# Superhydrophobic surfaces

Started by Lauren Hartle, Fall 2011.

Entry has been combined with Superhydrophobicity, Superhydrophicity (misspelled) and Superhydrophobic. (LH 2011)

## Introduction

A water droplet on a superhydrophobic surface has a contact angle of greater than 150 degrees and a very low roll-off angle. In nature and in man-made materials, this has been achieved with Structured Surfaces. Advantages of such surfaces include the ability to repel water and self-clean.[3] Industrial applications include "self-cleaning window glasses, paints, and textiles to low-friction surfaces for fluid flow and energy conservation."[2] To predict when a droplet will wet the surface and when it will be repelled, one can compare the contact angle for complete wetting to the contact angle for a droplet resting atop the surface structure with no penetration. When a droplet is in intimate contact with the surface, the Wenzel state, the following equation applies:

<math>\cos{\theta}_c = r \cos{\theta}</math>

where r is the ratio of the actual area to the projected contact area and <math>\theta</math> is the contact angle of the droplet on a flat surface of the same material.. When the droplet rests atop the surface structure, it is in the Cassie-Baxter state, modeled by:

<math>\cos{\theta}_{c} = \phi \left( cos \theta + 1 \right) - 1</math>

where <math>\phi</math> is the area fraction of the solid in contact with the liquid, and <math>\theta</math> is the contact angle of the droplet on a flat surface of the same material. Setting the two contact angles equal gives the following critical condition for the transition between the Cassie-Baxter and Wenzel states.

<math>cos \theta = \frac {\phi - 1}{r -\phi}</math>

By this prediction, a droplet resting atop surface structures might be a lower energy state than intimate contact when the cosine of the contact angle on a flat surface is less than the right hand side of the equation.

It is argued that surface structure can produce superhydrophobic effects, even on a hydrophilic surface. For example, lotus leaves have been shown to be superhydrophobic, despite the waxy, weakly hydrophilic coating on the surface.[1] It has been demonstrated that the surface of a lotus leaf is superhydrophobic in part due to the presence of hierarchical surface structures structures consisting of micro- and nano-scale features. [2] Figure 1 contrasts the state of a droplet on a flat surface and surfaces of increasingly complex structure.

Butterfly wings and parts of pitcher plants have observed superhydrophobic properties. Figure 2 shows an image of real lotus leaf structures at different scales from reference 2.

## References

[1]"Design parameters for superhydrophobicity and superoleophobicity". Anish Tuteja, Wonjae Choi, Gareth H. McKinley, Robert E. Cohen, and Michael F. Rubner. *MRS Bulletin* 33 (8), 752-758 (August 2008)

[2]"Fabrication of artificial Lotus leaves and significance of hierarchical structure for superhydrophobicity and low adhesion". Kerstin Koch, Bharat Bhushan, Yong Chae Jung and Wilhelm Barthlott. *Soft Matter*, 2009, 5, 1386–1393.

[3]"Self-cleaning materials: Lotus leaf inspired nanotechnology" Peter Forbes, *Scientific American* 30 July 2008.

## See also

Superhydrophobic surfaces in Effects of contact angles in Capillarity and wetting from Lectures for AP225.

## Keyword in References

Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity

Growth of polygonal rings and wires of CuS on structured surfaces