Surfactants and wetting at superhydrophobic surfaces: Water solutions and non
aqueous liquids
Michele Ferrari
⁎
, Francesca Ravera
CNR-Istituto per l'Energetica e le Interfasi, via De Marini 6, 16149 Genova, Italy
abstractarticle info
Available online 17 September 2010
Keywords:
Wetting
Surfactants
Superhydrophobic
Coating
Partitioning
Wetting studies regarding amphiphilic molecules and adsorption properties on highly water repellent solid
surfaces play key roles in research and technology, with increasing interest both in fundamental and
application fields. Nevertheless the wetting properties of aqueous surfactant solutions, non aqueous liquids or
immiscible phases on superhydrophobic (SH) solid surfaces have been so far rarely investigated.
In this work the authors give an overview on this topic reviewing the literature available together with
preliminary results concerning the influence of the distribution properties of surfactants between two
immiscible phases. Transition between wetting states can be also considered a possible development of these
studies based on switching mechanisms.
© 2010 Elsevier B.V. All rights reserved.
Contents
1. Introduction ...............................................................22
2. Surfactant adsorption at superhydrophobic surfaces ............................................24
2.1. Single surfactant solutions .....................................................24
2.2. Surfactant mixtures solutions ....................................................24
3. Non aqueous liquids at superhydrophobic surfaces .............................................26
4. Surfactants adsorption at SH surfaces in liquid–liquid systems........................................27
5. Conclusion ................................................................28
References ................................................................28
1. Introduction
Low energy surfaces defined with terms like superhydrophobic
(SH) or ultrahydrophobic usually are referred to substrates with a
water contact angle (CA) greater than 150°. Starting from the
observation of the Lotus effect© [1,2], the introduction of the idea to
enhance and exploit the self-cleaning properties of these surfaces has
pushed many researchers to develop plenty of techniques to obtain
SH surfaces, basically by controlling roughness and topography of low
energy surfaces.
As shown by different methods to enhance the water repellence of
a surface [3–5] by acting on its geometry or chemistry, great interest
has been then raised toward SH surfaces not only in fundamental
research, but also for a wide range of different practical applications
which are offered in fact by highly hydrophobic coatings ranging, as
an example, from protective films against corrosion to layers for solar
panels, cars and buildings glasses, and electronic devices.
The role of the roughness on the wettability properties of a solid
surface has been initially interpreted under the Wenzel [6,7] or the
Cassie–Baxter [8] models.
Modifying the Young's equation, the Wenzel approach was to
consider the liquid filling the space among the surface grooves linking
the experimental, apparent CA (θ′) and the thermodynamic equilib-
rium one (θ ) as follows:
cosθ
′
=rcosθ; ð1Þ
where r is the ratio between the true surface area and its horizontal
projected area. Since this condition provides hydrophobic surfaces
with CAs below 120°, it cannot give rise to superhydrophobicity.
Advances in Colloid and Interface Science 161 (2010) 22–28
⁎ Corresponding author.
E-mail address: m.ferrari@ge.ieni.cnr.it (M. Ferrari).
0001-8686/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cis.2010.09.002
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Advances in Colloid and Interface Science
journal homepage: www.elsevier.com/locate/cis