Fluid Mediated Unraveling of Thread Skeins*

Hagfish slime unravels within a fraction of a second to thwart a predator attack. Understanding the unraveling mechanism will create a pathway to produce synthetic thread skein analogs that uncoil to reveal a 1:1000 length expansion ratio. Here we report the scaling relations that relate hagfish thread properties (elasticity, diameter, length) to the hydrodynamic flows required for unraveling, which are stated in terms of relative flow velocities or strain rates and fluid viscosity. Modeling considered viscous hydrodynamic force can be responsible for rapid unraveling and is extended to include flow effects for larger length scale fibers where inertial effects of the fluid or fiber are considered (high Reynolds number regime). Our modeling shows that a minimum of 17s^-1 shear rate is required for natural skeins to unravel within 400ms. The collapse time of a single extended elastic thread was also considered, which is set by the ratio of fluid viscosity to the thread elasticity times a geometric factor. This modeling was then extended to consider a network of elastic elements via a poroelasticity model to obtain the collapse timescale of a slime network after deployment. Based on our findings, we provide a design criterion for synthetic skeins and the fluid viscosity and flow conditions required to unravel them. Experimental flow visualization of unraveling is pursued with a counter-rotating Couette flow setup in a rotational rheometer (MCR 702) with a single natural hagfish skein to support the hypothesis of viscous drag-dominated unraveling.