Sliding drops: unveiling the influence of defects on dynamic wetting
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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.