Silica Beads

Properties keywords:  solid, spherical, granular, uniform, permeable

Analogue keywords: granular flows, PDCs, crystals, fluidised flows, brittle crust

Common names: silica beads, glass beads, soda lime glass beads, ballotini

General Information:  Silica beads come in an extremely wide range of sizes, from powders less than a micron in diameter to spheres of several millimetres.  They are commonly used in industry for sandblasting and larger sizes are used in jewellery making.  Silica beads are manufactured to very high technical standards and as such are highly uniform.  They are also reusable.

Properties

Silica beads are used primarily because each type has a uniform size density, shape and composition.  They typically have a sphericity > 0.95 (Rowley, 2010), are hard and come in a variety of colours, in matte or metallic finishes.

Beads less than 100 micron are classed as powders, the behaviour of which differs greatly from granular materials (beads > 100 micron).  Silt sized and smaller particles behave more as a fluid whereas larger particles act as a granular solid (Iverson, 1997).  Powders are also affected by electrostatic interactions and humidity.

Silica powders, or flour, are characterised by high cohesion (300-400 Pa), an angle of repose of 40° and densities between 1050 and 1700 kg m-3 (Galland et al., 2015).  Silica grains, on the other hand, have low cohesion (67-137 Pa), an angle of repose of 20° and densities between 1500 and 2500 kg m-3 (Roche, 2012; Galland et al., 2015).

Silica beads are not affected by temperature.  They can be fluidised by liquid, in which case the interstitial liquid will dominate over friction-based particle interactions (Felix and Thomas, 2004).  Different sizes can be combined into polydisperse mixtures, however this can be difficult due to granular sorting mechanisms.

In contrast to sand, another common analogue material, silica beads, being artificial, are spherical and show high particle regularity.  However Roche (2012) noted that the behaviour of flows comprising glass beads and natural particles behaved similarly, suggesting that shape is a second order parameter.  Powders also have higher cohesion and less internal friction (Hoffman, 2010).

Applications

Due to having such well constrained physical properties, silica beads are ideal for combining analogue experiments with numerical modelling.  Their range in sizes also makes them useful for scaling to natural systems.

Glass beads have commonly been used as analogues for granular flows, including pyroclastic density currents (PDCs), block and ash flows, dome/edifice collapse, landslides, debris avalanches and destabilisation of slopes.  Different sizes and densities can be used to simulate the different particles found in granular flows i.e. ash, pumice and lithics.

Rowley (2010) used silica beads as pumice clast analogues in flume experiments to study the effect of sorting, reworking and deposition on PDC structure.  They found that simple models can replicate the behaviour and patterns of natural PDC deposition.  Roche et al. (2004) used ballotini to infer that dense fines-rich PDCs act like Newtonian fluid gravity currents when fluidised.  Larger beads can also act as useful markers of internal flow kinematics (Roche, 2012).

Other experiments have fluidised silica particles with water, in, for example, the experiments of Koyaguchi et al. (2009), who analysed the effect of turbulent flow intensity on tephra dispersion from ash clouds.  Quane and Russell (2005; 2006) used silica beads to show that welding of PDCs depends on temperature, compactional load, particle size and size distribution and glass rheology.

Aside from granular flows, silica beads have been used in many diverse analogue experiments.  Costa et al. (2006) used them as a porous layer with interstitial liquid to act as a vesiculating crystal mush at the base of a crystallising magma chamber or lava flow.  Cagnoli et al. (2002) performed shock tube experiments to study the behaviour of gas-particle flows in the conduit directly after fragmentation.  However they noted that the beads do not reflect irregular pyroclast shapes and that electrostatic forces between particles may effect experimental results.

Glass beads have also been used as analogues for crystals in studies of magma rheology and conduit dynamics (Mourtada-Bonnefoi and Mader, 2004; Mueller et al., 2010).

Silica flour has been shown to simulate the behaviour of the brittle crust as it exhibits analogous faulting styles (e.g. Galland et al., 2006).  As a result it has been used in experiments with air injections to simulate explosive fragmentation in the near vent region (Haug et al., 2013), and with golden syrup to study the propagation and inflation of dykes (Abdelmalak et al., 2012).

Limitations and tips for use

An issue with the use of silica beads is their simplified nature in comparison to real life systems.  Greater size and density variety exists in nature and their perfect uniformity does not capture some packing and porosity aspects of natural systems (Rowley, 2010).  Even polydisperse mixtures of beads do not reflect observed extremes in size and shape.  Furthermore, PDCs display constantly changing particle properties with time (Higashi and Sumita, 2009)

Granular sorting and flow mechanisms are greatly influenced by particle size, density and shape, and so care should be taken to ensure the specific particle properties accurately reflect the needs of the experiment.  Sizes < 50 microns are hard to fluidise at room temperature due to interparticle forces (Roche, 2012).

Fine beads may experience clumping, requiring the use of antistatic surfaces or aerosols (Iverson et al., 2004).  Silica beads may be toxic, as they can contain certain toxic chemicals and/or dyes.

Glass beads are brittle and can be a fragile material to work with.  Cheaper alternatives may also be found, for example, poppy seeds, mustard seeds, coriander seeds and wooden or ceramic beads.  All of these also have high sphericity, low friction and no electrostatic effect.

References

Abdelmalak MM, Mourgues R, Galland O, and Bureau D (2012) Fracture mode analysis and related surface deformation during dyke intrusion: results from 2D experimental modelling. Earth and Planetary Science Letters 359: 93–105

Cagnoli B, Barmin A, Melnic O, and Sparks RSJ (2002) Depressurization of fine powder in a shock tube and dynamics of fragmented magma in volcanic conduits. Earth and Planetary Science Letters 204: 101-113

Cimarelli C, Costa A, Mueller S, and Mader HM (2011) Rheology of magmas with bimodal crystal size distributions. Geochemistry Geophysics Geosystems 12: 7, 1-14

Costa A, Blake S and Self S (2006) Segregation processes in vesiculating crystallizing magmas. Journal of Volcanology and Geothermal Research 153: 287-300

Félix G and Thomas N (2004). Relation between dry granular flow regimes and morphology of deposits: formation of levées in pyroclastic deposits. Earth and Planetary Science Letters 221: (1-4), 197-213

Haug ØT, Galland O, Gisler GR (2013) Experimental modelling of fragmentation applied to volcanic explosions. Earth and Planetary Science Letters 384: 188–197

Higashi N and Sumita I (2009) Experiments on granular rheology: Effects of particle size and fluid viscosity. Journal of Geophysical Research 114: 1-18

Hofmann F (2010) The effect of material properties (sand vs. glass beads) in the structural developments of analogue Coulomb wedges, Bachelor Thesis, University of Ludwig- Maximilians, Munich

Iverson RM (1997) The physics of debris flows. Reviews in Geophysics 35: 245-296

Iverson RM, Logan M, and Denlinger RP (2004) Granular avalanches across irregular three-dimensional terrain: 2. Experimental tests. Journal of Geophysical Research 109: F01015

Koyaguchi T, Ochiai K, and Suzuki YJ (2009) The effect of  intensity of turbulence in umbrella cloud on tephra dispersion during explosive volcanic eruptions: Experimental and numerical approaches. Journal of Volcanology and Geothermal Research 186: 68-78

Mourtada-Bonnefoi CC and Mader HM (2004) Experimental observations of the effect of crystals and pre-existing bubbles in the dynamics and fragmentation of vesiculating flows. Journal of Volcanology and Geothermal Research 129: 83-97

Mueller S, Llewellin EW, and Mader HM, (2010) The rheology of suspensions of solid particles.  Proceedings of the Royal Society A 466: 2116, 1201-1228

Roche O, Gilbertson M, Phillips JC, and Sparks, RSJ (2002). Experiments on deaerating granular flows and implications for pyroclastic flow mobility. Geophysical research letters 29: (16), 40-1

Roche O, Gilbertson MA, Phillips JC, and Sparks RSJ (2004). Experimental study of gas-fluidized granular flows with implications for pyroclastic flow emplacement. Journal of Geophysical Research 109: B10, 1978–2012

Roche O, (2012) Depositional processes and gas pore pressure in pyroclastic fl ows: An experimental perspective. Bulletin of Volcanology 74: 1807–1820

Rowley PJ (2010) Analogue Modelling of Pyroclastic Density Current Deposition, PhD Thesis, Royal Holloway, University of London

Quane SL and Russell JK (2005) Welding: insights from high-temperature analogue experiments. Journal of Volcanology and Geothermal Research 142: 67-87

Quane SL and Russell JK (2006) Bulk and particle strain analysis in high-temperature deformation experiments. Journal of Volcanology and Geothermal Research 154: 63-73