Dalia G. Yablon

Mapping nanoscale elasticity and dissipation using dual frequency contact resonance AFM.

We report on a technique that simultaneously quantifies the contact stiffness and dissipation of an AFM cantilever in contact with a surface, which can ultimately be used for quantitative nanomechanical characterization of surfaces. The method is based on measuring the contact resonance frequency using dual AC resonance tracking (DART), where the amplitude and phase of the cantilever response are monitored at two frequencies on either side of the contact resonance. By modelling the tip-sample contact as a driven damped harmonic oscillator, the four measured quantities (two amplitudes and two phases) allow the four model parameters, namely, drive amplitude, drive phase, resonance frequency and quality factor, to be calculated. These mechanical parameters can in turn be used to make quantitative statements about localized sample properties. We apply the method to study the electromechanical coupling coefficients in ferroelectric materials and the storage and loss moduli in viscoelastic materials.

Viscoelastic Property Mapping with Contact Resonance Force Microscopy

We demonstrate the accurate nanoscale mapping of near-surface loss and storage moduli on a polystyrene-polypropylene blend with contact resonance force microscopy (CR-FM). These viscoelastic properties are extracted from spatially resolved maps of the contact resonance frequency and quality factor of the AFM cantilever. We consider two methods of data acquisition: (i) discrete stepping between mapping points and (ii) continuous scanning. For point mapping and low-speed scanning, the values of the relative loss and storage modulus are in good agreement with the time-temperature superposition of low-frequency dynamic mechanical analysis measurements to the high frequencies probed by CR-FM.

Loss tangent imaging: Theory and simulations of repulsive-mode tapping atomic force microscopy

An expression for loss tangent measurement of a surface in amplitude modulation atomic force microscopy is derived using only the cantilever phase and the normalized cantilever amplitude. This provides a direct measurement of substrate compositional information that only requires tuning of the cantilever resonance to provide quantitative information. Furthermore, the loss tangent expression incorporates both the lost and stored energy into one term that represents a fundamental interpretation of the phase signal in amplitude modulation imaging. Numerical solutions of a cantilever tip interacting with a simple Voigt modeled surface agree with the derived loss tangent to within a few percent.

Solvatochromism of Nile Red in Nonpolar Solvents

The absorbance and fluorescence spectra of Nile Red (NR) were examined in a series of nonpolar solvents comprising linear alkanes and a range of poly alpha olefins (PAO). These solvents span a 1000-fold range in viscosity and possess very similar dielectric constants and refractive index properties. A high-energy double peak with vibronic structure is observed in both fluorescence and absorbance spectra, possibly indicating that a locally excited (LE) state is accessed in these solvents. In addition, a red shift in peak position is observed with increasing refractive index; however, it is unaccompanied by any changes in Stokes shift. This shift is attributed to changes in the high-frequency polarizability of the solvent, which is a function of the refractive index. Finally, an increase in quantum yield with viscosity is also observed.

Measuring the loss tangent of polymer materials with atomic force microscopy based methods

Atomic force microscopy (AFM) quantitatively maps viscoelastic parameters of polymers on the nanoscale by several methods. The loss tangent, the ratio between dissipated and stored energy, was measured on a blend of thermoplastic polymer materials by a dynamic contact method, contact resonance, and by a recently developed loss tangent measurement by amplitude modulation AFM. Contact resonance measurements were performed both with dual AC resonance tracking and band excitation (BE), allowing for a reference-free measurement of the loss tangent. Amplitude modulation AFM was performed where a recent interpretation of the phase signal under certain operating conditions allows for the loss tangent to be calculated. The loss tangent measurements were compared with values expected from time–temperature superposed frequency-dependent dynamical mechanical curves of materials and reveal that the loss tangents determined from the BE contact resonance method provide the most accurate values.

Atomic Force Microscopy and Raman Spectroscopy Investigation of Additive Interactions Responsible for Anti-Wear Film Formation in a Lubricated Contact

Rational formulation of lubricants requires an understanding of additive interactions that impact antiwear film qualities such as thickness, topography, and friction. In an effort to understand the complex additive interactions responsible for formation of anti-wear and friction-reducing films, atomic force microscopy (AFM) in conjunction with Raman microscopy has been used to conduct a nanoscale investigation of the wear tracks formed by a high-frequency reciprocating rig (HFRR) in the presence of various commercial lubricant additives combinations. Of the additives examined, zinc dithiophosphate (ZnDTP)-based additives are found to be solely responsible for the formation of a thick (hundreds of nm) film that exhibits a pitted topography. Addition of a molybdenum-based friction modifier to the lubricant blend reduces the film thickness considerably and reacts to produce MoS 2 on the surface, suggesting an interaction with the zinc dithiophosphate–based additive that prevents antiwear film formation. Formation of MoS 2 , found only in the wear track, is consistent with a dramatic reduction of friction coefficient measured in the HFRR. Subsequent addition of borated dispersants to the lubricant reveals a further reduction in friction coefficient and a modest return of anti-wear film. However, addition of detergents to the formulation increases the friction coefficient and also promotes the formation of an anti-wear film. Nanoindentation measurements on the bulk properties of the anti-wear films determined that all of the anti-wear films had similar modulus and hardness measurements which were lower than that of the parent steel material, but did not correlate with the friction measurements obtained from the HFRR. This indicates that nanoscale measurements on material properties of the film are necessary to elucidate friction properties of the interface, and that these properties cannot be determined from macroscale measurements on the bulk film. Presented at the STLE Annual Meeting, in Toronto, Ontario, Canada. May 17–20, 2004 Review led by Bob Kauffman

An STM investigation of the adsorption of mixtures of fatty acids and substituted acids at the solution‐graphite interface

Scanning tunneling microscopy (STM) is used to investigate the adsorption on graphite of two-solute mixtures from phenyloctane solvent at room temperature. The first mixture is composed of the co-solutes hexadecanoic acid (HA) and 2-Br-hexadecanoic acid (2BrHA), while the second mixture is composed of co-solutes tetracosanoic acid (TA) and octadecanoic acid (OA). While the HA/2BrHA solution exhibits a miscible co-adsorbed surface structure, the TA/OA solution reveals a quasi-segregated surface structure where there is a sharp transition from full surface coverage by one solute to full coverage by the second as the composition of the solution mixture is varied. A one-dimensional Ising model is used to model the adsorption behavior of both systems, and it is shown that both the HA/2BrHA and OA/TA mixtures correspond to two different limits of this model. The HA/2BrHA mixture exhibits similar, weak interactions between adsorbed molecules, corresponding to one limit of the Ising model, which also corresponds to competitive Langmuir adsorption. Another limit of the Ising model is reached when there are significantly different interactions between the different co-adsorbates on the surface, a situation present in the OA/TA mixture resulting in a significantly different co-adsorption behavior.

Cantilever energy effects on bimodal AFM: phase and amplitude contrast of multicomponent samples

Bimodal atomic force microscopy (AFM) is a recently developed technique of dynamic AFM where a higher eigenmode of the cantilever is simultaneously excited along with the fundamental eigenmode. The effects of different operating parameters while imaging an impact copolymer blend of polypropylene (PP) and ethylene-propylene (E-P) rubber in bimodal mode are explored through experiments and numerical simulations. The higher mode amplitude and phase contrasts between the two components of the sample reverse at different points as the free amplitude of the higher eigenmode is increased. Three different regimes are identified experimentally depending on the relative contrast between the PP and the E-P rubber. It is observed that the kinetic energy and free air drive input energy of the two cantilever eigenmodes play a role in determining the regimes of operation. Numerical simulations conducted with appropriate tip-sample interaction forces support the experimental results. An understanding of these regimes and the associated cantilever dynamics will guide a rational approach towards selecting appropriate operating parameters.

Microcantilever sensors applications in the petroleum industry

Sensing applications in the petroleum industry pose unique challenges due to high viscosity and high refractive index of hydrocarbon fluids, which render many fluid-based techniques impractical. Real-time observation of adsorption, desorption, and competitive adsorption processes of long chain functionalized hydrocarbons onto activated cantilevers in static mode are shown. In addition, microcantilevers in dynamic mode are demonstrated to sense corrosion of very small (10ml) samples in fast (10 minutes) timescales.

Frequency Response of Microcantilevers in Viscous Fluids

This paper presents a new, general reconstruction algorithm that enables modeling of the experimental resonance spectrum of a prismatic microcantilever in a viscous fluid. A closed-form solution is obtained for the microcantilever frequency response from the equation of motion with fluid damping and internal friction terms, which allows direct calculation of the fluid damping and internal friction damping constants. In principle, the fluid damping constant has a simple relationship to viscosity thus potentially simplifying the process of obtaining viscosity from experimental data. Finally, the model is compared to experimental data.