Soft Matter and Complex Systems Seminar

1. Title: Hydrodynamic Levitation of Liquid Droplets on Rotating Surfaces: Experimental and CFD Analysis of Boundary Layer Interactions

Abstract: The interaction of liquid droplets with solid surfaces is a phenomenon of fundamental importance across numerous industrial processes, including spray coating, spray cooling, and cleaning applications. While thermal effects, such as the well-documented Leidenfrost effect, can induce droplet levitation via a vapor cushion, analogous non-wettable behavior can be achieved at ambient temperatures through hydrodynamic means. A moving surface submerged in a fluid generates a boundary layer capable of preventing direct contact between an impacting droplet and the surface itself. This hydrodynamic levitation has been observed in both low and high-velocity flow regimes.
This investigation presents a comprehensive experimental and computational fluid dynamics (CFD) analysis of the interaction between a liquid droplet and the boundary layer generated by a vertically rotating flat disk. The primary experimental objective was to determine the feasibility of achieving stable droplet levitation within both laminar and turbulent boundary layers. Furthermore, the study aimed to define the operational limits of this levitation, specifically by identifying the critical impact velocity beyond which a free-falling droplet penetrates the boundary layer and makes contact with the disk surface.
The computational portion of this work focuses on elucidating the complex flow field surrounding a levitating droplet. We analyze the mutual interaction between the primary rotating disk flow and the stationary droplet, quantifying the modifications to the base flow caused by the droplet's presence and the resultant aerodynamic forces. A key aspect of this analysis is explaining the origin of observed droplet shape oscillations during levitation by examining flow instabilities and their subsequent effect on the pressure distribution across the droplet's surface. This dual approach provides a detailed understanding of the underlying physics governing hydrodynamic droplet levitation.

Acknowledgments: This research was carried out with the support of the Interdisciplinary Centre for Mathematical and Computational Modelling University of Warsaw (ICM UW) under computational allocation no G100-2222. Research was funded by the Warsaw University of Technology within the Excellence Initiative: Research University (IDUB) programme.

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2. Title: Shaking Things Up: A Dynamic Oscillation Framework For Contact Angle Hysteresis Measurement

Abstract: This work presents a dynamic method for determining contact angle hysteresis (CAH) by placing droplets on a harmonically oscillating substrate, providing new kinetic insights into surface wettability. We designed a custom experimental setup featuring a lightweight, 3D-printed motion carriage actuated by a high-performance linear motor capable of sinusoidal oscillations with accelerations up to 9g, and equipped with a high-speed optical system for millisecond-scale imaging and analysis. Silicon wafers were used as substrates with Glaco superhydrophobic surface treatment, and deionised water was chosen as the working fluid due to its well-characterized and reproducible physicochemical properties, ensuring comparability and minimizing variability. The integrated imaging and analysis approach, including precise droplet deposition and a robust MATLAB processing pipeline, enabled accurate measurement of contact angle dynamics and improved uncertainty quantification. Results show this oscillation-based method effectively probes the thresholds required for depinning, advances the study of dynamic droplet mobility, and facilitates detection of local surface heterogeneities, outperforming conventional static and quasi-static CAH measurement techniques.