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1.  A nanometric cushion for enhancing scratch and wear resistance of hard films 
Scratch resistance and friction are core properties which define the tribological characteristics of materials. Attempts to optimize these quantities at solid surfaces are the subject of intense technological interest. The capability to modulate these surface properties while preserving both the bulk properties of the materials and a well-defined, constant chemical composition of the surface is particularly attractive. We report herein the use of a soft, flexible underlayer to control the scratch resistance of oxide surfaces. Titania films of several nm thickness are coated onto substrates of silicon, kapton, polycarbonate, and polydimethylsiloxane (PDMS). The scratch resistance measured by scanning force microscopy is found to be substrate dependent, diminishing in the order PDMS, kapton/polycarbonate, Si/SiO2. Furthermore, when PDMS is applied as an intermediate layer between a harder substrate and titania, marked improvement in the scratch resistance is achieved. This is shown by quantitative wear tests for silicon or kapton, by coating these substrates with PDMS which is subsequently capped by a titania layer, resulting in enhanced scratch/wear resistance. The physical basis of this effect is explored by means of Finite Element Analysis, and we suggest a model for friction reduction based on the "cushioning effect” of a soft intermediate layer.
PMCID: PMC4143114  PMID: 25161836
finite element analysis; hard coating; scanning probe microscopy; scratch; thin films; tribology
2.  Self-assembly at solid surfaces 
PMCID: PMC3257508  PMID: 22259766
3.  Dynamic nanoindentation by instrumented nanoindentation and force microscopy: a comparative review 
Viscoelasticity is a complex yet important phenomenon that drives material response at different scales of time and space. Burgeoning interest in nanoscale dynamic material mechanics has driven, and been driven by two key techniques: instrumented nanoindentation and atomic force microscopy. This review provides an overview of fundamental principles in nanoindentation, and compares and contrasts these two techniques as they are used for characterization of viscoelastic processes at the nanoscale.
PMCID: PMC3869246  PMID: 24367751
atomic force microscopy; loss modulus; nanoindentation; storage modulus; viscoelasticity
4.  Direct monitoring of opto-mechanical switching of self-assembled monolayer films containing the azobenzene group 
The potential for manipulation and control inherent in molecule-based motors holds great scientific and technological promise. Molecules containing the azobenzene group have been heavily studied in this context. While the effects of the cis–trans isomerization of the azo group in such molecules have been examined macroscopically by a number of techniques, modulations of the elastic modulus upon isomerization in self-assembled films were not yet measured directly. Here, we examine the mechanical response upon optical switching of bis[(1,1'-biphenyl)-4-yl]diazene organized in a self-assembled film on Au islands, using atomic force microscopy. Analysis of higher harmonics by means of a torsional harmonic cantilever allowed real-time extraction of mechanical data. Quantitative analysis of elastic modulus maps obtained simultaneously with topographic images show that the modulus of the cis-form is approximately twice that of the trans-isomer. Quantum mechanical and molecular dynamics studies show good agreement with this experimental result, and indicate that the stiffer response in the cis-form comprises contributions both from the individual molecular bonds and from intermolecular interactions in the film. These results demonstrate the power and insights gained from cutting-edge AFM technologies, and advanced computational methods.
PMCID: PMC3257510  PMID: 22259768
AFM; azobenzene; elastic modulus; molecular dynamics; nanomechanics; photoswitch; quantum mechanics computation; self-assembled monolayer
5.  Direct Visualization of Protease Action on Collagen Triple Helical Structure 
PLoS ONE  2010;5(6):e11043.
Enzymatic processing of extracellular matrix (ECM) macromolecules by matrix metalloproteases (MMPs) is crucial in mediating physiological and pathological cell processes. However, the molecular mechanisms leading to effective physiological enzyme-ECM interactions remain elusive. Only scant information is available on the mode by which matrix proteases degrade ECM substrates. An example is the enzymatic degradation of triple helical collagen II fragments, generated by the collagenase MMP-8 cleavage, during the course of acute inflammatory conditions by gelatinase B/MMP-9. As is the case for many other matrix proteases, it is not clear how MMP-9 recognizes, binds and digests collagen in this important physiological process. We used single molecule imaging to directly visualize this protease during its interaction with collagen fragments. We show that the initial binding is mediated by the diffusion of the protease along the ordered helix on the collagen ¾ fragment, with preferential binding of the collagen tail. As the reaction progressed and prior to collagen degradation, gelatin-like morphologies resulting from the denaturation of the triple helical collagen were observed. Remarkably, this activity was independent of enzyme proteolysis and was accompanied by significant conformational changes of the working protease. Here we provide the first direct visualization of highly complex mechanisms of macromolecular interactions governing the enzymatic processing of ECM substrates by physiological protease.
PMCID: PMC2886829  PMID: 20585385

Results 1-5 (5)