CBET Award Achievements
Notable Accomplishments from CBET Awards

Do the Locomotion!

Eric Lauga  -  Massachusetts Institute of Technology

Background:  A group of scientists and engineers from the Massachusetts Institute of Technology and the University of California - San Diego is studying snail locomotion.  Specifically, the researchers are investigating the different mechanisms employed in travel by water and land snails.

In order to move, land snails employ a mechanism called "adhesive locomotion".  As a land snail travels over rough terrain, the snail uses "adhesive locomotion" by transferring waves of shear stress (stress that is parallel to, or directed along, the organism) through a thin layer of mucus onto a solid surface underneath them.  Land snails, however, cannot crawl beneath an air-water (or free) surface using the same mechanism.  This effect is due to the fact that the free surface cannot sustain shear stress.

Interestingly, some water snails employ a more puzzling mode of locomotion:  crawling beneath the air-water surface (Figure 1).  Similar to their terrestrial counterparts, water snails secrete a thin mucus layer.  This mucus layer separates the foot of the water snail from the free surface.  So how are these water snails able to move?  The researchers seek the answer to this question as well as endeavor to provide a physical picture and mathematical model for this puzzling mode of locomotion.

Results:  The researchers observed that the freshwater snails, Sorbeoconcha Physidae, undulated their foot to deform the air-water interface (Figure 1).  Based on this observation, the investigators developed a mathematical model describing the lubrication flow within the thin mucus layer.

The results, displayed in Figure 2, highlight the importance of surface tension in this mode of locomotion.  As displayed in the graph, the ratio of viscous to surface tension forces is represented by the dimensionless Capillary number, Ca. The researchers found that under conditions of both extremely high and low Ca the propulsive motion force ceases, i.e. no movement.  With these findings, a high surface tension limit, or small Ca, is analogous to crawling on an undeformable surface.  In this case, the snail foot would lose traction on a surface, and the snails would simply "slip in place".  Under zero surface tension (large Ca) conditions, no pressure difference across the interface exists to drive a lubrication flow in the mucus layer generating no propulsive motion.  With a moderate Ca, or when the effects of surface tension are moderate, the undulating snail foot deforms the free surface and generates lubrication flows within the mucus layer, and, thus, the snail is able to propel itself forward.

Eric Lauga Image 1

Figure 1:  Snail (Sorbeoconcha Physidae) crawling beneath a free surface while the surface deforms.  The surface deformations are caused by the undulation of the snail's foot.
Credit:  David Hu and Brian Chan
            Massachusetts Institute of Technology
Eric Lauga Image 2

Figure 2:  Dimensionless propulsive force, Fprop, normalized by the number of undulatory waves generated by the snail, n, as a function of Capillary number, the ratio of viscous to surface tension forces.  The values of n range from 5 to 30 in increments of 5.
Credit:  Sungyon Lee
            Massachusetts Institute of Technology
Primary Strategic Outcome Goal:  (1) Discovery
  - Disciplinary/Interdisciplinary Research

Secondary Strategic Outcome Goal:  (2) Learning
  - K-12 Education
  - Graduate Education and Graduate Student Research
  - Public Understanding of Science and Lifelong Learning

This project addresses the strategic outcome goals, as described in the NSF Strategic Plan 2006-2011, of:

(1) Discovery:  In studying water snail locomotion, the researchers identified a key component of snail propulsion.  This knowledge could be helpful in biomedical and environmental applications using fluid-fluid interfaces.  Applications benefiting from this research include biofilm prevention, biodegradation, and eliminating surface-associated infections furthering environmentally-friendly alternatives to waste treatment and benefiting human health.

(2) Learning:  The researchers have identified a key mechanism which can easily be observed in a classroom setting with real snails, and provides educators in physics and biology an opportunity to introduce the concept of physical modeling to their students.  A plan for the PIs to publicly implement this is identified in the Broadening Participation paragraph below.

This highlight represents Broadening Participation.  The Pricipal Investigators engage in numerous outreach activities, one of which impacts a non-scientific audience.  The PIs plan to have an exhibit at the Boston Museum of Science, "Life at the Interface".  The PIs are in the process of adapting components of the research to an interactive museum setting.  These ideas include a "soap boat", pattern formation in spreading droplets, and simple mechanical swimmers (which can be placed in both low and high viscosity fluids).  The Boston Museum of Science receives visitors from various economic and ethnic backgrounds.

Impact on Industry and/or Society:  The researchers identified a key feature of water snail propulsion, which is the coupling of the surface deformation generated by the foot and the flow between the foot and the free surface.  Such strategies could be used in the design, fabrication and optimization of robotic systems capable of locomotion at fluid-fluid interfaces as well as improve our general understanding of biological locomotion near interfaces.

This knowledge could be helpful in biomedical and environmental applications using fluid-fluid interfaces.  An example of such an application could be the prevention of biofilm formation (bacteria growth) on a medical instrument thereby mitigating infections resulting from the surface contamination of medical instruments.

Program Officer:   William Schultz
NSF Award Number:   0624830
Award Title:   Life at the Interface: Biolocomotion Near Boundaries
PI Name:   Eric Lauga
Institution Name:   Massachusetts Institute of Technology
Program Element:   1443
NSF Investments:
  Understanding Complex Biological Systems (including
the interfaces of life, physical, and computational sciences)
CBET Nugget:   FY 2008

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This Nugget was Updated on 15 October 2008.