L8: The Mechanics of Adhesion used by the Gecko

Transcription

1 L8: The Mechanics of Adhesion used by the Gecko With help from Bo He Overview Gecko s foot structure Intermolecular force Measurement: 2-D MEMS sensor Gecko s adhesive mechanism Measurement results discussion Applications Summary

2 Background Many hypotheses - Suction: Gadow, Electrostatics: Schmidt, Friction: Madhendra, Micro-interlocking: Madhendra, Capillary wet adhesion Intermolecular force: van der Waals force Ref): Gecko s amazing feet Ref):

3 Gecko s foot structure Ref): Gecko s foot structure Ref):

4 Gecko foot structure Ref): Kellar Autumn, et al, Adhesive force of a single gecko foot-hair, Nature, 405, (2000) Adhesion force: van der Waals force Ref):

5 Van der Waals force An attractive force between atoms or molecules. Not the result of chemical bond formation, much weaker Responsible for some material properties: crystal structure, melting points, boiling points, surface tension, and densities. Van der Waals force Dipole-dipole force Dipole induced dipole force London dispersion force (induced dipole-induced dipole) Ref):

6 A tool: 2-D MEMS sensor Dual-axis piezoresistive cantilever 2-D MEMS sensor Ref): Chui, B. W., et al, Independent detection of vertical and lateral forces with a sidewall-implanted dual-axis piezoresistive cantilever, Appl. Phys. Lett. 72, (1998).

7 SEM of dual-axis cantilever Ref): Chui, B. W., et al, Independent detection of vertical and lateral forces with a sidewall-implanted dual-axis piezoresistive cantilever, Appl. Phys. Lett. 72, (1998). AFM images Vertical force image Lateral force image Ref): Chui, B. W., et al, Independent detection of vertical and lateral forces with a sidewall-implanted dual-axis piezoresistive cantilever, Appl. Phys. Lett. 72, (1998).

8 Line scan images Ref): Chui, B. W., et al, Independent detection of vertical and lateral forces with a sidewall-implanted dual-axis piezoresistive cantilever, Appl. Phys. Lett. 72, (1998). Direct force measurement Ref): Kellar Autumn, et al, Adhesive force of a single gecko foot-hair, Nature, 405, (2000)

9 2-D MEMS sensor: attachment Ref): Kellar Autumn, et al, Adhesive force of a single gecko foot-hair, Nature, 405, (2000) Attachment Experimental results Ref): Kellar Autumn, et al, Adhesive force of a single gecko foot-hair, Nature, 405, (2000)

10 Experimental results Attachment: With maximum preload: ~15µN Ref): Kellar Autumn, et al, Adhesive force of a single gecko foot-hair, Nature, 405, (2000) Detachment Experimental results Ref): Kellar Autumn, et al, Adhesive force of a single gecko foot-hair, Nature, 405, (2000)

11 Gecko adhesive mechanism A seta adhesion is ten times more effective than predicted from maximal estimates on whole animals. Orientation and loading effect. Attachment: preloading and parallel pulling Detachment: critical angle Ref): Kellar Autumn, et al, Adhesive force of a single gecko foot-hair, Nature, 405, (2000) Theoretical calculation Van der Waals force between 2 parallel surfaces AR F vdw = 2 6D Where A is the material-dependent Hamaker constant, about Joules Spatula R~0.2 µm, D~0.3nm 0.4µN for single spatula, spatula µn, within the range of direct force measurement results

12 Conclusions 1. Magnitude of forces measured within expected range of calculated van der Waals attraction. 2. Large adhesion is generated when the setae are first pushed against the surface and then dragged a small distance ~5um. 3. Adhesion forces increases with magnitude of preload. Applications What do you think of? Dry, self-cleaning glue: tape in space Others: adhesive microstructure to make insect trapping, a substrate to prevent adhesion at a specific position, micro manipulation tools without scratching surfaces, etc.

13 Further study Further study for the role of water adsorption or other types of water interaction (even tiny quantities of water could be sufficient to cause a capillary force that could be quite large at the nanoscale) Further data collection on spatula morphology, orientation, spacing and material properties. Summary Gecko s foot structure Intermolecular forces: van der Waals Experimental verification: tool and measurement results Promising applications Further study still needed

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