COLLOIDS AND INTERPARTICLE FORCES COLLOIDAL STABILITY: EFFECT ON DISPERSION GECKO ADHESION - "STICKY FEET" (From A. Kellar, et al. American Scientist, 2006, 124)HIERACHICAL STRUCTURE OF GECKO FEET (From A. Kellar, et al. American Scientist, 2006, 124)MOLECULAR ORIGINS OF ADHESION (From Autumn, et al. PNAS 2002 99, 19, 12252)THEORETICAL ASPECTS OF GECKO ADHESION (From Tian, et al. PNAS, 2006, 103, 51, 19320)3.052 Nanomechanics of Materials and Biomaterials Tuesday 03/20/07 Prof. C. Ortiz, MIT-DMSE I LECTURE 12: VAN DER WAALS FORCES AT WORK: GECKO FEET ADHESION Outline : LAST LECTURE : COLLOIDS AND INTERPARTICLE POTENTIALS ...................................................... 2 COLLOIDAL STABILITY : EFFECT ON DISPERSION ............................................................................ 3 GECKO ADHESION : STICKY FEET ........................................................................................................ 4 HIERARCHICAL STRUCTURE OF GECKO FEET ................................................................................... 5 ADHESIVE FORCE OF A SINGLE GECKO FOOT HAIR ......................................................................... 6 MOLECULAR MECHANISMS OF GECKO ADHESION............................................................................ 7 THEORETICAL ASPECTS OF GECKO SPATULA ADHESION ............................................................... 8 Objectives: To understand how weak van der Waals force can lead to enormous, reversible adhesion Readings: K. Autumn, American Scientist, 94 124 2006 and Tian, et al. PNAS 2006 103, 51, 19320 (posted on Stellar). Multimedia : K. Autumn, Discovery Channel movie (posted on stellar, 15 minutes). 13.052 Nanomechanics of Materials and Biomaterials Tuesday 03/20/07 Prof. C. Ortiz, MIT-DMSE COLLOIDS AND INTERPARTICLE FORCES -Definitions; Colloids, colloidal dispersion, colloidal inks; percolation z=2Rplanar surface spherez=Rz=0dzzxxzDD+z'2R-zR ()()222263 1MOLSFC SPHERESFC22212RDR6D~r ,W(D) ~D ,W(D) ~D−− −−−A=A=πρπρπρπρρSPHERE-SFCn-5SPHERE-SFC-4 AW(D) =n-2 n-3 (n-4)(n-5)-AW(D) (VDW, n = 6)=w(r)"Hamaker Constant" :A (interactions between the same material)A (i-192~ 10 JR6DW(D) RD6D∂=− =∂AAASPHERE-SFCSPHERE-SFCnteractions between different materials)-W(D) (VDW, n = 6)=-F(D) (VDW, n = 6) -Analytical formulas for VDW interactions for other geometries Lewis, J. Am. Ceram. Soc. 83 (10) 2341-59. 2000 -Colloidal stability, other long range forces; electrostatic double layer, steric, electrosteric, structural, depletion ++−VDW ELECTROSTATIC STERIC STRUCTURAL DEPLETIONDLVO TheoryW(D)= W(D) W(D) +W(D) W(D) W(D) 23.052 Nanomechanics of Materials and Biomaterials Tuesday 03/20/07 Prof. C. Ortiz, MIT-DMSE COLLOIDAL STABILITY: EFFECT ON DISPERSION "Depletion Interaction" : For entropic reasons the chains avoid the space between two close particles, or between a particle and a planar wall, and create an effective attraction among the colloid particles. Dispersed state : repulsive energy barrier >>kBT Weakly Flocculated : well depth ~ 2-20 kBT Strongly Flocculated : deep primary minimum -e.g. Dispersion of nanotubes Lewis, J. Am. Ceram. Soc. 83 (10) 2341-59. 2000 33.052 Nanomechanics of Materials and Biomaterials Tuesday 03/20/07 Prof. C. Ortiz, MIT-DMSE GECKO ADHESION - "STICKY FEET" (From A. Kellar, et al. American Scientist, 2006, 124) -attach and detach their toes in milliseconds to nearly every material (not Teflon!!!) -run on vertical and inverted, rough and smooth surfaces -gecko toes don't degrade, foul, or attach accidentally to the wrong spot→ like a pressure sensitive adhesive -they are self-cleaning and don't stick to each other -flatten their palm down and then unroll their toes; remove without any measurable force 43.052 Nanomechanics of Materials and Biomaterials Tuesday 03/20/07 Prof. C. Ortiz, MIT-DMSE HIERACHICAL STRUCTURE OF GECKO FEET (From A. Kellar, et al. American Scientist, 2006, 124) Macrostructure→ Mesostructure→ Microstructure→ Fine microstructure→ Nanostructure→ Nanostructure→ 53.052 Nanomechanics of Materials and Biomaterials Tuesday 03/20/07 Prof. C. Ortiz, MIT-DMSE ADHESIVE FORCE OF A SINGLE GECKO FOOT-HAIR (From A. Kellar, et al. Nature, 2000, 681) -Two front feet of a 50g Gecko can hold 2Kg, 20.1 N, 4.5 lbs - Wanted to measure individual seta adhesion to explain macroscopic forces; couldn't get this experiment to work for months, thinking about neural control, chemicals/proteins? started applying the sequence of motions that Gecko's use (mechanical program), perpendicular preload and then small rearward displacement - Measured force of individual seta 200 μN (can feel this) × 6.5 million setae on all feet = 1200 N, 269 lbs, 2 medium-sized humans!! only 3.5% of total possible adhesion needed to sustain the 2 Kg above, and < 0.04% to sustain body weight or 2000 of 6.5 million setae→overengineered, 3900% safety margin. - How do Gecko's ever take their feet off surfaces? Hair detaches automatically when angle between setal shaft and substrate is 30 degrees→ adhesive that is under mechanical control. 63.052 Nanomechanics of Materials and Biomaterials Tuesday 03/20/07 Prof. C. Ortiz, MIT-DMSE MOLECULAR ORIGINS OF ADHESION (From Autumn, et al. PNAS 2002 99, 19, 12252) Theories : × mechanical interlocking; nanoscale velcro hooking→ molecularly smooth Si wafers × suction cups→experiments done in vacuum × secretion of a protein adhesive→lack glandular tissue in toes × capillarity forces due to bridging water meniscus -van der Waals forces (short range) experiment hydrophilic (Si wafer) versus hydrophobic surfaces (GaAs, but is also polarizable) →Geckos stuck to both, hence concluded VDW interactions dominate -More dependent on geometry of structure rather than chemistry 73.052 Nanomechanics of Materials and Biomaterials Tuesday 03/20/07 Prof. C. Ortiz, MIT-DMSE THEORETICAL ASPECTS OF GECKO ADHESION (From Tian, et al. PNAS, 2006, 103, 51, 19320) Ff= Friction force FVDW= van der Waals force F(θ) = peeling force along spatula shaft FL= lateral component