Contact line relaxation of sessile drops on PDMS surfaces: A methodological perspective

Hypothesis: Characterization of contact angle hysteresis on soft surfaces is sensitive to the measurement protocol and might present adventitious time-dependencies. Contact line dynamics on solid surfaces is altered by the surface chemistry, surface roughness and/or surface elasticity. We observed a slow spontaneous relaxation of static water sessile drops placed on elastic surfaces. This unexpected drop motion reveals unresolved equilibrium configurations that may affect the observed values of contact angle hysteresis. Drop relaxation on deformable surfaces is partially governed by a viscoelastic dissipation located at the contact line. Experiments: In this work, we studied the natural relaxation of water drops formed on several smooth PDMS surfaces with different elastic moduli. We monitored in time the contact angle and contact radius of each drop. For varying the initial contact angle, we used the growing-shrinking drop method. Findings: We postulate that the so-called braking effect, produced by the surface deformability, affects the contact line velocity and in consequence, the contact angle measurements. We conclude that the wetting properties of elastic surfaces should be properly examined with reliable values of contact angle measured after drop relaxation. (C) 2020 Elsevier Inc. All rights reserved.

Automated summary

The study demonstrates that the characterization of contact angle hysteresis on soft surfaces is sensitive to the measurement protocol and may have adventitious time-dependencies. The slow spontaneous relaxation of water drops on elastic surfaces may affect the observed values of contact angle hysteresis. The wetting properties of elastic surfaces should be properly examined with reliable values of contact angle measured after drop relaxation.