Adhesion and relaxation of a soft elastomer on surfaces with skin like roughness

Abstract

For designing new skin adhesives, the complex mechanical interaction of soft elastomers with surfaces of various roughnesses needs to be better understood. We systematically studied the effects of a wide set of roughness characteristics, film thickness, hold time and material relaxation on the adhesive behaviour of the silicone elastomer SSA 7-9800 (Dow Corning). As model surfaces, we used epoxy replicas obtained from substrates with roughness ranging from very smooth to skin-like. Our results demonstrate that films of thin and intermediate thickness (60 and 160 µm) adhered best to a sub-micron rough surface, with a pull-off stress of about 50 kPa. Significant variations in pull-off stress and detachment mechanism with roughness and hold time were found. In contrast, 320 µm thick films adhered with lower pull-off stress of about 17 kPa, but were less sensitive to roughness and hold time. It is demonstrated that the adhesion performance of the silicone films to rough surfaces can be tuned by tailoring the film thickness and contact time.

Fig. 1

Topography of substrates used for adhesion testing, as characterized by stylus profilometry. (a)–(d) glass substrates (GS 1 and GS 2) and their epoxy replicas (ES 1 and ES 2), scans of 500 × 500 μm. (e)–(h) VitroSkin substrates (VS 1 and VS 2) and their epoxy replicas (ES 3 and ES 4), scans of 1000 × 1000 μm. Due to the large roughness range, the roughness scales differ.

Fig. 2

Roughness power spectra of the substrates used for adhesion testing. 1D power spectra of (a) the glass and VitroSkin master surfaces (GS 1–2; VS 1–2) and (b) the epoxy replica (ES 1–4) based on profilometer scans and generated with the Surface Topography Analyzer (http://contact.engineering/) (Jacobs et al., 2017.) (c) Resulting roughness parameters of all surfaces: average roughness R_a and average peak-to-valley distance R_z.

Fig. 3

Adhesion measurement results as function of film thickness and substrate roughness. (a) Pull-off stress as a function of the roughness parameter R_z and (b) pull-off stress, (c) work of separation and (d) maximum relative displacement for the films with three different thickness on four epoxy substrates. The hold time was 1 s.

Fig. 4

Exemplary pictures of the detachment mechanisms. Debonding of a “thin” (blue), “medium” (red) and “thick” (yellow) film from the epoxy substrates with increasing surface roughness. Finger-like cracks originating from the contact edge are observed in all cases. In thicker films, the fingers are coarser than in thinner films. Additional crack formation in the interior of the contact (cavitation) is seen in thinner films, especially in contact with substrates of higher roughness.

Fig. 5

Stress relaxation behaviour of the films on the different substrates. (a) Stress, normalized by the compressive pre-stress σ₀ of about 10 kPa, vs. hold time. Data shown are for the epoxy replica ES 2. Dots are experimental data, lines represents fits to Eq. (1). The values right to the curves represent the stress decreases at 120 s contact time relative to the initial pre-stress. (b) Time constants τ₁ (circles) and τ₂ (squares) obtained as a function of substrate and film thickness (see color code).

Fig. 6

Hold time effect on adhesion: Pull-off stress as a function of hold time for (a) thin (60 ± 10 μm), (b) medium (160 ± 25 μm) and (c) thick films (320 ± 30 μm) on the different epoxy substrates. The dashed lines are intended to guide the eye of the reader.