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Understanding the edge effect in wetting: a thermodynamic approach
1Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta T6G 2G8, Canada.
Langmuir : the ACS Journal of Surfaces and Colloids
|May 23, 2012
Summary
This study reveals the thermodynamic origin of the edge effect hindering sessile drop spreading. A free energy model explains how energy barriers at pillar edges dictate drop states, preventing complete collapse.
Area of Science:
- Thermodynamics
- Fluid Dynamics
- Surface Science
Background:
- The edge effect is recognized to impede sessile drop spreading.
- The precise thermodynamic mechanisms driving the edge effect remain incompletely understood.
Purpose of the Study:
- To develop a free energy model for investigating the energetic state of drops on single pillars.
- To elucidate the thermodynamic origin of the edge effect by analyzing free energy changes across pillar edges.
- To characterize different wetting behaviors based on edge geometry and liquid properties.
Main Methods:
- Development of a free energy model to analyze drop behavior on pillar geometries (upright to inverted frustum).
- Analysis of free energy levels before and after drop interaction with the pillar edge.
- Determination of four distinct wetting cases based on free energy plots and edge angles.
Main Results:
- Identified a free energy barrier near the pillar edge as crucial for determining drop states.
- Demonstrated that this barrier influences whether drops remain stable/metastable at the edge or collapse.
- Characterized drop behavior as stable, metastable, complete collapse, or intermediate sidewall positioning.
- Generated a wetting map correlating edge angle and intrinsic contact angle with observed wetting cases.
Conclusions:
- The developed thermodynamic model provides an energetic framework for understanding "re-entrant" structures.
- Results align with existing literature and enhance comprehension of Gibbs' inequality in wetting phenomena.
- The free energy barrier at edges is a key factor governing drop stability and spreading on pillars.
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