(671c) Effect of Surface Stress on Roughness and Adhesion of Soft Solids

Most established theories on adhesion of rough materials predict that higher roughness results in lower adhesion[1, 2] due to reduction of real area of contact and elastic energy stored in deformation and flattening of asperities. For example, Persson’s energy argument for adhesion of rough surfaces proposes that the elastic energy to flatten surface roughness is released on interfacial crack propagation, and thus should be subtracted from work of adhesion. Contrary to most established theories, Guduru [3] argues that roughness can actually strongly increase adhesion due to additional energy dissipation during intermittent crack trapping and unstable release. Surface stress can also affect deformation of a soft solid due to electrocapillarity by flattening the roughness[4]. Hui et. al. [5] theoretically account for the effect of surface stress flattening the soft solid on adhesion of rough materials. However, the effect of surface stress has not been investigated experimentally.

In this study we systematically investigate the effect of surface stress on adhesion of a rough soft solid. We perform adhesion testing with a thin polydimethylsiloxane (PDMS) beam sample on top of a thick gelatin sample using a double cantilever beam experiment with one cantilever fixed. We chose four configurations: smooth PDMS on smooth gelatin, rough PDMS on smooth gelatin, smooth PDMS on rough gelatin and rough PDMS on rough gelatin. Roughness of surfaces is characterized using power spectral density (PSD) curves obtained through interferometric height profiling[5, 6]. As shown in the attached figure, adhesion is characterized using the crack length from the rod that separates PDMS from the gelatin sample.

From our preliminary experiments we observe a smaller crack length for smooth PDMS- rough gelatin samples than for smooth PDMS-smooth gelatin ones. This indicates higher adhesion for the rougher sample. This result is in contradiction of the well-known Persson’s theory. We investigate two physical effects to explain our results. Crack trapping in the rougher sample lead to smaller crack length and higher adhesion for rough elastic solids. At the same time, energy to flatten the rough material modulates the surface roughness of the free gel surface and thus affects the adhesion. We will present experimental data investigating the role of surface stress in adhesion of rough and soft material systematically.

Uploaded Figure caption:

(A) Schematic diagram of double cantilever beam adhesion testing with thin PDMS (0.58 mm) beam on thick gelatin (4 mm) sample separated by a cylindrical rod of fixed diameter. Crack length, a, is used to estimate the adhesion between PDMS and gelatin sample with both smooth and rough surfaces. (B) Top view image of the experimental setup in which a motor moves the gelatin sample towards the fixed rod and motion of the crack edge is monitored. (C) Microscopic image taken with a 5x objective showing the crack edge between smooth PDMS and smooth gelatin samples.

References:

1. Fuller, K. and D. Tabor, The effect of surface roughness on the adhesion of elastic solids. Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences, 1975. 345(1642): p. 327-342.
2. Persson, B. and E. Tosatti, The effect of surface roughness on the adhesion of elastic solids. The Journal of Chemical Physics, 2001. 115(12): p. 5597-5610.
3. Guduru, P., Detachment of a rigid solid from an elastic wavy surface: theory. Journal of the Mechanics and Physics of Solids, 2007. 55(3): p. 445-472.
4. Style, R.W., et al., Elastocapillarity: Surface tension and the mechanics of soft solids. Annual Review of Condensed Matter Physics, 2017. 8(1): p. 99-118.
5. Hui, C.Y., et al., How surface stress transforms surface profiles and adhesion of rough elastic bodies. Proceedings of the Royal Society A, 2020. 476(2243): p. 20200477.
6. Jacobs, T.D., T. Junge, and L. Pastewka, Quantitative characterization of surface topography using spectral analysis. Surface Topography: Metrology and Properties, 2017. 5(1): p. 013001.