For centuries, the peculiar mechanical properties of the Batavian Tear, otherwise known as Prince Rupert’s Drop (PRD), has mesmerized scientists. Historically, the glass drop dates back to 1660, where it was brought to King Charles II of England by Prince Rupert from Germany (Brodsley, Frank, and Steeds, 1986). As its name suggests, the drop has a tadpole-like structure consisting of two prominent features: the bulbous head and a long, extending tail. Interestingly, this shape enables the head of Prince Rupert’s Drop to withstand immense forces, up to about 15 000 N, whereas a slight mishandle of the tail can result in instant disintegration (Brodsley, Frank, and Steeds, 1986; Aben et al., 2016). By investigating the material science of Prince Rupert’s Drop, the unique physical properties of the object can be better understood and employed in novel applications.
The process of forming a PRD is relatively simple. Typically, a soda-lime glass rod, which mainly consists of SiO2, Na2O, and CaO, is exposed to a flame and melted above a beaker containing cold water. Once the glass rod is red hot, molten glass drops are allowed to fall and cool in the water while still remaining attached to the original rod, albeit with a tail of decreasing diameter (Aben et al., 2016). Due to the rapid cooling of the outer glass layer and gradual solidification of the inner layer, residual stresses are locked into the structure of the glass drop. In essence, the crystalline configuration of the glass results in strong compressive stresses along the surface and tensile stresses in the interior of the drop (Aben et al., 2016).
In a study by Aben et al. (2016) that used tungsten carbide plates to test maximum stress loads, the head of a PRD was found to have surface compressive stresses up to 125 MPa and the tail had about 700 MPa of compressive stress. To determine this, the team observed the PRD using a method known as integrated photoelasticity, whereby the PRD was placed in an immersion liquid and a beam of polarized light was passed through the specimen, generating a view of the three-dimensional distribution of residual stresses (Figure 1) (Aben and Guillemet, 2012; Aben et al., 2016). Moreover, high-speed photos capturing the process of disintegration showed that cracks initiated in the tail, an area with higher internal tension, and propagated through the glass drop at speeds ranging from 1400 – 1900 m/s (Chandrasekar and Chaudhri, 1994).
From a biomechanical perspective, the presence of compressive and tensile stresses is integral in nature. For instance, avian eggshells can withstand large compressive loads that are applied along the major axis; this is the result of the shell’s geometry, which dissipates pressure over the surface of the egg (Hahn et al., 2017). When various avian eggs were placed in a compression testing apparatus, Hahn et al. (2017) found that fracturing occurred just under the loading area, where tensile stresses were highest. In this way, the structural integrity of both an egg and PRD is dependant on its composition; an egg breaks when radial tensile stress is equal to the tensile strength of calcium carbonate, and a PRD cracks when circumferential tensile stress overcomes residual surface compressive stress formed by the internal crystal structure (Hahn et al., 2017; Aben et al., 2016).
As a whole, the PRD epitomizes the duality between great strength and crushing fragility. With techniques like integrated photoelasticity, the distribution of residual stresses can be observed and used to determine the properties of the drop under various forces.While the PRD has had a long history with science, similar mechanical principles have been present in and continue to provide insight for other fields like biology.
References
Aben H., Guillemet C., 2012. Photoelasticity of Glass: Integrated Photoelasticity. Berlin, Heidelberg: Springer.
Aben, H., Anton, J., Õis, M., Viswanathan, K., Chandrasekar, S. and Chaudhri, M.M., 2016. On the extraordinary strength of Prince Rupert’s drops. Applied Physics Letters, [online] 109(23), p.231903. Available at: <http://aip.scitation.org.libaccess.lib.mcmaster.ca/doi/full/10.1063/1.4971339> [Accessed 12 Jan. 2018].
Brodsley, L., Frank, S.C., S, F.R. and Steeds, J.W., 1986. Prince Rupert’s drops. Notes Rec. R. Soc. Lond., [online] 41(1), pp.1–26. Available at: <http://rsnr.royalsocietypublishing.org/content/41/1/1> [Accessed 12 Jan. 2018].
Chandrasekar, S. and Chaudhri, M.M., 1994. The explosive disintegration of Prince Rupert’s drops. Philosophical Magazine Part B, [online] 70(6), pp.1195–1218. Available at: <http://www.tandfonline.com/doi/abs/10.1080/01418639408240284>
Hahn, E.N., Sherman, V.R., Pissarenko, A., Rohrbach, S.D., Fernandes, D.J. and Meyers, M.A., 2017. Nature’s technical ceramic: the avian eggshell. Journal of The Royal Society Interface, [online] 14(126), p.20160804. Available at: <http://rsif.royalsocietypublishing.org/content/14/126/20160804> [Accessed 13 Jan. 2018].
Keller, M., 2018. Prince Rupert’s Drops. [image online] Oberlin College. Available at: <http://www2.oberlin.edu/physics/catalog/demonstrations/mech/princerupertdrops.html> [Accessed 18 Jan. 2018].