Dynamic Force Microscopy Research Articles

Observation and Proposed Measurements of Three-dimensional Tortuous Capillary Pores with Depth for Hollow Fiber Hemoconcentrator Membrane Using Dynamic Force Microscopy

Observation and Proposed Measurements of Three-dimensional Tortuous Capillary Pores with Depth for Hollow Fiber Hemoconcentrator Membrane Using Dynamic Force Microscopy

Probing Dynamic Mechanical Analysis and Atomic Force Microscopy of Polypropylene/Kaolin Nanocomposite

The Dynamic mechanical analysis (DMA) and Atomic force microscopy (AFM) studies were conducted and evaluated on polypropylene/kaolin (P/K) nanocomposite treated with maleic anhydride (MA) and dicumyl peroxide (DCP) as additives in an in- situ process. Two-roll mill was used in compounding of the nanocomposites while moulding were done by injection moulding machine. Investigation in to the effect of K and MA/DCP on the nanocomposites (NCs) indicates that interfacial interactions between PP and K as filler was eminent. DMA analysis reveals an increase in the storage modulus which was at maximum significantly in P/K NC with 3 wt% and decrease in damping factor tan δ also at P/K NC of 3 wt%. The AFM study indicates that there was uniform and smooth surface roughness among the NCs. Thus, addition of MA/DCP on to P/K NC improves the reinforcing influence on the nanocomposites for better improvement.

Dynamic AFM on Viscoelastic Polymer Samples with Surface Forces

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 ...

Dynamical characterization of micro cantilevers by different excitation methods in dynamic atomic force microscopy.

An atomic force microscopy experimental setup was modified to analyze the differences between the piezoelectric excitation and the photothermal excitation (PTE) for three types of cantilevers, including two aluminum coated cantilevers and one uncoated single-crystalline silicon cantilever. The results show the PTE is a direct and localized excitation method to yield smooth and clean frequency spectra representing only the dynamics of the cantilever without the coupling with mechanical components. The cantilever can be easily excited for a high and controllable amplitude by the PTE method as compared to the piezoelectric excitation. The 1st and 2nd order flexural vibration amplitudes of the coated cantilever are easily and efficiently excited by the PTE method, mainly due to the bimetallic effect and a high photothermal efficiency. The energy conversion and absorption efficiency comparison has been analyzed for different cantilevers by the PTE method. The spurious effects can be avoided by the PTE method which clearly reflects dynamic characteristics of the cantilever, and the scanning image quality can be improved.

Kinetic control of molecular assembly on surfaces

It is usually assumed that molecules deposited on surfaces assume the most thermodynamically stable structure. Here we show, by considering a model system of dihydroxybenzoic acid molecules on the (10.4) surface of calcite, that metastable molecular architectures may also be accessed by choosing a suitable initial state of the molecules which defines the observed transformation path. Moreover, we demonstrate that the latter is entirely controlled by kinetics rather than thermodynamics. We argue that molecules are deposited as dimers that undergo, upon increase of temperature, a series of structural transitions from clusters to ordered striped and then dense networks, and finally to a disordered structure. Combining high-resolution dynamic atomic force microscopy experiments and density-functional theory calculations, we provide a comprehensive analysis of the fundamental principles driving this sequence of transitions. Our study may open new avenues based on kinetic control as a promising strategy for achieving tailored molecular architectures on surfaces.

Ferroelectric polymer nanocomposite alignment layer in twisted nematic liquid crystal devices for reducing switching voltage

ABSTRACTTwisted nematic liquid crystal device (TNLCD) was fabricated using a ferroelectric zinc oxide (ZnO)-doped polyimide alignment layer. The ferroelectric nanoparticle can produce a local electric field, which can trigger the orientation of liquid crystal molecule and reduces the switching voltage. The uniform dispersion of ferroelectric ZnO nanoparticles in the alignment layer was studied using field emission scanning electron microscopy and atomic force microscopy. The ferroelectric property of ZnO-doped polyimide was investigated using dynamic contact electrostatic force microscopy. An increased local electric field due to the presence of nano ZnO was confirmed with the help of scanning tunnelling microscopy. An augmentation of capacitance was observed with an increase in concentration, which substantiates the reduction of switching voltage of TNLCD with the modification of ferroelectric nanoparticle-doped alignment layer.

Scanning Dielectric Constant Microscopy for imaging single biological cells

A method is developed for obtaining nanoscale dielectric constant maps of any material, using dynamic Electrostatic Force Microscopy (EFM) in conjunction with an analytical model by Hudlet et al for the tip-apex force, modified for dielectric material; and is applied for visualizing dielectric constant variation at sub-cellular level. The method is simple and does not rely on high-end electronics required for sub-attofarad resolution for capacitance measurement or resource intensive computation for specific geometry, but can be applied on available commercial Atomic Force Microscope (AFM). The method is not restricted to obtaining dielectric constant at a single point but it provides the determination of lateral variation in dielectric constant in a sample. The dielectric constant maps obtained using the modified Hudlet tip-apex model are compared with those obtained using the parallel plate and partially-filled parallel plate models. The accuracy of the dielectric constant values depends on the validity of the analytical model used. The dielectric constant values obtained using the modified Hudlet tip-apex model are in agreement with the reported values.

α- & β-crystalline phases in polyvinylidene fluoride as tribo-piezo active layer for nanoenergy harvester

The manuscript introduces the use of non-electrically polled spin-coated thin polyvinylidene fluoride (PVDF) films as the active layers in a contact electrification-based nanoenergy harvester. The four-layered device utilizes both piezo and triboelectric effect coupled with electrostatic induction. The elucidation of potential generation during contact between crystalline phases ( α and β) of PVDF layer material is investigated in the manuscript. Fourier transform infrared–attenuated total reflectance spectroscopy is carried out to illustrate the α- and β-phases in PVDF pellet, prepared film as well as the film after contact. Dynamic contact mode electrostatic force microscopy (DC-EFM) along with atomic force microscopy is used for the evaluation of reverse piezoelectric, local ferroelectric, triboelectric voltage and adhesive energy of the PVDF films before–after contact process. Quantum chemical calculation is performed using density functional theory to explain possible electron transitions in the active layers between the cylindrically symmetric α-phase and electrical double layer charges in the β-phase of PVDF. The interface study of the film is also carried out both experimentally using DC-EFM and through quantum chemical calculations. The fabricated device with the hybrid piezo-tribo layer promises to be a simple and low-cost energy source for the next-generation self-powered electronic devices. The device can also be used as knock sensor in engines as well as a capacitor.

Modeling and Measuring Viscoelasticity with Dynamic Atomic Force Microscopy

The interaction between a rapidly oscillating atomic force microscope tip and a soft material surface is described using both elastic and viscous forces with a moving surface model. We derive the simplest form of this model, motivating it as a way to capture the impact dynamics of the tip and sample with an interaction consisting of two components: interfacial or surface force, and bulk or volumetric force. Analytic solutions to the piece-wise linear model identify characteristic time constants, providing a physical explanation of the hysteresis observed in the measured dynamic force quadrature curves. Numerical simulation is used to fit the model to experimental data and excellent agreement is found with a variety of different samples. The model parameters form a dimensionless impact-rheology factor, giving a quantitative physical number to characterize a viscoelastic surface that does not depend on the tip shape or cantilever frequency.

Time-frequency analysis of the tip motion in liquids using the wavelet transform in dynamic atomic force microscopy

The tip motion of the dynamic atomic force microscope in liquids shows complex transient behaviors when using a low stiffness cantilever. The second flexural mode of the cantilever is momentarily excited. Multiple impacts between the tip and the sample might occur in one oscillation cycle. However, the commonly used Fourier transform method cannot provide time-related information about these transient features. To overcome this limitation, we apply the wavelet transform to perform the time-frequency analysis of the tip motion in liquids. The momentary excitation of the second mode and the phenomenon of multiple impacts are clearly shown in the time-frequency plane of the wavelet scalogram. The instantaneous frequencies and magnitudes of the second mode are extracted by the wavelet ridge analysis, which can provide quantitative estimations of the tip motion in the second mode. Moreover, the relations of the maximum instantaneous magnitude (MIM) to the amplitude setpoint and the Young’s modulus of the sample surface are investigated. The results suggest that the MIM can be used to characterize the nanomechanical property of the sample surface at high amplitude setpoints.

In Situ Quantification the Complex Poisson's Ratio of Single Cells Using a Magnetic-Drive Dynamic Atomic Force Microscopy Approach

In this letter, a dynamic atomic force microscopy approach using a magnetic bead probe (MBP) is presented for in situ quantification of the complex Poisson's ratio of single cells. The MBP is fabricated as a double-bead probe comprising a polystyrene bead and a magnetic bead superimposed thereon. The polystyrene bead is molded into a flat bottom to apply accurate global force to individual cells. The magnetic bead is magnetized at a certain angle to the longitudinal axis of the cantilever, which can simultaneously bend and twist the probe at off-resonant frequencies up to tens of kiloHertz. This allows for the dynamic measurement of viscoelastic properties of a single cell in the directions of shear and stretching, obtaining its complex Poisson's ratio. The performance of the proposed method was tested by measuring the complex Poisson's ratio of individual cells at different excitation frequencies and compression ratios. The experimental results show that the developed method has a wide range of applications for the field of cell mechanics.

Flexible nanoenergy harvester using piezo-tribo functional polymer and carbon fibre as electrodes

Micro-energy harvesting has gained immense interest among the research community due to the requirement for self-powering of sensor units in unapproachable environments. Use of simple, low cost, flexible energy harvester aids generation of energy from ambient environments. The present work proposes two devices under test(DUT) with different electrodes namely copper(DUT1) and carbon fibre(DUT2), polymers Polyvinylidene fluoride (PVDF), Polydimethylsiloxane (PDMS) as a functional charge generating materials. The piezoelectric and triboelectric characterisation of polymer films is performed using Dynamic contact mode electrostatic force microscopy(DC-EFM). The proposed DUT1 and DUT2 generates an 8 Vpp and 9 Vpp during human finger tapping. The qualitative and quantitative analysis of DUT1 and DUT2 generated an output voltage of (8.5 V, 12.35 V) (high pressure),(13.5 V, 14.1 V) (285 rpm) respectively. The carbon fibre electrode based flexible device generated an output voltage of 9 V, 10 V and 2.275 V when subjected to biomechanical operations such as finger assisted tapping, press and release as well as bending operations. The device with copper electrode generated 0.32 nJ of energy and power density per unit area of 2.37 pW cm−2 in a single tap cycle. The DUT2 generated thrice the energy and power density than that of DUT1. The DUT2 thus promises to be an efficient, low-cost, flexible energy harvester with PVDF as well as PDMS polymers as a functional layer.

The characteristics of organic matter influence its interfacial interactions with MnO2 and catalytic oxidation processes

The influence of dissolved organic matter (DOM) properties on its interfacial interactions with MnO2 and on catalytic oxidation processes was studied by Time-Resolved Dynamic Light Scattering (TR-DLS) and Atomic Force Microscopy (AFM) under varied solution conditions. Four DOM fractions of different characteristics (e.g., SUVA, hydrophobic character, structural properties) were selected. Bared-MnO2 nanoparticles readily aggregated in NaCl and CaCl2 solutions. Classic DLVO Theory successfully described critical coagulation concentrations and aggregation behaviors. In NaCl solution, DOM adsorbed on MnO2 nanoparticles and provided electrosteric stabilization. The two DOM fractions of higher hydrophobic (HPO) character were more efficient in decreasing the aggregation rates. Enhanced MnO2 aggregation was observed at high Ca2+ concentrations due to charge screening and cation bridging between carboxyl groups in DOM structures. The addition of oxidant (H2O2) induced a high aggregation of bared-MnO2 nanoparticles, possibly due to the release of Mn2+ (i.e., complexation mechanisms) and generation of reactive species (O2−, HO2−, and H). Contrasted with their hydrophilic (HPI) counterparts, HPO isolates adsorbed on MnO2 significantly decreased the catalytic oxidation processes between H2O2/MnO2; suggesting a more efficient and stronger DOM coating. Interfacial forces measured by AFM, showed weaker interactions between HPI isolates and MnO2; suggesting unfavorable polar interactions. Conversely, the high adhesion forces between MnO2/HPO isolate would indicate stronger bonds and hydrophobic interactions. This study provided a nanoscale understanding of the impact of DOM characteristics on: a) performance of the MnO2 coated ceramic membranes in water treatment, and b) biogeochemical cycle of Mn-oxides in the environmental.

Wall slip of complex fluids: Interfacial friction versus slip length

Using a dynamic Surface Force Apparatus, we demonstrate that the notion of slip length used to describe the boundary flow of simple liquids, is not appropriate for viscoelastic liquids. Rather, the appropriate description lies in the original Navier's partial slip boundary condition, formulated in terms of an interfacial friction coefficient. We establish an exact analytical expression to extract the interfacial friction coefficient from oscillatory drainage forces between a sphere and a plane, suitable for dynamic SFA or Atomic Force Microscopy non-contact measurements. We use this model to investigate the boundary friction of viscoelastic polymer solutions over 5 decades of film thicknesses and one decade in frequency. The proper use of the original Navier's condition describes accurately the complex hydrodynamic force up to scales of tens of micrometers, with a simple "Newtonian-like" friction coefficient, not frequency dependent, and reflecting closely the dynamics of an interfacial depletion layer at the solution/solid interface.

Effect of the softer binder on the performance of repeated RAP binder

A softer binder as the rejuvenator was added into the repeated reclaimed asphalt pavement (RRAP) binder to study the rejuvenating mechanism. Consistency properties, chemical oxidation degree, rheological properties, microscopic morphology and mechanics were studied using consistency tests, fourier transform infrared spectroscopy (FTIR), dynamic shear rheometer (DSR) and atomic force microscope (AFM). With increase in rejuvenator concentration, consistency properties, rheological properties and micro mechanical properties were improved and the oxidation degree was declined. Meanwhile the microstructures changed significantly. Results show that properties of the RRAP binder can be restored with the addition of the softer binder and the appropriate proportion is 40%.

Flexible, ferroelectric nanoparticle doped polymer dispersed liquid crystal devices for lower switching voltage and nanoenergy generation

Flexible polymer dispersed liquid crystal (F-PDLC) devices were fabricated using transparent conducting ITO/PET film. Polymerization induced phase separation (PIPS) method was used for pure and ferroelectric BaTiO3 (BTO) and ZnO doped PDLC devices. The distribution of nanoparticles in the PDLC and the formation of micro cavities were studied using field emission scanning electron microscopy (FESEM). It was observed that the addition of ferroelectric BTO nanoparticles has reduced the threshold voltage (Vth) and saturation voltage (Vsat) of FNP-PDLC by 85% and 41% respectively due to the spontaneous polarization of ferroelectric nanoparticles. The ferroelectric properties of BTO and ZnO in the fabricated devices were investigated using dynamic contact electrostatic force microscopy (DC EFM). Flexing the device can generate a potential due to the piezo-tribo electric effect of the ferroelectric nanomaterial doped in the PDLC matrix, which could be utilized as an energy generating system. The switching voltage after multiple flexing was also studied and found to be in par with non-flexing situations.

Evaluating signal and noise spectral density of a qPlus sensor with an active feedback control

Q-control technique enables to actively change the quality factor of the probe oscillation in dynamic atomic force microscopy. The Q-control is realized by adding a self-feedback loop into the original actuation-detection system, in which a damping force with controllable damping coefficient in magnitude and sign is applied to the oscillating probe. While the applied force alters the total damping interaction and thus the overall ‘signal’ of the probe motion, the added feedback system changes the ‘noise’ of the motion as well. Here, we systematically investigate the signal, the noise, and the signal-to-noise ratio of the qPlus sensor under the active Q-control. We quantify the noise of the qPlus motion by measuring the noise spectral density, which is reproduced by a harmonic oscillator model including the thermal and the measurement noises. We show that the noise signal increases with the quality factor controlled, scaling as the square root of the quality factor. Because the overall signal is linearly proportional to the quality factor, the signal-to-noise ratio scales as the square root of the quality factor. The Q-controlled qPlus with a highly enhanced Q, up to 10,000 in air, leads to the minimum detectable force gradient of 0.001 N/m, which would enhance the capability of the qPlus sensor for atomic force microscopy and spectroscopy.

An optically-driven macroscopic cantilever

We report a simple experiment to demonstrate optically-driven resonance on a macroscopic object. Utilising dynamic force microscopy in ambient environment, the resonant motion of a cm-sized cantilever driven by an amplitude-modulated diode laser is directly observed. Our versatile setup involves a host of exciting modern experimental techniques that are suitable in the undergraduate laboratory.

Reducing detrimental electrostatic effects in Casimir-force measurements and Casimir-force-based microdevices

It is well known that residual electrostatic forces create significant difficulties to precise measurements of the Casimir force and to wide use of Casimir-operated microdevices. We experimentally demonstrate that with the help of Ar-ion cleaning of the surfaces it is possible to make electrostatic effects negligibly small as compared to the Casimir interaction. Our experimental setup consists of the dynamic atomic force microscope supplemented with an Ar-ion gun and argon reservoir. The residual potential difference between the Au-coated surfaces of a sphere and a plate was measured both before and after the in situ Ar-ion cleaning. It is shown that this cleaning decreases the magnitude of the residual potential by up to an order of magnitude and makes it almost independent on separation. The gradient of the Casimir force was measured using ordinary samples subjected to the Ar-ion cleaning. The obtained results are shown to be in good agreement with both previous precision measurements using the specially selected samples and with theoretical predictions of the Lifshitz theory. The conclusion is made that the suggested method of in situ Ar-ion cleaning is effective in reducing the electrostatic effects and therefore is a great resource for experiments on measuring the Casimir interaction and for Casimir-operated microdevices.

Lyapunov Estimator for High-Speed Demodulation in Dynamic Mode Atomic Force Microscopy

In dynamic mode atomic force microscopy (AFM), the imaging bandwidth is governed by the slowest component in the open-loop chain consisting of the vertical actuator, cantilever, and demodulator. While the common demodulation method is to use a lock-in amplifier (LIA), its performance is ultimately bounded by the bandwidth of the postmixing low-pass filters. This brief proposes an amplitude and phase estimation method based on a strictly positive real Lyapunov design approach. The estimator is designed to be of low complexity while allowing for high bandwidth. In addition, suitable gains for high performance are suggested such that no tuning is necessary. The Lyapunov estimator is experimentally implemented for amplitude demodulation and shown to surpass the LIA in terms of tracking bandwidth and noise performance. High-speed AFM images are presented to corroborate the results.