Dynamic
atomic force microscopy (dAFM) is widely used to characterize
polymer viscoelastic surfaces in the air/vacuum environments; however,
the link between the instrument observables (such as energy dissipation
or phase contrast) and the nanoscale physical properties of the polymer
surfaces (such as local viscoelasticity, relaxation, and adhesion)
remains poorly understood. To shed light on this topic, we present
a computational method that enables the prediction and interpretation
of dAFM observables on samples with arbitrary surface forces and linear
viscoelastic constitutive properties with a first-principles approach.
The approach both accelerates the computational method introduced
by Attard and embeds it within the tapping mode amplitude reduction
formula (or, equivalently, frequency modulation frequency shift/damping
formula) to recover the force history and instrument observables as
a function of the set point amplitude or Z distance.
The method is validated against other reliable computational codes.
The role of surface forces and polymer relaxation times on the phase
lag, energy dissipation, and surface deformation history is clarified.
Experimental data on energy dissipation in tapping mode/amplitude
modulation AFM (TM-AFM/AM-AFM) for different free amplitudes and set
point ratios are presented on a three-polymer blend consisting of
well-dispersed phases of polypropylene, polycarbonate, and elastomer.
An approach to experimental validation of the computational results
is presented and analyzed.
The simultaneous excitation and measurement of two eigenmodes in bimodal atomic force microscopy (AFM) during sub-micron scale surface imaging augments the number of observables at each pixel of the image...
Equal channel angular pressing is one of the attractive methods of severe plastic deformation techniques to fabricate ultrafine grained materials. In this study, eight different equal channel angular pressing configurations including the punch shape, sample preform and die geometry have been analyzed using finite element method to achieve materials with the high effective strain magnitude, better strain distribution uniformity and less required pressing force. The results show that the combination of punch shape and back slant specimen is the most efficient modification as compared to the conventional equal channel angular pressing process. Furthermore, investigations on various die channel and sample slant angles indicate that the configuration of punch shape and back slant sample with the die channel angle of 75°and sample slant angle of 30°is the optimum condition. Finally, experimental work has been performed to validate the results.
Quantitative atomic force microscopy (AFM) on soft polymers remains challenging due to the lack of easy-to-use computational models that accurately capture the physics of the interaction between the tip and sticky, viscoelastic samples. In this work, we enhance Attard’s continuum mechanics-based model, arguably the most rigorous contact model for adhesive viscoelastic samples, via three key enabling strategies. First, the original model’s formalism is rearranged to enable a fast and explicit solution of the model’s ordinary differential equations (ODEs). Second, the deformed surface is reconstructed using a complete set of optimized orthogonal basis functions as opposed to Attard’s original, computationally expensive radial discretization. Third, the model’s governing ODEs are solved using a multi-step numerical method to further stabilize the solution when using for soft and sticky samples. Implementing these enhancements, enhanced Attard’s model (EAM) is more stable, 3+ orders of magnitude faster, and equally accurate when compared to the original model. These facilitate EAM’s inclusion into simulations of various AFM operating modes. We demonstrate EAM based simulations of quasi-static force spectroscopy and amplitude modulation AFM approach curves on soft sticky polymer surfaces. On a typical desktop computer, simulation of an amplitude modulation approach curve with EAM takes less than a minute as compared to ≈15 h by the original Attard’s model. We expect EAM to be of interest to the AFM community because it facilitates the inclusion of rigorous models of tip-sample contact in simulations on polymer samples. EAM is available as part of the VEDA set of simulation tools deployed on nanoHUB.org cyber-infrastructure.
We demonstrate that vertically aligned carbon nanotubes (CNTs) can be precisely machined in a low pressure water vapor ambient using the electron beam of an environmental scanning electron microscope. The electron beam locally damages the irradiated regions of the CNT forest and also dissociates the water vapor molecules into reactive species including hydroxyl radicals. These species then locally oxidize the damaged region of the CNTs. The technique offers material removal capabilities ranging from selected CNTs to hundreds of cubic microns. We study how the material removal rate is influenced by the acceleration voltage, beam current, dwell time, operating pressure, and CNT orientation. Milled cuts with depths between 0–100 microns are generated, corresponding to a material removal rate of up to 20.1 μm3/min. The technique produces little carbon residue and does not disturb the native morphology of the CNT network. Finally, we demonstrate direct machining of pyramidal surfaces and re-entrant cuts to create freestanding geometries.
In this paper, a 3D conjugated heat transfer model for Nano-Encapsulated Phase Change Materials are 2.27, 1.81, 1.56 times higher than the ones with base fluid, respectively. However, with increasing bottom wall temperature, the Nusselt number first increases then decreases. The former is due to higher heat transfer capability of coolant at temperatures over the melting range of PCM particles due to partial melting of nanoparticles in this range. While, the latter phenomena is due 1 Corresponding Author.
2to the lower capability of NEPCM particles and consequently coolant in absorbing heat at coolant temperatures higher than the temperature correspond to fully melted NEPCM. It was observed that NEPCM slurry has a drastic effect on Euler number, and with increasing volume fraction and decreasing inlet velocity, the Euler number increases accordingly.
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