Babak Eslami

Selection of higher eigenmode amplitude based on dissipated power and virial contrast in bimodal atomic force microscopy

This paper explores the effect of the amplitude ratio of the higher to the fundamental eigenmode in bimodal atomic force microscopy (AFM) on the phase contrast and the dissipated power contrast of the higher eigenmode. We explore the optimization of the amplitude ratio in order to maximize the type of contrast that is most relevant to the particular study. Specifically, we show that the trends in the contrast range behave differently for different quantities, especially the dissipated power and the phase, with the former being more meaningful than the latter (a similar analysis can be carried out using the virial, for which we also provide a brief example). Our work is based on numerical simulations using two different conservative-dissipative tip-sample models, including the standard linear solid and the combination of a dissipation coefficient with a conservative model, as well as experimental images of thin film Nafion® proton exchange polymers. We focus on the original bimodal AFM method, where the hi...

Trade-offs in sensitivity and sampling depth in bimodal atomic force microscopy and comparison to the trimodal case

Summary This paper presents experiments on Nafion® proton exchange membranes and numerical simulations illustrating the trade-offs between the optimization of compositional contrast and the modulation of tip indentation depth in bimodal atomic force microscopy (AFM). We focus on the original bimodal AFM method, which uses amplitude modulation to acquire the topography through the first cantilever eigenmode, and drives a higher eigenmode in open-loop to perform compositional mapping. This method is attractive due to its relative simplicity, robustness and commercial availability. We show that this technique offers the capability to modulate tip indentation depth, in addition to providing sample topography and material property contrast, although there are important competing effects between the optimization of sensitivity and the control of indentation depth, both of which strongly influence the contrast quality. Furthermore, we demonstrate that the two eigenmodes can be highly coupled in practice, especially when highly repulsive imaging conditions are used. Finally, we also offer a comparison with a previously reported trimodal AFM method, where the above competing effects are minimized.

Imaging of surface nanobubbles by atomic force microscopy in liquids: influence of drive frequency on the characterization of ultrasoft matter.

Imaging of soft matter with atomic force microscopy (AFM) is challenging due to tip‐induced deformation, which convolutes with the measurement. The challenges are generally more serious in liquid environments due to a severe loss of sensitivity of the vibrating microcantilever to external forces, as well as due to the presence of undesirable mechanical resonances when piezoelectric excitation systems are used. Furthermore, the choice of imaging parameters can have a significant impact on the quality of the results, such that the customary practices used for tuning the cantilever are not always appropriate. Here we explore the influence of the chosen drive frequency on the imaging of ultrasoft matter, using surface gas nanobubbles on gold‐coated glass and highly oriented pyrolytic graphite (HOPG) as a test platform. We carry out single‐ and multifrequency AFM experiments using both the traditional amplitude‐peak method and the recently proposed phase‐slope‐peak method for tuning the cantilever, as well as piezoelectric and photothermal excitation, providing an extensive discussion on the factors governing the level of tip‐induced sample deformation and the quality of the phase contrast obtained for each of the methods. The general conclusion is that there is no “one‐size‐fits‐all” approach for tuning the cantilever for low‐impact tapping‐mode AFM, although rational optimization of the imaging process is generally possible, whereby the choice of the drive frequency plays a prominent role. The physical insight and guidelines provided here can be extremely useful for the gentle imaging of a wide range of biological and other soft materials in liquid environments. Microsc. Res. Tech. 80:41–49, 2017.

Evolution of nano-rheological properties of Nafion® thin films during pH modification by strong base treatment: A static and dynamic force spectroscopy study

Addition of a strong base to Nafion® proton exchange membranes is a common practice in industry to increase their overall performance in fuel cells. Here, we investigate the evolution of the nano-rheological properties of Nafion thin films as a function of the casting pH, via characterization with static and dynamic, contact and intermittent-contact atomic force microscopy (AFM) techniques. The addition of KOH causes non-monotonic changes in the viscoelastic properties of the films, which behave as highly dissipative, softer materials near neutral pH values, and as harder, more elastic materials at extreme pH values. We quantify this behavior through calculation of the temporal evolution of the compliance and the glassy compliance under static AFM measurements. We complement these observations with dynamic AFM metrics, including dissipated power and virial (for intermittent-contact-mode measurements), and contact resonance frequency and quality factor (for dynamic contact-mode measurements). We explain th...

Experimental approach for selecting the excitation frequency for maximum compositional contrast in viscous environments for piezo-driven bimodal atomic force microscopy

We propose a method for guiding the selection of the microcantilever excitation frequencies in low-quality-factor (liquid) bimodal amplitude-modulation atomic force microscopy (AFM). Within the proposed method, the compositional contrast frequency is selected based on maximizing the derivative of the phase shift with respect to the drive frequency, observed during a tuning curve. This leads to different frequency choices and significant differences in the observables with respect to the customary practice of selecting the drive frequencies based on the amplitude peaks in the tuning curve. We illustrate the advantages and disadvantages of our approach by imaging an atomically flat calcite surface with single-eigenmode tapping-mode AFM in water, but driving a higher eigenmode instead of the fundamental eigenmode, and by imaging a polytetrafluoroethylene thin film with bimodal AFM, also in water.

Imaging of viscoelastic soft matter with small indentation using higher eigenmodes in single-eigenmode amplitude-modulation atomic force microscopy

In this short paper we explore the use of higher eigenmodes in single-eigenmode amplitude-modulation atomic force microscopy (AFM) for the small-indentation imaging of soft viscoelastic materials. In viscoelastic materials, whose response depends on the deformation rate, the tip–sample forces generated as a result of sample deformation increase as the tip velocity increases. Since the eigenfrequencies in a cantilever increase with eigenmode order, and since higher oscillation frequencies lead to higher tip velocities for a given amplitude (in viscoelastic materials), the sample indentation can in some cases be reduced by using higher eigenmodes of the cantilever. This effect competes with the lower sensitivity of higher eigenmodes, due to their larger force constant, which for elastic materials leads to greater indentation for similar amplitudes, compared with lower eigenmodes. We offer a short theoretical discussion of the key underlying concepts, along with numerical simulations and experiments to illustrate a simple recipe for imaging soft viscoelastic matter with reduced indentation.

Acoustic and atomic force microscopy characterization of microbubbles with varying shell chemistry

Applications of microbubbles (MBs) in diagnostic and therapeutic interventions critically depend on their stability and scattering properties. The shell chemistry of MBs defines these properties. We investigated the effects of shell chemistry on the size, abundance, acoustic response, and mechanical properties of MBs by varying the poly(ethylene glycol) (PEG) molar ratio (0 to 100%) in a two-lipid (DPPC and DPPE-PEG2000) component shell formulation. Increasing PEG concentration from 0% to 10% resulted in an increase in the number of MBs by at least 10-fold, with adverse effects upon further increases. Microbubbles made with 5–10% PEG generated the strongest fundamental as well as nonlinear (subharmonic and second harmonic) components at the excitation frequency of 2.25 MHz. We used interfacial rheological models to determine the mechanical properties of MB shells as functions of PEG concentration using experimentally measured attenuation values. We also employed atomic force microscopy (AFM) to perform th...

Smart Fasteners and Their Application in Flanged Joints

Fasteners are widely used in many industrial applications. Their function in many cases is to provide a leak-proof joint at the flange interface. To accomplish this function, fasteners must be clamped with appropriate force. In practice, it is difficult to measure such forces, intermittently or continuously. While measurement using load cells or strain gages is an available option, it tends to be expensive or infeasible due to the constraints imposed by the application. When tightening the fasteners initially or during maintenance, a less accurate method of specifying the bolt-tightening torque for achieving the necessary bolt force is widely followed in industry. These torque values are calculated using published design correlations [1]. Many factors affect such calculations: friction between the threads and collar and flange, age of the fasteners, assumptions about rigidity of the clamped components, being a few among those. Since specific values applicable for individual application are not always be known, fasteners are often over-tightened, resulting in increased stresses in the assembly elements or under tightened, resulting in leakages at the flange interfaces. In the current paper, we introduce the concept of smart fasteners that can visually indicate the tension they are subjected to, and validate it for a widely used industrial fastener size. Results from numerical and experimental studies conducted are presented for UNC 1/2 -13 × 4 1/4″ smart fasteners. Lastly, relationship between bolt-tightening torque and color intensity of the smart fasteners is provided.Copyright