Direct Force Measurements Research Articles

Characteristic Features of Hybrid Bridge Structure Models

This work discusses the classification of bridge structures and their models. Examples of hybrid models are also indicated in the case of homogeneous and mixed structures. Where hybrid design and model cases are presented, an algorithm for the analysis of contact interactions between subsystems is provided. The soil and shell structure is adopted as an example of analyses, where in its characteristic feature, differentiating it from classic bridges, is the large influence of both the soil backfill and the road surface as load-bearing elements of the structure. There are two structural subsystems present in the model of the soil-shell structure: a corrugated sheet coating and a soil backfill with a surface. The interaction between them is modeled as a contact interaction, i.e. forces with a direction normal and tangent to the line of the circumferential band of the shell. The paper presents an algorithm of their analysis, resulting from the outcomes of tests with the use of strain gauges, with the load being a vehicle changing its position. The advantage of the algorithm is that it takes into account the actual physical properties of the soil in the backfill layers and the contact layer of the coating with the soil. Direct contact force measurements along the entire length of the circumferential strip are very difficult. In practice, pressure gauges are used to determine normal interactions.

Direct measurement of thermophoretic and photophoretic force acting on hot micromotors with optical tweezers

Synthetic microparticles present exciting features owing to their customizable light-matter interaction. We hereby report on the optical trapping of two artificial plasmonic microparticles: one with isotropic nanoparticles covering the surface (homogeneous particle) used as a hot Brownian particle and an anisotropic Janus microparticle, half coated with a gold nano-layer. The homogeneous particle decorated with plasmonic nanoparticles on the surface displays features of hot Brownian dynamics as well as photophoretic motion along z dimension in the optical trap. A dielectric particle was used as a reference particle because it acts as a cold particle with only the gradient force affecting it. In general, Janus particles orient in the trap with the dielectric part in the trap center. Plasmonic gold nanostructures absorb the light energy and produce heat; the photothermal forces significantly affect the optical trapping. These hot microspheres display temperature and Janus orientation dependent position distribution significantly different from cold (purely dielectric) microparticles. The developed method allows for the first time direct determination of the photophoretic (thermal force along light propagation direction) and thermophoretic force (light propagation direction independent force) acting on the respective particles, which opens new paths for analysis and control of micromachines.

Surface Forces and Structure in a Water-in-Salt Electrolyte.

Water-in-salt electrolytes are a fascinating new class of highly concentrated aqueous solutions with wide electrochemical stability windows that make them viable as aqueous battery electrolytes. However, the high ion concentration of water-in-salt electrolytes means that these systems are poorly understood when compared to more dilute electrolyte solutions. Here, we present direct surface force measurements across thin films of a water-in-salt electrolyte at several concentrations. We find that the electrolyte adopts a layered structure at charged interfaces composed of a nanostructure of a hydrated cation and nonaqueous anion-rich domains. These observations will aid in the interpretation of capacitance and double-layer behavior of water-in-salt electrolytes with consequences for their use in energy storage devices.

Design and Calibration of a Tri-Directional Contact Force Measurement System

In low pressure turbine stages, adjacent blades are coupled to each other at their tip by covers, called shrouds. Three-dimensional periodic contact forces at shrouds strongly affect the blade vibration level as energy is dissipated by friction. To validate contact models developed for the prediction of nonlinear forced response of shrouded blades, direct contact force measurement during dynamic tests is mandatory. In case of shrouded blades, the existing unidirectional and bi-directional contact force measurement methods need to be improved and extended to a tri-directional measurement of shroud contact forces for a comprehensive and more reliable validation of the shroud contact models. This demands an accurate and robust measurement solution that is compatible with the nature and orientation of the contact forces at blade shrouds. This study presents a cost effective and adaptable tri-directional force measurement system to measure static and dynamic contact forces simultaneously in three directions at blade shrouds during forced response tests. The system is based on three orthogonal force transducers connected to a reference block that will eventually be put in contact with the blade shroud in the test rig. A calibration process is outlined to define a decoupling matrix and its subsequent validation is demonstrated in order to evaluate the effectiveness of the measurement system to measure the actual contact forces acting on the contact.

Direct measurement of individual optical forces in ensembles of trapped particles

Optical tweezers are a powerful tool to hold and manipulate particles on the microscale. The ability to measure tiny forces enables detailed investigations, e.g., of the mechanical properties of biological systems. Here we present a generally applicable method to simultaneously measure all components of the force applied to a specific particle in a trapped ensemble, or to a specific site of an extended object. This holographic force measurement relies on a detailed analysis of a single interference pattern formed in the far field to recover amplitude and phase of the field. It requires no information about size, shape, or optical properties of the particles and can be scaled to many traps—we show individual force measurements for up to 10 particles. In addition, we demonstrate force measurements when stretching a red blood cell, held directly by four traps. This method opens up a wealth of new opportunities made possible by localized quantitative force measurements in complex biological settings.

Pedestrian-Induced Load Identification from Structural Responses Using Genetic Algorithm with Numerical and Experimental Validation

A reliable pedestrian load model is a prerequisite for the accurate calculation of human-induced structural vibration. In recent years, many researchers have proposed pedestrian load models based on direct force measurements, such as force plates. However, direct measurement techniques often suffer from incapability when applied to real structures in operation and can hardly be used to measure crowd loads. In this paper, an inverse load identification method is proposed to extract pedestrian vertical load from structural responses. Through a genetic algorithm, the pedestrian’s pacing frequency, dynamic load factors, and phase angles in the Fourier-series model are identified from structural acceleration responses. The proposed algorithm is further investigated for the identification of multiple-pedestrian load parameters, where structural displacement responses are used to give an equivalent number of pedestrians. Numerical examples demonstrate that the pedestrian load parameters are estimated with high accuracy and robustness against noise and modeling errors. A sensitivity analysis is given to explain the different estimation accuracies among the parameters. Finally, the proposed method is validated through an experimental test, showing its practicality for identifying pedestrian loads in real structures.

Inertial drag and lift forces for coarse grains on rough alluvial beds measured using in-grain accelerometers

Abstract. Quantifying the force regime that controls the movement of a single grain during fluvial transport has historically proven to be difficult. Inertial micro-electromechanical system (MEMS) sensors (sensor assemblies that mainly comprise micro-accelerometers and gyroscopes) can used to address this problem using a “smart pebble”: a mobile inertial measurement unit (IMU) enclosed in a stone-like assembly that can measure directly the forces on a particle during sediment transport. Previous research has demonstrated that measurements using MEMS sensors can be used to calculate the dynamics of single grains over short time periods, despite limitations in the accuracy of the MEMS sensors that have been used to date. This paper develops a theoretical framework for calculating drag and lift forces on grains based on IMU measurements. IMUs were embedded a spherical and an ellipsoidal grain and used in flume experiments in which flow was increased until the grain moved. Acceleration measurements along three orthogonal directions were then processed to calculate the threshold force for entrainment, resulting in a statistical approximation of inertial impulse thresholds for both the lift and drag components of grain inertial dynamics. The ellipsoid IMU was also deployed in a series of experiments in a steep stream (Erlenbach, Switzerland). The inertial dynamics from both sets of experiments provide direct measurement of the resultant forces on sediment particles during transport, which quantifies (a) the effect of grain shape and (b) the effect of varied-intensity hydraulic forcing on the motion of coarse sediment grains during bedload transport. Lift impulses exert a significant control on the motion of the ellipsoid across hydraulic regimes, despite the occurrence of higher-magnitude and longer-duration drag impulses. The first-order statistical generalisation of the results suggests that the kinetics of the ellipsoid are characterised by low- or no-mobility states and that the majority of mobility states are controlled by lift impulses.

Direct measurement of interaction force between hydrophilic silica surfaces in triblock copolymer solutions with salt by atomic force microscopy

Triblock copolymers composed of polyethylene oxide (PEO) and polypropylene oxide (PPO) are used in various fields as nonionic surfactants. In this study, we measured interaction forces between untreated hydrophilic silica surfaces in solutions with two typical triblock copolymers, Pluronic P123 (PEO20PPO70PEO20) and F127 (PEO99PPO65PEO99), in the presence of 1 mM and 500 mM NaCl using atomic force microscopy (AFM). In solutions at the copolymer concentration of 1 µM, which is below the critical micelle concentration (CMC), the measured interaction forces were monotonically repulsive in the presence of 1 mM NaCl, which suggested the brush-like conformation of copolymers on the surfaces. When the concentration of NaCl was increased to 500 mM, interaction forces became attractive, which indicated the bridging of adsorbed polymers onto surfaces, the strength of which varied depending on the affinity and adsorption density of copolymers. The interactions at the copolymer concentration of 1 mM, which were above the CMC of both copolymers, were steric repulsions between adsorbed micelles on the surfaces with 1 mM of NaCl. For 500 mM of NaCl, an attractive jump after a steric repulsion was observed only in the force curve for P123, which inferred that the displacement of micelles from the surfaces was presumably due to a decrease in the strength of adsorption caused by the dehydration of EO groups. These results indicated that the length of the EO group considerably affected the interactions.

Mean wind loads on equilateral triangular lattice tower under skewed wind loading

Compared with lattice towers with square or rectangle cross sections, triangular lattice towers have the advantage of smaller foundation footprint. Triangular lattice towers have been widely used in transmission towers and communication towers, yet only a few studies have been conducted on their wind loads. Therefore, a series of wind tunnel tests on a triangular lattice tower were conducted using direct force measurements in this study. Effective drag coefficients and effective skewed wind load factors normalized with the “true” windward projected areas were proposed and obtained from the test results. Piecewise functions were suggested to calculate the effective skewed wind load factors and skewed wind load factors. It is found that the skewed wind load factors show an approximate “W” profile which the British standard formula does not fit. The effective skewed wind load factors show a strong regularity and are only related to yaw angles and solidity ratios. The analytical effective skewed wind load factors have a good agreement with fitting results with a maximum error of 3.1%. The recommended formula for skewed wind load factors is concise and can effectively illustrate the changing tendency of drag coefficient curves.

Surface Energy of the Titan Aerosol Analog “Tholin”

Abstract The photochemical haze produced in the upper atmosphere of Titan plays a key role in various atmospheric and surface processes on Titan. The surface energy, one important physical property of the haze, is crucial for understanding the growth of the haze particles and can be used to predict their wetting behavior with solid and liquid species on Titan. We produced Titan analog haze materials, so-called “tholins,” with different energy sources and measured their surface energies through contact angle and direct force measurements. From the contact angle measurement, we found that the tholins produced by cold plasma and UV irradiation have a total surface energy around 60–70 mJ m−2. The direct force measurement yields a total surface energy of ∼66 mJ m−2 for plasma tholin. The surface energy of tholin is relatively high compared to common polymers, indicating its high cohesiveness. Therefore, the Titan haze particles would likely coagulate easily to form bigger particles, while the haze-derived surface sand particles would need a higher wind speed to be mobilized because of the high interparticle cohesion. The high surface energy of tholins also makes them easily wettable by Titan’s atmospheric hydrocarbon condensates and surface liquids. Thus, the haze particles are likely good cloud condensation nuclei for hydrocarbon clouds (methane and ethane) to nucleate and grow. And if the haze particles are denser compared to the lake liquids, they will likely sink into the lakes instead of forming a floating film to dampen the lake surface waves.

A time domain method for wheel-rail force identification of rail vehicles

Using dynamic load identification technology to identify the wheel-rail force of rail vehicles is an economical and convenient method for indirect measurement. In this paper an analytical load identification method is proposed. Firstly, based on the theory of structural dynamics, the Duhamel integral is applied, combined with the characteristics of the rail vehicle system, to derive the analytical load identification method. Secondly, the correctness of the method is verified by a multi-degree-of-freedom system with exact solution. Next, a detailed vehicle dynamics simulation model is established as a virtual test vehicle and source of virtual measurements of the vehicle responses (axlebox accelerations). Different operating conditions are considered, and the virtual acceleration measurements are used as input to the load identification method for indirect measurement of the wheel-rail forces. Finally, the identified vertical and lateral wheelset force, vertical force of the left and right wheels, derailment coefficient and wheel unloading rate are compared with the corresponding time-histories from the virtual test-vehicle simulations. A good agreement is found with all of their correlation coefficients above 0.8. This result has important implications on the safety evaluation of vehicle curve negotiation by using acceleration measurements only, without the need for direct force measurements.

Structural and Double Layer Forces between Silica Surfaces in Suspensions of Negatively Charged Nanoparticles.

Direct force measurements between negatively charged silica microparticles are carried out in suspensions of like-charged nanoparticles with atomic force microscopy (AFM). In agreement with previous studies, oscillatory force profiles are observed at larger separation distances. At smaller distances, however, soft and strongly repulsive forces are present. These forces are caused by double layer repulsion between the like-charged surfaces and can be quantitatively interpreted with the Poisson-Boltzmann (PB) model. However, the PB model must be adapted to a strongly asymmetric electrolyte to capture the nonexponential nature of these forces. Thereby, the nanoparticles are modeled as highly charged co-ions, while the counter ions are monovalent. This model permits extraction of the effective charge of the nanoparticles, which is well comparable to the one obtained from electrophoresis. The PB model also explains the presence of a particle-free layer close to the interface.

Single‐Molecule Interactions between Heme Proteins and Carboxylic Groups in Various Chemical Environments

AbstractNoncovalent chemical interactions, including electrostatic interactions, hydrogen bonding, and hydrophobic interactions, constitute crucial driving forces in the bottom‐up assembly of multifunctional molecular complexes and nanomaterials. Electrostatic interactions in chemical environments are generally considered as non‐specific interactions, which pose notable challenges for the control and direct measurement of electrostatic forces at the single‐molecule level. Here, we report a model system consisting of cytochrome c and molecules containing carboxyl groups, and use electrochemical AFM force spectroscopy to navigate the electrostatic forces of the system under various chemical conditions. The effects of solution pH, ion strength, and redox states of cytochrome c on electrostatic interactions as well as control over these parameters are systematically studied. This work provides a useful approach for investigating intermolecular electrostatic interactions at the single‐molecule level.

Measuring the systolic and the diastolic properties of single human pluripotent stem-cell cardiomyocyte for drug testing and disease modeling

Abstract Background The advent of human pluripotent stem cell–derived cardiomyocytes (hPSC-CMs) provided exciting tools for cardiovascular physiological studies, disease modeling and drug testing applications. Current platforms for studying the mechanical properties of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) as single-cells do not measure forces directly, require numerous assumptions, and cannot study cell mechanics at different loading conditions. Objective To establish a novel platform to assess the active and passive mechanical properties of single-cell hPSC-CMs at different loading conditions and to demonstrate the potential of this approach for drug testing and disease modeling applications. Methods and results To allow morphological maturation, hPSC-CMs were treated with Tri-iodo-thyronine hormone, dexamethasone and Insulin-like growth factor-1. The hPSC-CM were then lifted and attached to a highly sensitive optical-force transducer and a piezoelectric length controller and electrically-stimulated. The attached hPSC-CM remained intact and contractile allowing evaluation of their passive and active mechanical properties. Utilizing this technique, single-cell hPSC-CMs exhibited positive length-tension (Frank-Starling) relationships, and appropriate inotropic, klinotropic, and lusitropic changes in response to treatment with isoproterenol. The unique potential of the approach for drug testing and disease modeling was exemplified by treating the cells with doxorubicin (a potential cardiotoxic anti-cancer agent) and omecamtiv mecarbil (a positive ionotropic drug currently in stage 3 clinical trial). The results of these studies recapitulated the drugs' known actions to suppress (doxorubicin) and augment (omecamtiv mecarbil at low dose) cardiomyocyte contractility. Finally, novel insights were gained regarding the cellular effects of these drugs as doxorubicin treatment led to cellular mechanical alternans and high doses of omecamtiv mecarbil suppressed contractility and worsened the cellular diastolic properties. Conclusion A novel method that allows direct active and passive force measurements from single hPSC-CMs at different loading conditions for the first time was established and validated. Our results highlight the potential implications of this novel approach for pharmacological studies and disease modeling studies. Funding Acknowledgement Type of funding source: Public grant(s) – EU funding. Main funding source(s): European Research Council

Leading edge vortex formation and detachment on a flat plate undergoing simultaneous pitching and plunging motion: Experimental and computational study

This study focuses on the formation and detachment of a leading edge vortex (LEV) appearing on an airfoil when its effective angle of attack is dynamically changed, inducing additional forces and moments on the airfoil. Experimental measurements of the time-resolved velocity field using Particle Image Velocimetry (PIV) are complemented by a computational study using an URANS (Unsteady Reynolds-Averaged Navier–Stokes) framework. In this framework a transition-sensitive Reynolds-stress model of turbulence, proposed by Maduta et al. (2018), which combines the near-wall Reynolds-Stress model by Jakirlic and Maduta (2015) and a phenomenological transition model governing the pre-turbulent kinetic energy by Walters and Cokljat (2008), is employed. Combined pitching and plunging kinematics of the investigated flat plate airfoil enable the effective inflow angle to be arbitrarily prescribed. A qualitative assessment of flow fields and a quantitative comparison of LEV characteristics in terms of its center position and circulation as well as an investigation of the mechanism causing the vortex to stop accumulating circulation revealed close agreement between the experimental and simulation results. Further considerations of the lift contribution from the pressure and suction side of the airfoil to the overall lift indicates that the qualitative lift evolution is reproduced even if the pressure side contribution is neglected. This reveals important characteristics of such airfoil dynamics, which can be exploited in future experimental studies, where direct aerodynamic force and moment measurements are greatly inhibited by dominating inertial forces.

Topography-Dependent Effective Contact Line in Droplet Depinning.

The mobility of a fakir state droplet on a structured surface is fundamentally determined by the effective length of a microscopic contact line. However, it is largely unknown how the surface topography determines the effective contact line length. Based on the direct measurement of droplet adhesion force and the visualization of contact line, this work shows that effective contact line length is topography dependent as opposed to prior notion. On pored surfaces, contact line is not distorted, and the effective length approaches the droplet apparent perimeter regardless of pore dimensions. On pillared surfaces, the distortion of contact line is significantly dependent on the packing density of the pillar structures so that the effective length is as small as a pillar diameter on densely packed pillars and as large as a pillar perimeter on sparsely-packed pillars, while changing linearly between the two extremes.

Colloidal stability of cellulose nanocrystals in aqueous solutions containing monovalent, divalent, and trivalent inorganic salts

Aggregation kinetics and surface charging properties of rod-like sulfated cellulose nanocrystals (CNCs) have been investigated in aqueous suspensions containing monovalent, divalent, or trivalent inorganic salts. Electrophoresis and time-resolved dynamic light scattering (DLS) were used to characterize the surface charge and colloidal stability of the CNCs, respectively. The surface charge and aggregation kinetics of the sulfated CNCs were found to be independent of solution pH (pH range 2–10). For the monovalent salts (CsCl, KCl, NaCl, and LiCl), the critical coagulation concentration (CCC) followed the order of Cs+ < K+ < Na+ < Li+, which follows the direct Hofmeister series, indicating specific interaction of the cations with the CNCs surface. The experimental aggregation kinetics of CNCs were in very good agreement with predictions based on the classic Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. A Hamaker constant of 3.6 × 10−20 J for the CNCs in aqueous medium was derived, for the first time, from the colloidal stability curves with monovalent salts. This value is consistent with a previous value determined by direct force measurements for cellulose surfaces in aqueous solutions. For the divalent salts (MgCl2, CaCl2, and BaCl2), the CCC values followed the order Mg2+ > Ca2+ > Ba2+, which is in the reverse order of the counterion ionic size. For the trivalent salts (LaCl3, AlCl3, and FeCl3), the CNCs suspension was destabilized much more effectively. The observed complex stability curves with AlCl3 and FeCl3 are attributed to charge neutralization and charge reversal imparted by the adsorption of aluminum and ferric hydrolysis species on the CNC surface. The significant charge reversal induced by the ferric hydrolysis species led to the restabilization of suspensions. Our results on the colloidal stability of CNCs are of central importance to the nanotechnology and materials science communities working on various applications of CNCs.

Direct Measurement of Contraction Force in Cardiac Tissue Construct in 2D-Plane Using Dual Axis Cantilever Sensor

In this work, we present a technique for a dual axis contraction force measurement of human cell based cardiac tissue constructs. The cardiac tissue constructs consist of a vascular-like network and induced pluripotent stem cell derived cardiomyocytes. Before the force measurements, the cardiac tissue constructs were detached from the culture substrate to allow less restricted contraction. The in-house prepared force sensors are composed of piezoelectric sensing elements and a metallic cantilever for contacting the cardiac tissue constructs. A dedicated measurement platform with embedded signal processing software is used for data acquisition from the sensors. Dual axis force sensor results are compared with our previously developed single axis force sensor technique. Additionally, the proposed dual axis force measurement system can measure two-dimensional displacement trajectories of the cantilever probe tip. We propose a pattern matching method for classification of the captured cardiac contraction cycle patterns and for extracting anomalies in the measured cycles. We demonstrate both single and dual axis peak cardiac construct contraction force measurement results in the ranges of 3.4 - $6.7~\mu \text{N}$ and 9.4 - $10.6~\mu \text{N}$ , respectively. The relative standard deviation of the peak contraction force results varied between 1.0 and 4.1% in eight captured 60 second measurement sequences.

Nanomechanics of π-cation-π interaction with implications for bio-inspired wet adhesion

Cation-π interactions play a vital role in modulating various biological processes, e.g., potassium-selective channel, protein folding and adhesion of marine organism. Previous studies mainly focus on binary cation-π interaction, whereas due to the complexity of biological systems and surrounding environments, a single cation is often in close proximity with more than one π-conjugated unit, which could exhibit essentially different binding behavior. Herein, the first experimental evidence of ternary π-cation-π interaction is reported through direct nanomechanical force measurement in a model π-conjugated poly(catechol) (PC) system coexisting with K+. Ternary π-cation-π interactions can bridge π-conjugated moieties, resulting in robust adhesion and promoting PC assembly and deposition. Particularly, these ternary complexes are discovered to transit to binary binding pairs by increasing K+ concentration, undermining adhesion and assembly due to lack of bridging. The π-cation-π binding strength follows the trend of NMe4+ > K+ > Na+ > Li+. Employing the π-cation-π interaction, a deposition strategy to fabricate π-conjugated moiety based adhesive coatings on different substrates is realized. Our findings provide useful insights in engineering wet adhesives and coatings with reversible adhesion properties, and more broadly, with implications on rationalizing biological assembly.

Direct force measurement of critical detachment force between a particle and an air bubble using dynamic interaction force apparatus

Bubble-particle detachment is the main factor to limit the recovery of coarse particle in flotation. A dynamic interaction force apparatus (DIFA) was designed to investigate the bubble-particle detachment process. The DIFA could directly measure the detachment force and visually observe the interfacial behavior during a particle detaching from an air bubble. The detachment process was composed of three sub-processes: bubble stretching process, bubble sliding process, and bubble necking process. Calculated detachment force showed good agreement with measurement force. The critical detachment force, defined as the maximum detachment force during detachment process, was proposed to determine whether a particle completely detached from a bubble. The Bond number was modified using the critical detachment force. The modified Bond number, which considers the bubble-particle detachment process, was more appropriate to evaluate the bubble-particle aggregate stability. It was found that the bubble-particle aggregate stability was enhanced by increasing surface hydrophobicity, whereas the stability was weakened by increasing particle size. The weak stability of bubble-particle aggregate results in a low recovery in coarse particle flotation.