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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 44: Wetting, Fluidics and Liquids at Interfaces and Surfaces II (joint session CPP/DY)
CPP 44.2: Talk
Thursday, March 30, 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 Θa, and receding contact angle Θr 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 Ff (U)=F0+µ w Caα 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.