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Oscillating Magnetic Drop: How to Grade Water-Repellent Surfaces

Department of Physics, Florida State University, Tallahassee, FL 32306, USA
Biocolloid and Fluid Physics Group, Applied Physics Department, Faculty of Sciences, University of Granada, 18071 Granada, Spain
Author to whom correspondence should be addressed.
Coatings 2019, 9(4), 270;
Received: 19 February 2019 / Revised: 3 April 2019 / Accepted: 17 April 2019 / Published: 21 April 2019
(This article belongs to the Special Issue Superhydrophobic Coatings for Corrosion and Tribology)
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Evaluation of superhydrophobic (SH) surfaces based on contact angle measurements is challenging due to the high mobility of drops and the resolution limits of optical goniometry. For this reason, some alternatives to drop-shape methods have been proposed such as the damped-oscillatory motion of ferrofluid sessile drops produced by an external magnetic field. This approach provides information on surface friction (lateral/shear adhesion) from the kinetic energy dissipation of the drop. In this work, we used this method to compare the low adhesion of four commercial SH coatings (Neverwet, WX2100, Ultraever dry, Hydrobead) formed on glass substrates. As ferrofluid, we used a maghemite aqueous suspension (2% v/v) synthesized ad hoc. The rolling magnetic drop is used as a probe to explore shear solid–liquid adhesion. Additionally, drop energy dissipates due to velocity-dependent viscous stresses developed close to the solid–liquid interface. By fitting the damped harmonic oscillations, we estimated the decay time on each coating. The SH coatings were statistically different by using the mean damping time. The differences found between SH coatings could be ascribed to surface–drop adhesion (contact angle hysteresis and apparent contact area). By using this methodology, we were able to grade meaningfully the liquid-repelling properties of superhydrophobic surfaces. View Full-Text
Keywords: water-repellent surfaces; ferrofluid drop; magnetic field; damped harmonic oscillation water-repellent surfaces; ferrofluid drop; magnetic field; damped harmonic oscillation

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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).

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Goncalves Dos Santos, A.; Montes-Ruiz Cabello, F.J.; Vereda, F.; Cabrerizo-Vilchez, M.A.; Rodriguez-Valverde, M.A. Oscillating Magnetic Drop: How to Grade Water-Repellent Surfaces. Coatings 2019, 9, 270.

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