Capillary ripples in thin viscous films

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Abstract

Capillary ripples in thin viscous films are important features of coating and lubrication flows. Here, we present experiments based on digital holographic microscopy, measuring with nanoscale resolution the morphology of capillary ripples ahead of a viscous drop spreading on a prewetted surface. Our experiments reveal that upon increasing the spreading velocity, the amplitude of the ripples first increases and subsequently decreases. Above a critical spreading velocity, the ripples even disappear completely and this transition is accompanied by a divergence of the ripple wavelength. These observations are explained quantitatively using linear wave analysis, beyond the usual lubrication approximation, illustrating that new phenomena arise when the capillary number becomes of the order of unity.

Original languageEnglish
Pages (from-to)430-440
Number of pages11
JournalJournal of fluid mechanics
Volume880
Early online date9 Oct 2019
DOIs
Publication statusPublished - 10 Dec 2019

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ripples
Lubrication
lubrication
Microscopic examination
Experiments
Coatings
Wavelength
unity
divergence
microscopy
coatings
approximation
wavelengths

Keywords

  • UT-Hybrid-D
  • capillary waves
  • thin films
  • capillary flows

Cite this

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Capillary ripples in thin viscous films. / Jalaal, Maziyar; Seyfert, Carola; Snoeijer, Jacco H.

In: Journal of fluid mechanics, Vol. 880, 10.12.2019, p. 430-440.

Research output: Contribution to journalArticleAcademicpeer-review

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AB - Capillary ripples in thin viscous films are important features of coating and lubrication flows. Here, we present experiments based on digital holographic microscopy, measuring with nanoscale resolution the morphology of capillary ripples ahead of a viscous drop spreading on a prewetted surface. Our experiments reveal that upon increasing the spreading velocity, the amplitude of the ripples first increases and subsequently decreases. Above a critical spreading velocity, the ripples even disappear completely and this transition is accompanied by a divergence of the ripple wavelength. These observations are explained quantitatively using linear wave analysis, beyond the usual lubrication approximation, illustrating that new phenomena arise when the capillary number becomes of the order of unity.

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