Sliding drops: unveiling the influence of defects on dynamic wetting

Abstract

The friction and dynamics of sliding droplets on hydrophobic surfaces are fundamental to nu merous industrial and natural processes, ranging from self-cleaning coatings to microfluidic systems. This thesis investigates the microscopic origins of energy dissipation and Contact Angle Hysteresis (CAH) by focusing on the real-time interaction between the triple-phase con tact line (CL) and sub-microscopic surface defects. To address this problem, the research is structured around two distinct experimental campaigns. In the first campaign, detailed in Chapter 3, we utilize a functionalized droplet probe in an Atomic Force Microscope (AFM) to quantify interactions at the nanoscopic scale. By attaching a picoliter-scale droplet (≈30 µm diameter) to a tipless cantilever, we measure the lateral friction forces encountered by the CL as it interacts with isolated, well-defined defects. These measurements validate a linear-force model for pinning, demonstrating that the Joanny-de Gennes formalism remains an accurate description of the mechanical restorative forces at these sub-microscopic scales. The second campaign, presented in Chapter 4, employs a tilted-plate setup coupled with high-speed reflection microscopy to investigate macroscopic drop behavior. Capable of record ing at frame rates up to 100,000 fps, this system provides the temporal resolution necessary to observe rapid contact line fluctuations and ”stick-slip” dynamics on various hydrophobic substrates. The resulting high-speed datasets are processed through an automated image anal ysis pipeline, incorporating Canny edge detection, morphological transformations, and iterative polynomial fitting, to extract the precise coordinates of the CL. We quantify the magnitude of pinning through the Deviation (δ), defined as the local distance between the physical CL and its ideal, unperturbed state. The synthesis of these investigations reveals a significant ”Hysteresis Gap”. Statistical analysis across various hydrophobic coatings (PFOTS, PFOMS, Teflon, and PS) shows no direct correlation between the density of resolvable nanoscopic defects and the measured macroscopic hysteresis. Even when accounting for sub-diffraction contact line perturbations down to a 50 nm limit, the energy dissipated by individual pinning events remains significantly lower than the total energy associated with macroscopic drop friction. These findings imply that while defects may influence contact angle hysteresis, this effect does not exceed half a degree, a value that is considerably lower than the total hysteresis observed on the studied surfaces.