A novel technique for simultaneous velocity and interface profile measurements on micro-structured surfaces

Bolognesi, Guido; Cottin-Bizonne, Cécile; Guène, Elhadji Mama; Teisseire, Jérémie; Pirat, Christophe

doi:10.1039/c2sm26781k

Cited by 17 publications

(27 citation statements)

References 19 publications

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“…A number of studies show that this is hardly strictly true also in simple systems [35,36,37,38]. However, the present results show that a detailed description of the surface pattern is not required unless the particle is very close to the wall.…”

Section: Resultscontrasting

confidence: 40%

Mobility tensor of a sphere moving on a superhydrophobic wall: application to particle separation

Pimponi

Chinappi

Gualtieri

et al. 2013

Microfluid Nanofluid

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The paper addresses the hydrodynamic behavior of a sphere close to a micro-patterned superhydrophobic surface described in terms of alternated no-slip and perfect-slip stripes. Physically, the perfect-slip stripes model the parallel grooves where a large gas cushion forms between fluid and solid wall, giving rise to slippage at the gas-liquid interface. The potential of the boundary element method (BEM) in dealing with mixed no-slip/perfect-slip boundary conditions is exploited to systematically calculate the mobility tensor for different particle-to-wall relative positions and for different particle radii. The particle hydrodynamics is characterized by a non trivial mobility field which presents a distinct near wall behavior where the wall patterning directly affects the particle motion. In the far field, the effects of the wall pattern can be accurately represented via an effective description in terms of a homogeneous wall with a suitably defined apparent slippage. The trajectory of the sphere under the action of an external force is also described in some detail. A "resonant" regime is found when the frequency of the transversal component of the force matches a characteristic crossing frequency imposed by the wall pattern. It is found that, under resonance, the particle undergoes a mean transversal drift. Since the resonance condition depends on the particle radius the effect can in principle be used to conceive devices for particle sorting based on superhydrophobic surfaces.

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Section: Resultscontrasting

confidence: 40%

Mobility tensor of a sphere moving on a superhydrophobic wall: application to particle separation

Pimponi

Chinappi

Gualtieri

et al. 2013

Microfluid Nanofluid

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show abstract

“…3 shows the transverse profiles of the boundaries of the excluded volumes for both M1 and M2. As reported elsewhere, 32 the absolute position of the measured boundaries is accurate within a tracer diameter (namely, 0.3 μm), whose size is also shown in figure for comparison. The distance between the actual interfaces and the excluded volume boundary depends on the thickness of the tracer depletion layer.…”

Section: B Local Effective Slip Lengthmentioning

confidence: 76%

“…The number N of images per plane is chosen so that at the minimum detectable speed (about 30 μm/s) the Brownian motion component of tracer velocity is negligible (less than 1%) with respect to the average component. As a consequence of that, 32 we recorded N = 4000 frames at focal planes nearby the silicon surface and N = 3200 anywhere else.…”

Section: Velocity Measurementsmentioning

confidence: 99%

“…We recently introduced a novel technique 32 to measure the liquid-air and liquid-solid interface relative positions and shapes with a resolution of tens of nanometers. Such a measurement is performed by post-processing the same fluorescent microscopy images used for the μ-PIV velocity analysis.…”

Section: Interface Detectionmentioning

confidence: 99%

“…Micro-structured surfaces with micro-grooves parallel to the flow direction or with pillars and holes would be much more difficult to be studied in that experimental configuration. In this respect, we recently developed a novel technique 32 to perform simultaneous velocity and interface profile measurements on SHS. That technique has no restriction on the SHS orientation and it has been successfully used to study even very complicated surface pattern like ellipse-shaped pillars.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Evidence of slippage breakdown for a superhydrophobic microchannel

Bolognesi¹,

Cottin-Bizonne²,

Pirat³

2014

Physics of Fluids

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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.

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Pressure effects on water slippage over silane-coated rough surfaces: pillars and holes

Gentili

Bolognesi

Giacomello

et al. 2014

Microfluid Nanofluid

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Nanoscale roughness in combination with surface chemistry modification can achieve large liquid slippage due to entrapment of air pockets in the surface asperities (superhydrophobic Cassie state). However, pressure increase may result in the collapse of the gaseous enclaves into the fully wet Wenzel state, and in the irreversible loss of the desirable superhydrophobic features. In order to explain the subtle effects of pressure, an extensive simulative campaign based on full-atoms molecular dynamics and aimed at reproducing a realistic superhydrophobic system comprising water, solid walls, and hydrophobic coating with octadecyltrichlorosilane is addressed in comparison with a continuum approach. The dramatic impact of pressure on slippage is twofold: on the one hand, it influences the shape of the liquid/vapor interface, thus indirectly affecting the flow. This is a purely geometrical effect well captured in principle by the continuum description. On the other hand, pressure controls the complex interplay between water and hydrophobic chains close to the triple line where it significantly alters the liquid structure, thus modifying apparent slippage. This is an intrinsically atomistic phenomenology that cannot be predicted at the continuum level. The combination of the two effects is found to explain the peculiar response of slippage to the applied pressure at the nanoscale, that may look at first sight in contradiction with most microscale literature results

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A novel technique for simultaneous velocity and interface profile measurements on micro-structured surfaces

Cited by 17 publications

References 19 publications

Mobility tensor of a sphere moving on a superhydrophobic wall: application to particle separation

Mobility tensor of a sphere moving on a superhydrophobic wall: application to particle separation

Evidence of slippage breakdown for a superhydrophobic microchannel

Pressure effects on water slippage over silane-coated rough surfaces: pillars and holes

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