Abstract
Motivated by roughness-induced adhesion enhancement (toughening and strengthening) in low modulus materials, we study the detachment of a sphere from a substrate in the presence of both viscoelastic dissipation at the contact edge, and roughness in the form of a single axisymmetric waviness. We show that the roughness-induced enhancement found by Guduru and coworkers for the elastic case (i.e. at very small detachment speeds) tends to disappear with increasing speeds, where the viscoelastic effect dominates and the problem approaches that of a smooth sphere. This is in qualitative agreement with the original experiments of Guduru’s group with gelatin. The cross-over velocity is where the two separate effects are comparable. Viscoelasticity effectively damps roughness-induced elastic instabilities and makes their effects much less important.Graphical
Highlights
It is well known that adhesion of hard solids is difficult to measure at macroscopic scales, and Fuller and Tabor [1] proved that even in low modulus materials, a ∼ 1 m of roughness destroy adhesion almost completely, despite van der Waals adhesive forces are quite strong, giving the so-called “adhesionPage 3 of 11 127 paradox” [2]
We indicate with P0 = 3∕2 w0R the JKR value of pull-off for the smooth sphere
A possible explanation could be the effect observed in the present paper, namely that there is no multiplicative effect of roughness-induced adhesion enhancement and viscoelasticity-induced adhesion enhancement
Summary
It is well known that adhesion of hard solids is difficult to measure at macroscopic scales, and Fuller and Tabor [1] proved that even in low modulus materials (they used rubbers with E ∼ 1 MPa ), a ∼ 1 m of roughness destroy adhesion almost completely, despite van der Waals adhesive forces are quite strong, giving the so-called “adhesionPage 3 of 11 127 paradox” [2]. Dahlquist [3, 4], while working at 3 M, proposed a criterion largely used in the world of adhesives, namely that the elastic Young modulus should be smaller than ∼ 1 MPa to achieve stickiness even in the presence of roughness This clearly is just a rough indication, but Tiwari [5] finds for example that the work of adhesion (at a given retraction speed) is reduced of a factor 700 for a rubber in contact with a rough hard sphere when the rubber modulus is E = 2.3 MPa (because of a certain roughness) but is increased for the same roughness by a factor 2 when the rubber has modulus E = 0.02 MPa. The threshold does not change much even if we consider nanometer scale roughness, as in the recent results of Dalvi et al [6] for pull-off of PDMS hemispheres having four different elastic moduli against different roughened plates: Dahlquist’s criterion seems to work surprisingly well, as while there is little effect of roughness for the three cases of low modulus up to near E = 2 MPa, roughness has strong effect both during approach and retraction for the high modulus material ( E = 10 MPa), where the hysteresis may be due partly to viscoelastic effects. For the 3 low modulus materials, roughness almost systematically increases the work of adhesion rather than decreasing it as for the high modulus material, for a given retraction speed
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
