become more likely and more severe.
So another adjustment to minimize dilatancy and dilatant blockages is to reduce the solids content of suspensions and forming bodies.
Interparticle Chemistry
For those who believe that higher solids contents are always better, interparticle chemistries are also important. The reason is that most processes require suspensions and forming bodies to have certain viscosities. So as solids contents are raised, viscosities increase. The solution to the problem of increasing viscosities as solids contents are raised is to deflocculate the suspensions with additive chemicals. Deflocculants are used to cause particles to repel. Many deflocculants increase the electrostatic charges on the surfaces of particles so the particles repel one another. (Like-charges repel.) As suspensions are deflocculated, viscosities tend to decrease.
The onset of dilatancy (see "Dilatancy", Ceramic Processing E-zine, Volume 1 Number 1) moves to lower shear rates when solids contents are increased and when suspensions are deflocculated. Raising solids contents and deflocculating may allow one to increase solids contents at the same level of viscosity, but the onset of dilatancy will move to lower shear rates when these two compensating adjustments are used. The combination of these two adjustments can bring dilatancy problems into systems which have never before had such problems. Be careful when adjustments are made in this direction.
Similarly, when these two adjustments are made in the opposite direction, the onset of dilatancy moves to higher shear rates. When solids contents are decreased and when suspensions are flocculated, viscosities can be maintained, but dilatancy problems move out towards higher shear rates. The combination of these two adjustments can move dilatancy problems totally out of the shear rate range of a process system.
Another solution to dilatancy problems, therefore, is to increase the flocculation state of suspensions and forming bodies.
High Shear Rates
Dilatancy and dilatant blockages occur at high shear rates. Obviously, high is a relative term. But when dilatant problems are plaguing a process, shear rates can always be reduced. This may be an unpopular solution with management because production rates may be reduced when shear rates are lowered. But this is frequently a necessary, but always valid, adjustment for dilatancy problems.
Lowering process shear rates is the easiest and most immediate solution to dilatancy problems. Just remember: When dilatancy problems occur, SLOW DOWN!!!
Irregularly Shaped Particles
Non-spherical particles also tend to exacerbate dilatancy problems. Elongated (fibrous) particles very easily become entangled with one another. This means that the potential for dilatancy and dilatant blockages increases as particles become more non-spherical, and especially as particles become more elongated.
This is not something that can normally be adjusted in any suspension with dilatant problems, but it should be kept in mind when irregularly shaped particles are used in a process. The other adjustments and considerations will be necessary to successfully produce and use suspensions of irregularly shaped particles.
Summary
Dilatancy and dilatant blockages are produced when particle/particle collisions dominate during flow of process suspensions and forming bodies. To reduce dilatant problems when they occur, particle size distributions can be adjusted to produce better packing, solids contents can be decreased, interparticle chemistries can be adjusted towards more flocculated systems, and when all else fails, process speeds (shear rates) can be reduced.
Dilatant problems can never be totally eliminated because that would require all particles to be removed from the systems. If all particles were removed, we would no longer have ceramic suspensions or forming bodies. Since this is not possible, we must all learn to live with dilatancy. But dilatant problems can be reduced and prevented by making the adjustments discussed in this article.
Miscellany
Please suggest topics for future columns. I look forward to hearing them. Thanks to those of you who have made such suggestions.
Until next time ...
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