Decoupling of the liquid response of a superhydrophobic quartz crystal microbalance

Paul Roach, Glen McHale, Carl Evans, Neil Shirtcliffe, Michael Newton

Research output: Contribution to journalArticlepeer-review

41 Citations (Scopus)

Abstract

Recent reports using particle image velocimetry and cone-and-plate rheometers have suggested that a simple Newtonian liquid flowing across a superhydrophobic surface demonstrates a finite slip length. Slippage on a superhydrophobic surface indicates that the combination of topography and hydrophobicity may have consequences for the coupling at the solid-liquid interface observed using the high-frequency shear-mode oscillation of a quartz crystal microbalance (QCM). In this work, we report on the response of a 5 MHz QCM possessing a superhydrophobic surface to immersion in water-glycerol mixtures. QCM surfaces were prepared with a layer of SU-8 photoresist and lithographically patterned to produce square arrays of 5 mu m diameter circular cross-section posts spaced 10 mu m center-to-center and with heights of 5,10, 15, and 18 mu m. Non-pattemed layers were also created for comparison, and both non-hydrophobized and chemically hydrophobized surfaces were investigated. Contact angle measurements confirmed that the hydrophobized post surfaces were superhydrophobic. QCM measurements in water before and after applying pressure to force a Cassie-Baxter (non-penetrating) to Wenzel (penetrating) conversion of state showed a larger frequency decrease and higher dissipation in the Wenzel state. QCM resonance spectra were fitted to a Butterworth-van Dyke model for the full range of water-glycerol mixtures from pure water to (nominally) pure glycerol, thus providing data on both energy storage and dissipation. The data obtained for the post surfaces show a variety of types of behavior, indicating the importance of the surface chemistry in determining the response of the quartz crystal resonance, particularly on topographically structured surfaces; data for hydrophobized post surfaces imply a decoupling of the surface oscillation from the mixtures. In the case of the 15 mu m tall hydrophobized post surfaces, crystal resonance spectra become narrower as the viscosity -density product increases, which is contrary to the usual behavior. In the most extreme case of the 18 mu m tall hydrophobized post surfaces, both the frequency decrease and bandwidth increase of the resonance spectra are significantly lower than that predicted by the Kanazawa and Gordon model, thus implying a decoupling of the oscillating surface from the liquid, which can be interpreted as interfacial slip
Original languageEnglish
Pages (from-to)9823-9830
JournalLangmuir
Volume23
Issue number19
DOIs
Publication statusPublished - 11 Sep 2007

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