Dynamic Atomic Force Microscopy Research Articles

Rheological and Microstructural Investigation of the Effects of Rejuvenators on Reclaimed Asphalt Pavement Bitumen by DSR and AFM

Utilization of RAP in asphalt pavements may lead to severe quality problems, mostly due to asphalt aging phenomenon. Within the scope of this study, the use of rejuvenators to improve the properties of RAP is investigated. Two different types of waste oils together with a commercial rejuvenator are used. The rejuvenated bitumen samples are evaluated from both rheological and microstructural aspects. In addition, mixture testing also employed to investigate the effectiveness of used rejuvenators. Experimental studies on bitumen samples included conventional and Dynamic Shear Rheometer (DSR) testing and Atomic Force Microscopy (AFM) imagings. The results unveiled that, the utilization of rejuvenators can significantly improve the quality of recycled asphalt and threatened the aged bitumen quality. Missing bee-structures of aged bitumen samples evolve in a different type of surface topography while the mixtures performance evolves in terms of workability. The rejuvenation process provides viscous behavior to aged bitumen. The Rejuvenation Index (RI) which is a dimensionless value, simply evolves this fact. While the RI of the non-rejuvenated aged bitumen is taken as 1 (one) as per RI definition, the neat bitumen RI is calculated as 0.79, the RI of waste oils are calculated as 0.74 and 0.79 while the commercial sample RI is calculated as 0.60. Among the rejuvenators, waste oils showed similar results with almost same volume of peri- and para-phase since the commercial rejuvenator behaved more similar to neat bitumen. The rejuvenated samples workability performance improves using rejuvenating additives. Binder and mixture testing demonstrated high consistency and conformity with each other in terms of performance and aging characteristics. The results proved that, waste oils can be used as rejuvenating agents within RAP. Rejuvenators can significantly restore the properties of aged RAP bitumen and increase the amount of RAP in bituminous mixtures.

Mode Coupling in Dynamic Atomic Force Microscopy

Increasing the signal-to-noise ratio in dynamic atomic force microscopy plays a key role in nanomechanical mapping of materials with atomic resolution. In this work, we develop an experimental procedure for increasing the sensitivity of higher harmonics of an atomic-force-microscope cantilever without modifying the cantilever geometry but instead by utilizing dynamical mode coupling between its flexural modes of vibration. We perform experiments on different cantilevers and samples and observe that via nonlinear resonance frequency tuning we can obtain a frequency range where strong modal interactions lead to 7-fold and 16-fold increases in the sensitivity of the 6th and 17th harmonics while reducing sample indentation. We derive a numerical model that captures the observed physics and confirms that nonlinear mode coupling is the reason for the increase of the amplitude of higher harmonics during tip-sample interactions.

Biocarbon reinforced polypropylene composite: An investigation of mechanical and filler behavior through advanced dynamic atomic force microscopy and X-ray micro CT

Polymer composites were manufactured using biocarbon particles as a reinforcing filler to improve the mechanical and thermal properties. However, a detailed examination of dispersion and agglomeration of filler is essential to correlate the filler matrix and the filler and filler interactions with the mechanical properties of the product. We investigated the variations of mechanical, agglomeration behaviour of fillers, and thermal properties of polypropylene coconut shell biocarbon composites. PP CSB composites were prepared by melt mixing process varying the CSB content using a Brabender mixer. The nanomechanical mapping of the composites studied using Atomic Force Microscopy revealed an increase in Youngs modulus from 1.6 to 2.9 GPa when CSB loading increased from 0 to 20 weight percentage. The dispersion and agglomeration of CSB filler in the PP matrix were investigated using 3D reconstructed images with the help of X-ray micro CT, and a dedicated 3D reconstruction software. The thermal stability of the PP CSB composites also improved with an increase in CSB content in PP.

Mapping cellular nanoscale viscoelasticity and relaxation times relevant to growth of living Arabidopsis thaliana plants using multifrequency AFM

The shapes of living organisms are formed and maintained by precise control in time and space of growth, which is achieved by dynamically fine-tuning the mechanical (viscous and elastic) properties of their hierarchically built structures from the nanometer up. Most organisms on Earth including plants grow by yield (under pressure) of cell walls (bio-polymeric matrices equivalent to extracellular matrix in animal tissues) whose underlying nanoscale viscoelastic properties remain unknown. Multifrequency atomic force microscopy (AFM) techniques exist that are able to map properties to a small subgroup of linear viscoelastic materials (those obeying the Kelvin-Voigt model), but are not applicable to growing materials, and hence are of limited interest to most biological situations. Here, we extend existing dynamic AFM methods to image linear viscoelastic behaviour in general, and relaxation times of cells of multicellular organisms in vivo with nanoscale resolution (~80 nm pixel size in this study), featuring a simple method to test the validity of the mechanical model used to interpret the data. We use this technique to image cells at the surface of living Arabidopsis thaliana hypocotyls to obtain topographical maps of storage E′ = 120–200 MPa and loss E″ = 46–111 MPa moduli as well as relaxation times τ = 2.2–2.7 µs of their cell walls. Our results demonstrate that (taken together with previous studies) cell walls, despite their complex molecular composition, display a striking continuity of simple, linear, viscoelastic behaviour across scales–following almost perfectly the standard linear solid model–with characteristic nanometer scale patterns of relaxation times, elasticity and viscosity, whose values correlate linearly with the speed of macroscopic growth. We show that the time-scales probed by dynamic AFM experiments (microseconds) are key to understand macroscopic scale dynamics (e.g. growth) as predicted by physics of polymer dynamics.

Micro/Nanoscale Study on the Effect of Aging on the Performance of Crumb Rubber Modified Asphalt

In order to explore the variation of crumb rubber modified asphalt (CR) before and after aging at the micro- and nanoscales, the molecular dynamics simulation and atomic force microscopy (AFM) asphalt microstructure scanning were carried out on CR before and after aging. The molecular dynamics energy, radial distribution function (RDF), surface microstructure image, roughness, and other test results of the CR before and after aging were compared and analyzed. The results show that the molecular dynamic energy of the CR after aging increases; the asphaltene-asphaltene RDF is decreased after aging; and the AFM surface microstructure images of asphalt before and after aging do not change significantly, but quantitative analysis by roughness theory shows that aging makes the microscopic surface of the CR more uniform and gentle. By analyzing the changes of asphalt properties before and after aging at two scales, it can be found that there is a certain relationship between the properties obtained at different scales. The reasons and mechanisms for the influence of microstructure on aging are obtained by analyzing this relationship.

Soft sample deformation, damage and induced electromechanical property changes in contact- and tapping-mode atomic force microscopy

Probe-induced soft sample damage in atomic force microscopy (AFM), as well as the resulting alteration of local mechanical and electrical properties of the material are explored, specifically comparing contact-mode and intermittent-contact-mode imaging methods. In our experiments, performed on conductive polymer films, induced changes are present in contact-mode imaging while they are negligible or absent in tapping-mode imaging. To understand this result, a viscoelastic parameter extraction is performed, which suggests that permanent sample deformation can readily occur for tip-sample interactions with a duration on the timescale of contact-mode interactions. Using the extracted viscoelastic parameters, a dynamic AFM simulation is conducted, which suggests that the material responds more elastically with reduced or absent sample damage in tapping-mode AFM, due to the higher rate of mechanical deformation and shorter timescales.

On the frequency dependence of viscoelastic material characterization with intermittent-contact dynamic atomic force microscopy: avoiding mischaracterization across large frequency ranges.

Atomic force microscopy (AFM) is a widely use technique to acquire topographical, mechanical, or electromagnetic properties of surfaces, as well as to induce surface modifications at the micrometer and nanometer scale. Viscoelastic materials, examples of which include many polymers and biological materials, are an important class of systems, the mechanical response of which depends on the rate of application of the stresses imparted by the AFM tip. The mechanical response of these materials thus depends strongly on the frequency at which the characterization is performed, so much so that important aspects of behavior may be missed if one chooses an arbitrary characterization frequency regardless of the materials properties. In this paper we present a linear viscoelastic analysis of intermittent-contact, nearly resonant dynamic AFM characterization of such materials, considering the possibility of multiple characteristic times. We describe some of the intricacies observed in their mechanical response and alert the reader about situations where mischaracterization may occur as a result of probing the material at frequency ranges or with probes that preclude observation of its viscoelastic behavior. While we do not offer a solution to the formidable problem of inverting the frequency-dependent viscoelastic behavior of a material from dynamic AFM observables, we suggest that a partial solution is offered by recently developed quasi-static force–distance characterization techniques, which incorporate viscoelastic models with multiple characteristic times and can help inform dynamic AFM characterization.

Multiparametric analytical quantification of materials at nanoscale in tapping force microscopy

Atomic force microscopy (AFM) is a powerful technique for accurate, reliable and non-destructive imaging and characterization of materials at the nanoscale. Among the numerous AFM methods, amplitude modulation or tapping mode AFM (AM-AFM) is an established method for imaging and characterization for most commercial AFM systems. Despite its high spatial resolution and sensitivity, quantitative characterization by AM-AFM lag behind other advanced AFM methods as far as quantification of materials properties is concerned. In this paper a fully analytical multiparametric approach for AM-AFM is proposed which simultaneously quantifies the Hamaker constant and viscoelastic properties of materials. The main advantage of the proposed method lies in the inclusion of adhesion to calculate viscoelasticity, which makes it superior to the current equations used in the AFM community. The accuracy of the proposed method is validated by several simulations and experiments and comparison with nanoindentation results, which strongly support its candidacy as a method of choice for material properties quantification by dynamic AFM.

Error estimation and enhanced stiffness sensitivity in contact resonance force microscopy with a multiple arbitrary frequency lock-in amplifier (MAFLIA)

In contact resonance force microscopy and related dynamic atomic force microscopy methods, an accurate description of the real-time cantilever dynamics is essential to the mapping of local material properties, such as viscoelasticity, piezo response, and chemical composition. Stiffness and damping variations of the tip-sample contact result in variations in the cantilever’s resonance frequency and quality factor as it scans a sample of interest. When measuring characteristics of the resonance, generally, there is a tradeoff between full spectral coverage, best obtained by sweeping the amplitude versus frequency response in the time or frequency domain, and high-speed information, obtained by observing the cantilever response at one or two discrete frequencies, that may be required to track a resonance frequency that changes spatially. Here, we introduce a new option for performing contact resonance force microscopy with a low-cost multifrequency lock-in amplifier system with up to eight simultaneous independent excitation and detection frequencies. We demonstrate how the multifrequency approach can measure contact resonance frequency, quality factor, amplitude, and phase during imaging, with high precision and error estimation, without the need for frequency-tracking feedback. We show, using a wood composite sample, that this multifrequency approach can determine resonance frequency and quality factor, and associated uncertainty. This ability to estimate uncertainty of resonance parameters is not possible with 1 and 2 frequency methods. We further utilize the multifrequency lock-in to develop a novel means of increasing the stiffness range for highly sensitive nanomechanical sensing by dividing the eight lock-in frequencies to monitor two or four simultaneous eigenmodes, each of which is optimized for sensitivity in a particular stiffness regime. Overall, we show how multifrequency lock-in amplifiers with observation frequency chosen to coincide with an expected eigenmode’s contact resonance can benefit the characterization of strongly heterogeneous samples, while maintaining fast measurement speed.

AFM Microcantilever With a Collocated AlN Sensor-Actuator Pair: Enabling Efficient Q-Control for Dynamic Imaging

This manuscript presents a novel microcantilever with an embedded piezoelectric sensor-actuator pair for dynamic atomic force microscopy (AFM). The transducer pair is constructed from a two-layered AlN stack. Stacking the piezoelectric transducers in this manner leads to a minimal feedthrough from actuation to sense electrode, granting a high dynamic range frequency response for dynamic mode AFM. The cantilever’s design allows for dual mode operation at 1st and 2nd resonance modes. High resolution tapping mode imaging results are reported, while the cantilever is operated at these modes. A feedback control loop is used to modify quality factor of the 1st mode of the cantilever, using a positive position feedback (PPF) controller. A faster response time is achieved by reducing the ${Q}$ -factor, enabling the cantilever to track the topography at a higher scan rate. Results of bimodal AFM imaging are reported, using amplitude changes of the 1st mode for surface topography, while material properties are encoded in phase changes of the 2nd mode. [2020-0096]

Isolation and characterization of cellulose nanocrystals from Cucumis sativus peels

Cucumber (Cucumis sativus) peels waste being a potential cellulosic sources, were used for extracting cellulose nanocrystals (CNCs), and characterized in the present study. Firstly, the cucumber peels were purified chemically through acid, alkali, and bleaching treatments for cellulose isolation. Later obtained cellulose was acid (60 wt% H2SO₄) hydrolyzed at 45 ℃ for 45 min to obtain CNC45 suspension, and again 10 min sonicated for CNC45-S10 suspension. The effect of sonication on the particle size of CNC45, and CNC45-S10 were investigated with Dynamic light Scattering and Atomic force microscopy. The microstructural changes, thermal, and crystalline properties of resulting fibers and CNC45 were analysed after each treatments through scanning electron microscopy, thermo-gravimetric analyser, and X-ray diffraction respectively. The acid-hydrolysed CNC45 from cucumber peels showed rod-like shape with high crystallinity (74.1 %), excellent thermal stability (>200 °C), and negative zeta potential values (<−30 mV), and CNC45 can be used as potential nanofillers.

Chemical and multi-scale material properties of recycled and blended asphalt binders

Motivated with sustainability and economic factors, there is a rapid increase in the use of recycled asphalt pavement (RAP) material in the construction of new asphalt pavements. This necessitates the development of better understanding of the interaction between recycled and virgin binders, as well as studying the influence of this interaction on the properties of the blended binder. This paper provides in-depth evaluation of these properties at various scales using chemical, mechanical and rheological tests.This study involved the use of binders with different proportions of RAP (0, 15, 25, 35 and 100%). The chemical characterization of these binders was carried out using Fourier Transform Infrared Spectroscopy (FTIR), and the results showed that higher peak levels of CO and SO bonds were observed with the increase in RAP binder content. Atomic Force Microscopy (AFM) imaging of these binders showed that the surface roughness increased with an increase in RAP content especially when the RAP proportion exceeded 25%. The multiple stress creep recovery (MSCR) test was used to measure the rutting resistance, and the linear amplitude sweep test (LAS) was used to evaluate the fatigue resistance. There was slight improvement in rutting resistance with an increase in RAP content; however, the fatigue resistance degraded with the addition of RAP. The moduli/stiffness of virgin, RAP and blended binders was determined using Dynamic Shear Rheometer (DSR), Nanoindentation (NI), and AFM. The moduli obtained from these three techniques increased exponentially and at similar rates up to 25% of RAP. However, the dynamic modulus from DSR increased at a higher rate than the nano-scale moduli at higher RAP contents. This finding suggests that the nano-scale interactions were not influenced by the increase in RAP content as the DSR measurements of dynamic modulus and fatigue life values imply. The nano-scale understanding of blended binder offers opportunities for better engineering and accommodation of RAP in asphalt mixtures.

Inorganic nanoparticle-blended polymer nanofilm and its wettability improvement: Film grading and dewetting cause analysis

Dewetting of polymer thin film coated on a solid substrate at higher temperature than the glass transition is one of the major problems restricting its functionality. While nanoparticle inclusion is considered as a simple but effective solution, influence of directly measurable film characteristics on polymer chain dynamics has been a crucial topic. Here, using dynamic atomic force microscopy (DAFM)-based nano mechanical analysis, we present a systematic investigation of inorganic nanoparticle-filled polymeric nanofilms designed for heat-resistant coating based on pinning effect of the interfacial nanoparticle layer by varying concentrations of the zinc oxide (ZnO) nanofiller and the polystyrene (PS) polymer. Performing morphological analysis from dewetting hole occurrence and surface roughness together with size and distribution of the protruding surface aggregates, not only dewetting and non-dewetting films are distinguished, but also the optimal film can be identified. Dewetting suppression mechanism has been proposed on the basis of height profiles of individual dewetting holes, relative interfacing energies, and relation between local stiffness and polymeric chain crowding, suggesting effect from polymeric canopies incorporated with the nanoparticles. This work sheds more light on improving the thermal-proof coating and provides a general protocol for film grading and dewetting inspection at the nanoscale.

High Dynamic Range AFM Cantilever With a Collocated Piezoelectric Actuator-Sensor Pair

We present a novel approach to fundamentally eliminate electrical feedthrough (cross-talk) signal in self-sensing piezoelectric microcantilevers, used in dynamic mode atomic force microscopy (AFM). The probe is an Si microcantilever on which a two-layer piezoelectric stack transducer is microfabricated. The top transducer in the stack functions as an actuator, while the bottom transducer is used as a deflection sensor. This arrangement leads to a collocated actuator-sensor pair that co-exist on the same surface area. We report experimental results that establish the sensor’s ability to capture the full dynamic range of the cantilever with minimal feedthrough from the actuator. [2019-0143]

Atomic force microscopy phase imaging of epitaxial graphene films

Dynamic mode atomic force microscopy phase imaging is known to produce distinct contrast between graphene areas of different atomic thickness. But the intrinsic complexity of the processes controlling the tip motion and the phase angle shift excludes its use as an independent technique for a quantitative type of analysis. By investigating the relationship between the phase shift, the tip-surface interaction, and the thickness of the epitaxial graphene areas grown on silicon carbide, we shed light on the origin of such phase contrast, and on the complex energy dissipation processes underlying phase imaging. In particular, we study the behavior of phase shift and energy dissipation when imaging the interfacial buffer layer, single-layer, and bilayer graphene regions as a function of the tip-surface separation and the interaction forces. Finally, we compare these results with those obtained on differently-grown quasi free standing single- and bilayer graphene samples.

Electroviscous effects on the squeezing flow of thin electrolyte solution films

We present a detailed analysis of the electroviscous effect in the squeeze out of thin electrolyte films confined between two charged surfaces. The two charged surfaces consist of a curved surface and a flat surface, which closely simulate the tip–substrate configuration in the force measurement of electrolyte solutions with dynamic atomic force microscopy. In the lubrication limit, we find the analytical solution of the electroviscous-effect-modified squeezing flow field in the thin electrolyte film confined between the tip and substrate by solving the Nernst–Planck–Poisson/Navier–Stokes equation under the justified condition of pseudo-steadiness. We also derive the solution of the tip–substrate interaction, which comprises of a conservative electric double layer (EDL) force and an electroviscous-effect-enhanced dissipative hydrodynamic force. The current work focuses on the dissipative hydrodynamic force since the conservative EDL force has been well described by the well-known Derjaguin–Landau–Verwey–Overbeek theory. We introduce a power-law index ( $n$ ) for the tip surface, which enables an unprecedented quantitative characterization of the tip profile effect on the electroviscous effect. We observe a seemingly counter-intuitive effect that, for a given tip–substrate separation, the electroviscous effect is the strongest at one particular value of the zeta ( $\unicode[STIX]{x1D701}$ ) potential and diminishes as the $\unicode[STIX]{x1D701}$ potential departs from this value. We reveal the counterion conductivity of the EDL to be the governing factor for the electroviscous effect under a given $\unicode[STIX]{x1D701}$ potential. In addition to enhancing the dissipative hydrodynamic interaction force, the electroviscous effect modifies the velocity profiles in the thin electrolyte solution films such that they are much sharper.

Simulation of Subnanometer Contrast in Dynamic Atomic Force Microscopy of Hydrophilic Alkanethiol Self-Assembled Monolayers in Water

Atomic resolution imaging of surfaces in liquid environments using atomic force microscopy (AFM) is challenging in terms of both reproducibility and measurement interpretation. To understand the origins of these challenges, we used molecular dynamics simulations of AFM on hydrophilic self-assembled monolayers (SAMs) in water. The force on the model AFM tip was calculated as a function of lateral and vertical position relative to the SAM surface. The contributions of the water and SAMs to the overall force were analyzed, and the former was correlated to the water density distribution. Then, dynamic AFM was modeled by oscillating the tip at a driving amplitude. It was found that the contrast between amplitudes at different lateral positions on the surface was dependent on the vertical position of the tip. Lastly, amplitude maps were produced for two vertical positions at constant height, and the ability to capture atomic resolution was related to the force on the tip. These results offer an explanation for the observed instability in atomic scale imaging using AFM and more generally provide insight into the contrast mechanisms of surface images obtained in liquid environments.

Atomic Force Microscopy Imaging of Crystalline Sucrose in Alcohols.

Imaging nanometer- or molecule-scale topography has been achieved by dynamic atomic force microscopy (AFM) when a solid object of interest is damaged by vacuum exposure or electron irradiation. Imaging in a liquid offers a means to remove contaminations from the surface scanned using the microscope tip when the object is soluble to the surrounding liquid, typically water. In the present study, we attempted to take topographic images of crystalline sucrose. A problem arose due to the high solubility of this compound to water. Cantilever oscillation could not be excited in the saturated, viscous aqueous solution. By using n-hexanol instead of water, the solubility in the solvent and thus viscosity of the solution were reduced sufficiently to excite cantilever oscillation. Single-height steps and sucrose molecules were recognized in the images and thereby recorded on the (001)-oriented facets of sucrose crystals. Furthermore, two-dimensional distribution of liquid-induced force pushing or pulling the tip was mapped on planes perpendicular to the hexanol–sucrose interface. Observed uneven force distributions indicated liquid hexanol structured on the corrugated surface of sucrose. The viscosity tuning demonstrated here, which is not limited to hexanol instead of water, extends the range of liquid–solid interfaces to be probed by dynamic AFM.

Effect of polyether/polyester polyol ratio on properties of waterborne two-component polyurethane coatings

In order to obtain waterborne two-component polyurethane (WB 2K-PUR) with good mechanical properties as well as excellent corrosion resistance, a series of hydroxyl-based polyurethane (HO-WPU) were synthesized using isophorone diisocyanate (IPDI), dimethylolpropionic acid (DMPA), and different proportions of polyether polyol (PTMEG) and polyester polyol (P1012) in this paper. Then WB 2K-PUR coatings were prepared with water-dispersible isocyanates (WDPI) as curing agent. The influence of the proportion of polyester polyols and polyether polyols in soft segments on coatings properties was studied. The characteristics of the coatings were analyzed by Fourier transform infrared spectroscopy (ATR-IR), contact angle, differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and atomic force microscopy (AFM). The results showed that the mechanical properties and corrosion resistance of polyester and polyether polyols with appropriate proportion are improved significantly. The mechanism tended to be that with the addition of polyester, more hydrogen bonds were formed between the molecules, which increased the compactness of the coating as a physical crosslinking point. However, the ester groups have a strong affinity to water molecules, so the hydrophilicity of the coating was increased by excessive polyester polyols, and the shielding effect on the corrosion medium was reduced. With the inclusion of 40 % (g/g) P1012 in polyols, the coating has satisfied comprehensive properties.

Effect of ultraviolet (UV) aging on rheology properties and microstructure of polyurethane (PU) modified asphalt

In order to investigate the effect of polyurethane (PU) modifier on the anti-UV aging ability of asphalt, in this study, the base asphalt and PU modified asphalts were subjected to short-term UV and long-term UV aging. After that, three tests, including physical property test (penetration and ductility), dynamic shear rheology test (DSR) and atomic force microscopy (AFM) test were used to evaluate the effect of UV aging on the performance of PU modified asphalt. The test results show that the change rate of softening point and ductility of PU modified asphalt is lower than that of base asphalt after aging. The PU modifier reduces the effect of UV aging on the complex modulus G* and phase angle δ of the asphalt. In addition, AFM was used to observe the morphology of asphalt before and after UV aging. The results showed that the addition of PU modifier can hinder the movement of small molecules in the asphalt, leading to the microstructure of the asphalt more stable. Through statistical analysis of the height and modulus of asphalt surface morphology, it is found that the effect of UV aging on the surface height and modulus of PU modified asphalt is less than that of the base asphalt. This indicates that the PU modifier could significant improve the UV aging properties of asphalt binders.