Pull-off Force Research Articles

Investigations of the Adhesive Contact Behavior of Elastic Layered Media With Surface Roughness

A deterministic adhesive model for the contact between an elastic layered medium with surface roughness and a smooth elastic microsphere was developed on the basis of the Lennard–Jones surface force law. Through numerical simulations, the adhesive contact behavior of the layered medium with the measured three-dimensional (3D) surface topography was comparatively analyzed with that of the homogeneous medium. Furthermore, the contact characteristics of the layered medium with pre-assigned roughness parameters were investigated with the aid of a computer-generated technique for simulating surface roughness. Results showed that the pull-off force for the contact problem involving rough surfaces was influenced by the contact location, and the average value for the contact between an alumina (SiO2) microsphere and a diamond-like carbon/silicon (DLC/Si)-layered medium was smaller than that for the contact between a SiO2 microsphere and a Si homogeneous half-space. In addition, the effect of the diamond-like carbon (DLC) layer on reducing adhesion was smaller than that of the surface roughness. Finally, the average pull-off force for a DLC/Si-layered medium with computer-generated surface roughness rapidly decreased; however, it eventually became almost unchangeable with the increase in the root-mean-square (RMS) deviation.

The role of adhesion in contact mechanics.

Adhesive (e.g. van der Waals) forces were not generally taken into account in contact mechanics until 1971, when Johnson, Kendall and Roberts (JKR) generalized Hertz' solution for an elastic sphere using an energetic argument which we now recognize to be analogous to that used in linear elastic fracture mechanics. A significant result is that the load-displacement relation exhibits instabilities in which approaching bodies 'jump in' to contact, whereas separated bodies 'jump out' at a tensile 'pull-off force'. The JKR approach has since been widely used in other geometries, but at small length scales or for stiffer materials it is found to be less accurate. In conformal contact problems, other instabilities can occur, characterized by the development of regular patterns of regions of large and small traction. All these instabilities result in differences between loading and unloading curves and consequent hysteretic energy losses. Adhesive contact mechanics has become increasingly important in recent years with the focus on soft materials (which generally permit larger areas of the interacting surfaces to come within the range of adhesive forces), nano-devices and the analysis of bio-systems. Applications are found in nature, such as insect attachment forces, in nano-manufacturing, and more generally in industrial systems involving rubber or polymer contacts. In this paper, we review the strengths and limitations of various methods for analysing contact problems involving adhesive tractions, with particular reference to the effect of the inevitable roughness of the contacting surfaces.

Application of SMA spring tendons for improved grasping performance

Shape memory alloy (SMA)-based soft actuators and grippers have generally used SMA wires due to design restrictions, limited actuation force and poor cooling performance of SMA springs. This work demonstrates that SMA springs can be used for high performance soft actuation by positioning them externally as a tendon with the force transmitted to the polymeric matrix using a tendon. Through improvements by using active cooling through fans, a spring longer than the matrix and a matrix with a trapezoidal cross-section, the actuator was capable of cyclic actuation at 0.25 Hz, a bending angle of 270° and a tip force of 0.21 N. The same actuator design was used to build a soft robotic gripper capable of grasping objects with a wide range of shapes and weights, and its maximum pull-off force was measured to be 30 N. The gripper was also capable of holding a weight of 2 kg. The increased design freedom of the proposed design would also enable further improvements to the bending and grasping performance of the actuator, and the use of SMA-based soft actuation to more complex designs and a wider range of robotic applications.

Pull-off and friction forces of micropatterned elastomers on soft substrates: the effects of pattern length scale and stiffness.

The adhesiveness of biological micropatterned adhesives primarily relies on their geometry (e.g., feature size, architecture) and material properties (e.g., stiffness). Over the last few decades, researchers have been mimicking the geometry and material properties of biological micropatterned adhesives. The performance of these biomimetic micropatterned adhesives is usually tested on hard substrates. Much less is known about the effect of geometry, feature size, and material properties on the performance of micropatterned adhesives when the substrate is deformable. Here, micropatterned adhesives of two stiffness degrees (Young’s moduli of 280 and 580 kPa) were fabricated from poly(dimethylsiloxane) (PDMS) and tested on soft poly(vinyl alcohol) (PVA) substrates of two stiffness degrees (12 and 18 kPa), and on hard glass substrates as a reference. An out-of-the-cleanroom colloidal lithographic approach was successfully expanded to fabricate adhesives with two geometries, namely dimples with and without a terminal layer. Dimples without a terminal layer were fabricated on two length scales, namely with sub-microscale and microscale dimple diameters. The cross section of samples with a terminal layer showed voids with a spherical shape, separated by hourglass-shaped walls. These voids penetrate the terminal layer, resulting in an array of holes at the surface. We found that on soft substrates, generally, the size of the dimples did not affect pull-off forces. The positive effects of sub-microscale features on pull-off and friction forces, such as defect control and crack trapping, as reported in the literature for hard substrates, seem to disappear on soft substrates. The dimple geometry with a terminal layer generated significantly higher pull-off forces compared to other geometries, presumably due to interlocking of the soft substrate into the holes of the terminal layer. Pull-off from soft substrates increased with the substrate stiffness for all tested geometries. Friction forces on soft substrates were the highest for microscale dimples without a terminal layer, likely due to interlocking of the soft substrate between the dimples.

Boundary Element Analyses on the Adhesive Contact between an Elastic Cylinder and a Rigid Half-Space

Boundary element method is used to analyze the adhesive contact between an elastic cylinder and a rigid half-space. Lennard-Jones potential is used for the surface traction. In the past, the simulation for the adhesive contact between cylinders usually used parabolic approximation for cylinder surface, and used line loading acting on a half-space. Since line loading may cause infinite deformation, only contact half-width/load relation and pull-off force can be obtained. In this paper, the adhesive contact between an exact elastic cylinder and a rigid half-space is investigated. The S-shaped load-approach curve and the whole solution are obtained. Using the load-approach curves, the pull-off force, pull-off distance and jump-in distance are obtained. The effects of Tabor parameter and radius are investigated. The result is compared with the numerical simulation for the adhesive contact between an elastic parabolically approximated cylinder and a rigid half-space and the two-dimensional JKR model. For large Tabor parameters, two-dimensional JKR model can approximate the adhesive contact. For small Tabor parameters, two-dimensional Bradley model can approximate the adhesive contact. The radii do affect the load-approach relation for large Tabor parameters, and have very small effects for small Tabor parameters. A semi-rigid cylinder model is proposed. This model can predict the load-approach curves for small Tabor parameters and can predict the jump-in distance for large Tabor parameters. In addition, a modified load-approach relation for two-dimensional JKR model is proposed. This relation can approximate the load-approach relation and predict the pull-off distance for large Tabor parameters. It is also found that the radius does not affect the pull-off force.

Change in Pull-off force by increasing CNT content in silicone/ CNT composite sheet

Change in Pull-off force by increasing CNT content in silicone/ CNT composite sheet

Adhesion of compliant spheres: an experimental investigation

The detachment of soft adhesive spheres is strongly affected by the displacement rate. Nonetheless, the classical Johnson, Kendall & Roberts (JKR) theory is usually adopted to describe both the loading and unloading process, with the implicit assumption of defining different values of the elastic modulus and interfacial adhesion energy for the two phases.In this work, we use a different approach to model the unloading phase, usually characterized by viscoelastic dissipation. Specifically, experimental data are fitted with the numerical model proposed by Muller for the contact of viscoelastic spheres. In such way, by exploiting a mixed JKR-Muller model, we are able to quantify the energy loss due to elastic and viscoleastic adhesion hysteresis.Moreover, we find that the pull-off force is order of magnitude larger than the JKR prediction and increases with the detachment rate, while the effect of the maximum load reached before unloading is, as expected, negligible.

Computer Simulation of the Effect of Wetting Conditions on the Solvation Force and Pull-Off Force of Water Confined between Two Flat Substrates

Modeling experimental results of pull-off versus relative humidity obtained by atomic force microscopy (AFM) has suggested that the classic interaction force models (van der Waals and capillarity) ...

Atomic force spectroscopy on the rough surface of micro/nanoporous fibers using a sharp tip

Atomic Force Spectroscopy (AFS) was conducted on the rough surface of the electrospun porous cellulose acetate fibers to assess the intrinsic hydrophilicity through the molecular interactions between solid surface of the material and the silicon tip of Atomic Force Microscope (AFM). The pull-off force measurement was carried out by a sharp tip rather than a colloidal tip that offers the benefits of nano scale contact area, such as detecting local changes of hydrophilicity in heterogeneous substrates and/or chemically modified surfaces, through precisely selected points from high resolution images. The data were filtered in two steps to eliminate the force-distance curves which were affected by the errors arisen from viscoelastic deformations, drifts, and multicontacts between tip and sample. The proposed refinement procedure would provide an exciting opportunity to overcome the major limitations of interpreting AFS data obtained from nanorough surfaces and advances our knowledge of wetting properties of these surfaces.

Numerical Study of Soft Colloidal Nanoparticles Interaction in Shear Flow.

The mechanical behavior of nanoparticle assemblies depends on complex particle interactions that are difficult to study experimentally. Depending on the nanoparticle morphology, these interactions could lead to adhesive and elastic-plastic behavior during contact deformation. The aim of this research is to study the effect of contact interactions between polymer nanoparticles and their impact on the macroscopic properties of formed aggregates. For this purpose, the discrete element method (DEM) was used to develop an interaction model combining elastic-plastic deformation and adhesion to study the behavior of spherical polymeric nanoparticles. Initially, a pair of particles interacting in the normal direction was simulated to evaluate the effect of adhesion and plastic deformation in the pull-off force of the contact. Based on these results, the simulations were extended to a dispersed system of nanoparticles, in which multibody interactions become dominant. Considering the aggregation between the nanoparticles induced by a shear flow, we performed an analysis of the number of aggregates and aggregates size in time to characterize the strength of clusters formed during the process. The simulation results showed that the interaction strength upon breakage of the clusters, correlating with the aggregates size, depends on the nanoparticle's softness. In this way, we verified that the type of contact interaction directly influences the macroscopic mechanical response of nanoparticle assemblies. Therefore, our model represents a new way of predicting the mechanical behavior of polymer nanoparticle systems and of optimizing it by adjusting primary particle properties.

Modeling and characterization of cohesion in fine metal powders with a focus on additive manufacturing process simulations

The cohesive interactions between fine metal powder particles crucially influence their flow behavior, which is important to many powder-based manufacturing processes including metal additive manufacturing (AM). The present work proposes a novel modeling and characterization approach for micron-scale metal powders, with a focus on characteristics of importance to powder bed AM. The model is based on the discrete element method (DEM), and the considered particle-to-particle and particle-to-wall interactions involve frictional contact, rolling resistance and cohesive forces. Special emphasis lies on the modeling of cohesion. The proposed adhesion force law is defined by the pull-off force resulting from the surface energy of powder particles in combination with a van-der-Waals force curve regularization. The model is applied to predict the angle of repose (AOR) of exemplary spherical Ti-6Al-4 V powders, and the surface energy value underlying the adhesion force law is calibrated by fitting the corresponding angle of repose values from numerical and experimental funnel tests. To the best of the authors' knowledge, this is the first work providing an experimental estimate for the effective surface energy of the considered class of metal powders. By this approach, an effective surface energy of 0.1mJ/m2 is found for the investigated Ti-6Al-4 V powder. This value is considerably lower than typical experimental values for flat metal contact surfaces, which range from 30 − 50mJ/m2. Thus, factors such as surface roughness, surface chemistry and potential surface oxidation have crucial influence on bulk power behavior. Moreover, the present study demonstrates that a neglect of the related cohesive forces leads to a drastical underestimation of the AOR and, consequently, to an insufficient representation of the bulk powder behavior.

A shear assay study of single normal/breast cancer cell deformation and detachment from poly-di-methyl-siloxane (PDMS) surfaces

This paper presents the results of a combined experimental and analytical/computational study of viscoelastic cell deformation and detachment from poly-di-methyl-siloxane (PDMS) surfaces. Fluid mechanics and fracture mechanics concepts are used to model the detachment of biological cells observed under shear assay conditions. The analytical and computational models are used to compute crack driving forces, which are then related to crack extension during the detachment of normal breast cells and breast cancer cells from PDMS surfaces that are relevant to biomedical implants. The interactions between cells and the extracellular matrix, or the extracellular matrix and the PDMS substrate, are then characterized using force microscopy measurements of the pull-off forces that are used to determine the adhesion energies. Finally, fluorescence microscopy staining of the cytosketelal structures (actin, micro-tubulin and cyto-keratin), transmembrane proteins (vimentin) and the ECM structures (Arginin Glycine Aspartate – RGD) is used to show that the detachment of cells during the shear assay experiments occurs via interfacial cracking between (between the ECM and the cell membranes) with a high incidence of crack bridging by transmembrane vinculin structures that undergo pull-out until they detach from the actin cytoskeletal structure. The implications of the results are discussed for the design of interfaces that are relevant to implantable biomedical devices and normal/cancer tissue.

Contact mechanics of a gel under constrained swelling

Gels have been widely explored in many engineering applications, in which they often swell with mechanical constraints. In this paper, we study mechanics of contact between a constrained swelling gel and a rigid spherical probe. We first derive an analytical form of the surface Green's function of a laterally constrained swelling gel occupying a half space. We then obtain an analytical form of the relationship between the indentation force and indentation depth for a spherical rigid indenter pressing onto a laterally constrained swelling gel, with and without taking account of the adhesion. Our theory also uncovers the influences of the constrained swelling ratio of the gel on the contact area (as well as its eccentricity), pull-off force and critical distance at separation. The essential parts of the analytical results are validated by finite element simulations. The results obtained in this article may help to better interpret experimental measurements from indentation tests of swollen gels.

Friction force evaluation for grasping in minimally invasive surgery

Abstract This study investigates direct and indirect measurement techniques for a typical laparoscopic operation in an ex vivo experiment. The direct measurement technique involves obtaining contact forces at the tissue-tool level. The indirect measurement technique involves acquiring contact forces without compromising the contact surface. The two techniques are compared in terms of the friction force. This estimation is important to ensure that grasping is stable so that tissue sliding is avoided. The results demonstrate that direct measurement is better at measuring the pinch force and estimating the friction force because of the alteration of the force center. However, integration of the tactile sensor into the tool tip increases the pull-off force by approximately 44%.

Environmental stress tolerance and immune response for the small abalone hybrids

Recently, mass mortality affected the cultured small abalone, Haliotis diversicolor diversicolor, which was the dominant cultured abalone species in southern China. Prior studies revealed that survivorship varied significantly between different stocks and crosses. However, the immunological basis for differences in susceptibility has not been well understood to date. Herein, low temperature, air exposure tolerance tests, and pull-off force measurement were assessed in the three groups (Japan, Taiwan, and their Hybrid stock). The results showed that the critical thermal minimum (CTMin) at 50% was 15.6 °C for the Taiwan stock, 12.1 °C for the Japan stock, and 13.2 °C for the Hybrid stock. Upon air exposure challenge, 100% abalones from the Taiwan group died after 8 h at 24 °C, while the survival rate in the Japan and Hybrid groups were 37.8% and 29.4%, respectively. The detachment stress for the Japan group was 42.3 kPa, which was 2.78-fold and 1.43-fold higher compared to the Taiwan and Hybrid groups, respectively. Variation in susceptibility to disease may be based on the effectiveness of the innate immune responses. Therefore, total hemocyte count, phagocytosis, respiratory burst, superoxide dismutase activity, acid phosphatase activity, alkaline phosphatase activity, and myeloperoxidase activity were determined for the healthy abalones in each group. Positive mid-parent heterosis on immunological parameters was consistent with the prior knowledge on the positive mid-parent heterosis for survival rate, which indicated the improvement on immune reaction and disease resistance through hybridization methods. The current study will be useful in efficient design of breeding programs for the development of sustainable abalone aquaculture.

Changes in surface tension of saliva in Down syndrome.

Surface tension in saliva might contribute to the maintenance of upper airway patency. The present study aimed to determine whether salivary surface tension is altered in patients with Down syndrome who are predisposed to upper airway collapse. We used the pull-off force technique to measure surface tension in samples (100 μL) of saliva collected from twenty-three male patients with Down syndrome and twenty-three healthy males (controls). p < 0.05 was considered to indicate significance. Salivary surface tension was significantly lower in the patients than in the controls (57.3 ± 4.9 vs. 60.3 ± 4.7 mN/m; p = 0.039). Age and surface tension positively correlated in the patients (p = 0.001). The lower surface tension of saliva in patients with Down syndrome might compensate for an anatomical predisposition towards upper airway collapsibility and other risk factors. The function of surface tension in saliva might be altered due to aging in such patients.

Delamination of a rigid punch from an elastic substrate under normal and shear forces

Delamination of rigid objects from an elastic substrate with finite thickness is a fundamental problem underlying applications such as marine fouling release coatings or anti-icing coatings. Most existing theoretical studies assume that delamination is driven by forces normal to the substrate surface, while in practice the delamination force may also include shear components that are parallel to the substrate surface. In this work, we consider a model system where a rigid cylindrical punch is detached from an elastic substrate under normal force, shear force or both. Our focus is to determine the pull-off force and to reveal the delamination mechanics under various geometrical and loading conditions, specifically the substrate thickness and the position and angle of the delamination force. To gain theoretical insights, we first study a plane strain model where a long rigid strip is adhered to an elastic half-space, and obtain an analytical solution revealing how the pull-off force depends on the loading position and angle. Moreover, we develop a three-dimensional finite element model to simulate the delamination of a rigid cylindrical punch from an elastic substrate with finite thickness. Three delamination modes are identified from finite element results: Mode-I crack propagation, Mode-II crack propagation, and interface cavitation. For the first two modes, we obtain empirical formulas to calculate the pull-off force using adhesion energy, substrate modulus, contact radius and substrate thickness. We also find that the analytical solution derived from the plan strain model can serve as a qualitative guide to estimate the effect of loading position and angle on the pull-off force.

Adhesive contact of rough brushes.

The adhesive contact between a rough brush-like structure and an elastic half-space is numerically simulated using the fast Fourier transform (FFT)-based boundary element method and the mesh-dependent detachment criterion of Pohrt and Popov. The problem is of interest in light of the discussion of the role of contact splitting in the adhesion strength of gecko feet and structured biomimetic materials. For rigid brushes, the contact splitting does not enhance adhesion even if all pillars of the brush are positioned at the same height. Introducing statistical scatter of height leads to a further decrease of the maximum adhesive strength. At the same time, the pull-off force becomes dependent on the previously applied compression force and disappears completely at some critical roughness. For roughness with a subcritical value, the pressure dependence of the pull-off force qualitatively follows the known theory of Fuller and Tabor with moderate modification due to finite size effect of the brush.

Adhesive contact problems for a thin elastic layer: Asymptotic analysis and the JKR theory

Contact problems for a thin compressible elastic layer attached to a rigid support are studied. Assuming that the thickness of the layer is much less than the characteristic dimension of the contact area, a direct derivation of asymptotic relations for displacements and stress is presented. The proposed approach is compared with other published approaches. The cases are established when the leading-order approximation to the non-adhesive contact problems is equivalent to contact problem for a Winkler–Fuss elastic foundation. For this elastic foundation, the axisymmetric adhesive contact is studied in the framework of the Johnson–Kendall–Roberts (JKR) theory. The JKR approach has been generalized to the case of the punch shape being described by an arbitrary blunt axisymmetric indenter. Connections of the results obtained to problems of nanoindentation in the case that the indenter shape near the tip has some deviation from its nominal shape are discussed. For indenters whose shape is described by power-law functions, the explicit expressions are derived for the values of the pull-off force and for the corresponding critical contact radius.

Adhesive forces between two cleaved calcite surfaces in NaCl solutions: The importance of ionic strength and normal loading

The mechanical strength of calcite bearing rocks is influenced by pore fluid chemistry due to the variation in nano-scale surface forces acting at the grain contacts or close to the fracture tips. The adhesion of two contacting surfaces, which affects the macroscopic strength of the material, is not only influenced by the fluid chemistry but also by the surface topography. In this paper, we use Atomic Force Microscope (AFM) to measure the interfacial forces between two freshly cleaved calcite surfaces in CaCO3-saturated solutions with varying NaCl concentration. We show that calcite contacts become stronger with increasing NaCl concentration (>100 mM), as a result of progressively weaker secondary hydration and increasing attraction due to instantaneous ion-ion correlation. Moreover, we discuss the effect of normal applied force (Fn) and surface roughness on the measured adhesion forces (Fad). We show that the measured pull-off force (adhesion) is linearly correlated with the magnitude of Fn, where an increase in applied force results in increased adhesion. This is attributed to a larger number of contacting surface asperities and thus increase in real contact area and the contact-bond strength. We discuss that the possible variation in local topography at contacts, together with strong dependence on ionic strength of the solution, can explain the inconsistent behavior of calcite rocks in NaCl solutions.