Multiple time scale dynamics in the breakdown of superhydrophobicity

Pirat, Christophe; Sbragaglia, Mauro; Peters, A.M.; Borkent, B.M.; Lammertink, Rob G.H.; Weßling, Matthias; Lohse, Detlef

doi:10.1209/0295-5075/81/66002

Cited by 56 publications

(62 citation statements)

References 27 publications

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“…Both polymers are hydrophobic with similar static contact angles, measured to be 120 o for flat PDMS and 127.4 o ± 2.6 o for flat porous PVDF (OCA 20, Dataphysics). These large contact angles can help microstructured PDMS and PVDF surfaces sustain compositewetting Cassie-Baxter (CB) state with air pockets underneath a still water droplet [24,25]. From our experiences, sometimes the non-wetting state can be maintained, i.e., the case of apparent slippage, for our imposed flow rates.…”

Section: Methodsmentioning

confidence: 75%

Quantifying effective slip length over micropatterned hydrophobic surfaces

Tsai¹,

Peters²,

Pirat³

et al. 2009

Physics of Fluids

169

134

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We employ micro-particle image velocimetry (µ-PIV) to investigate laminar micro-flows in hydrophobic microstructured channels, in particular the slip length. These microchannels consist of longitudinal micro-grooves, which can trap air and prompt a shear-free boundary condition and thus slippage enhancement. Our measurements reveal an increase of the slip length when the width of the micro-grooves is enlarged. The result of the slip length is smaller than the analytical prediction by Philip et al. [1] for an infinitely large and textured channel comprised of alternating shear-free and no-slip boundary conditions. The smaller slip length (as compared to the prediction) can be attributed to the confinement of the microchannel and the bending of the meniscus (liquid-gas interface). Our experimental studies suggest that the curvature of the meniscus plays an important role in microflows over hydrophobic micro-ridges.

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Section: Methodsmentioning

confidence: 75%

Quantifying effective slip length over micropatterned hydrophobic surfaces

Tsai¹,

Peters²,

Pirat³

et al. 2009

Physics of Fluids

169

134

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show abstract

“…First, by assuming a flat interface, we have neglected an additional mechanism for a dissipation connected with the meniscus curvature [30][31][32]. Second, we ignore a possible transition toward an impaled (Wenzel) state that can be provoked by additional pressure in the liquid phase [33,34].…”

Section: General Theorymentioning

confidence: 99%

Drag force on a sphere moving toward an anisotropic superhydrophobic plane

2011

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We analyze theoretically a high-speed drainage of liquid films squeezed between a hydrophilic sphere and a textured superhydrophobic plane that contains trapped gas bubbles. A superhydrophobic wall is characterized by parameters L (texture characteristic length), b1 and b2 (local slip lengths at solid and gas areas), and φ1 and φ2 (fractions of solid and gas areas). Hydrodynamic properties of the plane are fully expressed in terms of the effective slip-length tensor with eigenvalues that depend on texture parameters and H (local separation). The effect of effective slip is predicted to decrease the force as compared with what is expected for two hydrophilic surfaces and described by the Taylor equation. The presence of additional length scales, L, b1, and b2, implies that a film drainage can be much richer than in the case of a sphere moving toward a hydrophilic plane. For a large (compared to L) gap the reduction of the force is small, and for all textures the force is similar to expected when a sphere is moving toward a smooth hydrophilic plane that is shifted down from the superhydrophobic wall. The value of this shift is equal to the average of the eigenvalues of the slip-length tensor. By analyzing striped superhydrophobic surfaces, we then compute the correction to the Taylor equation for an arbitrary gap. We show that at a thinner gap the force reduction becomes more pronounced, and that it depends strongly on the fraction of the gas area and local slip lengths. For small separations we derive an exact equation, which relates a correction for effective slip to texture parameters. Our analysis provides a framework for interpreting recent force measurements in the presence of a superhydrophobic surface.

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“…As the fakir state is typically the preferred one, it would be important to understand the robustness of this state, and the mechanisms leading to collapse to the Wenzel state, which may dramatically change the hydrophobicity. This issue has recently attracted a lot of interest, and both exp-1 perimental [11,13,14] and numerical [14][15][16] techniques have been utilized to elucidate the transition between different superhydrophobic states. These studies have been conducted on microstructured surfaces decorated with regular patterns of posts.…”

mentioning

confidence: 99%

Impalement transitions in droplets impacting microstructured superhydrophobic surfaces

Hyväluoma

Timonen

2008

Europhys. Lett.

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Liquid droplets impacting a superhydrophobic surface decorated with micro-scale posts often bounce off the surface. However, by decreasing the impact velocity droplets may land on the surface in a fakir state, and by increasing it posts may impale droplets that are then stuck on the surface. We use a two-phase lattice-Boltzmann model to simulate droplet impact on superhydrophobic surfaces, and show that it may result in a fakir state also for reasonable high impact velocities. This happens more easily if the surface is made more hydrophobic or the post height is increased, thereby making the impaled state energetically less favourable.

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Multiple time scale dynamics in the breakdown of superhydrophobicity

Cited by 56 publications

References 27 publications

Quantifying effective slip length over micropatterned hydrophobic surfaces

Quantifying effective slip length over micropatterned hydrophobic surfaces

Drag force on a sphere moving toward an anisotropic superhydrophobic plane

Impalement transitions in droplets impacting microstructured superhydrophobic surfaces

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