Evidence of slippage breakdown for a superhydrophobic microchannel

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.

Effective slip lengths for immobilized superhydrophobic surfaces

“…An immediate observation is that, as φ increases, the slip associated with a no-shear surface decreases while the slip associated with a no-slip surface increases. In two experiments, referred to as M1 and M2, Bolognesi et al (2014) observed that a more depressed groove associated with experiment M1 is associated with a larger slip length than experiment M2, where the interface invaded the groove to a lesser degree. As those authors pointed out, this is precisely not what is expected if the meniscus is a no-shear surface.…”

“…As those authors pointed out, this is precisely not what is expected if the meniscus is a no-shear surface. Bolognesi et al (2014) went on to suggest that the observed behaviour is more consistent with the meniscus being a no-slip surface. From figure 3 of Bolognesi et al (2014), we fitted our theoretical profiles to the experimental meniscus profiles by visually estimating the geometrical parameters d/c = 1.5 µm/16 µm for experiment M1 and d/c = 0.8 µm/16 µm for experiment M2.…”

“…For longitudinal flow, Crowdy (2016) has derived a formula for the effective slip length for exactly circular no-shear menisci. The effective slip length in (1.4) of Crowdy (2016) is plotted as a function of φ in figure 4 along with the new slip length result (1.5) for a no-slip surface with the same fixed value of 2c/L = 0.5, corresponding to the no-shear fraction used in the experiments of Bolognesi et al (2014). (Formula (1.4) of Crowdy (2016) is known to be accurate to within 1 %-2 % for this normalized groove width.)…”

“…These authors also suggested that their observed results were more consistent with immobilized no-slip free surfaces than those exhibiting zero shear. Bolognesi et al (2014) suggested surfactants or other surface contaminants as possible causes for this surface immobilization. In a very recent study, Peaudecerf et al (2017) focused on examining the effect of surfactants on hydrodynamic slip in longitudinal flows along unidirectionally patterned surfaces.…”

“…One of their principal conclusions from this study of Poiseuille flow through the microchannel was that the air-water interface more closely resembles a no-slip boundary than a shear-free one, and that a Wenzel state can have better friction reducing properties than the Cassie state. In a joint experimental and numerical study of a longitudinal flow scenario, Bolognesi, Cottin-Bizonne & Pirat (2014) tracked the effects of the meniscus shape, and the liquid-air interfacial friction properties, to examine the surface drag reduction properties. In this way, they were able to test the no-shear assumption on the menisci.…”