SKM 2023 – wissenschaftliches Programm

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DY: Fachverband Dynamik und Statistische Physik

DY 36: Wetting, Fluidics and Liquids at Interfaces and Surfaces I (joint session CPP/DY)

DY 36.2: Vortrag

Donnerstag, 30. März 2023, 09:45–10:00, MER 02

Sliding drops: towards a universal law of friction — Xiaomei Li¹, Francisco Bodziony², Mariana Yin², Holger Marshall², Rüdiger Berger¹, and •Hans-Jürgen Butt¹ — ¹Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz — ²Computational Multiphase Flows, Technische Universität Darmstadt, Alarich-Weiss-Straße 10, 64287 Darmstadt

Liquid drops moving on tilted surfaces are an everyday phenomenon and are important for many industrial applications. Still, it is not possible to predict their velocity. To make a step forward in quantitative understanding, we measured the velocity U, width w, length, advancing Θ_a, and receding contact angle Θ_r of liquid drops sliding down inclined flat surfaces. By solving the equation of motion, we determined the friction force versus slide velocity for different hydrophobic surfaces. The friction force acting on moving drops of polar and non-polar liquids with viscosities ranging from 10⁻³ to 1 Pa s can empirically be described by F_f (U)=F₀+µ w Ca^α for the whole relevant velocity range. Here, Ca = Uη/γ is the capillary number, in which η is the viscosity and γ the surface tension of the liquid. The friction coefficient µ is in the range of 1 - 3 N/m for all liquid/surface combinations. For viscosities above 0.006 Pa s, we find α = 1.0. Bulk and wedge viscous dissipation can fully account for the velocity-dependent friction force. These results were confirmed by direct numerical diffuse-interface simulations of the flow pattern inside sliding drops. We demonstrate that the Furmidge-Kawasaki equation, is also valid in the dynamic case.