Crawling beneath the free surface: Water snail locomotion

TL;DR

This paper models how water snails can propel themselves beneath the free surface by deforming it, using a lubrication model to analyze mucus flow and surface interactions, revealing conditions for effective propulsion.

Contribution

It introduces a lubrication model for water snail locomotion beneath the free surface, showing how surface deformation enables propulsion despite shear stress limitations.

Findings

Propulsive force depends on Capillary number.

Surface deformation generates curvature pressures and lubrication flows.

Force vanishes at very high or low Capillary numbers.

Abstract

Land snails move via adhesive locomotion. Through muscular contraction and expansion of their foot, they transmit waves of shear stress through a thin layer of mucus onto a solid substrate. Since a free surface cannot support shear stress, adhesive locomotion is not a viable propulsion mechanism for water snails that travel inverted beneath the free surface. Nevertheless, the motion of the freshwater snail, Sorbeoconcha physidae, is reminiscent of that of its terrestrial counterparts, being generated by the undulation of the snail foot that is separated from the free surface by a thin layer of mucus. Here, a lubrication model is used to describe the mucus flow in the limit of small amplitude interfacial deformations. By assuming the shape of the snail foot to be a traveling sine wave and the mucus to be Newtonian, an evolution equation for the interface shape is obtained and the resulting propulsive force on the snail is calculated. This propulsive force is found to be non-zero for moderate values of Capillary number but vanishes in the limits of high and low Capillary numbers. Physically, this force arises because the snail's foot deforms the free surface, thereby generating curvature pressures and lubrication flows inside the mucus layer that couple to the topography of the foot.