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Water Drop Friction on Superhydrophobic Surfaces
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology.ORCID iD: 0000-0002-5444-7276
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.ORCID iD: 0000-0001-8622-0386
2013 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 29, no 29, 9079-9089 p.Article in journal (Refereed) Published
Abstract [en]

To investigate water drop friction on superhydrophobic surfaces, the motion of water drops on three different superhydrophobic surfaces has been studied by allowing drops to slide down an incline and capturing their motion using high-speed video. Two surfaces were prepared using crystallization of an alkyl ketene dimer (AKD) wax, and the third surface was the leaf of a Lotus (Nelumbo Nucifera). The acceleration of the water droplets on these superhydrophobic surfaces was measured as a function of droplet size and inclination of the surface. For small capillary numbers, we propose that the energy dissipation is dominated by intermittent pinning-depinning transitions at microscopic pinning sites along the trailing contact line of the drop, while at capillary numbers exceeding a critical value, energy dissipation is dominated by circulatory flow in the vicinity of the contacting disc between the droplet and the surface. By combining the results of the droplet acceleration with a theoretical model based on energy dissipation, we have introduced a material-specific coefficient called the superhydrophobic sliding resistance, b(sh). Once determined, this parameter is sufficient for predicting the motion of water drops on superhydrophobic surfaces of a general macroscopic topography. This theory also infers the existence of an equilibrium sliding angle, beta(eq), at which the drop acceleration is zero. This angle is decreasing with the radius of the drop and is in quantitative agreement with the measured tilt angles required for a stationary drop to start sliding down an incline.

Place, publisher, year, edition, pages
2013. Vol. 29, no 29, 9079-9089 p.
Keyword [en]
Alkyl ketene dimer, Capillary numbers, Circulatory flow, Quantitative agreement, Sliding resistance, Super-hydrophobic surfaces, Superhydrophobic, Theoretical models
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-127767DOI: 10.1021/la401152bISI: 000322416700008Scopus ID: 2-s2.0-84880624496OAI: oai:DiVA.org:kth-127767DiVA: diva2:646538
Funder
Swedish Foundation for Strategic Research
Note

QC 20130909

Available from: 2013-09-09 Created: 2013-09-05 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Fundamentals of Wetting and Mechanical Durability of Superhydrophobic Coatings
Open this publication in new window or tab >>Fundamentals of Wetting and Mechanical Durability of Superhydrophobic Coatings
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In paper I the friction between three different superhydrophobic surfaces and water drops were investigated using high‑speed video. The surfaces were two based on a hydrophobic wax and the third was the leaf of a Lotus (Nelumbo Nucifera). The acceleration of water drops was measured as a function of drop size and surface inclination. For small capillary numbers it was shown that the dissipation was dominated by pinning‑depinning transitions along the trailing contact line. A parameter called the superhydrophobic sliding resistance bsh has been introduced. The motion of drops on superhydrophobic surfaces of a general macroscopic topography can be predicted provided that bsh and the drop size are known. This theory also infers the existence of an equilibrium sliding angle, beq, at which the drop acceleration is zero.

The effect of line‑shaped defects on the motion of water drops on superhydrophobic surfaces were also investigated using high‑speed video in paper II. It was shown that the motion of the drop in the vicinity of the defect can be approximated by a damped harmonic oscillator. Whether a drop got trapped or not while traversing the defect was determined by the incident speed and the characteristics of the oscillator. In systems with low viscous dissipation it is possible to predict the trapping speed as well as the exit speed using a simple work‑energy consideration.

The resistance of wax based superhydrophobic coatings subjected to different types of mechanical damage were investigated in paper III. Scratch tests were performed using atomic force microscopy (AFM) and rubbing with an index finger. Coatings were also subjected to compression with a silicone rubber stamp. The effect of impacting water drops was also investigated. A load of 12 nN was enough to remove the coating from the substrate. The coatings remained superhydrophobic at compression pressures up to 59 kPa but the superhydrophobic properties were lost after only one stroke with a finger. The coatings resisted at least 200 000 impacts of falling water drops without losing their superhydrophobic properties.

In paper IV superhydrophobic coatings were fabricated in a semi‑continuous process, where an alkyl ketene dimer (AKD) was dissolved in supercritical carbon dioxide (scCO2) and sprayed onto the substrate. Several different substrates such as: glass, aluminium, paper, poly (ethylene terephthalate) (PET) and poly (tetrafluoroethylene) (PTFE) were successfully coated. The most efficient spray process, considering surface properties and mass of extracted AKD, was obtained at the lowest temperature investigated, 67 °C, and the highest pressure evaluated in this study, 25 MPa. The influence of the pre‑expansion conditions (p, T) on the surface temperature (at a spray distance of 3 cm) was also shown to be negligible.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. xvii, 60 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2014:58
National Category
Polymer Chemistry Materials Chemistry
Research subject
Fibre and Polymer Science
Identifiers
urn:nbn:se:kth:diva-156402 (URN)978-91-7595-375-5 (ISBN)
Public defence
2014-12-18, D3, Lindstedtsvägen 5, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research
Note

QC 20141202

Available from: 2014-12-02 Created: 2014-11-27 Last updated: 2016-06-02Bibliographically approved

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Pettersson, TorbjörnWågberg, Lars

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