Pinning and wicking in regular pillar arrays

Area of Science:

• Physics
• Materials Science
• Fluid Dynamics

Background:

• Understanding liquid behavior in confined geometries is crucial for microfluidic devices and material design.
• Existing models for liquid meniscus behavior in pillar arrays often rely on simplified thermodynamic or geometric assumptions.

Purpose of the Study:

• To investigate the pinning and wicking of liquid menisci in square pillar arrays.
• To compare numerical simulations with experimental wetting data.
• To identify limitations of current models and explain observed phenomena like re-entrant pinning.

Main Methods:

• Numerical energy minimization to model meniscus behavior.
• Experimental investigation of liquid wicking in pillar arrays.
• Analysis of energy landscapes to understand meniscus shape transitions.

Main Results:

• Thermodynamic and geometric models are insufficient for predicting wicking onset.
• High aspect ratio square pillars exhibit a re-entrant pinning regime with increasing density.
• This re-entrant behavior is explained by energy barriers between different meniscus shapes.
• Experimental validation confirms re-entrant behavior for square pillars but not for circular ones (contact angle θ0 = 47°).

Conclusions:

• Current simplified models do not fully capture the complex liquid meniscus behavior in dense pillar arrays.
• The re-entrant pinning regime in square pillars is a consequence of energy barriers and topological changes in the meniscus.
• Experimental results validate the numerical findings, emphasizing the importance of pillar geometry in wicking phenomena.