Johnson-Kendall-Roberts Research Articles

The effect of JKR surface energy on the flowing property of fresh concrete in DEM simulation

The effect of Johnson–Kendall–Roberts (JKR) surface energy on the flowing property of fresh concrete was numerically studied. The fresh concrete was simplified into coarse aggregate and mortar, both described as the particles. The Hertz-Mindlin with JKR contact model was employed to describe the constitutive relationship of fresh concrete. The contact parameters were calibrated through experiment and numerical simulation of multiple flow tests. The impact of these contact parameters on the residual height (Hr) of fresh concrete after slumping was examined using a slump test. The significance degree of mortar–mortar contact parameters on the Hr was studied using an orthogonal test. The effect of the surface energy ( γ ) on the flow index (Bm) and flow time (Tv) of fresh concrete was studied intensively based on the L-box flow and V-funnel tests. The mortar–mortar contact parameters, γ and rolling friction coefficient (μ r), most significantly impacted the Hr. The mortar–mortar surface energy ( γ m-m) significantly impacted Bm and Tv compared to the aggregate–mortar and aggregate–aggregate surface energies (( γ a-m) and ( γ a-a), respectively). The effect of γ a-a, γ a-m, and γ m-m on Bm was interconnected. Under appropriate parametric values, the Bm was the most stable, and the workability of fresh concrete was the best. In addition, as γ increased, Hr and Bm decreased and Tv increased. The flowability of the fresh concrete worsened.

A unified treatment of axisymmetric adhesive contact for piezoelectric materials

Nanoindentation technique has been widely applied to characterize the electromechanical properties of piezoelectric materials, where the adhesion effect induced by different surface forces becomes very prominent. The indenter tip should have more general profile rather than just three common simple shapes (flat, cone and sphere) in practical testing. In this study, the classical Johnson-Kendall-Roberts (JKR) and Maugis-Dugdale (M-D) models are extended to investigate the adhesion behaviors of a piezoelectric half-space indented by the power-law shaped punches with different electric properties, whose shape index is denoted as n. The JKR-n and M-D-n adhesion models for both the electrically conducting and electrically insulating punches are set up by means of Griffith energy balance. The Derjaguin-Muller-Toporov-n (DMT-n) adhesion solutions are derived from the corresponding M-D-n adhesion models as the limiting cases. The generalized Tabor parameter and interfacial adhesion strength applicable to piezoelectric materials are defined for the first time, with the former being used to describe the transition behavior from JKR-n model to DMT-n model. The impacts of the electric loading and the shape index on adhesion behaviors are discussed in detail. It is found that the pull-off force increases with the electric potential, which reveals the adhesion strengthening effect caused by electric loading. The shape index has a prominent influence on the pull-off force, interfacial adhesion strength, contact radius at pull-off moment, indentation depth and contact radius at self-equilibrium state, and the transition behavior from JKR-n model to DMT-n model. The results derived from this work not only are helpful to understand the contact behaviors of piezoelectric materials at micro/nano scale, but also provide a theoretical basis for nanoindentation technique in evaluating material properties of piezoelectric mediums.

Discrete Element Modelling of a Bulk Cohesive Material Discharging from a Conveyor Belt onto an Impact Plate

The discrete element method (DEM) has become the numerical method of choice for analysing and predicting the behaviour of granular materials in bulk handling systems. Wet-and-sticky materials (WSM) are especially problematic, resulting in build-up and blockages. Furthermore, due to the large number of particles in industrial-scale applications, it is essential to decrease the number of particles in the model by increasing their size (upscaling or coarse graining). In this study, the accuracy with which upscaled DEM particles can model the discharge of a cohesive material from a belt conveyor onto an inclined impact plate was investigated. Experimentally, three sand grades (particle size distributions, PSDs) were used, each in a dry (non-cohesive) state and with three levels of moisture-induced cohesion. The effects of the modelled PSDs on the material flow, build-up on the plate, the peak impact force and the residual weight were investigated. Although a linear cohesion contact model was mostly used, the results were also compared to that of the Johnson–Kendall–Roberts (JKR) and simplified JKR (SJKR) models. It was found that the general profile of the pile (build-up) could be accurately modelled, but using a more accurate (but still upscaled) PSD improved the results. The impact force and the residual weight on the plate could be accurately modelled (error <15%) if the particle size was not excessively scaled. The maximum acceptable scaling factor was found to be a geometric factor of the bulk measure of interest, and not a factor of the physical particle size. Furthermore, with an increase in cohesion, the bulk measures such as the thickness of the discharge stream and the height of the material build-up increased, which meant that the maximum acceptable scale factor also increased. The results are valuable for future accurate and efficient modelling of large industrial scale applications of WSMs.

Static Wetting of a Liquid Droplet on a Soft Elastic Substrate

Abstract A refined spherical cap model, combined with an elastic foundation model for the elastic substrate, is proposed to study the static wetting of a liquid droplet on a soft elastic substrate. The strain energy of the substrate is evaluated by the Johnson–Kendall–Roberts (JKR) model, and the increase of the surface energy of the substrate outside the contact zone is calculated based on the elastic foundation model. The total potential energy of the droplet-substrate system is given in terms of four geometrical parameters: the contact radius, the contact angle of the droplet, the deflection angle inside the contact zone, and the maximum downward displacement of the substrate surface at the contact zone center. The equilibrium state is determined based on the stationary condition of total potential energy. The present model reduces to the Young’s equation for a rigid substrate and to the Neumann’s triangle for a liquid-like substrate. Three equations are given to determine the liquid droplet shape in terms of surface energies and substrate’s elastic modulus. Reasonable agreement with existing experimental data and simulation results shows that the present model with derived formulas has the potential to catch the role of substrate’s elastic deformation on static wetting and fill the gap between the Young’s equation and the Neumann’s triangle for a soft elastic substrate.

A force–displacement relation based on the JKR theory for DEM simulations of adhesive particles

Studying the collective behavior of adhesive particles with the discrete element method (DEM) requires well-founded force–displacement relations (force models). While the Johnson–Kendall–Roberts (JKR) theory reliably predicts the dependence of the contact radius a on the force F, it has remained a challenge in this framework to obtain a straightforward force–displacement relation F(δ) with physically meaningful parameters to calculate the force F from the displacement δ. We derive a novel force–displacement relation from the JKR theory as a composition of functions F(δ)=(F∘a∘λ)(δ), with the intermediate functions contact radius a(λ) and effective adhesive contact radius λ(δ). We also analyze contact geometry errors in the Hertz and JKR models, derive the exact contact centroid to accurately calculate contact torques and relative tangential velocities, and propose a smoothed JKR model to avoid discontinuities in force and energy. We find that incorrect torques and relative tangential velocities are obtained when the stiffness quotient is neglected because it affects the exact location of the contact centroid. We also find that contact geometry errors can become non-negligible for nanoparticles due to their large relative contact size. In addition, we show from bouncing ball simulations that the JKR models with a nominal coefficient of restitution derived for the Hertz model result in higher damping and thus reduced actual coefficients of restitution. Our analysis serves as a foundation for contact-mechanics-based force models for DEM simulations of adhesive particles to investigate their collective behavior in the future.

A spheropolyhedral-based discrete element lattice Boltzmann method for simulation of non-spherical adhesive particulate flow

Non-spherical particulate flow is commonly encountered in nature, industry and our daily life. But up to now, there still lack effective model to describe the adhesive elastic collision between non-spherical microparticles. To solve this problem, a spheropolyhedral-based discrete element method (DEM) combined with Johnson-Kendall-Roberts (JKR) model is proposed. The contact between non-spherical particles is attributed to the contacts between three geometry features, including vertice-vertice (V-V), vertice-edge(V-E), vertice-face (V-F) and edge-edge (E-E). The JKR adhesive model is modified to calculate the contact force of the contacting geometry features based on the actual contact area. Meanwhile, the lattice Boltzmann coupled with the immersed moving boundary (LB-IMB) method is employed to simulate the fluid flow and fluid-particle interaction. The novel spheropolyhedral-based DE-LBM is verified to be accurate and effective by several experiments and the data in references. Finally, an application test for simulating the packing process of non-spherical microparticles is presented. The method well predicts the packing structure of both adhesive and non-adhesive non-spherical particles, meaning that it can be easily extended to simulate the massive non-spherical microparticles with high effectiveness and efficiency. Program summaryProgram Title: adhesive contact force model for non-spherical particleCPC Library link to program files:https://doi.org/10.17632/vswvv7ppz2.1Developer's repository link:https://github.com/QIAN-C/adhesive-force-model-for-non-sphericalLicensing provisions: GNU General Public License v3Programming language: C++Nature of problem: Various methods have been proposed to handle the complex shapes of non-spherical particles in the discrete element method. However, there is still a lack of effective models for calculating the contact force between the non-spherical fine particles, particularly the adhesive forces.Solution method: Based on the sphere-polygon algorithm and the JKR model, this program has developed a contact force model that can accurately calculate adhesive forces between non-spherical fine particles within the framework of the open source library MechSys (URL: http://mechsys.nongnu.org/index.shtml).

Mechanics of heterogeneous adhesive contacts

Contact of elastic solids with heterogeneous interface energy of adhesion is considered. The known results on the asymptotic analysis of the quasi-static crack propagation along with the Griffith-type approach (based on the energy balance) and the Irwin-type approach (based on the concept of the contact stress intensity factor) are applied to generalize the Johnson–Kendall–Roberts (JKR) theory of adhesive contact. Under the assumption of slightly heterogeneous adhesion, the effective interface energy is determined depending on the approach used. The asymptotic equivalence between the three derived models, including that based on the mixed Griffith–Irwin-type approach, has been established. The leading-order asymptotic approximation for the pull-off force is derived. The cases of the ray-shaped and periodically-patterned adhesion are considered in detail.

碎玉米秸秆离散元模型参数标定

For the problem of lacking reliable parameters during simulation analysis of the crushed corn stalk (CCS) micro-comminution process with gas-solid coupling method, the simulation parameters are calibrated using a combination of physical measurements and virtual experiments with CCS as the research object. The intrinsic parameters of CCS are obtained by physical experiment, and the stacking test is carried out by cylinder lifting method, and the actual angle of repose(AoR) is obtained as 44.12° by fitting the boundary line with Matlab software; the discrete element model of CCS is established, and the virtual stacking test is carried out by EDEM software; CCS-CCS dynamic friction coefficient, CCS-CCS static friction coefficient and JKR (Johnson-Kendall-Roberts) surface energy are identified as the parameters with significant effects on the AoR by Plackett-Burman test; the steepest climb test is leveraged to determine the center of response surface analysis; the quadratic polynomial regression model of the simulation parameters and the AoR is established by the Box-Behnken test using the AoR as the evaluation index, and the optimal combination of the significant parameters are obtained as follows: the CCS-CCS dynamic friction coefficient is 0.55, the CCS-CCS static friction coefficient is 0.14, and the JKR surface energy is 0.12; the AoR verification test is conducted based on the optimal combination of the significant parameters The results show that the AoR of the CCS is 43.82°, which is 0.68% of the actual AoR, indicating that the parameter combination is reliable. The data obtained in this research can provide corresponding simulation parameters for CCS discrete element simulation and the development of straw micro-crushing equipment.

Modeling the adhesion of spherical particles on rough surfaces at nanoscale

In this paper, the Van der Waals force acting between a spherical nanoparticle and a surface with random distribution of asperities has been simulated using the Johnson-Kendall-Roberts (JKR) contact mechanics model. To predict the adhesion force in different mediums and under different contact conditions, it is necessary to calculate the adhesion force in micro/nano systems. For this purpose, four different cases of surface roughness have been studied. The geometries of these rough surfaces have been defined taking into account the two key parameters of asperity radius and asperity height, and the necessary equations to determine the contact area and the magnitude of the adhesion force have been derived. Based on the geometries of the surface cap/trough, the interaction force between a spherical nanoparticle and a rough substrate has been obtained by applying Van der Waals theory. Then, the adhesion force for spherical nanoparticle has been modeled with respect to its height above the rough surface. Finally, the behavior of nanoparticles with different radii on four different rough substrates have been simulated and analyzed. The simulation results show that more precise adhesion forces can be obtained by modelling the surface roughness at the nanoscale. For small distances between spherical nanoparticle and the substrate, the adhesion force determined with the new roughness models is larger for rough substrates than for smooth ones.

Buckling and Interfacial Deformation of Fluorescent Poly(N-isopropylacrylamide) Microgel Capsules

Hollow microgels are fascinating model systems at the crossover between polymer vesicles, emulsions, and colloids as they deform, interpenetrate, and eventually shrink at higher volume fraction or when subjected to an external stress. Here, we introduce a system consisting of microgels with a micrometer-sized cavity enabling a straightforward characterization in situ using fluorescence microscopy techniques. Similarly to elastic capsules, these systems are found to reversibly buckle above a critical osmotic pressure, conversely to smaller hollow microgels, which were previously reported to deswell at high volume fraction. Simulations performed on monomer-resolved in silico hollow microgels confirm the buckling transition and show that the presented microgels can be described with a thin shell model theory. When brought to an interface, these microgels, that we define as microgel capsules, strongly deform and we thus propose to utilize them to locally probe interfacial properties within a theoretical framework adapted from the Johnson-Kendall-Roberts (JKR) theory. Besides their capability to sense their environment and to address fundamental questions on the elasticity and permeability of microgel systems, microgel capsules can be further envisioned as model systems mimicking anisotropic responsive biological systems such as red blood and epithelial cells thanks to the possibility offered by microgels to be synthesized with custom-designed properties.

Comprehensive Measurement of the Adhesion between Ice or Glass and Model PDMS Coatings

Ice accretion is an unavoidable phenomenon that endangers the performance of outdoor infrastructure and vehicles at low temperatures. To combat ice accretion, the use of anti‐icing coatings has been one of the most appealing approaches because of their simplicity. Silicone elastomers have been increasingly employed due to their natural hydrophobicity, simple preparation, and low price. However, most of the characterization of silicone‐based polymers and other anti‐icing coatings has been done using unproven techniques that often do not directly relate to materials’ properties. In this work, the adhesion between glass and ice and four PDMS‐based elastomers has been studied by a combination of a macroscale shear test, a microscale technique (the Johnson–Kendall–Roberts (JKR) approach), and a commonly used push‐off test. Results are obtained at different temperatures and, importantly, different test velocities. The shear tests yield an energy release rate (the material's property related to adhesion) in the range of 1–10 J m−2, whereas the quasistatic JKR test yields values in the 0.1–0.5 J m−2 range for glass/PDMS interfaces. Ice/PDMS interfaces are found to have larger energy release rates in shear tests and lower values in quasistatic JKR tests. Both differences can be attributed to differences in interfacial crack dynamics.

An analytical self-consistent model for the adhesion of Gibson solid

A full self-consistent model (FSCM) for the adhesive contact between an axisymmetric rigid punch and a Gibson solid (an incompressible, linear graded elastic material) is established, which gives a self-consistent relation between the surface gap and interaction. Power-law shaped indenters and the Lennard–Jones interaction law are studied as representative cases, and the self-consistent equation is expressed in a dimensionless form with two independent parameters, namely the shape index and the Tabor number. The self-consistent equation for Gibson solid is a higher order polynomial equation with respect to the surface gap, which differs from the nonlinear integral equation for power-law graded elastic material. By taking the surface gap as the independent variable instead of the radius, the self-consistent equation is solved explicitly, which gives the first explicit form of the solution to an FSCM. When the Tabor number exceeds a critical value, jump-in instability and adhesion hysteresis occur, and the folding phenomenon that the Gibson solid surface is flipped and folded along the radial direction is observed. The critical Tabor number is determined in an explicit form and it is found to be independent of the surface index. The extended Maugis–Dugdale (M–D) model for Gibson solid is invalid when the Tabor number is large enough and the extended Johnson–Kendall–Roberts (JKR) model does not present the soft limit, due to their assumption of simple contact. An asymptotic solution is derived for the soft limit of the FSCM, which gives a power-law asymptotic relation between the dimensionless pull-off force and the Tabor number. This study provides a self-consistent toy model for the adhesive contact of Gibson solid and may deepen the understanding on the adhesion of graded materials.

A multi-asperity adhesive contact model for catheter and vascular artery contact in endovascular surgery.

Contact behaviors of medical devices, such as guidewires and catheters, are critical in endovascular surgeries. In this work, a new method to predict adhesive contact force between catheter and vascular artery is presented. Multi-asperity adhesion on the surface of vascular artery, deformation of asperity and deformation of vascular substrate are all considered. The single asperity behavior is described with Johnson-Kendall-Roberts (JKR) contact model. The multi-asperity behavior is based on Greenwood-Williamson (GW) asperity model. Vascular substrate is considered as elastic bulk substrate and its deformation is determined with Hertzian pressure from asperity on a circular region on the elastic half space. The model shows that the deformation of vascular substrate accounts for the majority of the total contact deformation and significantly affects the predicted contact force. The model is verified with published experimental data. The comparison shows that the model produces very accurate prediction of contact force between catheter and vascular artery when the contact force is compressive. Parametric analysis based on asperity topography is carried out. The analysis shows that the diameter of the circular region of the interface between asperity and vascular substrate has more significant effect on the estimation of contact force than the radius of asperity. Further validation of prediction accuracy of the model under experiment is needed.

基于坍落度试验的湿粘性水田泥浆颗粒离散元参数分析及标定

In order to obtain the discrete element contact parameters of wet and viscous paddy field mud particles, an accurate numerical simulation model was constructed. Firstly,, the paddy field mud with an average particle size of 0.2 mm was taken as the research object, and the basic physical parameters and rheological behavior laws were obtained through physical measurements and rotational rheological tests. Based on the slump test, combined with the particle scaling theory and Johnson-Kendall-Roberts (JKR) model, and the slump and slump-flow values were taken as response values, the Plackett-Burman test, the steepest climb test and the Box-Behnken test were designed by Design-Expert software to complete the simulation parameters calibration. The optimal significance parameters are as follows: 0.096 J/m2 for the JKR surface energy of mud, 0.13 for the mud-mud restitution coefficient and 0.6 for the mud-steel static friction coefficient. Finally, the mud slump and fluidity verification tests showed that the relative errors between the simulation values and the physical values of slump and slump-flow are 1.73% and 0.42%, and the average error of torque is 2.47%, and the parameters are accurate and reliable. The calibration method can accurately construct the discrete element model of wet-viscous particles, which provides basic data and technical guidance for the coupling mechanism of paddy machinery-mud.

Simulation and Validation of Discrete Element Parameter Calibration for Fine-Grained Iron Tailings

In order to improve the calculation efficiency of a discrete element EDEM (Discrete Element Method) numerical simulation software for micron particles, the particle model is linearly enlarged. At the same time, the parameters of the amplified particles were calibrated according to the Hertz-Mindlin with JKR (Johnson-Kendall-Roberts) contact model to make the amplified particles have the same particle flow characteristics as the actual particles. Actual tests were utilized to gather the angle of repose of the microfine iron tailings, which was then used as a reference value for response surface studies based on the JKR contact model from six factors connected to the fine iron tailings particles. The Plackett-Burman test was used to identify three parameters that had a significant effect on the rest angle: static friction factor; rolling friction factor; and JKR surface energy. The Box-Behnken experiment was used to establish a second-order regression model of the rest angle, and the significant parameters and the optimized parameters were: surface energy JKR coefficient 0.459; particle-particle static friction coefficient 0.393; and particle-particle dynamic friction coefficient 0.393, with a dynamic friction coefficient between particles of 0.106. By entering the parameters into the discrete element program, the angle of repose generated from the simulations was compared with the real test values, and the error was 1.56%. The contact parameters obtained can be used in the discrete element simulation of the amplified particles of fine-grained iron tailings, providing an EDEM model reference for the numerical simulation of fine-grained iron tailings particles. There is no discernible difference between the actual and simulated angles.

The Effect of Adhesion on Indentation Behavior of Various Smart Materials

The nanoindentation technique plays a significant role in characterizing the mechanical properties of materials at nanoscale, where the adhesion effect becomes very prominent due to the high surface-to-volume ratio. For this paper, the classical adhesion theories were generalized to study the contact behaviors of various piezoelectric materials indented by conical punches with different electric properties. With the use of the Hankel integral transform, dual integral equations, and superposing principle, the closed-form solutions of the physical fields for the Johnson-Kendall-Roberts (JKR) and Maugis-Dugdale (M-D) models were obtained, respectively. The contribution of the electrical energy to the energy release rate under the conducting punch was taken into consideration. The relationships between the contact radius, the indentation load, and the indentation depth were set up using the total energy method for the JKR model and the Griffith energy balance for the M-D model, respectively. Numerical results indicate that increasing the half cone angle of the conical punch enhances the adhesion effect, which can significantly affect the accuracy of the results of characterization in nanoindentation tests. It was found that the effect of electric potential on adhesion behaviors is sensitive to different material properties, which are not revealed in the existing studies of axisymmetric adhesive contact of piezoelectric materials and multiferroic composite materials. The load-displacement curves under conical punches with different half cone angles have very different slopes. These results indicate that the half cone angle has a prominent effect on the characterization of mechanical properties of piezoelectric solids in nanoindentation tests.

Modelling of paddy soil using the CFD-DEM coupling method

Soil dynamics has been the core of studying the mechanism of soil-tool interaction. However, it is very challenging to study the dynamics of paddy soil that contains a significant amount of water. In this study, a paddy soil model, namely coupling model was developed using the CFD-DEM coupling method. In the coupling model, spherical particles with Johnson-Kendall-Roberts (JKR) surface energy were used to represent paddy soil. The Volume of Fluid (VOF) gas-liquid two-phase flow model was used to represent the characteristics of the water in paddy field. A test and a simulation of soil rotation were performed to calibrate the critical model parameter of soil particle, JKR surface energy. The calibrated value was 2.5 J·m−2, and the relative average error between the simulation and the test was 1.27 %. To verify the accuracy of the coupling model, an experiment of a fertilizer banding opener was conducted in a paddy soil in laboratory conditions. Also, the interaction of the opener with paddy soil was simulated using the coupling model. Then, soil disturbance characteristics and the soil resistance force of the opener were compared between the experiment and model prediction. When using the coupling model to predict, the relative errors were less than 10 % for soil disturbance characteristics and less than 15 % for soil resistance force. This suggests that the model developed using the CFD-DEM coupling method is able to predict the dynamics of soil-tool interaction in paddy fields with reasonably good accuracies.

Numerical Study on Particulate Fouling Characteristics of Flue with a Particulate Fouling Model Considering Deposition and Removal Mechanisms

Due to a large amount of particulate matter in industrial flue gas, the formation of particulate deposits on the flue wall will increase the instability of equipment operation, which needs to be solved urgently. In this paper, a numerical investigation on the characteristics of particulate deposition and removal in the furnace flue was carried out for waste heat and energy recovery. This research adopted a comprehensive fouling model combined with the discrete phase model (DPM) which was performed by the CFD framework and extended by user-defined functions (UDFs). Firstly, the particulate deposition and removal algorithms were proposed to develop the judgment criterion of particle fouling based on the Grant and Tabakoff particle–wall rebound model and the Johnson–Kendall–Roberts (JKR) theory. This model not only considered the particles transport, sticking, rebound, and removal behaviors, but also analyzed the deposition occurring through the multiple impactions of particles with the flue wall. Then, the influence of furnace gas velocity, particle concentration, and inflection angle α of the tee section on the particulate fouling were predicted. The results show that the furnace gas velocity, particle concentration, and flue structure have significant effects on particle fouling and distribution, and the particle fouling mainly occurs in the blind elbow section and the tee sections of the flue. In addition, the fouling mass of particles decreases with an increasing furnace gas velocity and the decrease in particle concentration. Lastly, the fouling mass of particles decreases with the increase in the inflection angle α of the tee section, and the location of particle fouling gradually transfers from the blind elbow section to the tee section.

Filtration characteristics of a binary multi-layer granular bed filter based on CFD-DEM coupling simulation

The coupled CFD-DEM method with the JKR (Johnson-Kendall-Roberts) model for describing the contact adhesion of dust to filter particles (FPs) is used to simulate the distribution pattern of dust particle deposition in the granular bed filter (GBF) with multi-layer media. The minimum inlet flow velocity must meet the requirement that the contact probability between dust and FPs is in the high contact probability region. The air flow forms vortices on the leeward side of the FPs and changes abruptly at the intersection of different particle size FPs layers. Dust particles form large deposits at the intersection of the first and second layers and the different particle size filter layers. Dual element multilayer GBF can further optimize the bed structure by interlacing filter layers with different particle sizes. Compared with single particle size multi-layer GBF, the bed pressure drop is reduced by 40.24%–50.65% and the dust removal efficiency is increased by 21.93%–55.09%.

Determination of the rate-dependent adhesion of polydimethylsiloxane using an atomic force microscope

A JKR (Johnson-Kendall-Roberts) formalism is used to extract the Young modulus, the contact radius, and the energy release rate simply from the retraction curve of a borosilicate glass colloidal probe from a polydimethylsiloxane (PDMS) surface using an atomic force microscope. PDMS samples ranged from perfectly elastic to those with incipient viscoelasticity. The dependence of the release rate energy with the crack speed is verified using a fracture mechanics-based method.