Cornstarch

Properties keywords:  non-Newtonian fluid, shear thickening

Analogue keywords: cooling lava flow

Common names: Cornflour (UK), maize starch, maizena, ‘oobleck’

General Information:  Cornstarch is starch derived from the corn kernel. It is a white, tasteless and odourless hygroscopic powder.  It is non-toxic, cheap, easy to make and readily available.  For use as an analogue material it is mixed with water in ratios of 1 part water to 1-2 parts cornstarch.

Cornstarch is a popular food ingredient, often used as a thickening agent.  It is also used to make corn syrup and as an anti-caking or anti-stick agent.

Properties

Cornstarch suspensions display highly unusual rheological properties.  They are highly non-Newtonian and shear thickening (dilatant, Figure 1), whereby suspensions behave like a fluid at low strain-rates and like a solid at high strain-rates (Balmforth et al., 2005; 2007; Ternik et al., 2006).  They also fracture and bend in a brittle manner.

Figure 1. Stress vs strain-rate plot for cornstarch and water mixture showing shear thickening rheology (Ternik et al., 2006)

Suspensions have a relaxation timescale, similar to viscoelastic fluids, yet they do not behave as a viscoelastic fluid (Balmforth et al., 2005).  In fact, there is no existing rheological model for cornstarch suspensions and no comprehensive understanding of their fracture properties, although Balmforth et al. (2007) found suspensions to be power law fluids with an exponent of n=3-4.

The material has a Poissons ratio of 0.5 (Muller and Dahm, 2000), a negligible yield stress of approximately 0.5 Pa (Balmforth et al., 2005) and a density of approximately 1200 kg m-3 (Muller and Dahm, 2000).

Applications

The use of cornstarch suspensions as an analogue material is very limited, due to its complex and uncharacterised rheology. 

It has predominantly been used to replicate the fracture patterns in a cooling lava flow by dessicating cornstarch and water using an external heat source (Muller 1998; Muller and Dahm, 2000; Goehring et al., 2006).  These studies have aided in understanding the columnar jointing observed in basaltic lava flows, for example, dessicated cornstarch shows two distinct stages of fracture generation.

It is thought to be an adequate analogue for cooling basalt due to its fine-grained granular structure and the fact that columnar jointing occurs at a Peclet number (a dimensionless number which measures the ratio of advective to diffusive effects) of approximately 0.2 in the cornstarch mixture and an almost equivalent 0.15 in lava (Goehring et al., 2006).  However, cracks in lava significantly enhance water transport, which is not the case in cornstarch mixtures, and heat transport is via non-linear diffusion in the analogue material, but linear in lava (Goehring et al., 2006). 

A study using cornstarch suspensions to investigate roll waves at low Reynolds number found that the material behaves similarly to flowing granular layers and shows high frequency vibrations at the crests of roll waves, which may be similar to the generation of acoustic waves during flow of fluidized granular media (Balmforth et al., 2005).

Limitations and tips for use

As mentioned above, lack of knowledge regarding the rheological and fracture properties of cornstarch suspensions makes it an unfavourable analogue material. 

It has been shown that its properties can change with time, i.e. the suspension may separate after 1 or 2 days and behave differently even when remixed.  This may be due to evaporation of the water and/or swelling of the starch grains (Balmforth et al., 2005).  It is also thought that the suspension may be inhomogeneous due to an uneven particle distribution, resulting in layers of contrasting viscosity (Balmforth et al., 2005).

References

Balmforth NJ, Bush JWM, and Craster RR.(2005). Roll waves on flowing cornstarch suspensions. Physics Letters A, 338: 479-484

Balmforth NJ, Craster RV, Perona P, Rust AC, and Sassi R (2007) Viscoplastic dam breaks and the Botswick consistometer. Journal of Non-Newtonian Fluid Mechanics 142: 63-78

Goehring L and Morris SW (2006) Experimental investigation of the scaling of columnar joints. Physical review E 74: 036115

Müller G (1998) Starch columns: Analog model for basalt columns. Journal of Geophysical Research 103: B7, 15239-15253

Müller G and Dahm T (2000) Fracture morphology of tensile cracks and rupture velocity. Journal of Geophysical Research 105: 723-738

Smith JV (1997) Shear thickening dilatancy in crystal-rich flows. Journal of Volcanology and Geothermal Research 79: 1-8

Ternik P, Marn J, and Žunič Z (2006) Non-Newtonian fluid flow through a planar expansion: shear-thickening fluids. Journal of Non-Newtonian Fluid Mechanics 135: 136-148