Open Access

Wettability Switching Techniques on Superhydrophobic Surfaces

  • Nicolas Verplanck1,
  • Yannick Coffinier1, 2,
  • Vincent Thomy1Email author and
  • Rabah Boukherroub1, 2
Nanoscale Research Letters20072:577

DOI: 10.1007/s11671-007-9102-4

Received: 27 June 2007

Accepted: 22 October 2007

Published: 13 November 2007




Microfluidic Superhydrophobic surfaces Wettability switching Electrowetting


Biological surfaces, like lotus leaves, exhibit the amazing property for not being wetted by water leading to a self cleaning effect. The lotus leaves capability to remain clean from dirt and particles is attributed to the superhydrophobic nature of the leaves surface. The latter is composed of micro and nano structures covered with a hydrophobic wax, creating a carpet fakir, where water droplets attained a quasi spherical shape. In order to mimic these properties, artificial superhydrophobic surfaces have been prepared by several means, including the generation of rough surfaces coated with low surface energy molecules [16], roughening the surface of hydrophobic materials [79], and creating well-ordered structures using micromachining and etching methods [10, 11].



Surface Wetting


Figure 1


The contact angle of a liquid on a surface according to the surface tension is given by the relation of Young (1). The surface tension, noted γ, is the tension which exists at the interface of two systems (solid/liquid, liquid/liquid, solid/gas). It is expressed in energy per unit of area (mJ m−2), but can also be regarded as a force per unit of length (mN m−1). From this definition, it is possible to identify three forces acting on the three phase contact line: γ LG (liquid surface stress/gas), γ LS (liquid/solid surface stress) and γ SG (solid surface stress/gas). The three forces are represented in Fig. 2.
Figure 2


At the equilibrium state:


Concretely, following the rule of Zisman [13, 14], wetting surfaces are surfaces of high energy (500–5,000 mN m−1), where the chemical binding energies are about an eV (ionic, covalent, metal connections). The wetting materials are typically oxides (glass), metal oxides,… On the other hand, nonwetting surfaces are characterized by low surface energy (10–50 mN m−1). For these materials, the binding energies are about kT (ex: crystalline substrates and polymers) [15].


The hysteresis of a surface is related to its imperfections. Indeed, the formula of Young considers that there is only one contact angle, the static contact angle, noted θ 0. However, this configuration exists only for perfect surfaces. Generally, surfaces present imperfections related to physical defects like roughness or to chemical variations. The static contact angle thus lies between two values called advanced angle, noted θ A, and receding angle, noted θ R. The difference between these two angles (θ Aθ R) is called hysteresis. While this force is opposed to droplet motion, the smaller hysteresis is, the more it will be easy to move the liquid droplet. Concretely, these angles can be measured thanks to the shape of a droplet on a tilted surface (Fig. 3).
Figure 3

Advancedθ A and receding θ R angles of a liquid droplet on a tilted surface

Wetting on Superhydrophobic Surfaces: Wenzel and Cassie–Baxter States

Figure 4




Figure 5



Figure 6



Wenzel (1936)

Figure 7



Cassie–Baxter (1944)

Figure 8

Planar surface composed of two different and chemically heterogeneous materials

The examined surface consists of two materials; each one has its own surface energy, characteristic contact angle, and occupies a definite fraction of the surface. If material 1 is hydrophobic and material 2 is replaced by air, a drop in contact with each of the two phases (solid and air) forms respective contact angles θ E and 180°, whereas the fractions of respective surfaces are Φ s and (1 − Φ s ). Considering a displacement dx of the three phase contact line, the change of energy dE could be expressed by:
It is to be noted that the apparent angle θ* is included in the interval [θ 1,θ 2]. Figure 9 illustrates the behavior of the apparent Young angle according to the Cassie–Baxter relation (5).
Figure 9


Figure 10

Coexistence of two superhydrophobic modes. With feeble hydrophobicity (cosθ c < cosθ < 0), the apparent contact angle is theoretically given by the relation of Wenzel while for strong hydrophobicity (cosθ < cosθ c), the apparent contact angle follows the relation of Cassie–Baxter. However, in practice, an average hydrophobicity generally involves a metastable configuration of Cassie–Baxter (dotted lines)

However, when a drop is deposited on a rough surface, a Cassie–Baxter regime occurs even when θ < θ c (for water, θ < 120°) [2729]. This state is metastable, i.e. by applying a pressure to the drop, for example, it is possible to reach the Wenzel regime: stable and displaying an important hysteresis [30]. This state is problematic, in particular in microfluidic microsystems where the displacement of a drop with a hysteresis of 100° is not easily realizable. An ideal configuration is the rolling ball or fakir effect i.e. the Cassie–Baxter state.


The first assumptions on this double roughness were brought by Bico [31], Herminghaus [32] and many other teams [33, 34]. According to the work of Bico, this double roughness would avoid placing the drop in the Wenzel state; small asperities will trap air and as a consequence the drop will be in an intermediate configuration between Wenzel and Cassie–Baxter [21] (Fig. 11).
Figure 11

Apparent contact angle on a surface with two different roughness scales

where θ is the angle of Young,θ 1*, Φ s1 and Φ a1 are respectively the angle, the solid fraction of surface and the fraction of air surface with nanometric roughness, and θ 2*, Φ s2 and Φ a2 are respectively the angle, the solid fraction of surface and the fraction of air surface with micrometric roughness (Fig. 11). From Eq. 7, the double roughness amplifies the superhydrophobic surface property. If, for example, two roughnesses are homothetic, they have the same fraction of surface Φ s and the equation of Cassie–Baxter becomes:

When Φ s < 1, cosθ* is smaller than in the case of a simple roughness, the contact angle increases.

Preparation of Superhydrophobic Surfaces


Wettability Switching Techniques on Superhydrophobic Surfaces

Carbon Nanotubes Anisotropic Structures



Figure 12


Figure 13


Figure 14










Dual Temperature/pH

Figure 15

(a) When the pH and/or temperature is varied the CAs reversibly change. (b) Temperature and pH dependence of water CAs for P(NIPAAm-co-AAc) thin films. Water CAs change at different temperatures for a modified substrate at pH values of 2 (□), 4 (), 7 (), 9 () and 11 (), respectively. Reprinted with permission from [46]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA



Figure 16


Figure 17


Figure 18


Figure 19


Figure 20



Theory and History

Figure 21


where ε r is the permittivity of the dielectric layer, ε 0, the electric permittivity of the vacuum, x, the length of the capacitor and E, its thickness. By applying the principle of virtual work, the force per transverse unit of length is deduced:



Optical Applications of EWOD


Figure 22


Figure 23


Figure 24


Figure 25


EWOD for Microdroplets Displacement

In order to displace microdroplets and to realize microfluidic basic operations (merging, creating droplets), the EWOD system needs to have two plans: a base composed of electrodes for displacement and a counter-electrode (instead of a needle). A general diagram of the two plans microsystem is shown in Fig. 26. Initially, no voltage is applied between the electrodes and the counter-electrode, and whatever the place where the drop is placed, the contact angle is the contact angle of the drop θ 0. When a voltage is applied on an electrode under the drop, the contact angle of the three phase contact line in contact with this electrode decreases to reach a value θ d and thus the radius of curvature Rd of the meniscus increases. The contact angle on the rest of the substrate is always the contact angle to balance θ 0 and the associated radius of curvature R0 is lower than the radius of curvature Rd.
Figure 26


where P g is the pressure on the left side in the drop whereas P d is the pressure on the right side. These two values are determined by the following expressions:
where P a is the atmospheric pressure, R the ray of the drop in the transverse direction, R 0 , the radius of curvature of the left meniscus and R d , the radius of curvature of the right meniscus. Thus,
We found:
Starting from Eq. 19, the driving force F m , which allows displacement, can be deduced (per unit of length):


Lab-on-chip Applications


Figure 27





Nonreversible Electrowetting on Superhydrophobic Surfaces

Figure 28



Figure 29



We found:



Reversible Electrowetting on Superhydrophobic Surfaces

Figure 30


Table 1



Time (min)

Pressure (T)

Length (μm)


































Figure 31






Authors’ Affiliations

Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN)
Institut de Recherche Interdisciplinaire (IRI)


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