Mechanical Properties Of Soft Materials Research Articles

Axisymmetric Compression of a Circular Particle Raft Driven by the Diffusion of Surfactant.

Particle rafts are a new kind of soft matter formed by self-organization on the interface, which possesses mechanical properties between fluid and solid, and they have been widely used in many industrial fields. In the present study, the compression experiment of a circular particle raft is first performed, where an SDS (sodium dodecyl sulfate)-coated metal ring is placed around its periphery. When the surfactant diffuses, the particle raft shrinks, and its shrinkage ratio increases with the increase in the surfactant concentration, where the experimental results are consistent with the numerical simulation. Next, the relationship between the initial surface tension difference of SDS and the radius shrinkage of the particle raft is obtained by dimensional analysis. In what follows, the diffusion model is built to quantify the diffusion process of SDS at the liquid-gas interface, and then the analytical concentration solution of the concentration of SDS at the periphery of particle raft is given. The particle raft is viewed as an elastic circular plate under the action of the radial pressure, which originates from the surface tension difference, which has been verified by the experimental result. These explorations cast a new light on how to apply loads to measure mechanical properties of soft matter, which also provide some inspirations on the design of microsensors and microfluidics.

Stiffness and pre-stretching estimation from indentation test of hyperelastic membrane

Obtaining precise data on the mechanical properties of soft materials, often in the form of thin, pre-tensioned membranes, is crucial, especially when conventional testing methods have limited applicability. The article focuses on an innovative method for estimating the Young's modulus and pre-stretching the membranes using the indentation method, assuming an isotropic, incompressible hyperelastic material. The parameters were estimated for two models of indenter-membrane contact: with perfect slip and without slip. Experimental tests were performed for pre-stretched latex membranes glued to a stiffer ring to minimize the effect of attachment on membrane deformation. In order to validate the estimation method, the predictions of the model with the determined mechanical parameters were compared with the results of indentation tests with a liquid layer under the membrane. The discussion shows the consistency of the estimation results with the literature results for small latex deformations, and indicates the advantages and numerous limitations of the approach, especially related to the choice of the material model.

Accurate measurement of mechanical properties of soft materials by introducing transition layers into test samples

Soft materials loaded by conventional clamps are susceptible to damage caused by local stress concentrations, which leads to inaccurate measurement of strength, toughness, and other important properties. Herein, we propose a facile approach to solve the problem by introducing transition layers to test samples. The effectiveness of the proposed method is demonstrated by experiments and numerical analyses. It is found that the introduction of transition layers can stably and accurately reveal the stress-stretch relationship of soft materials by using the conventional hard clamps. What is more, the transition layers have almost no effect on the fracture toughness measurement for pure soft material samples and those with transition layers. These results should be of great significance for the measurement of various materials, and can serve as a design guidance of various soft material structures in blooming applications.

A Novel Microindenter to Assess the Mechanical Properties of Soft Materials

A Novel Microindenter to Assess the Mechanical Properties of Soft Materials

A New Approach for Measuring Viscoelastic Properties of Soft Materials Using the Dynamic Response of a Spherical Object Placed at the Sample Interface

BackgroundThere are several techniques to characterize the mechanical properties of soft materials, such as the indentation method and the method based on the application of a spherical object placed inside the sample. The indentation systems usually yield the elastic properties of materials and their mathematical models do not consider the inertia of the sample involved in motion and radiation damping, while placing an object inside the sample is not practical and this procedure can alter the mechanical properties of the sample for the method based on the application of a bubble/sphere placed inside the sample.ObjectiveA new approach for the identification of the viscoelastic properties of soft materials using the dynamic response of a spherical object placed at the sample interface was proposed.MethodsThe spherical object placed at the sample interface was pressed using an electromagnet and the dynamic response of the spherical object was tracked using a high-speed camera, while the dynamic response of the spherical object placed at the sample interface was estimated using a comprehensive analytical model. The effects of the shear modulus, viscosity, Poisson’s ratio and density of the soft sample, the radius and density of the spherical object and the damping due to radiation were considered in this mathematical model. The shear modulus and viscosity of the soft sample were determined by matching the experimentally identified and theoretically estimated responses of the spherical object.ResultsThe shear moduli and viscosities of the three phantoms with the gelatin mass ratios of 0.20, 0.25 and 0.29 were measured to be 3450, 4300 and 4950 Pa and 12.5, 14.0 and 15.0 Pa⋅s, respectively. The shear modulus and viscosity of the phantom increases as the gelatin mass ratio increases. The frequency of oscillations of the hemisphere placed at the phantom interface increases as the gelatin mass ratio increases due to stiffness increase.ConclusionsAfter matching the experimental and theoretical steady-state displacements and amplitudes of oscillations of the hemisphere at the sample interface, the comparison of the experimentally identified and theoretically predicted frequency of oscillations further confirmed the identified material properties of the samples. The approach presented here is expected to provide valuable information on material properties in biomedical and industrial applications.

In-situ shearing process observation system forsoft materials via transmission electron microscopy.

We developed an in-situ shear test system suitable for transmission electron microscopy (TEM) observations, which enabled us to examine the shear deformation behaviours inside soft materials at nanoscale resolutions. This study was conducted on a nanoparticle-filled rubber to investigate its nanoscale deformation behaviour under a large shear strain. First, the shear deformation process of a large area in the specimen was accurately examined and proven to exhibit an almost perfect simple shear. At the nanoscale, voids grew along the maximum principal strain during shear deformation. In addition, the nanoscale regions with rubber and silica aggregates exhibited deformation behaviours similar to the global shear deformation of the specimen. Although the silica aggregates exhibited displacement along the shearing directions, rotational motions were also observed owing to the torque generated by the local shear stress. This in-situ shear deformation system for TEM enabled us to understand the nanoscale origins of the mechanical properties of soft materials, particularly polymer composites. Graphical Abstract.

Elasticity of whole blood clots measured via Volume Controlled Cavity Expansion

Measuring and understanding the mechanical properties of blood clots can provide insights into disease progression and the effectiveness of potential treatments. However, several limitations hinder the use of standard mechanical testing methods to measure the response of soft biological tissues, like blood clots. These tissues can be difficult to mount, and are inhomogeneous, irregular in shape, scarce, and valuable. To remedy this, we employ in this work Volume Controlled Cavity Expansion (VCCE), a technique that was recently developed, to measure local mechanical properties of soft materials in their natural environment. Through highly controlled volume expansion of a water bubble at the tip of an injection needle, paired with simultaneous measurement of the resisting pressure, we obtain a local signature of whole blood clot mechanical response. Comparing this data with predictive theoretical models, we find that a 1-term Ogden model is sufficient to capture the nonlinear elastic response observed in our experiments and produces shear modulus values that are comparable to values reported in the literature. Moreover, we find that bovine whole blood stored at 4 °C for greater than 2 days exhibits a statistically significant shift in the shear modulus from 2.53 ± 0.44 kPa on day 2 (N = 13) to 1.23 ± 0.18 kPa on day 3 (N = 14). In contrast to previously reported results, our samples did not exhibit viscoelastic rate sensitivity within strain rates ranging from 0.22 – 21.1 s−1. By surveying existing data on whole blood clots for comparison, we show that this technique provides highly repeatable and reliable results, hence we propose the more widespread adoption of VCCE as a path forward to building a better understanding of the mechanics of soft biological materials.

Fishing Net‐Inspired Mutiscale Ionic Organohydrogels with Outstanding Mechanical Robustness for Flexible Electronic Devices

AbstractMechanically robust and electrically conductive organohydrogels/hydrogels are increasingly required in flexible electronic devices, but it remains a challenge to achieve organohydrogels/hydrogels with integrated high performances. Herein, inspired by the geometric deformability and robustness of fishing nets, multiscale ionic organohydrogels with outstanding isotropic mechanical robustness are developed. The organohydrogels are prepared by introducing polyacrylamide (PAM) hydrogel, Zn2+ and a binary solvent of glycerol‐water into a crosslinked fibrous mat which is electrospun from poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA). Because of the unique structure, the resultant organohydrogels, being mentioned as PAA‐PVA/PAM/Zn2+ organohydrogels, exhibit outstanding tensile strength (9.45 MPa), high stretchability, excellent anti‐fatigue property, skin‐like mechanical behaviors and ionic conductivity. Importantly, the organohydrogels are promising in flexible electronic devices capable of operating properly over a wide temperature range and under harsh mechanical conditions, such as mechanical‐electrical signal transducing materials in flexible mechanosensors and robust electrolytes in zinc ion hybrid supercapacitors. Not only the multiscale design strategy will provide a clue to improve the mechanical properties of soft materials, but also the organohydrogels offer promising materials for future flexible electronic devices.

Design, testing and control of a smart haptic interface driven by 3D-printed soft pneumatic actuators for virtual reality-based hand rehabilitation

This work presents the main steps of design and testing of a novel haptic interface and adaptive admittance control scheme for optimal regulation of the human–machine interaction in hand rehabilitation mediated by a smart system in virtual–reality environment. The prototype development is the result of an integrated HW/SW design and, moreover, the advantages from additive manufacturing techniques and mechanical properties of soft materials are exploited for the realization steps. Indeed, to make the interface smart, a network of piezo-resistive force sensors is embedded into the user’s command interface and the acquired signals are used for the adaptive regulation of human–machine interaction. Another distinctive feature of the haptic interface, which enables to identify this latter as a smart system, is the interaction control based on the estimation of the user’s intention within a novel scheme of adaptive admittance control. The enhanced training process in rehabilitation assisted by the haptic interface and virtual environment has been experimentally validated during a series of goal-directed tasks. The improvement of the motor performance of the user under the assistance of the adaptive admittance control has been experimentally evaluated. Further results show that the rehabilitation system supports the quantitative assessment of the robustness of the motor learning performance of the hand under the generation of haptic disturbances.

Studying the high-rate deformation of soft materials via laser-induced membrane expansion.

Cavitation is a phenomenon that occurs when the internal pressure of a material exceeds the resistance to deformation provided by the surrounding medium. Several measurements, such as the blister test, bubble inflation, and cavitation rheology, take advantage of this phenomenon to measure the local mechanical properties of soft materials at relatively low deformation rates. Here, we introduce a new measurement called laser-induced membrane expansion (LIME) that measures the shear modulus of a thin membrane at high strain rates (≈106 s-1 to 108 s-1) by using laser ablation to rapidly expand a thin (tens of microns) elastomeric membrane. To demonstrate the capabilities of this measurement, we use LIME to study the mechanical properties of poly(dimethylsiloxane) (PDMS) membranes at several thicknesses (from 10 μm to 60 μm) and crosslink densities. We find that the shear modulus of the PDMS measured by LIME was weakly dependent on the crosslink density, but was strongly strain rate dependent with values ranging from 106 Pa to 108 Pa. This measurement platform presents a new approach to studying the mechanical properties of soft but thin materials over a range of deformation rates.

Using laser-induced breakdown spectroscopy to characterize the surface mechanical properties of soft materials

The ratio of ionic to atomic lines with surface rigidity implying the feasibility of characterizing soft surface mechanical properties using LIBS.

Tailoring Rate and Temperature-Dependent Fracture of Polyether Networks with Organoaluminum Catalysts

Polyethers are ubiquitous in engineering and biomedical applications because their oxygen-rich backbone allows them to interact with a variety of polar small molecules such as ions, gases, and pharmaceuticals. These materials are also able to sustain large reversible deformations when cross-linked at the molecular scale, leading to an interesting combination of functional and mechanical properties. We synthesized two families of polyether networks by organoaluminum-catalyzed ring-opening copolymerization of ethyl glycidyl ether (EGE) monomer and 1,4-butanediol diglycidyl ether (BDGE) cross-linker and explored the relationship between network architecture and fracture properties. The key result is that living copolymerizations, as enabled by a chelate of triethylaluminum with dimethylaminoethanol, afford access to a critical cross-link density, νx ≈ 3 × 1025 chains/m3, and loss tangent, tan(δ) ≈ 0.09, at which fracture is dominated by chain scission rather than friction. Such control over the fracture resistance of polyether networks unveils the potential of living copolymerizations to design the functional and mechanical properties of soft materials.

Editorial: Exploring the mechanical properties of soft materials at multiple length scales

Editorial: Exploring the mechanical properties of soft materials at multiple length scales

Passive and Active Microrheology for Biomedical Systems.

Microrheology encompasses a range of methods to measure the mechanical properties of soft materials. By characterizing the motion of embedded microscopic particles, microrheology extends the probing length scale and frequency range of conventional bulk rheology. Microrheology can be characterized into either passive or active methods based on the driving force exerted on probe particles. Tracer particles are driven by thermal energy in passive methods, applying minimal deformation to the assessed medium. In active techniques, particles are manipulated by an external force, most commonly produced through optical and magnetic fields. Small-scale rheology holds significant advantages over conventional bulk rheology, such as eliminating the need for large sample sizes, the ability to probe fragile materials non-destructively, and a wider probing frequency range. More importantly, some microrheological techniques can obtain spatiotemporal information of local microenvironments and accurately describe the heterogeneity of structurally complex fluids. Recently, there has been significant growth in using these minimally invasive techniques to investigate a wide range of biomedical systems both in vitro and in vivo. Here, we review the latest applications and advancements of microrheology in mammalian cells, tissues, and biofluids and discuss the current challenges and potential future advances on the horizon.

Multiscale Microrheology Using Fluctuating Filaments as Stealth Probes.

The mechanical properties of soft materials can be probed on small length scales by microrheology. A common approach tracks fluctuations of micrometer-sized beads embedded in the medium to be characterized. This approach yields results that depend on probe size when the medium has structure on comparable length scales. Here, we introduce filament-based microrheology using high-aspect-ratio semiflexible filaments as probes. Such quasi-1D probes are much less invasive than beads due to their small cross sections. Moreover, by imaging transverse bending modes, we simultaneously determine the micromechanical response of the medium on multiple length scales corresponding to the mode wavelengths. We use semiflexible single-walled carbon nanotubes as probes that can be accurately and rapidly imaged based on their stable near-IR fluorescence. We find that the viscoelastic properties of sucrose, polyethylene oxide, and hyaluronic acid solutions measured in this way are in good agreement with those measured by conventional micro- and macrorheology.

Poisson’s ratio measurement through engraving the grid pattern inside poly(dimethylsiloxane) by ultrafast laser

This work demonstrates an application of ultrafast laser processing to engrave a grid pattern for evaluating mechanical properties. No observable changes in the material properties caused by laser engraving were confirmed if the grid pattern is engraved with laser energy of 0.21 μJ/pulse and pulse repetition rate of 1 kHz. The shape changes of grid pattern directly reveal whether the deformation accompanied with stretching is uniaxial or not. By analyzing the optical images of the grid pattern, Poisson ratio, Young’s modulus, and shear modulus of poly(dimethylsiloxane) (PDMS) with a weight percent of curing agent of 9% were estimated to be 0.49989 ± 0.00111, 1.45 ± 0.01 MPa, and 0.725 MPa, respectively. The comparison of the current results with the literature values suggests that the proposed method is appropriate to provide quantitative mechanical properties of soft materials with high accuracy.

Meso/macro-scale ultra-soft materials' mechanical property evaluation device and testbed.

Ultra-soft materials find applications in biomedical devices, sensors and actuators, robotics, and wearable electronics. The mechanical properties of soft materials are often determined using nanoindentation and atomic force microscope techniques, which provide localized properties at a small-scale length. There is a need to evaluate the meso/macro-scale properties of ultra-soft materials to develop integrated devices made of the same. Metallic probes in the existing macroscale equipment cannot be used as they can pierce through the soft materials and fail to capture the associated adhesion forces, resulting in inaccurate values. This study has developed a meso/macro-scale mechanical testing platform to characterize ultra-soft materials accurately. This probe can be adapted to any mechanical testing load frame with a small load cell to capture the adhesion forces during the approach and detachment segments of soft materials' indentation. The integrated camera with the probe enables overcoming the challenge of surface detection and capturing the pull-on and pull-off events. Indentation tests on soft materials with varying stiffness (e.g., high-fat yogurt, chicken breast, aloe Vera, toothpaste, gelatin, and a chocolate bar) were conducted using a 10mm stiff flat-end polymer probe. A variation of the Johnson-Kendall-Roberts technique was adopted to account for adhesion forces and compute stiffness. Our results suggest that the novel device and methodology can measure mechanical stiffness in the extensive range of 0.5kPa to a few MPa with high reproducibility at the macro-scale length. The validation was carried out using a commercially available nanoindenter for soft materials.

Lift forces on three-dimensional elastic and viscoelastic lubricated contacts

When an object suspended in fluid moves past a soft substrate it experiences a lift force due to the deformability of the substrate. Here, we find this lift force analytically for a general elastic or viscoelastic substrate. For oscillatory relative motion, the lift force is time-dependent and its temporal Fourier modes carry signatures of the viscous and elastic components of the substrate response. The theory may be useful to characterize the mechanical properties of soft materials without solid-to-solid contact via lift force measurements.

Measurement of quasi-static and dynamic mechanical properties of soft materials based on instrumented indentation using a piezoelectric cantilever

Instrumented indentation is a powerful in-situ mechanical characterization method and has been intensively investigated in recent years. In this work, we proposed an instrumented indentation method for soft materials based on a piezoelectric bimorph cantilever where the indentation force (F) is measured by a strain gauge and the indentation depth (h) is monitored by a piezoelectric motor driven displacement platform. The quasi-static mechanical properties of the soft samples are obtained by using the F-h curve and the dynamic properties are extracted from the contact resonance curve of the piezoelectric cantilever which is measured by using electromechanical impedance (EMI). Firstly, the working principle of this indentation method for quasi-static and dynamic mechanical property measurement is presented. Then, both the quasi-static and dynamic indentation testing were conducted on a silica gel sample, and the testing results fit well with that obtained using tensile testing. Later, the viscoelastic properties of a Polydimethylsiloxane (PDMS) sample was tested which is strongly dependent on the indentation force. Finally, modulus imaging was conducted on a silica gel sample containing a stiffer elastomer sphere using the local contact stiffness (LCS) as the indicator, and the elastomer can be clearly revealed. The proposed piezoelectric cantilever based instrumented indentation method is very convenient for mechanical characterization of soft materials and may be widely used in near future.

Indentation probe with optical fibre array‐based optical coherence tomography for material deformation

We present a new optomechanical probe for mechanical testing of soft matter. The probe consists of a micromachined cantilever equipped with an indenting sphere, and an array of 16 single‐mode optical fibres, which are connected to an optical coherence tomography (OCT) system that allows subsurface analysis of the sample during the indentation stroke. To test our device and its capability, we performed indentation on a PDMS‐based phantom. Our findings demonstrate that Common Path (CP)‐OCT via lensed optical fibres can be successfully combined with a microindentation sensor to visualise the phantom's deformation profile at different indentation depths and locations in real time.Lay DescriptionThis work presents a new approach to simultaneously perform micro‐indentation experiments and OCT imaging. An optical fiber array‐based sensor is used to develop a new hybrid tool where micro‐indentation is combined with optical coherence tomography. The sensor is therefore capable of compressing a sample with a small force and simultaneously collecting OCT depth profiles underneath and around the indentation point. This method offers the opportunity to characterize the mechanical properties of soft materials and simultaneously visualize their deformation profile. The ability to integrate OCT imaging with indentation technology is promising for the non‐invasive and precise characterization of different soft materials.