Gelatine
Properties keywords: elastic, brittle, high strength, gel, transparent
Analogue keywords: lithosphere, elastic/brittle crustal rock
Common names: Gelatine, Hydrolysed collagen
General Information: Gelatine is a naturally occurring biopolymer. It is a colourless, brittle, edible material derived from animal or plant collagen. It forms a gel when dissolved in water whose properties can be extensively varied by altering the concentration of dissolved gelatine in the mixture. For the purpose of analogue experiments we consider gelatine here only in its gel-state.
Gelatine is soluble in water so easy to clean, non-toxic and low cost. It is commonly used in the food, cosmetic, photographic and pharmaceutical industries.
Properties
Whereas sand is the weak end member for rock analogues, gelatine in its gel state is the strong end member (Galland et al., 2015). It has a low rigidity (shear modulus 50 – 10000 Pa) and can therefore deform significantly under gravity. Gelatine gel behaves in a purely elastic-brittle manner at laboratory timescales (s to m) and room temperature (McLeod and Tait, 1999; Bons et al., 2001; Kavanagh et al., 2013; Galland et al., 2015). It therefore fails through the formation of mode I, tensile opening fractures. However, at the tips of intruding gelatine it can be locally viscoelastic (Galland et al., 2015).
Gelatine is solid-like (a gel) between approximately 4 - 40°C (Di Guiseppe et al., 2009). It freezes at the same temperature as water. Gelatine is a highly flexible material because its properties can be controlled by varying the concentration of gelatine in water. Density, viscosity and stiffness (Young’s modulus) all depend on gel concentration (Di Guiseppe et al., 2009, Kavanagh et al., 2013). Young’s modulus has a linear relationship with gel concentration (Kavanagh et al., 2013).
Viscosity also depends on temperature and density, which is similar to that of water. This means different forms of gelatine can be used to model different crustal compositions or strengths (Kavanagh et al., 2015). Gelatine can be layered as it will not mix once solidified. Because of the similar density to water, intrusion of water into gelatine cannot be driven by buoyancy and instead requires an internal fluid pressure to drive propagation (Kavanagh et al., 2009).
It is a homogeneous, isotropic material with a bulk modulus of 2.2 GPa, again similar to that of water, and a large Poisson’s ratio of 0.5 (Rivalta et al., 2005). Gelatine is colourless and so very useful for imaging models in 3D and for easily tracing crack propagation through the material. It can also be formed into moulds (e.g. Acocella and Tibaldi, 2005). Another advantage of gelatine is that it is birefringent when strained (e.g. Dupré and Lagarde 1997; Dupré et al. 2010) and as a result strained areas can be viewed easily in crossed polarizers (Taisne and Tait, 2011).
Figure 1. a) A linear relationship between fracture toughness and Young’s modulus reveals the pure elastic, brittle behaviour of gelatine (Menand and Tait, 2002). b) Controlled stress vs strain tests for different concentrations of pig skin gelatine, from (Di Guiseppe et al., 2009).
Applications
Due to its elastic nature and high strength, gelatine is a suitable analogue for natural host rocks with high cohesion, i.e. well-consolidated sedimentary rocks or crystalline basement rocks (e.g. Galland et al., 2015). It is not a suitable analogue for sub-lithospheric material as its Reynolds number is too high (Di Guiseppe et al., 2009).
It is commonly used to simulate the shallow crust and lithosphere and the propagation of dykes and sills into crustal rock. Studies have injected dyed water or air into gelatine to observe the ascent and propagation of dykes (Takada, 1990; Bons et al., 2001; Menand and Tait, 2002; Walter and Troll, 2003; Kavanagh et al., 2006; Kervyn et al., 2009), including how different volcano collapse scars affect this process (Acocella and Tibaldi, 2005).
Numerous studies have injected magma analogues into strength-layered gelatine and observed the formation of sill-like structures at the interface with the rigid layer (Menand and Tait, 2001; Rivalta et al., 2005; Kavanagh et al., 2006). Compressive vertical stress has also been shown to create sills in gelatine (Menand 2011).
Other analogue experiments have used gelatine to study the role of viscosity (Koyaguchi and Takada, 1994; McLeod and Tait, 1999), country rock (Kervyn et al., 2009) overpressurization (Canon-Tapia and Merle, 2006), and edifice load (Muller et al., 2001; Watanabe et al., 2002) on dyke development and propagation. Hot wax can be injected into cooler gelatine to model the cooling of dykes in the crust (Taisne and Tait, 2011).
Limitations and tips for use
The major limitation to using gelatine as a crustal analogue is that its cohesion and strength are too high to adequately simulate weak rock. As a result it does not display shear failure at low stresses (Mathieu et al., 2005). It cannot be used, therefore, to show crustal deformation due to magma intrusion and it is also unlikely to result in the formation of cone sheets (Galland et al., 2015). Crustal rocks also show some inelasticity during dyke emplacement, rendering gelatine an inaccurate analogue due to its dominantly elastic behaviour. Cohesive granular materials may be a more realistic analogue for crustal rock (e.g. Galland et al., 2015).
Furthermore the large Poisson’s ratio of gelatine means that the relative deformation and opening of fractures will be significantly larger in gelatine than in rock (Rivalta et al., 2005). The mode of dyke propagation also differs from natural rock, in that dykes move by hydraulic fracturing in gelatine but due to viscous drag from the magma in nature (Kervyn et al., 2009).
To make gelatine it should be diluted in hot water (60 – 80°C) until completely dissolved. The required water temperature depends on gelatine type and can be obtained from product data sheets. This is necessary to avoid lumps or structural discontinuities in the gel. It must then be left for 48 hours to cool and solidify and for the Young’s modulus to develop (Menand and Tait, 2002; Kavanagh et al., 2013). Although the Young’s modulus evolves with time (Kavanagh et al., 2013), many experiments are of short enough duration (e.g. < 20 minutes) for it to be essentially constant. A thin layer of silicon oil or vegetable oil can be poured on top to prevent water evaporation during solidification.
Gelatine can break down over time and its strength and viscosity will be affected by extreme pH, bacteria and prolonged heating over 40°C. Temperature and pH should therefore be kept constant. It should be kept in a sealed container when not in use. A previous study added 0.1 % sodium hypochlorite to the gel to prevent fungal growth (Menand and Tait, 2002). Any waiting time between the preparation of the gelatine and the experimental run must be constant to ensure repeatable mechanical properties of experiments.
References
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Di Guiseppe E, (2009) Gelatins as rock analogs: A systematic study of their rheological and physical properties. Tectonophysics 473 (3–4): 391403
Dupré JC, Valle V, Jarny S, and Monnet P (2010) Fringe analysis by phase shifting technique for birefringent fluid studies. Optics and Lasers in Engineering 48 (1): 37–42
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Takada A (1990) Experimental study on propagation of liquid-filled crack in gelatine: shape and velocity in hydrostatic stress condition. Journal of Geophysical Research 95: 8471-8481
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