Experimental investigation into the combined effects of roughness
and stiffness in soft-material adhesion. 

Abhijeet Gujrati1, Siddhesh Dalvi2, Ali Dhinojwala2, Tevis D. B. Jacobs1

 

1Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA

2Department of Polymer Science, The University of Akron, Akron, OH

Accurately predicting the adhesion of soft materials on hard surfaces remains a significant challenge. Many models have been proposed, often based on sinusoidal or fractal-like surfaces; however, the experimental validation for these models remains limited. A primary reason for this is that adhesion in these contacts depend on many factors, including surface chemistry, surface topography, mechanical properties, and testing conditions. Further, because most surfaces have roughness across many length scales, the measured values of roughness parameters are often highly dependent on measurement size and technique.

 

Here we performed adhesion tests using hemispherical lenses of polydimethylsiloxane (PDMS) with varying elastic modulus in contact with surfaces of polycrystalline diamond with varying roughness. The topography of the surfaces was pre-characterized across multiple length scales, to enable the calculation of both scalar and spectral roughness parameters. The contact tests were performed with in situ measurements of the load-dependent contact size. This enabled independent measurement of behavior during loading, during unloading, and at the point of pull-off. We analyze results in light of a multi-scale model [Persson & Tosatti, J. Chem. Phys., 115, 5597 (2001)]. While the behavior upon approach is accurately captured by this model, showing a monotonically decreasing trend with roughness and modulus, the pull-off behavior is not, instead showing a maximum value at an intermediate roughness. Finally, we will discuss the sensitivity of model predictions to how the topography is measured and to which assumptions are made.