[Paper Review] Experimental evidence of slippage breakdown for a superhydrophobic surface in a microfluidic device

Guido Bolognesi, Cécile Cottin-Bizonne+1 more|arXiv (Cornell University)|Jun 12, 2014

Surface Modification and Superhydrophobicity3 citations

TL;DR

This study experimentally demonstrates that liquid-air interfaces on superhydrophobic surfaces in microfluidic devices can sustain non-zero shear stresses, violating the widely assumed no-shear boundary condition. Using combined fluorescence microscopy and digital image processing, the authors measure both velocity profiles and meniscus shapes, revealing that interfacial friction—potentially due to contaminants or thermal effects—significantly reduces effective slip, undermining the friction-reduction benefits of superhydrophobicity even in the stable Cassie state.

ABSTRACT

A full characterization of the water flow past a silicon superhydrophobic surface with longitudinal micro-grooves enclosed in a microfluidic device is presented. Fluorescence microscopy images of the flow seeded with fluorescent passive tracers were digitally processed to measure both the velocity field and the position and shape of the liquid-air interfaces at the superhydrophobic surface. The simultaneous access to the meniscus and velocity profiles allows us to put under a strict test the no-shear boundary condition at the liquid-air interface. Surprisingly, our measurements show that air pockets in the surface cavities can sustain non-zero interfacial shear stresses, thereby hampering the friction reduction capabilities of the surface. The effects of the meniscus position and shape as well as of the liquid-air interfacial friction on the surface performances are separately assessed and quantified.

Motivation & Objective

• To investigate the validity of the no-shear boundary condition at liquid-air interfaces on superhydrophobic surfaces in microfluidic channels.
• To quantify the combined effects of meniscus deformation and interfacial friction on effective slip length.
• To determine whether stable Cassie-state air pockets can still lead to significant frictional resistance in microflows.
• To resolve the discrepancy between theoretical predictions and experimental measurements of slip length on superhydrophobic surfaces.
• To assess the role of surface contamination and laser-induced thermal effects in generating interfacial shear stress.

Proposed method

• Employed a microfluidic device with a silicon superhydrophobic surface featuring longitudinal micro-grooves to create a stable Cassie state.
• Used fluorescence microscopy with passive tracers to measure the 2D velocity field across the flow channel.
• Applied digital image processing to simultaneously reconstruct the meniscus shape and position at the liquid-air interface.
• Calculated global effective slip length by averaging velocity profiles over periodic unit cells containing one liquid-air and one liquid-solid interface.
• Correlated local meniscus geometry (curvature, penetration depth) with measured interfacial shear stress to isolate friction contributions.
• Used a custom experimental setup with co-planar focal plane alignment to enable simultaneous flow and meniscus imaging.

Experimental results

Research questions

• RQ1 Can liquid-air interfaces on superhydrophobic surfaces in microfluidic devices sustain non-zero shear stresses, challenging the no-shear boundary condition?
• RQ2 How do meniscus shape and position affect the effective slip length in superhydrophobic microchannels?
• RQ3 To what extent does interfacial friction at the liquid-air interface compromise the friction-reduction potential of superhydrophobic surfaces?
• RQ4 What are the relative contributions of meniscus deformation and interfacial shear stress to reduced slippage in the Cassie state?
• RQ5 Are surface contaminants or laser-induced thermal effects plausible causes of measurable interfacial friction in microfluidic experiments?

Key findings

• The liquid-air interface on the superhydrophobic surface exhibits measurable non-zero shear stress, violating the standard no-shear boundary condition.
• The effective slip length was found to be significantly lower than theoretical predictions, with global slip lengths showing up to 40% variation across different measurement spots.
• Meniscus protrusion into the flow channel—due to local shape and position—was identified as a major contributor to reduced slippage, with penetration depth directly correlated to slip length reduction.
• Interfacial friction, possibly caused by contaminant particles or laser-induced thermal effects, was found to be a significant source of resistance, comparable in magnitude to the detrimental effects of meniscus deformation.
• Even in the stable Cassie state, the presence of flat or slightly deformed menisci does not guarantee effective slip, as interfacial friction can dominate.
• The study provides the first direct experimental evidence that non-deformed liquid-air interfaces are insufficient to ensure friction reduction, as interfacial shear stress can still limit performance.