Silicone Putty

Properties keywords:  non-Newtonian fluid, shear thickening (dilatant), viscoelastic

Analogue keywords: silicate melt, low viscosity layers/reservoirs

Common names:   Silly Putty, Polydimethylsiloxane (PDMS), Silastic Gum

General Information:  Silicone putty, commercially known as Silly Putty (a trademark of Crayola LLC), is made by combining silicone oil with inert fillers to form the stiff end member of silicone polymers.  Its rheology is complex, and distinct from silicone oils.  Silicone putty is a common analogue due to being low cost, easily attainable and non-toxic.

Properties

Silicone putty displays viscoelastic behaviour, whereby it behaves like an elastic solid when a high stress is applied on a short timescale, and a viscous liquid when a low stress is applied on a long timescale (ten Grotenhuis et al., 2002). Viscosity also increases as strain-rate increases i.e. it is a dilatant (shear thickening) material (Figure 1a).

Many forms approximate a Bingham fluid, that is, they possess a yield strength, of approximately 300 Pa at 18°C, which must be overcome for the putty to flow (Dixon and Summers, 1995; 1996).  At low strain-rates, between 10-4 and 10-2 s-1, silicone putty approximates a power-law rheology with a flow index of ~ 7 (dilatant behaviour), but at high strain-rates, over 1 s-1, the rheological behaviour is approximately linearly viscous (Dixon and Summers, 1995; 1996). 

Some manufactured putties, however, have shown Newtonian flow behaviour at strain-rates between 10-6 and 10-2 s-1 (Vendeville et al., 2002).  The degree of non-Newtonian behaviour is thought to be related to filler content (Weijermars 1985; ten Grotenhuis et al., 2002).

Viscosity, which is dependent on temperature, is typically on the order of 104 Pas at room temperature (Weijermars 1985; Merle and Borgia, 1996; Vendeville et al., 2002; Byrne et al., 2014). Bulk density has been measured as approximately 1200 kg m-3 but it can be varied by increasing/decreasing filler content (ten Grotenhuis et al., 2002; Vendeville et al., 2002; Byrne et al., 2014). 

The rigidity (shear modulus G) of silicone putty is 2.6 x 105 Pa (Galland et al., 2015).  Silicone putty can be purchased in opaque or transparent forms.

Figure 1. a) Stress against strain-rate graph for silicone putty, from Dixon and Summers (1985) showing dilatant rheology and b) Experiments by Merle and Donnadieu (2000), showing intrusion of silicone putty into a sand volcanic edifice.

Applications

Because silicone putty acts as a viscous liquid over long timescales and is fully mouldable, it is very commonly used as an analogue for layers or resevoirs of low strength, low viscosity material within the Earth’s crust.  At low strain-rates it behaves in a ductile manner over geologic timescales (ten Grotenhuis et al., 2002; Merle and Borgia, 1996).

It is often used in conjunction with sand, which replicates brittle sections of the crust, in studies including: the formation of volcano flank terraces on Mars (Byrne et al., 2014); lithospheric extension due to an intruding magma body (Callot et al., 2001); caldera collapse and dome resurgence (Acocella et al., 2000; Roche et al., 2000); and volcano spreading due to a ductile substratum (Merle and Borgia, 1996).

It has also been employed in experiments of dyke or sill emplacement e.g. Canon Tapia and Merle (2006), where it was injected into gelatin, an analogue for the country rock to analyse dyke nucleation patterns.  Merle and Donnadieu (2000) used silicone putty as a viscous intrusion in a study of volcanic edifice deformation (Figure 1b), revealing the importance of magma rheology on the growth of lateral bulges, like that which characterized the 1980 eruption of Mount St Helens.

Silicone putty has also been used to give insight into flow processes occurring within conduits during explosive volcanic eruptions (Ichihara et al., 2002; Taddeucci et al., 2006).  For example, shock tube experiments by Taddeucci et al. (2006) took advantage of the tendency of putty to fracture in a brittle manner at very high strain-rates in order to study diffusion driven bubble expansion.

Limitations and tips for use

Silicone putty is highly adaptable for a wide range of conditions, however, because it displays different rheological behaviour at different stress and strain-rate conditions, care must be taken to ensure that experimental conditions are relevant for the type of behaviour you wish to replicate.

If using putty in experiments at elevated temperatures, be aware that the viscosity decrease with increasing temperature is recoverable, but a long time is needed for the material to regain its original stiffness.  Silicone putty may require resting time before use in order for trapped gas bubbles to be removed.

Models where sand and silicone are used together have limitations whereby, when scaled, the silicone represents magma with a viscosity between 1016 and 1017 Pas, much greater than that of natural magmas.  Galland et al. (2015) suggest the use of lower viscosity analogues for studies of magmatic sheet intrusions.

Silicone putty may not have identical properties between batches, and chemical composition and flow properties may vary widely with manufacturer. 

Commercially available silicone putty is non-toxic.  It can adhere to clothes and hair but dissolves in contact with alcohol.

References

Acocella V, Cifelli R, and Funiciello R (2000) Analogue models of collapse calderas and resurgent domes. Journal of Geophysical Research 104: 81-96

Byrne PK, Holohan EP, Kervyn M, Van Wyk de Vries B, and Troll VR (2014) Analogue modelling of volcano flank terrace formation on Mars. Geological Society of London Special Publications 401: 185-202

Callot JP, Grigne C, Geoffroy L, and Brun JP (2001) Development of volcanic passive margins: Two-dimensional laboratory models. Tectonics 20: 148-159

Dixon JM and Summers JM (1985) Recent developments in centrifuge modelling of tectonic processes: equipment, model construction techniques and rheology of model materials. Journal of Structural Geology 7: 83 – 102 

Dixon JM and Summers JM (1986) Another word on the rheology of silicone putty: Bingham. Journal of Structural Geology 8: 593 – 595

Galland O, Holohan E, van Wyk de Vries B, and Burchardt S (2015) Laboratory modelling of volcano plumbing systems: a review.  Advances in Volcanology.  Springer Berlin Heidelberg. 1-68

Ichihara M, Rittel D, and Sturtevant B (2002) Fragmentation of a porous viscoelastic material: Implications to magma fragmentation. Journal of Geophysical Research 107: B10, 1-14

Merle O and Borgia A (1996) Scaled experiments of volcanic spreading. Journal of Geophysical Research 101: 13805-13817

Merle O and Donnadieu F (2000) Indentation of volcanic edifices by ascending magma. Geological Society of London Special Publications 174: 43-53

Naus-Thijssen F (2007) Dynamics of Igneous Systems: Modelling. University of Maine. http://www.geology.um.maine.edu/user/Felice Naus Thijssen/FDmodule/html%20pages/modeling.html

Roche O, Druitt TH, and Merle O (2000) Experimental study of caldera formation. Journal of Geophysical Research 105: 395-416

Taddeucci J, Spieler O, Ichihara M, Dingwell DB, and Scarlato P (2006) Flow and fracturing of viscoelastic media under diffusion-driven bubble growth: An analogue experiment for eruptive volcanic conduits. Earth and Planetary Science Letters 243: 771-785

ten Grotenhuis SM, Piazolo S, Pakula T, Passchier CW, and Bons PD (2002) Are polymers suitable rock analogs? Tectonophysics 350: 35-47

Vendeville B, Cobbold PR, Davy P, Brun JP, and Choukroune P (2002) Physical models of extensional tectonics at various scales.  In: Extensional Tectonics: Faulting and related processes, Part 2. The Geological Society, London: 171-183

Weijermars R (1986) Flow behaviour and physical chemistry of bouncing putties and related polymers in view of tectonic laboratory applications. Tectonophysics 124(3–4): 325–358