Contact angle hysteresis with a twist – Stretched droplets on elastic substrates

• Water drops adhering to elastic PDMS surfaces show reproducible shape hysteresis under reversible stretching and relaxation. • The hysteresis can be influenced by modifying topography (fs-laser-processing) and surface chemistry (functionalisation). • The adhesion behaviour is related to time-dependent wetting transitions and the presence of permanent air inclusions. • Advancing and receding contact angles and strain-dependent contact angle hysteresis are found within the droplet shape cycle. The dynamic wetting behaviour of liquids on deformed surfaces involves a complex interplay of surface tension, substrate elasticity, and droplet adhesion. Nevertheless, conventional methods are often incapable to fully characterise stimuli-responsive surfaces, based on mechano-responsive topography changes under permanent droplet contact. As measuring the contact angle hysteresis is essential for evaluating a surface’s wettability, alternative methods for investigating the strain-dependent change of the droplet shape and its contact angles are required. For this purpose, we observed the wetting behaviour of attached droplets during reversible substrate stretching (up to 200 %) and relaxation using elastic polydimethylsiloxane with different laser-processed microstructures and chemically functionalised surfaces. The associated droplet shape hysteresis within the range of less than 5° to over 70° shows a significant dependence on the topography and surface chemistry. Pinning and deformation of the three-phase contact line are primarily influenced by permanent air inclusions or complete surface wetting. Comparisons with well-established wetting models and conventional hysteresis measurements confirm that this strain-dependent method provides reliable and reproducible insights into droplet-surface interactions on deformable substrates. These findings enhance the understanding of dynamic wetting processes on elastic materials during reversible mechano-responsive modification and support the development of advanced droplet manipulation strategies for smart surface applications.