3.052 Nanomechanics of Materials and Biomaterials Thursday 02/15/07 Prof. C. Ortiz, MIT-DMSE I LECTURE 4: FORCE-DISTANCE CURVES

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1 I LECTURE 4: FORCE-DISTANCE CURVES Outline : LAST TIME : ADDITIONAL NANOMECHANICS INSTRUMENTATION COMPONENTS... 2 PIEZOS TUBES : X/Y SCANNING... 3 GENERAL COMPONENTS OF A NANOMECHANICAL DEVICE... 4 HIGH RESOLUTION FORCE SPECTROSCOPY (HRFS) Force-Distance Raw Data... 5 Force-Distance Data Conversion... 6 Chemical Force Microscopy (CFM)... 7 Macromolecular Adhesion : Cartilage Aggrecan... 8 Lateral Force Microscopy (LFM)... 9 Objectives: To understand high resolution force spectroscopy data; i.e. how it is converted from raw data, interpretation of different regions, and different types (i.e. normal, lateral, & chemically specific) Readings: Course Reader Document Multimedia : Watch movie Introduction to AFM by Asylum Research, Inc., and the Force curve animation from NCState. 1

2 LAST TIME : ADDITIONAL NANOMECHANICS INSTRUMENTATION COMPONENTS High resolution displacement detection : Optical Lever (Beam) Deflection Technique High resolution displacement control : "piezoelectric materials" : material which exhibits a change in dimensions in response to an applied voltage due to dipole alignment Lateral Force Microscopy (LFM) V A+C -V B+D mirror A B V A+ B -V C+D C D Normal Force Microscopy 4-quadrant (NFM) position sensitive photodiode laser beam cantilever ε is linearly proportional to electric field strength : ε =d Ε j ij i ΔL ε j = = strain (m/m = unitless) L ij i o d = strain coefficients or sensitivity (m/volt) E = electric field strength (Volt/m) i = direction of applied field, j = direction of strain 1,2,3 = normal axes ; 4, 5, 6 = shear + Poisson's ratio Ld U L = d Δ o 31 3 where d = wall thickness, U = operating voltage probe tip D+ΔD d +Z D ~ -Z electrodes voltage applied -X +Y +X electrodes L o sample L o +ΔL z (1) y (3) x (2) connecting wires 2

3 PIEZO TUBES X/Y SCANNING single axis positioner side view of monolithic tube z- piezo (top) x/y (bottom) scanner (side view) : apply voltages to different sides of tube left x-segment right x-segment U=0 outer electrode inner electrode z y z y- segment front tube contracts radially (small amount) -expands much more in z-direction due to high aspect ratio tube expands radially lowers height tube contracts radially height increases tube tilts - reverse voltage- scan in opposite direction - same for y-axis positioning -Coupling - if you tell the piezo tube to move in x-direction, it will also move a bit in y and z, x is coupled to y and z Figure by MIT OCW. -Another approach : individual "piezo stacks" with flexures in a "nested design" (Introduction to AFM by Asylum Research, Inc. (Quicktime Movie)- Pset 2 3

4 GENERAL COMPONENTS OF A NANOMECHANICS DEVICE I. high-resolution force transducer III. displacement detection system δ sample II. high-resolution displacement control z 4

5 HIGH RESOLUTION FORCE SPECTROSCOPY EXPERIMENT (HRFS): RAW DATA approaching A. tip and sample out of contact B. attractive interaction pulls tip down towards surface C. tip jumps to surface D. tip and sample / z-piezo move in unison (δ=cantilever deflection) δ 1 =0 δ 2 <0 δ 3 <0 δ 4 >0 G. tip and sample out of contact F. tip releases from surface E. attractive force keeps tip in contact with surface D. tip and sample / z-piezo move in unison retracting Photodiode Sensor Output, s (V) 0 0 jump-to-contact adhesion no interaction repulsive regime attractive regime z-piezo Deflection, z (nm) - Measure sensor output (Volts) vs. z-piezo displacement/deflection - See animation on the MIT Server (Force curve animation from NC State). 5

6 HIGH RESOLUTION FORCE SPECTROSCOPY EXPERIMENT: CONVERTED F-D DATA x-axis conversion D Δδ Δz repulsive regime attractive regime k c Force, F (nn) sample 0 piezo D= Δz-Δδ Tip-Sample Separation Distance, D (nm) Contact vs. Noncontact region Zero x-axis position chosen (x=0) : by baseline far away from sample Zero y-axis position chosen (D=0) : as region of apparent infinite slope (artifact of soft spring, stiff sample) Jump to contact region : region of mechanical instability, cantilever moving too fast to collect data, lose all data in this region Adhesion force : maximum force needed to separate two bodies, determined by surfaces force/ intermolecular interactions; sources; hydration capillary forces in air, noncovalent interactions, polymer interactions, etc. 6

7 CHEMICAL FORCE MICROSCOPY (CFM) Vezenov DV, Noy A, Rosznyai LF, Lieber CM J. Am. Chem. Soc. Image removed due to copyright restrictions. See Vezenov DV, Noy A, Rosznyai LF, Lieber CM J. Am. Chem. Soc. Courtesy of The University of Sheffield Polymer Centre. Used with permission. 7

8 MEASURING MACROMOLECULAR ADHESION : CARTILAGE AGGRECAN Cartilage aggrecan is a very unique "bottle-brush" macromolecule that is largely responsible for the mechanical properties and health of cartilage tissue in our joints. (*podcasts later on in the semester on this topic, unpublished data by L. Han) Tip Motion H Image removed due to copyright restrictions. Diagram of cartilage aggrecan. Force (nn) Approach Retract Distance (nm) 8

9 LATERAL FORCE MICROSCOPY (LFM) stick slip shear of aggrecan HIGH NORMAL LOAD (F~20nN) stick slip stick slip Lateral Force (nn) M NaCl μ Ι = 0.10±0.01 μ II = 0.44±0.03 (I) (II) Normal Force (nn) -Measure shear / friction coefficient nanotribology - study of friction and wear. -Linear dependence of lateral force on normal force between OH-SAM and aggrecan changed upon the point of full penetration of aggrecan layer by the nanosized probe tip. -At the same height, larger lateral forces were observed at lower IS, due to stronger shear resistance 9