Effect of Interface Wettability and Roughness on Sliding Friction Behavior of PDMS

When the contact interface of visco-elastomers is between wet and dry, a new wetting condition higher than the dry friction is always captured, which is called the tacky regime. The maximum friction coefficient in this state is termed as the friction peak. In this manuscript, the influences of wettability (θ) and surface roughness (Ra) of glass slide surfaces on the friction peak were investigated using a custom-built in-situ optical microtribometer. Different glass slides were chosen as the base including transparent glass slide, teflon (PTFE) tape + glass slide and polypropylene (BOPP) + glass slide, which named as sample A, B and C, respectively. The contact angles of sample A, B and C were 38°, 97° and 100°, and the surface roughness of sample A, B and C were 1.420 μm, 1.315 μm and 0.312 μm, respectively. As the soft surface, a PDMS hemesphere with the ratio of the curing agent to the base of 1:5 was produced under water on a glass plate. The diameter of the resulting PDMS sphere was 3.58 mm with the contact angle of 110°. Two kinds of sliding experiments were conducted between the three samples and the PDMS hemisphere including dry condition and wet to dry condition. It was found that in the dry condition when the surface roughnesses of the glass slide were similar, the friction between the PDMS hemisphere and the surface with the smaller contact angle was larger; when the contact angles of the glass slide surfaces were similar, the friction of the surface with the smaller surface roughness was larger. In the friction experiments where the contact interface changed from wet to dry, the maximum friction coefficients of the sample A, B and C were 1.20, 1.02 and 1.25, respectively. In order to compare the tribological behaviors of the three samples in the tacky regime, the increased percentage of the maximum friction coefficient compared with the dry friction coefficient was defined as the relative increased percentage, ∆μ%, and the increased percentage of hysteresis displacement in the tacky regime compared with the dry condition was defined as ∆S%. The result showed that the ∆μ% of the three samples were 15%, 70%, 28% and ∆S% of the three samples were 20%, 130%, 40%. It was found that there was a good linear relationship between the maximum increase percentage of friction coefficient (Δμ%) and the increase percentage of hysteresis displacement (ΔS%) comparing with those in the dry condition. The increase of friction coefficient and hysteresis displacement might be related to the residual droplets between the contact surfaces. It was conjectured that the wettability and surface roughness of the glass slide surfaces would affect the shape and number of liquid bridge generated in the contact interfaces, thus affecting the increase of the friction peak in the tacky regime. When the surface roughness of the smooth glass slide surface was similar, the friction peak of the surface with smaller contact angle was lower. In this condition, the hydrophilic surface was more unfavorable to the formation of liquid bridge between the asperities, thus reduced the maximum friction peak. When the contact angle was close, the friction peak of the surface with smaller surface roughness was lower. We hypothesis that for the smoother glass slide surface, the real contact area with the PDMS hemisphere was larger, which promoted dewetting of water film in the contact interfaces, resulting in the reduction of capillary adhesion in the tacky regime, and thus weakened the friction peak. For smooth hydrophobic surfaces, micro-scale roughness might promote the friction peak in the tacky regime. We proposed that the friction peak in the tacky regime might be affected by the interface dewetting behavior, the number and the shape of the micro-liquid bridges in the sliding process, and the specific mechanism needed to be studied and discussed further.