Friction Coefficients for Droplets on Solids: The Liquid-Solid Amontons' Laws.
Glen McHale1, Nan Gao2, Gary G Wells1
1Institute for Multiscale Thermofluids, School of Engineering, The University of Edinburgh, Edinburgh EH9 3FB, U.K.
Langmuir : the ACS Journal of Surfaces and Colloids
|March 30, 2022
Summary
Researchers defined droplet friction coefficients analogous to solid friction laws. They found static friction relates to contact angle hysteresis and kinetic friction to dynamic contact angle differences, aiding droplet motion control.
Area of Science:
- Physics
- Materials Science
- Surface Science
Background:
- Dry friction laws, established by Amontons, involve static and kinetic coefficients. Liquid droplets on surfaces also exhibit friction, with recent efforts to define coefficients and regimes.
- Contact lines of liquids on solids experience pinning, and friction concepts are applied to droplet motion, though formal friction coefficients are recent.
Purpose of the Study:
- To define coefficients of friction for liquid droplets on solid surfaces.
- To establish a droplet analogy of Amontons' laws of friction, replacing normal load force with normal surface tension force.
- To provide a framework for understanding and controlling droplet motion on surfaces.
Main Methods:
- Utilized surface free energy considerations to analyze frictional forces at liquid-solid contact lines.
- Defined static and kinetic friction coefficients for droplets based on pinning force and dynamic contact angles.
- Developed a droplet analogy of Amontons' first and second laws and validated it against existing equations and literature data.
Main Results:
- The frictional force per unit length of a contact line is proportional to the normal component of the surface tension force.
- The coefficient of static friction is proportional to contact angle hysteresis.
- The coefficient of kinetic friction is proportional to the difference between dynamic advancing and receding contact angles.
Conclusions:
- Established liquid-solid Amontons' laws consistent with Furmidge's equation, providing a new conceptual framework for droplet friction.
- Reported friction coefficients for various liquid-solid systems, demonstrating the applicability of the developed laws.
- The findings offer insights for designing surfaces like superhydrophobic and slippery liquid-infused porous surfaces (SLIPS) to control droplet motion.
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