Time-resolved Force Research Articles

Fast temperature-responsive nanocomposite PNIPAM hydrogels with controlled pore wall thickness: Force and rate of T-response

Fast thermoresponsive porous hydrogels based on the PNIPAM–silica nanocomposite were prepared and optimized in order to obtain increased force (pressure) response to temperature stimuli. This was achieved by increasing pore wall thickness via changing the monomers concentration during the hydrogels’ cryogenic synthesis. The mechanically strong and compact pore structure of the best gel made it possible to increase the force (pressure) response 3 times and the shear modulus 4 times, while the volume ratio of swollen to deswollen state remained unchanged, and the kinetics of the T-response was only moderately slower than in the case of the analogous ultra-fast responsive soft gels. The porous structure of the hydrogels (SEM), the dispersion and particle size of the nanofiller in the pore walls (TEM), as well as the swelling behavior and time-resolved force response to T-change are discussed in this work.

Scanning probe microscopy of solar cells: From inorganic thin films to organic photovoltaics

Abstract

Analysis of time-resolved interaction force mode AFM imaging using active and passive probes

We present an in-depth analysis of time-resolved interaction force (TRIF) mode imaging for atomic force microscopy (AFM). A nonlinear model of an active AFM probe, performing simultaneous topography and material property imaging on samples with varying elasticity and adhesion is implemented in Simulink®. The model is capable of simulating various imaging modes, probe structures, sample material properties, tip-sample interaction force models, and actuation and feedback schemes. For passive AFM cantilevers, the model is verified by comparing results from the literature. As an example of an active probe, the force sensing integrated readout and active tip (FIRAT) probe is used. Simulation results indicate that the active and damped nature of FIRAT provides a significant level of control over the force applied to the sample, minimizing sample indentation and topography error. Active tip control (ATC) preserves constant contact time during force control for stable contact while preventing the loss of material property information such as elasticity and adhesive force. Simulation results are verified by TRIF mode imaging of the samples with both soft and stiff regions.

Unsteady aspect of the electrohydrodynamic force produced by surface dielectric barrier discharge actuators

The time-resolved electrohydrodynamic force produced by single dielectric barrier discharge (DBD) actuators used for airflow control is computed from electric wind velocity measurements. Two actuator designs are investigated: a plate-to-plate and a wire-to-plate surface DBD because each of them produces a different discharge current. Results show that: (1) the high voltage active electrode shape plays a key role in the plasma physics, (2) the body force is highly unsteady with fluctuations up to about ten times its time-averaged value, and (3) the typical plate-to-plate DBD produces a positive force during the positive half-cycle and a negative force during the negative half-cycle when both cycles result in a positive force with the wire-to-plate DBD.

Nanoscale investigation of the electrical properties in semiconductor polymer–carbon nanotube hybrid materials

The morphology and electrical properties of hybrids of a semiconducting polymer (namely poly(3-hexylthiophene) P3HT) and carbon nanotubes are investigated at the nanoscale with a combination of Scanning Probe Microscopy techniques, i.e., Conductive Atomic Force Microscopy (C-AFM) and time-resolved Current Sensing Force Spectroscopy Atomic Force Microscopy (CSFS-AFM, or PeakForce TUNA™). This allows us to probe the electrical properties of the 15 nm wide P3HT nanofibers as well as the interface between the polymer and single carbon nanotubes. This is achieved by applying controlled, low forces on the tip during imaging, which allows a direct comparison between the morphology and the electrical properties at the nanometre scale.

Understanding Thermodynamic Competitivity between Biopolymer Folding and Misfolding under Large-Scale Intermolecular Interactions

Cooperativity is a hallmark of spontaneous biopolymer folding. The presence of intermolecular interactions could create off-pathway misfolding structures and suppress folding cooperativity. This raises the hypothesis that thermodynamic competitivity between off-pathway misfolding and on-pathway folding may intervene with cooperativity and govern biopolymer folding dynamics under conditions permitting large-scale intermolecular interactions. Here we report direct imaging and theoretical modeling of thermodynamic competitivity between biopolymer folding and misfolding under such conditions, using a two-dimensional array of proton-fueled DNA molecular motors packed at the maximal density as a model system. Time-resolved liquid-phase atomic force microscopy with enhanced phase contrast revealed that the misfolding and folding intermediates transiently self-organize into spatiotemporal patterns on the nanoscale in thermodynamic states far away from equilibrium as a result of thermodynamic competitivity. Computer simulations using a novel cellular-automaton network model provide quantitative insights into how large-scale intermolecular interactions correlate the structural dynamics of individual biomolecules together at the systems level.

Vortex wake, downwash distribution, aerodynamic performance and wingbeat kinematics in slow-flying pied flycatchers

Many small passerines regularly fly slowly when catching prey, flying in cluttered environments or landing on a perch or nest. While flying slowly, passerines generate most of the flight forces during the downstroke, and have a 'feathered upstroke' during which they make their wing inactive by retracting it close to the body and by spreading the primary wing feathers. How this flight mode relates aerodynamically to the cruising flight and so-called 'normal hovering' as used in hummingbirds is not yet known. Here, we present time-resolved fluid dynamics data in combination with wingbeat kinematics data for three pied flycatchers flying across a range of speeds from near hovering to their calculated minimum power speed. Flycatchers are adapted to low speed flight, which they habitually use when catching insects on the wing. From the wake dynamics data, we constructed average wingbeat wakes and determined the time-resolved flight forces, the time-resolved downwash distributions and the resulting lift-to-drag ratios, span efficiencies and flap efficiencies. During the downstroke, slow-flying flycatchers generate a single-vortex loop wake, which is much more similar to that generated by birds at cruising flight speeds than it is to the double loop vortex wake in hovering hummingbirds. This wake structure results in a relatively high downwash behind the body, which can be explained by the relatively active tail in flycatchers. As a result of this, slow-flying flycatchers have a span efficiency which is similar to that of the birds in cruising flight and which can be assumed to be higher than in hovering hummingbirds. During the upstroke, the wings of slowly flying flycatchers generated no significant forces, but the body-tail configuration added 23 per cent to weight support. This is strikingly similar to the 25 per cent weight support generated by the wing upstroke in hovering hummingbirds. Thus, for slow-flying passerines, the upstroke cannot be regarded as inactive, and the tail may be of importance for flight efficiency and possibly manoeuvrability.

High-Resolution Nanomechanical Mapping Using Interferometric-Force-Sensing AFM Probes

In this paper, we demonstrate high-resolution mapping of composite surfaces based on their nanomechanical properties by employing an atomic force microscope probe that can resolve high-frequency tip-sample interaction forces in tapping-mode atomic force microscopy (AFM) (TM-AFM). Time-resolved force measurements are achieved by a small integrated interferometric high-bandwidth grating force sensor at the end of the cantilever beam. The probes are batch fabricated using optical lithography and are used in an AFM system with no further modification. The fabricated devices are characterized and grating-force-sensor signals and their measured spectra confirm that probes with integrated high-bandwidth force sensors can successfully resolve high-frequency tip-sample interaction forces. We utilize the capability of our probes to form images by recording the amplitude of the higher harmonics of the time-resolved tip-sample interaction forces with lock-in detection. These images show that our probes can successfully map, with high spatial resolution, the mechanical contrast due to different materials and structural differences in composite surfaces.

Inhibition of myosin II triggers morphological transition and increased nuclear motility

We investigate the effect of myosin II inhibition on cell shape and nuclear motility in cultures of mouse radial glia-like neural progenitor and rat glioma C6 cells. Instead of reducing nucleokinesis, the myosin II inhibitor blebbistatin provokes an elongated bipolar morphology and increased nuclear motility in both cell types. When myosin II is active, time-resolved traction force measurements indicate a pulling force between the leading edge and the nucleus of C6 cells. In the absence of myosin II activity, traction forces during nucleokinesis are diminished below the sensitivity threshold of our assay. By visualizing the centrosome position in C6 cells with GFP-centrin, we show that in the presence or absence of myosin II activity, the nucleus tends to overtake or lag behind the centrosome, respectively. We interpret these findings with the help of a simple viscoelastic model of the cytoskeleton consisting active contractile and passive compressed elements.

Modeling, design, and analysis of interferometric cantilevers for time-resolved force measurements in tapping-mode atomic force microscopy

Cantilevers with interferometric high bandwidth force sensors can resolve nonlinear tip-sample interaction forces in tapping-mode atomic force microscopy. In this paper, we provide a detailed analysis of time-resolved force measurements using such cantilever. We first model the probe as a coupled spring-mass system and investigate its steady state dynamics under tapping-mode imaging conditions. Next, we analyze the optical response of the interferometric force sensor: Diffraction patterns as a function of tip displacement are obtained both analytically and by simulations. Finally, the frequency response of the force sensor is calculated, and the effects of the sensor geometry variations on the sensor mechanical response are analyzed.

Single-dielectric barrier discharge plasma actuator modelling and validation

Single-dielectric barrier discharge (SDBD) plasma actuators have gained a great deal of world-wide interest for flow-control applications. With this has come the need for flow-interaction models of plasma actuators that can be used in computational flow simulations. SDBD plasma actuators consist of two electrodes: one uncovered and exposed to the air and the other encapsulated by a dielectric material. An AC electric potential is supplied to the electrodes. When the AC potential is large enough, the air in the region over the encapsulated electrode ionizes. The ionized air in the presence of the electric field results in a space–time dependent body force vector field. The body force is the mechanism for flow control. This study describes a semi-empirical model that has been developed to capture the dynamic nature of the local air ionization and time-dependent body force vector distribution. Validation of the model includes comparisons to experimentally measured space–time charge distribution and the time-resolved and time-averaged body force. Two flow simulations are then used to further validate the SDBD plasma actuator model. These involved an impulsively started plasma actuator in still air, and the flow around a circular cylinder in which plasma actuators were used to suppress the Karman vortex street. In both cases, the simulations agreed well with the experiments.

Combined quantitative ultrasonic and time-resolved interaction force AFM imaging

The authors describe a method where quantitative ultrasonic atomic force microscopy (UAFM) is achieved during time-resolved interaction force (TRIF) imaging in intermittent contact mode. The method uses a calibration procedure for quantitative UAFM. It improves elasticity measurements of stiff regions of surfaces while retaining the capabilities of the TRIF mode for topography, adhesion, dissipation, and elasticity measurements on soft regions of sample surfaces. This combination is especially advantageous when measuring and imaging samples with broad stiffness range in a nondestructive manner. The experiments utilize an active AFM probe with high bandwidth and the UAFM calibration is performed by measuring the magnitude of the time-resolved UAFM signal at a judiciously chosen frequency for different contact stiffness values during individual taps. Improved sensitivity to stiff surface elasticity is demonstrated on a special sample. The results show that combining UAFM with TRIF provides 2.5 GPa (5%) standard deviation on the silicon surface reduced Young's modulus, representing 5× improvement over using only TRIF mode imaging.

水滴烧蚀多脉冲激光推进性能

Time-resolved force sensing technique was applied to the study on propulsive characteristics of water droplets for multi-pulse TEA (transversely excited at atmospheric pressure) CO2 laser propulsion. Laser-driven blast waves and associated flow dynamics in impulse generation processes of ablation of water droplets were studied through schlieren visualization. Experimental results show that coupling coefficient and specific impulse decrease as the interval between laser pulses and pulse numbers increase. The maximum speed of blast wave in the opposite and same direction of laser propagation is 10 km/s and 7 km/s, respectively. Time-resolved force and schlieren visualization results indicate that vaporization and blast waves dominate the impulse generation processes of water droplets for las

Imaging Local Trap Formation in Conjugated Polymer Solar Cells: A Comparison of Time-Resolved Electrostatic Force Microscopy and Scanning Kelvin Probe Imaging

We study local photooxidation and trap formation in all-polymer bulk-heterojunction organic photovoltaics (OPVs) using both time-resolved electrostatic force microscopy (trEFM) and conventional scanning Kelvin probe microscopy (SKPM). We create electron-trapping defects at known locations by locally photooxidizing blends of poly[(9,9′-dioctylfluorene-alt-(bis(N,N′-(4-butylphenyl))-bis(N,N′-phenyl-1,4-phenylenediamine)] and poly[9,9′-dioctylfluorene-alt-1,4-benzothiadiazole]. We then compare the local surface photovoltage shifts measured via SKPM and the changes in local photoinduced charging rates measured via trEFM with changes in the performance of macroscopic photodiodes that have been exposed to similar photooxidation. We find that the trEFM charging rate images can identify local photooxidation and trap formation with much better sensitivity than conventional SKPM images. In addition, the changes in the trEFM charging rates correlate well with the external quantum efficiencies of the macroscopic photodiodes. In contrast, the SKPM images not only are less sensitive to trap formation but also show a more complicated response. We conclude that trEFM is well suited to studying local trap formation in organic solar cells and caution that SKPM data by itself can be difficult to interpret on OPV films, especially when materials have been exposed to photooxidation.

New SPM techniques for analyzing OPV materials

Organic solar cells hold promise as an economical means of harvesting solar energy due to their ease of production and processing. However, the efficiency of such organic photovoltaic (OPV) devices is currently below that required for widespread adoption. The efficiency of an OPV is inextricably linked to its nanoscale morphology. High-resolution metrology can play a key role in the discovery and optimization of new organic semiconductors in the lab, as well as assist the transition of OPVs from the lab to mass production. We review the instrumental issues associated with the application of scanning probe microscopy (SPM) techniques such as photoconductive atomic force microscopy and time-resolved electrostatic force microscopy that have been shown to be useful in the study of nanostructured organic solar cells. These techniques offer unique insight into the underlying heterogeneity of OPV devices and provide a nanoscale basis for understanding how morphology directly affects OPV operation. Finally, we discuss opportunities for further improvements in scanning probe microscopy to contribute to OPV development.

Effects of laser energy density on impulse coupling coefficient of laser ablation of water for propulsion

Time-resolved force sensing and intensified charge-coupled device (ICCD) imaging techniques were applied to the study of the effects of laser energy density on impulse coupling coefficient of laser ablation of water for propulsion. A Transversely Excited at Atmospheric pressure (TEA) CO2 laser operated at 10.6 μm, 30 J pulse energy was used to ablate water contained in a quadrate quartz container. Net imparted impulse and coupling coefficients were derived from the force sensor data and relevant results were presented for various laser energy densities. ICCD imaging was used in conjunction with the dynamic force techniques to examine the dependencies on laser energy density. Results showed that the impulse coupling coefficient could reach a maximum value when laser energy density was about 105 J/m2, and it would increase before laser energy got to this point and would decrease after this point, and ICCD imaging supplied important phenomenon to explain this variation, which were water ablation before laser energy density got to 105 J/m2 and laser-induced air-breakdown with water as an induction when laser energy density was higher than 105 J/m2.

The weight of a falling chain, revisited

A vertically hanging chain is released from rest and falls due to gravity on a scale pan. We discuss the various experimental and theoretical aspects of this classic problem. Careful time-resolved force measurements allow us to determine the differences between the idealized problem and its implementation in the laboratory. We observe that, in spite of the upward force exerted by the pan on the chain, the free end at the top falls faster than a freely falling body. Because a real chain exhibits a finite minimum radius of curvature, the contact at the bottom results in a tensional force, which pulls the falling part downward.

Photogenerated Exciton Dissociation in Highly Coupled Lead Salt Nanocrystal Assemblies

Internanocrystal coupling induced excitons dissociation in lead salt nanocrystal assemblies is investigated. By combining transient photoluminescence spectroscopy, grazing incidence small-angle X-ray scattering, and time-resolved electric force microscopy, we show that excitons can dissociate, without the aid of an external bias or chemical potential gradient, via tunneling through a potential barrier when the coupling energy is comparable to the exciton binding energy. Our results have important implications for the design of nanocrystal-based optoelectronic devices.

The effect of set point ratio and surface Young’s modulus on maximum tapping forces in fluid tapping mode atomic force microscopy

There is great interest in using proximal probe techniques to simultaneously image and measure physical properties of surfaces with nanoscale spatial resolution. In this regard, there have been recent innovations in generating time-resolved force interaction between the tip and surface during regular operation of tapping mode atomic force microscopy (TMAFM). These tip/sample forces can be used to measure physical material properties of surface in an analogous fashion to the well-established static force curve experiment. Since its inception, it has been recognized that operation of TMAFM in fluids differs significantly from that in air, with one of the major differences manifested in the quality factor (Q) of the cantilever. In air, Q is normally on the order of 200–400, whereas in fluids, it is of the order of approximately 1–5. In this study, we explore the impact of imaging parameters, i.e., set point ratio and free cantilever oscillation amplitude, on time varying tip-sample force interactions in fluid TMAFM via simulation and experiment. The numerical AFM model contains a feedback loop, allowing for the simulation of the entire scanning process. In this way, we explore the impact of varying the Young’s modulus of the surface on the maximum tapping force.

Exploration of AFM Imaging Artifacts Occurring at Sharp Surface Features When Using Short Carbon Nanotube Probes and Possible Mitigation With Real-Time Force Spectroscopy

We present an overview of four types of imaging artifacts that can occur during characterization of sharp sample topographies with intermittent contact atomic force microscopy (AFM) when using short nanotube probes (<100 nm in length). We discuss the causes behind these artifacts, as well as their implications in the context of nanomanufacturing, and explore theoretically their mitigation using AFM techniques that can perform simultaneous imaging and spectroscopy. In particular, we focus on the experimentally validated spectral inversion method [Stark et al., 2002, “Inverting Dynamic Force Microscopy: From Signals to Time-Resolved Interaction Forces,” Proc. Natl. Acad. Sci. U.S.A., 99, pp. 8473–8478; Sahin et al., 2007, “An Atomic Force Microscope Tip Designed to Measure Time-Varying Nanomechanical Forces,” Nat. Nanotechnol., 2, pp. 507–514] and on a recently proposed dual-frequency-modulation method [Chawla and Solares, 2009, “Single-Cantilever Dual-Frequency-Modulation Atomic Force Microscopy,” Meas. Sci. Technol., 20, p. 015501], which has been demonstrated within computational simulations and is under experimental implementation in our laboratory. We discuss the capabilities and limitations of each of these approaches as well as possible areas of future development.