Dynamic wetting and gas viscosity effects

Benkreira, Hadj; Ikin, J.B.

doi:10.1016/j.ces.2009.11.019

Cited by 27 publications

(35 citation statements)

References 7 publications

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“…A similar paradox is encountered for air entrainment by rapidly advancing contact lines, where a liquid advances over a surface that it partially wets [2,23,39,[62][63][64]. Once again, it was found that depressurizing the gas leads to a significant increase of the threshold of air entrainment [23,65]. This contradicts the classical viewpoint that, for given wettability, the contact line speed depends mainly on the liquid properties as ∼ γ/η ℓ [9,12,21,39], with minor influence of the gaseous phase.…”

Section: Introductioncontrasting

confidence: 40%

“…A pressure reduction does not affect the dynamical viscosity of a gas [71]. However, as also mentioned in [23,65], it does increase the mean free path ℓ mfp by a factor ℓ mfp ∼ p atm /p. Since under atmospheric conditions ℓ mfp ≈ 70 nm, the mean free path is pushed well into the micron range when pressure is reduced by a factor 100.…”

Section: Interpretation and Discussionmentioning

confidence: 97%

“…We explain this by the influence of the air flow when the local angle of the interface is close to π. Can such a scenario explain the observed increase of entrainment speed when depressurizing the air [23,65]? A pressure reduction does not affect the dynamical viscosity of a gas [71].…”

Section: Interpretation and Discussionmentioning

confidence: 99%

“…When the plate is moving beyond a critical speed, the meniscus can no longer equilibrate to a steady state. In the case of receding contact lines (withdrawing plate) this leads to the deposition of a liquid film [3, 12-14, 18, 74, 75], while air will be entrained for advancing contact lines [2,23,39,[62][63][64][65]76].…”

Section: Formulationmentioning

confidence: 99%

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Dynamical wetting transition: liquid film deposition and air entrainment

Chan¹

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Section: Introductioncontrasting

confidence: 40%

Section: Interpretation and Discussionmentioning

confidence: 97%

Section: Interpretation and Discussionmentioning

confidence: 99%

Section: Formulationmentioning

confidence: 99%

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Dynamical wetting transition: liquid film deposition and air entrainment

Chan¹

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“…Recently, Benkreira and Khan (2008) and Benkreira and Ikin (2010) demonstrated in a series of dip coating experiments in an environment comprising gas other than air that the viscosity of the very thin gas film entrained by the moving substrate also played a critical role. Using helium at very low pressures (~25mPa), they measured gas entrainment speeds that were much higher (as much as 3 times larger) than the air entrainment speed under atmospheric pressure.…”

Section: Introductionmentioning

confidence: 99%

Slot coating minimum film thickness in air and in rarefied helium

Benkreira

Ikin

2016

Chemical Engineering Science

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This study assesses experimentally the role of gas viscosity in controlling the minimum film thickness in slot coating in both the slot over roll and tensioned web modes. The minimum film thickness here is defined with respect to the onset of air entrainment rather than rivulets, the reason being that rivulets are an extreme form of instabilities occurring at much higher speeds. The gas viscosity effects are simulated experimentally by encasing the coaters in a sealed gas chamber in which various gases can be admitted. An appropriate choice of two gases was used to compare performances: air at atmospheric pressure and helium at sub-ambient pressure (25mbar), which we establish has a significantly lower "thin film" viscosity than atmospheric air. A capacitance sensor was used to continuously measure the film thickness on the web, which was ramped up in speed at a fixed acceleration whilst visualizations of the film stability were recorded through a viewing port in the chamber. The data collected show clearly that by coating in rarefied helium rather that atmospheric air we can reduce the minimum film thickness or air/gas entrainment low-flow limit. We attribute this widening of the stable coating window to the enhancement of dynamic wetting that results when the thin film gas viscosity is reduced. These results have evident practical significance for slot coating, the coating method of choice in many new technological applications, but it is their fundamental merit which is new and one that should be followed with further data and theoretical underpinning.

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