Capillary adhesion between elastic solids with randomly rough surfaces

Persson, B. N. J.

doi:10.1088/0953-8984/20/31/315007

Cited by 97 publications

(109 citation statements)

References 46 publications

(96 reference statements)

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“…The effect of adhesion has only been determined for a limited range of parameters, and we hope that our predictions will be compared with future results from this model. Persson has also presented a theory of capillary adhesion (38) that includes a calculation of the distribution of surface separations and may be able to predict A att .…”

Section: Discussionmentioning

confidence: 99%

Contact between rough surfaces and a criterion for macroscopic adhesion

Pastewka

Robbins

2014

Proc. Natl. Acad. Sci. U.S.A.

265

238

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At the molecular scale, there are strong attractive interactions between surfaces, yet few macroscopic surfaces are sticky. Extensive simulations of contact by adhesive surfaces with roughness on nanometer to micrometer scales are used to determine how roughness reduces the area where atoms contact and thus weakens adhesion. The material properties, adhesive strength, and roughness parameters are varied by orders of magnitude. In all cases, the area of atomic contact is initially proportional to the load. The prefactor rises linearly with adhesive strength for weak attractions. Above a threshold adhesive strength, the prefactor changes sign, the surfaces become sticky, and a finite force is required to separate them. A parameter-free analytic theory is presented that describes changes in these numerical results over up to five orders of magnitude in load. It relates the threshold adhesive strength to roughness and material properties, explaining why most macroscopic surfaces do not stick. The numerical results are qualitatively and quantitatively inconsistent with classical theories based on the Greenwood−Williamson approach that neglect the range of adhesion and do not include asperity interactions.surface roughness | contact mechanics S urfaces are adhesive or "sticky" if breaking contact requires a finite force. At the atomic scale, surfaces are pulled together by van der Waals interactions that produce forces per unit area that are orders of magnitude larger than atmospheric pressure (1). This leads to strong adhesion of small objects, such as Gecko setae (2, 3) [capillary forces may also contribute to Gecko adhesion in humid environments (4, 5)] and engineered mimics (6), and unwanted adhesion is the main failure mechanism in microelectromechanical systems with moving parts (7). Although tape and gecko feet maintain this strong adhesion at macroscopic scales, few of the objects we encounter are sticky. Indeed, our world would come to a halt if macroscopic objects adhered with an average pressure equal to that from van der Waals interactions.The discrepancy between atomic and macroscopic forces has been dubbed the adhesion paradox (8). Experiments show that a key factor underlying this paradox is surface roughness, which reduces the fraction of surface atoms that are close enough to adhere (8-11). Quantitative calculations of this reduction are extremely challenging because of the complex topography of typical surfaces, which have bumps on top of bumps on a wide range of scales (12, 13). In many cases, they can be described as self-affine fractals from a lower wavelength λ s of order nanometers to an upper wavelength λ L in the micrometer to millimeter range (10,14).The traditional Greenwood−Williamson (GW) (15) approach for calculating nonadhesive contact of rough surfaces approximates their complex topography by a set of spherical asperities of radius R. The distribution of asperity heights is assumed to be either exponential or Gaussian, and the long-range elastic interactions between different asperities...

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Section: Discussionmentioning

confidence: 99%

Contact between rough surfaces and a criterion for macroscopic adhesion

Pastewka

Robbins

2014

Proc. Natl. Acad. Sci. U.S.A.

265

238

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“…This is due to a force in the capillary action which pulls the surfaces together [15,35]. However, this is not the case when one of the materials is soft.…”

Section: Capillary Adhesionmentioning

confidence: 99%

“…Persson [15] analysed this situation for a hard, rough surface (R q = 6 µm) contacting a smooth, elastically soft solid at a nominal applied pressure of 0.1 MPa, where the Poisson's ratio of the soft solid was 0.5 and the Young's modulus varied from 3 to 300 MPa. Persson [15] found that for elastically soft solids (not in this case skin, but Persson refers to a number of case studies involving biological systems in this work including tree frog feet), when the water level on the surface decreases, there is a large increase in the area of contact.…”

Section: Capillary Adhesionmentioning

confidence: 99%

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Understanding the Friction Mechanisms Between the Human Finger and Flat Contacting Surfaces in Moist Conditions

et al. 2010

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Human hands sweat in different circumstances and the presence of sweat can alter the friction between the hand and contacting surface. It is therefore important to understand how hand moisture varies between people, during different activities and the effect of this on friction.In this study a survey of fingertip moisture was done. Friction tests were then carried out to investigate the effect of moisture.Moisture was added to the surface of the finger, the finger was soaked in water, and water was added to the counter-surface; the friction of the contact was then measured. It was found that the friction increased, up until a certain level of moisture and then decreased. The increase in friction has previously been explained by viscous shearing, water absorption and capillary adhesion. The results from the experiments enabled the mechanisms to be investigated analytically. This study found that water absorption is the principle mechanism responsible for the increase in friction, followed by capillary adhesion, although it was not conclusively proved that this contributes significantly. Both these mechanisms increase friction by increasing the area of contact and therefore adhesion. Viscous shearing in the liquid bridges has negligible effect. There are, however, many limitations in the modelling that need further exploration.

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“…As dimensions shrink into the nanoscale, capillary forces become increasingly important and can be the dominant source of adhesion between surfaces [11][12][13]. For example, they often prevent micro-or nanoelectromechanical systems from functioning under ambient conditions or lead to damage in their fabrication processes [14].…”

Section: Introductionmentioning

confidence: 99%

Capillary adhesion at the nanometer scale

Cheng

Robbins

2014

Phys. Rev. E

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Molecular dynamics simulations are used to study the capillary adhesion from a nonvolatile liquid meniscus between a spherical tip and a flat substrate. The atomic structure of the tip, the tip radius, the contact angles of the liquid on the two surfaces, and the volume of the liquid bridge are varied. The capillary force between the tip and substrate is calculated as a function of their separation h. The force agrees with continuum predictions based on macroscopic theory for h down to ∼5 to 10 nm. At smaller h, the force tends to be less attractive than predicted and has strong oscillations. This oscillatory component of the capillary force is completely missed in the macroscopic theory, which only includes contributions from the surface tension around the circumference of the meniscus and the pressure difference over the cross section of the meniscus. The oscillation is found to be due to molecular layering of the liquid confined in the narrow gap between the tip and substrate. This effect is most pronounced for large tip radii and/or smooth surfaces. The other two components considered by the macroscopic theory are also identified. The surface tension term, as well as the meniscus shape, is accurately described by the macroscopic theory for h down to ∼1 nm, but the capillary pressure term is always more positive than the corresponding continuum result. This shift in the capillary pressure reduces the average adhesion by a factor as large as 2 from its continuum value and is found to be due to an anisotropy in the pressure tensor. The component in the plane of the substrate is consistent with the capillary pressure predicted by the macroscopic theory (i.e., the Young-Laplace equation), but the normal pressure that determines the capillary force is always more positive than the continuum counterpart.

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Capillary adhesion between elastic solids with randomly rough surfaces

Cited by 97 publications

References 46 publications

Contact between rough surfaces and a criterion for macroscopic adhesion

Contact between rough surfaces and a criterion for macroscopic adhesion

Understanding the Friction Mechanisms Between the Human Finger and Flat Contacting Surfaces in Moist Conditions

Capillary adhesion at the nanometer scale

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