On a recent stickiness criterion using a very simple generalization of DMT theory of adhesion

In the contact of rough surfaces, contact occurs on smaller and smaller scales, the well-known Tabor adhesion parameter decreases and the so-called Derjaguin–Muller–Toporov (DMT) theory is the appropriate limit. Fuller and Tabor developed 40 years ago a model based on asperities and JKR theory, and more recently the author developed an asperity theory using asperities and DMT theory in the form given by Maugis. Both lead to adhesion parameters which do not depend on the range of attractive forces, in contrast to the parameter recently suggested by Pastewka and Robbins (PNAS, 111(9), 3298–3303, 2014). As it is well known from random process theory that contact of rough surfaces can be described reasonably well by asperity summits at least for low bandwidths, the Pastewka–Robbins DMT model and stickiness criterion should correspond in the limit case of a spherical contact. We therefore consider this limit case, and show that Pastewka–Robbins DMT model introduces a dependence on range of attractive forces, or on Tabor parameter, which is not correct for the sphere, and therefore may be incorrect also in general.     

The Jump-to-Contact Distance in Atomic Force Microscopy Measurement

The jump-to-contact phenomenon of atomic force microscopy measurement is investigated. The force-approach relation for the adhesive contact based on the Lennard-Jones potential with the Derjaguin approximation is analyzed. For a small Tabor parameter, the force-approach relation is similar to that with the van der Waals force between two rigid spheres. For a large Tabor parameter, the force-approach relation is similar to that with the van der Waals force between two deformable spheres. Empirical formulas for the approaching part of the force-approach curve are proposed. The jump-to-contact distance can be obtained by using the semi-empirical formulas. The jump-to-contact distance for a fixed grips device and for large Tabor parameter is also obtained.     

An analytical approach for the adhesion of a semi-infinite elastic body in contact with a sinusoidal rigid surface under zero external pressure

An analytical approach for the adhesion of a semi-infinite elastic body in contact with a sinusoidal rigid surface under zero external pressure is presented. Although Johnson (Int. J. Solids Struct. 32, 423 (1995)) has proposed an analytical solution for a slightly wavy surface, while Zilberman and Persson (Solid State Commun. 123, 173 (2002); J. Chem. Phys. 118, 6473 (2003)) have given a numerical solution for a highly wavy surface by considering the curvature of the contact area in the calculation of the interfacial term of the total energy, our solution is not only for small amplitude of roughness (i.e., the slightly wavy surface as Johnson's) but also for large amplitude of roughness (i.e., the highly wavy surface as of Zilberman and Persson). Our solution considers the curvature of the contact area as do Zilberman and Persson. Our results which are obtained for the total energy and equilibrium condition of the system agree with both Johnson's and Zilberman and Persson's results. The effects of the material constants and the surface roughness on the adhesion are clearly expressed and discussed.     

刚性波纹面与柔性波纹面传热及流动特性

采用大涡模拟方法,对流体在刚性及柔性波纹面上的流动与传热特性进行了数值研究。结果表明:对于刚性波纹面,当振幅与波长比值(a/λ)较小时,波谷区域无回流现象;当a/λ增大至0.03时,波谷区域出现回流区,且回流区域随波幅的增加而增大;波纹面上坡部位沿展向出现较高Nusselt数的斑块;当a/λ从0.01增至0.04时,时均Nusselt数提高了近63.5%,综合系数增加了0.5112。对于柔性波纹面,其压力分布也呈现周期性变化;且在上坡位置也出现较大Nusselt数斑块;当a/λ从0.01增至0.04时,时均Nusselt数值提高了173.1%,综合系数增加了1.1232。与刚性波纹面相比,在消耗相同流体输送泵功时,柔性波纹面具有更好的传热效果。     

Revisit the adhesive contact between a nanoscale rigid sphere and an elastic half-space: considering the effect of sphere size

The current paper revisits the adhesive contact between a rigid nanoscale sphere and an elastic half-space. Using analytical solution for traction and a modified numerical scheme, a new simulation is proposed. The adhesive contact is simulated faster and more easily than previous researches. The effect of sphere size is investigated. A modified spherical JKR model and spherical DMT model are proposed. The results are compared with spherical JKR, spherical DMT, and rigid sphere model. The load–approach and contact radius–load relations can be predicted by the modified spherical JKR model for sphere radius larger than 50?. Finally, approximation equations for pull-off force vs. Tabor parameter and jump-in distance vs. Tabor parameter are proposed.     

A random process asperity model for adhesion between rough surfaces

A simple asperity model using random process theory is developed in the presence of adhesion, using the Derjaguin, Muller and Toporov model for each individual asperity. A new adhesion parameter is found, which perhaps improves the previous parameter proposed by Fuller and Tabor which assumed identical asperities – the model in all his variants for the radius always gives a finite pull-off force, as in Fuller and Tabor, and contrary to the exponential asperity height distribution, where the force is either always compressive, or always tensile. It is shown that a model with spheres having a radius only dependent on height is a reasonable approximation with respect to models having also a distribution of radius curvatures – the three models differ considerably, as opposed to the adhesionless case where these details did not matter. The important surface parameters in the theory determining the pull-off force are the three moments m₀, m₂, m₄. The asymptotic form of the model at large separation is solved in closed form. As the theoretical pull-off of aligned asperities having the same radius (the average value) increases with the square root of the Nayak bandwidth of the roughness, and as asperity models are known to describe less well the surface at large bandwidth parameters, the limit behavior at large bandwidths remains uncertain.     

Numerical Simulation and Analysis of Particle Mixing and Conduction in Wavy Drums

A thermal discrete element method (DEM) is used to simulate particle mixing and heat conduction inside wavy drums to explore the effects of wavy walls. Sinusoidal configurations with different waves on the walls are simulated. The Lacey mixing index is applied to analyze the mixing characteristics. The driven forces from the wavy wall, either positive/negative or effective driven forces, are analyzed to explain the mechanisms of mixing enhancement in the wavy drum. A new control parameter is proposed to explain the mechanism of mixing enhancement. It is found that a locally oscillating effect exists in wavy drums, which is imparted on the bulk rotating motions of particles and enhances the characteristics of particle mixing and heat conduction significantly. Except over large wave numbers and rotating speeds when the flow regime is deteriorated for mixing, the wavy drum is generally beneficial for mixing augmentation as well as conduction enhancement.     

Estimating work of adhesion using spherical contact between a glass lens and a PDMS block

This study proposes a method of estimating the thermodynamic work of adhesion using the spherical contact between a glass lens and a polydimethylsiloxane (PDMS) block. An equivalent stiffness of the measurement system is determined prior to the experimental measurements. Parameters such as force, contact radius, and displacement are measured during the contact processes that consist of a loading process and an unloading process. The elastic modulus of the PDMS is determined from the measured parameters. Hysteresis is observed in the contact process, showing that the process is in a non-equilibrium state. However, an equilibrium instant exists when the contact area attained a maximum value. The work of adhesion can be determined from the strain energy release rate using the parameters of the instant. The estimated work of adhesion is in the range estimated by another method. The limitation of the conventional method is also presented. The proposed method is suggested to be applicable to interface characterization in a practical adhesive contact with soft materials.     

Adhesive contact and rolling of a rigid cylinder under the pull of gravity on the underside of a smooth-surfaced sheet of rubber

The mechanical approach of the rolling contact of a rigid cylinder under the flat and smooth inclined surface of a natural rubber sample, if molecular attraction forces act, allows one to demonstrate, contrary to our first intuition, that the rolling of the cylinder occurs even if its weight per unit axial length is much greater than the critical value for the static equilibrium under a horizontal surface, as previously studied. It is shown that the maximum value of the rolling force per unit length is 50 times the static equilibrium force measured under a horizontal surface.

The rolling force varies as an n power function of the rolling speed and the value n = 0.55 previously determined in recent rebound and rolling experiments with the same rubber-like material is found again. Moreover, contact length measurements confirm theoretical predictions: the contact length varies as the n/3 = 0.183 power function of the rolling speed in a large range of velocities.     

Nonlinear Interaction between a Gas Flow and a Wavy Liquid Film: Taking into Account Drop Detachment or Precipitation

Methods are proposed for calculating the parameters of the nonlinear interaction between a gas flow and a wavy liquid film with regard for various mechanisms of drop detachment from or drop precipitation onto the liquid film surface. Expressions are obtained for the wave amplitude and the film thickness that allow for drop detachment or precipitation.     

Adhesion Induced Flow of a Soft Polyester-Polydimethylsiloxane Copolymer Substrate Over Micrometer and Submicrometer Size Spherical Particles: Observations of Anomalously Large Menisci, Interparticle Bridging and Particle Encapsulation

Observations made with a scanning electron microscope (SEM) provided direct evidence for a soft polyester-polydimethylsiloxane block copolymer substrate undergoing extensive surface-force-induced plastic deformation upon contact with micrometer or submicrometer size spherical particles. Anomalously large menisci were detected at the particle /substrate interfaces. Moreover, the substrate material appeared to bridge or encapsulate the particles. The heights of the contact menisci between the 2.2 micrometer radius polystyrene beads and the substrate were found to be approximately 0.4 micrometers; those between 3.6 micrometer radius glass spheres and the substrate were approximately 0.5 micrometers. The heights of the observed menisci were found to be large compared with the values calculated using Tabor's analysis (D. Tabor, J. Colloid Interface Sci. 58, 2 (1977)) based on the elastic model proposed by Johnson, Kendall and Roberts (K. L. Johnson, K. Kendall, and A. D. Roberts, Proc. R. Soc. London A. 324, 301 (1971)). These results suggest that the surface-force-induced tensile stresses may have exceeded the elastic limit of the substrate or that the surface material has an unexpectedly low surface modulus.     

The Effects of Surface Waviness and Length on Electrokinetic Transport in Wavy Capillary

An electrokinetic model for a wavy capillary has been developed. Poisson‐Nernst‐Planck and Navier‐Stokes equations constitute the model that governs fluid and ionic fluxes and electric potential distribution inside the capillary. In the present paper, a finite wavy cylindrical capillary with a large reservoir at both capillary ends is analyzed using finite element method. The model is used primarily to examine the influence of capillary surface waviness on the electrokinetic transport behaviours. Different frequencies and amplitudes of the wavy surface are considered to investigate the influence of surface waviness on electrokinetic transport. Fluctuations in potential and ionic concentration distribution increase with the increase in either amplitude or frequency of the capillary surface waviness. However, for higher frequencies the fluctuation diminishes for all surface waviness amplitudes. It is observed that for any irregularity in the capillary surface results in higher salt rejection. Salt rejection is found to be dependent on capillary axial length as well as flow velocity. A critical Peclet number, beyond which salt rejection attains a constant steady value, dictates maximum salt rejection.     

Numerical Simulation of the Adhesive Contact between a Slightly Wavy Surface and a Half-Space

In this work, the adhesive contact between a slightly wavy surface and a half-space has been investigated numerically. The surface traction was described by the Lennard–Jones potential with the Derjaguin's approximation. The deformation was first obtained by using the formula for the line contact and then, using the arc-length continuation algorithm, the relation between the contact half-width and the total force per asperity was obtained. The pull-off forces were then determined. The numerical simulation presented in this paper can be used to simulate all adhesive contacts ranging from the JKR contact to a rigid contact.     

Adhesion of two elastic conforming solids with a single interface gap

The adhesive contact between a half-space with a single surface micro-groove and a flat half-space is investigated. Surface interaction is described by the Maugis–Dugdale adhesion model. The contact problem is reduced to a singular integral equation for a height of the interface gap, which is solved analytically. For evaluating widths of the gap and the adhesion zone, a system of two transcendental equations is obtained, which is solved numerically. Three stable equilibrium states are found. The gap width-applied pressure curves are characterized by discontinuities and hysteresis. The effects of the maximum groove height and the adhesive stress on the adhesive contact are studied. It is shown that the adhesion hysteresis is greater for smaller grooves and larger adhesive stresses.     

Surface modification of poly(ethylene terephthalate) fiber by excimer light

Changes in the surface topography and chemical structure on the surfaces of poly(ethylene terephthalate) (PET) fibers and films caused by the irradiation using an excimer laser beam and excimer lamp light were monitored. The SEM (scanning electron microscopy) observation suggests that a wavy shape was produced by irradiation with the excimer laser beam, while such a wavy shape was not observed when the excimer lamp light was used. The XPS (X-ray photoelectron spectroscopy) analysis of the fiber surface suggests that the O/C intensity ratio was reduced by irradiation using the laser beam, whereas this ratio gradually increased with irradiation with the lamp light. This difference is attributed to the difference in the number of photons in the laser beam, which was much higher than that in the lamp light, although the laser beam and lamp light had the same wavelength and energy. As for wettability to water, the contact angle was smallest for PET irradiated by the excimer lamp light. For adhesion studies, the PET fabric was first coated with an epoxy acrylate solution, irradiated with excimer light, dipped into RFL (resorcinol-formaldehyde-latex) adhesive, and the peel strength to rubber sheet was examined. When the irradiation by the excimer laser beam was compared with that with the excimer lamp light, the laser irradiation showed a good adhesion property even for rubber vulcanization carried out for a long time at high temperature.     

Adhesion between side surface of an elastic beam and flat surface of a rigid body

A theory of adhesion between an elastic beam and a rigid body is proposed using linear beam theory. Normalized force between the elastic beam and the rigid body considering adhesion of the side surface of the elastic beam is investigated theoretically. Adhesion of an elastic beam is important to analyze gecko adhesion, and peeling mechanism of an adhered film. This adhesion is also important in design of grip-and-release devices. The force between an elastic beam and a rigid body is investigated by considering shear force and total energy, and is obtained as a function of the displacement of the elastic beam. The proposed theory is different from Kendall’s thin-film peeling theory in terms of the elastic energy. The proposed theory considers bending elastic energy, whereas Kendall’s theory considers extension elastic energy. Two different contacts, line contact and area contact, are taken into account to discuss the loading and unloading processes in terms of the relation between the force and the displacement. Non-dimensional parameter, which relates to the work of adhesion and the specifications of the elastic beam, is introduced to explain the normalized maximum tensile force.     

Tailoring intrinsic hydrophobicity and surface energy on rough surface via low-T Cassie–Wenzel wetting transition method

Wettability is an important parameter of micro/nanostructured composites. The measurement of apparent contact angle is strongly affected by surface roughness, which induces some challenges to study the intrinsic hydrophobicity correlating to the nature of chemistry. Carbon-Nafion composites exhibited about 30° decrease in apparent contact angle from 30 to 10°C due to the condensation of water vapor into cavities, suggesting a significant Cassie–Wenzel wetting transition phenomenon. The focus of this work has been on the first-time use of a low-T Cassie–Wenzel wetting transition method to evaluate Young's (ideal) contact angle and surface free energy. A maximum Young's contact angle (113°) and minimum total surface energy (12 mJ/m²) were determined at Nafion content of 70 wt%, indicating the orientation effect that sulfonate groups in Nafion preferentially pointed toward polar carbon. This approach provided the reasonable prediction of intrinsic hydrophobicity, especially when a rough solid surface is not easily wetted by liquids.     

Rebound dynamics of two droplets simultaneously impacting a flat superhydrophobic surface

In this letter, we investigate the rebound dynamics of two equally sized droplets simultaneously impacting a superhydrophobic surface via lattice Boltzmann method (LBM) simulations. We discover three rebound regimes depending on the center-to-center distance between the two droplets: a complete-coalescence-rebound (CCR) regime, a partial-coalescence-rebound (PCR) regime, and a no-coalescence-rebound (NCR) regime. We demonstrate that all the rebound regimes are closely associated with dynamic behaviors of the formed liquid ridge or bridge between the two droplets. We also present the contact time in the three regimes. Intriguingly, although partial coalescence takes place, the contact time is still dramatically shortened in the PCR regime, which is even smaller than that of single-droplet impact. These findings provide new insights into the contact time of multiple-droplet impact and thereby offering useful guidance for some applications such as anti-icing, self-cleaning, and so forth.     

Detachment of particles from surfaces by thermocapillary flows induced by a moving laser beam

The thermocapillary flows produced by heating with the moving laser beam in a thin liquid layer on a light-absorbing solid surface have recently been recognized to be effective for the removal of particulate impurities from the surface. We performed the comparative analysis of adhesion and thermocapillary removal forces acting on particles attached to the solid surface. A simplified hydrodynamic model was used to evaluate the velocity field of the thermocapillary flow in the liquid layer covering particles. Hydrodynamic forces such as the drag force, the lifting force induced by the shear flow far away from the receding contact line and the maximal lifting force caused by the transversal velocity in the layer near the receding contact line were identified as the cleaning forces, which are determined by the temperature gradient along the surface. It was found out that the lifting force is not enough to overcome the adhesion force and detach particles from the solid surface. The drag and the maximal lifting forces were shown to be responsible for removal of particles by using the proposed method. A reasonable qualitative agreement between experimental results and the quantitative force analysis was observed.     

Fabrication of superhydrophobic surfaces with tunable water droplet adhesion force by a two-step method

In this paper, we present a convenient approach to prepare hierarchical structured superhydrophobic coatings with tunable adhesion force, composed of micro-size glass beads, nano-size SiO₂ particles and epoxy resin. Surfaces of two types with different roughness were fabricated, one type is only with single-scale roughness demonstrating lotus effect with low sliding angle, the other type is hierarchically micro-nano-structured roughness exhibiting petal effect with high adhesion force. The surface roughness is pivotal for controlling the wetting behavior and regulating the contact angle including the contact angle hysteresis. Varying the density of micro-size glass beads could adjust the roughness of the surface, which means the adhesion force of the prepared surface could be easily controlled based on the proposed method. Through variation of glass beads’ amount, the surface could be designed to pin the water droplet with different adhesion force when the surface turned upside down. The surface wettability, surface morphology, adhesion force of the prepared samples are investigated and mechanism of the Cassie-to-Wenzel state transition are discussed in detail. Furthermore, the convenient method provides a possibility for controlling surface morphology, composition and corresponding surface adhesion which could be applied to various substrates such as tile, wood, steel and fabric.