Micro bulk rheology of complex fluids, the non-linear properties of snail mucus

C. Clasen¹, R. Ewoldt², W.-M Kulicke¹, G.-H. McKinley²

¹ Institute for Technical and Macromolecular Chemistry, University of Hamburg, Germany

² Hatsopoulos Microfluids Laboratory, Massachusetts Institute of Technology, Cambridge USA.

Investigating the material properties of biological liquids often hinges on the ability of measuring the viscoelastic properties of very small quantities of the fluid on length scales relevant to their actual application in nature. A number of techniques have been developed for measuring the microrheological response of ml samples of complex fluids by probing the local linear response of the system via particulate probes or AFM techniques. In contrast, in this paper we present bulk rheological investigations of minute amounts of fluid under both linear and nonlinear deformation conditions.

Utilizing a flexure-based microgap rheometer (FMR) we measure the rheological properties of slug and snail pedal mucus for film thicknesses of 20-40 micrometer that are representative of the natural slime carpet employed by the snails for locomotion. The complex physically-crosslinked structure of the aqueous mucin gel that is secreted during locomotion results in an effective yield stress, which allows part of the snail foot to glide forward over a viscous fluid region while the remaining portions remain elastically-attached to the substrate in a yielded state. The time and strain-dependent structural changes of these complex mucus gels are presented in the form of Pipkin diagrams which constitute a readily-accessible fingerprint of the viscoelastic properties of the slime carpet for a specified deformation history.