Polyelectrolyte adsorption at the solid-liquid interface favors receding contact line instability

Abstract

Controlling the motion of non-Newtonian drops on surfaces is crucial for applications ranging from inkjet printing to biomedical devices and food processing. While the macroscopic behavior of viscoelastic drops sliding on tilted hydrophobic surfaces has been characterized, showing reduced velocities and elongation compared to Newtonian fluids, the underlying microscopic mechanisms remain poorly understood. To address this gap, we developed a high-speed, high-resolution reflection microscope that enables direct visualization of the contact line of sliding drops. We used water/soluble polyelectrolyte solutions based on polyacrylamide and let drops sliding on hydrophobic substrates composed of Teflon AF- and PDMS-coated glass slides. The substrate tilting angle was varied between 20° and 45°. We reveal how viscoelasticity influences the dynamics of the receding contact line and drop motion. Our experiments demonstrate that viscoelasticity can destabilize the receding contact line, triggering filament formation. This instability previously observed in the coating of thin viscoelastic films, is reported here for the first time in sliding drops. We further highlight the critical role of polymer charge in this process: while cationic and non-ionic polymers promote filament formation, anionic polymers do not, a difference we attribute to the distinct wetting properties of the solutions. In conclusion, we clarify the interplay between rheology, surface interactions, and drop dynamics.

Figures (9)

• Figure 1: a) Schematics of the setup used for reflection microscopy of sliding drops. b-c) Typical images obtained respectively from the side-view and bottom-view camera.
• Figure 2: Side-view images of the drops and reflection microscope images of the receding contact line for a tilting angle of 40° for the anionic polymer (respectively a,d), the non-ionic polymer (b,e), and the cationic polymer (c,f). The dark part correspond to a liquid/solid interface while the brighter part correspond to the air/solid interface.
• Figure 3: (a) Effective capillary number as a function of the tilting angle and (b) contact angle hysteresis as a function of the capillary number for the anionic (orange), non-ionic (green), and cationic (red) polymers. Error bars were computed based on the standard deviation of the averaged velocity.
• Figure 4: (a) Length $L$ and (b) wavelength $\lambda$ of the filaments as a function of tilting angle for the non-ionic (green) and cationic (red) polymer. Error bars correspond to standard deviation.
• Figure 5: Schematic of the rear side of a sliding drop representing polymer adsorption at the solid-liquid interface depending on its charge: (a) anionic and (b) cationic polyelectrolyte.