Kaolin Slurry
Properties keywords: non-Newtonian, viscoplastic, Bingham fluid
Analogue keywords: lava flows, lava domes, mudflows, lahars
Common names: kaolin-water suspension, kaolinite (Al2Si2O4(OH)4), China clay
General Information: Kaolin is a very fine-grained white powdered clay, comprising the hydrous aluminium silicate mineral kaolinite, which is readily dispersed in water. It is low cost and readily available.
The main use of kaolin is in the paper industry and chinaware. Kaolin and water slurries are commonly used in construction and engineering and, as a result, their properties are very well constrained.
Properties
Kaolin-water slurry is a viscoplastic fluid with a yield stress arising from particle interactions within the suspension. It is often described as a Bingham fluid yet its rheology can be more accurately described by a Herschel-Bulkley model (Figure 1), as it shows a non-linear dependence on strain-rate (Hulme, 1974; Huang and Garcia, 1998; Balmforth et al., 2000).
The concentration of kaolin powder in water affects the physical properties of the suspension. A higher ratio of kaolin increases the yield stress (Figure 1, Huang and Garcia, 1998; Shen, 1998; Balmforth et al., 2000) and changing the pH, by adding acids or bases, affects inter-particle attractions and thus suspension viscosity (Nuntiya and Prasanphan, 2006). In this way, kaolin slurries are highly adaptable and able to reproduce a range of different viscosities, including the textural endmembers of basaltic lava flows: a’a’ and pahoehoe.
Figure 1. Rheological data for a 1.2:1 kaolin:water suspension and constitutive models for Herschel-Bulkley and Bingham fluids (a), from Balmforth et al. (2000) and (b) graph showing effect of kaolin concentration on yield stress, from Huang and Garcia (1998).
Density of kaolin slurries used in experiments have ranged from approximately 1200 – 1500 kg m-3 (Table 1), and yield stress from 14-200 Pa, both depending on kaolin concentration (Huang and Garcia, 1998; Shen, 1998; Balmforth et al., 2000). Kaolin is chemically stable up to 537 °C. Kaolin slurry does not have a temperature-dependent viscosity.
Properties of kaolin-water suspensions of different concentrations, from Shen (1998).
Applications
Kaolin slurry is used as an analogue for lava flows and domes and may also be appropriate for use in lahar experiments (Huang and Garcia, 1998). Crystals present in lavas contribute to the development of yield stress and a viscoplastic rheology, making a Bingham-type fluid a good analogue (e.g. Pinkerton and Wilson, 1994).
Blake (1990) and Balmforth et al. (2000) used kaolin-water slurries to study the effects of yield stress on the morphology of lava domes, in isothermal flow experiments. They found that surface fractures and asymmetrical features typical of lava domes were adequately replicated by the material.
Hulme (1974) observed levee formation at the sides of an unconfined flow of kaolin slurry, and deduced that the non-Newtonian rheology of a Bingham fluid has more control on flow morphology than cooling rate. Pinkerton and Wilson (1994) describe experiments where the material was subjected to forced evaporation in order to simulate cooling of a lava flow.
Kaolin and PEG wax
The above experiments are all isothermal and therefore do not accurately replicate the effects of cooling and solidifying on lava flow dynamics. Kaolin slurries can, however, be easily combined with PEG wax (see PEG page), in order to combine the yield stress of a kaolin slurry with the temperature dependence and solidifying properties of PEG, thereby effectively replicating the major rheological properties of lava.
Several studies have used kaolin-PEG mixtures as an analogue for solidifying lava domes and found that the difference in yield strength between the internal lava and the cooling crust is an important factor in the formation and rheology of lava domes (Griffiths and Fink, 1997; Fink and Griffiths, 1998; Lyman et al., 2005; Lyman and Kerr, 2006).
Limitations and tips for use
As mentioned above, kaolin-water slurries are only appropriate for use in isothermal experiments and should be combined with a temperature dependent material in order to include the cooling effects important for lava fluid dynamics.
Kaolin is non-toxic, however inhalation of the powder should be avoided and face protection is recommended.
When preparing kaolin-water solutions it is advised to mix the slurry two days before the experiment and again immediately prior to the experiment, ensuring no separation occurs.
References
Balmforth NJ, Burbidge AS, Craster RV, Salzig J, and Shen A (2000) Visco-plastic models of isothermal lava domes. Journal of Fluid Mechanics 403: 37-65
Blake S (1990) Viscoplastic models of lava domes, in Lava Flows and Domes: Emplacement Mechanisms and Hazard Implications, edited by J. H. Fink. Springer, New York: 88 – 126
Fink JH and Griffiths RW (1998) Morphology, eruption rates and rheology of lava domes: Insights from laboratory models. Journal of Geophysical Research, 103: 527-545
Griffiths RW (2000) The dynamics of lava flows. Annual Review of Fluid Mechanics 32: 477-518
Griffiths RW and Fink JH (1997) Solidifying Bingham extrusions: a model for the growth of silicic lava domes. Journal of Fluid Mechanics 347: 13-36
Huang X and Garcia MH (1998) A Herschel-Bulkley model for mud flow down a slope. Journal of Fluid Mechanics 374: 305-333
Hulme G (1974) The interpretation of lava flow morphology. Geophysical Journal of Royal Astronomical Society 39, (2): 361-383
Lyman AW, Kerr RC, and Griffiths RW (2005) Effects of internal rheology and surface cooling on the emplacement of lava flows. Journal of Geophysical Research 110: 1-16
Lyman AW and Kerr RC (2006) Effect of surface solidification on the emplacement of lava flows on a slope. Journal of Geophysical Research 111: B05206
Maciel GF, Santos HK, and Ferreira FO (2009) Rheological analysis of water clay compositions in order to investigate mudflows developing in canals. Journal of Brazilian Society of Mechanical Sciences and Engineering 31: 64-74
Nuntiya A and Prasanphan S (2006) Rheological behaviour of kaolin suspensions. Chiang Mai Journal of Science 33 (3): 271-281
Pinkerton H and Wilson L (1994) Factors controlling the lengths of channel-fed lava flows. Bulletin of Volcanology 56: 108-120
Shen AQ (1998) Mathematical and analog modeling of lava dome growth. University of Illinois, Urbana-Champaign