Oscillating Magnetic Drop: How to Grade Water-Repellent Surfaces
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AuthorGoncalves Dos Santos, Angelica; Montes Ruiz-Cabello, Francisco Javier; Vereda, Fernando; Cabrerizo Vílchez, Miguel Ángel; Rodríguez Valverde, Miguel Ángel
Water-repellent surfacesFerrofluid dropMagnetic fieldsDamped harmonic oscillation
Goncalves Dos Santos, A., Montes-Ruiz Cabello, F. J., Vereda, F., Cabrerizo-Vilchez, M. A., & Rodriguez-Valverde, M. A. (2019). Oscillating Magnetic Drop: How to Grade Water-Repellent Surfaces. Coatings, 9(4), 270.
SponsorshipThis research was financed by the State Research Agency (SRA) and European Regional Development Fund (ERDF) through the project MAT2017-82182-R. Fernando Vereda acknowledges financial support from MAT 2016-78778-R and PCIN-2015-051 projects (Spain).
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.