Worms in Jell-O: Using photoelastic stress analysis to measure burrowing forces

Muddy marine sediments are elastic materials through which worms and other animals extend burrows by fracture. Gelatin has similar fracture properties to muds, and worms burrow readily through this transparent analog. Polarized light shows that the water-filled burrow has an elongated, tongue-depressor-shaped crack. Photoelastic stress analysis was used to measure forces applied by worms burrowing in gelatin. These results, combined with modeling, show that stress intensity factors at the crack tip reach the fracture toughness, validating the method of burrow extension by fracture. Additionally, these measured forces and understanding of the mechanics of burrowing allowed for quantification of the energetic cost of burrowing, which is much lower than previously thought. Stress visualization in gelatin has also been used to test instruments to measure fracture toughness and stiffness of marine sediments. These properties govern burrowing behaviors and may be important metrics to quantify the ecosystem engineering impacts of animals living in sediments.