The Effect of Atomic-Scale Roughness on the Adhesion of Nanoscale Asperities: A Combined Simulation and Experimental Investigation

Jacobs, Tevis D. B.; Ryan, Kathleen E.; Keating, Pamela L.; Grierson, David S.; Lefever, Joel A.; Turner, Kevin T.; Harrison, Jeffrey S.; Carpick, Robert W.

doi:10.1007/s11249-012-0097-3

Cited by 121 publications

(134 citation statements)

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“…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)(9)(10)(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).…”

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confidence: 99%

Contact between rough surfaces and a criterion for macroscopic adhesion

Pastewka

Robbins

2014

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

275

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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mentioning

confidence: 99%

Contact between rough surfaces and a criterion for macroscopic adhesion

Pastewka

Robbins

2014

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

275

238

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“…2). Of course the roughness may affect the effective radius that should be used in extracting w [48].…”

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Contact area of rough spheres: Large scale simulations and simple scaling laws

Pastewka

Robbins

2016

Applied Physics Letters

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We use molecular simulations to study the nonadhesive and adhesive atomic-scale contact of rough spheres with radii ranging from nanometers to micrometers over more than ten orders of magnitude in applied normal load. At the lowest loads, the interfacial mechanics is governed by the contact mechanics of the first asperity that touches. The dependence of contact area on normal force becomes linear at intermediate loads and crosses over to Hertzian at the largest loads. By combining theories for the limiting cases of nominally flat rough surfaces and smooth spheres, we provide parameter-free analytical expressions for contact area over the whole range of loads. Our results establish a range of validity for common approximations that neglect curvature or roughness in modeling objects on scales from atomic force microscope tips to ball bearings. *

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“…14,15 The effect of surface roughness was accounted for using the modified-Rumpf model 16,17 since it is best suited for solid materials with surface roughness values less than 2 nm. 17 In this model, the roughness is approximated as a single hemispherical asperity on a flat surface; the radius of the asperity depends only on the RMS roughness of the surface.…”

confidence: 99%

“…17 In this model, the roughness is approximated as a single hemispherical asperity on a flat surface; the radius of the asperity depends only on the RMS roughness of the surface. 15,17 The equation combining the DMT and modified-Rumpf model is 13…”

confidence: 99%

“…22 The results for the implanted and annealed sample indicate an oxide thickness of ∼2 nm with a stoichiometry of RuO 2 at the surface, confirming the accuracy of the SRIM simulations. Using the DMT model, 14,15 we have also estimated the effects of other parameters such as the contact radius (a), sample deformation (δ), and contact pressure (P) underneath the tip during measurement. The contact radius (a) for the AFM tip is given by the following expression: 15…”

Section: W Admentioning

confidence: 99%

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