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Materials for soft machines

Materials for soft machines. Zhigang Suo Harvard University. Hard Machines. Soft Machines. octopus. Mäthger, Hanlon, Kuzirian – Marine Biological Laboratory, Woods Hole MA. S quid changes color. Expanding chromatophores by contracting radial muscles.

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Materials for soft machines

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  1. Materials for soft machines Zhigang Suo Harvard University

  2. Hard Machines

  3. Soft Machines

  4. octopus Mäthger, Hanlon, Kuzirian – Marine Biological Laboratory, Woods Hole MA

  5. Squid changes color Expanding chromatophores by contracting radial muscles Mathger, Denton, Marshall, Hanlon, J. R. Soc. Interface 6, S149 (2009)

  6. Adaptive Optics • Many stimuli cause deformation. • Deformation affects optics. Wilbur, Jackman, Whitesides, Cheung, Lee, Prentiss Elastomeric optics Chem. Mater. 1996, 8, 1380-1385 Aschwanden, Stemmer Optics letters 31, 2610 (2006)

  7. Dielectric elastomer Reference State Current State Dielectric Elastomer Compliant Electrode Pelrine, Kornbluh, Pei, Joseph High-speed electrically actuated elastomers with strain greater than 100%. Science 287, 836 (2000).

  8. gel = network + solvent solvent gel reversible network Solid-like: long polymers crosslink by strong bonds. Retain shape Liquid like: polymers and solvent aggregate by weak bonds. Enable transport

  9. Swelling packers in oil wells A project with Schlumberger

  10. Osmosis solution solvent membrane

  11. Neutral gel Paul Flory Flory, Rehner, J. Chem. Phys., 11, 521 (1943)

  12. Diaper Sodium polyacrylate

  13. - - Super Absorption - - gel external solution Electroneutrality Donnan equilibrium

  14. Ion exchanger

  15. - - Ion exchange - - gel external solution Donnan equilibrium

  16. Gels regulate flow in plants Missy Holbrook Zwieniecki, Melcher, Holbrook, Hydrogel control of xylem hydraulic resistance in plants Science 291, 1095 (2001)

  17. Self-regulating fluidics David Beebe Responsive to Physiological variables: • pH • Salt • Temperature • light • Many stimuli cause deformation. • Deformation affects flow. Beebe, Moore, Bauer, Yu, Liu, Devadoss, Jo, Nature 404, 588 (2000)

  18. pH-sensitive hydrogel

  19. Dissociation of an acid water R R C O C O OH acid base proton Constant of acidic dissociation

  20. pH-sensitivehydrogel 20

  21. Donnan equilibrium Electroneutrality Conservation of particles Constant of acidic dissociation Ricka, Tanaka,Macromolecules17, 2916 (1984). Marcombe, Cai, Wei, Zhao, Lapusta, Suo, Submitted

  22. Free swelling

  23. Constrained swelling

  24. Blocking force

  25. Inhomogeneous deformation Swelling gels blocked by hard materials Need to formulate boundary-value problems

  26. Electric potential Electrochemical potential Material B Gibbs (1878) electrode • Battery does work on electrons, Fdq • Pump does work on ions, mdM • Helmholtz free energy of system, F Ions, dM equilibrium electrons, dq electroneutrality battery, F pump, m electrode Material A

  27. General principles Deformation gradient Adding volumes Electroneutrality Conservation of particles Independent variables Free energy function Condition of equilibrium

  28. Material model Free energy of the gel Electrochemical potentials of particles in the external solution

  29. Finite element method Condition of equilibrium Legendre transformation Marcombe, Cai, Hong, Zhao, Lapusta, Suo, submitted

  30. Core-shell structure

  31. Channel wall B D A pillar gel FEM results pH = 2 pH = 6

  32. Soft Active Materials (SAM) Soft:large deformation in response to small forces (e.g., rubbers, gels) Active: large deformation in response to diverse stimuli (e.g., electric field, temperature, pH, salt) Stimuli cause deformation. Deformation does something useful. Stimuli pH, E, T, C… SAM deforms Something useful optics, flow…

  33. Summary • Convergence of two worlds (soft biology, hard engineering). • Soft active materials (SAMs) have many uses (soft robots, adaptive optics, self-regulating fluidics, oil recovery, energy harvesting, drug delivery, tissue regeneration, low-cost diagnosis…). • Science of SAMs is interesting and challenging (mechanics, electricity, and chemistry). • The field is wide open (fabrication, computation, imagination).

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