Superhydrophobic Surfaces

M.S. Bobji, R.N. Govardhan
Department of Mechanical Engineering
Indian Institute of Science
Bangalore

Hydrophobicity

Hydrophobicity refers to the physical property of a material that repels a mass of water. The word is coined from two Greek word Hydro (water) and Phobic (fear). Some of the common natural hydrophobic materials are waxes, oil and fats. Some house hold hydrophobic materials are Teflon tape and Carbon black.

Hydrophobic interaction originates from the van der Walls Interaction between water molecules and atoms or molecules of the hydrophic substances. It is characterized by surface energy of the substance in relation with the surface tension (or surface energy) of water.

Contact Angle

Contact Angle

A drop of water sitting on a surface will assume a spherical surface under negligible of gravitational force (a small drop for example). The contact angle of the water drop can be related to surface energies through Young's equation as,

cos θ_Y= ( γ_SG - γ_SL ) / γ_LG

If, θ_Y is greater than 90^o then the surface is called as hydrophobic and if Y is less than 90^o then the surface is called as hydrophilic surface. Maximum contact angle of 120^o has been obtained for inert surfaces like Teflon (Polytetrafluoroethylene –PTFE).

Rough Surfaces

"Wenzel" state

Surfaces are rough unless extreme care is taken while creating those surfaces. Only cleaved surfaces are smooth down to atomic scale. When a liquid drop sits on such a surface then the contact angle is modified. For most kind of roughness the contact angle will satisfy Wenzel equation,

cos θ^* = r cos θ_Y

where r is the roughness factor a ratio of the actual surface area to project area. r is greater than 1 for rough surfaces and reaches value of about 1.2 to 1.3 for fractured surfaces. However the roughness factor can be increased to very large values by various machining techniques. In nature many leaves (lotus, paper mulberry) and insects have dense hair like structures that can give rise to high roughness factors. For the Wenzel equation to be valid, water has to penetrate into the roughness.

"Cassie" state

The contact angle, θ^*, will be higher than the Young's contact angle if the surface is hydrophobic and will be lesser is it is hydrophilic. If the water fails to penetrate the roughness of the surface then air gets trapped in between. The contact angle in such a condition is given by Cassie- Baxter relation,

cos θ^* = φ_s( cos θ_Y +1) -1

with φ_s being the fraction of surface in contact with water. For φ_s less than 1, θ^* will always be greater than θ_Y! Hairy structures like those on the Lotus leaves can create high contact angle, provided the air trapped within them. This means that if you have dense hair on your head (like me!) then light rain will not wet your scalp. Try it out.

Superhydrophobic surfaces

Super hydrophobic - Contact angle > 140^o

By controlling texture (roughness) of the surface, it is possible to control the contact surface area, φ_s. Thus, contact angle,θ^*, can be made to be larger than 120^o, obtained for smooth Teflon surfaces. When the contact angle is greater than 140^o then the surfaces may be called as superhydrophobic. There has been no clear definition emerging yet as some researchers contend that the superhydrophoic surface should be characterised by other criteria as well, for example, rolling of water drops and low hysteresis angles.

Lotus effect

Lotus leaves exhibit superhydrophobicity. Water simple rolls off the leaf surface and the surface tension of the water pulls any dirt that might have been accumulated on the surface. This ability to remain clean and the ability to not get wet from the surrounding water has given rise to many a similies in Indian literature! (See Arunn’s Notebook )

Lotus leaf has tiny protuberances on its surface. It also has a waxy coating whose surface energy is such that on a smooth surface the contact angle with water is about 75o. ( Y T Cheng et al 2006 Nanotechnology, 17,1359 ) The contact angle on the lotus measured with freshly deposited water drop is about 140o. Please not that the contact angle measurement (by sessile drop method) is highly subjective and especially difficult for superhydrophic surfaces as the water drop rolls off even for a small disturbances. Mostly ig the drop is sitting on the surface quiet enough to enable measurement would mean that the water is stuck on local inhomogeneity.

To quote from Langmuir 2009, 25(20), 12120–12126

“Superhydrophobic surfaces have been realized by controlling surface energy and surface roughness. Superhydrophobic properties such as contact angles greater than 150^o and high drop mobility result from the Cassie state of wetting. In the Cassie state, a water drop is in contact with only a fraction of the solid surface (φ_s) at the summits of the rough surface, resulting in a composite interface.2 Over the remaining fraction (1 - φ_s) of the interface, the water is in contact with the entrapped air. The contact angle depends on the fraction s and is given by the Cassie-Baxter relation. The air fraction (1 - φ_s) plays an important role in determining the superhydrophobic properties of rough hydrophobic surfaces. Keeping this in mind, the roughness/texture of the surface can be suitably designed for specific applications.”