Superhydrophobicity — The Lotus Effect - Lesson - www.TeachEngineering.org

 superhydrophobic surfaces and the "lotus effect." Water spilled on a superhydrophobic surface does not wet the surface, but simply rolls off. Additionally, as water moves across the superhydrophobic surface, it picks up and carries away any foreign material, such as dust or dirt. Students learn how plants create and use superhydrophobic surfaces in nature and how engineers have created human-made products that mimic the properties of these natural surfaces. They also learn about the tendency of all superhydrophobic surfaces to develop water droplets that do not roll off the surface but become "pinned" under certain conditions, such as water droplets formed from condensation. They see how the introduction of mechanical energy can "unpin" these water droplets and restore the desirable properties of the superhydrophobic surface.

Engineering Connection

Inspired by nature, the self-cleaning superhydrophobic products being engineered today are destined to become important and novel products in our everyday lives. Some products include: coatings that reduce the water drag on boats and protect surfaces and equipment exposed to fresh or salt water, building materials that remain clean with little maintenance and are better protected from wear from the elements, and textiles that are resistant to staining and/or remain dry when submerged in waterSuperhydrophobic Surfaces

When the lotus plant emerges on the surface of a pond or lake, it remains pristine despite its muddy environment. Flower Breaking the Surface, Palace Museum, Beijing, China. Painting by Yun Shouping, 17th century, Qing Dynasty.

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In some Asian religions, the lotus plant is revered as a symbol of purity. The roots of the lotus plant take hold in the muddy bottoms of ponds and riverbeds. From there, thick stems rise above the water's surface and issue giant, pristine leaves and flowers. The leaves remain clean despite the water and mud on which they rest. Even water refuses to stick to the leaves of the lotus plant. Instead, it beads on the surface and rolls off at the slightest disturbance (see Figure 2).

Figure 2. Leaves of Indian Lotus, Nelumbo nucifera, at Lotus Pond, Hyderabad, India. Water does not wet the lotus leaves, but instead forms beads of water that easily roll off its edges. In this computer-generated microscopic image of a lotus leaf with water droplets, notice the double structure of the leaf and the protrusions from the leaf covered in a rough, waxy material, which creates its superhydrophobic surface.

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Scientists have found that the basis for both of these properties (self-cleaning and water-repellent) lies in the rough structure of the surface of the lotus leaves. The lotus leaf has a series of protrusions on the order of 10 μm (1.0 x 10- 5 m) high covering its surface. Each protrusion is itself covered in bumps of a hydrophobic, waxy material that are roughly 100 nm (1 x 10-7 m) in height. When water droplets are applied to the lotus leaf, they sit lightly on the tips of the hydrophobic protrusions as if on a bed of nails (see Figure 2). This combined structure traps a layer of air in between the surface of the leaf and the water droplet. Hence, the water is not allowed to wet the surface and is easily displaced (see Figure 3).

Figure 3. (left) The unique roughness of superhydrophobic surface forces water droplets into spherical drops that minimize contact with the surface, rolling off easily and carrying away dirt and other debris, leaving the surface clean. (right) Water drops roll off a screen door treated with a superhydrophobic powder coating developed at Oak Ridge National Laboratory. Even though the mesh is porous, water drops do not pass through due to the extreme water repellency of the powder coating.

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The surface of a lotus leaf is an example of superhydrophobicity. On a superhydrophobic surface, the contact angle is greater than 150o, meaning almost no wetting of the surface by the liquid takes place. This leads to the second property associated with lotus plants — the ability to stay spotlessly clean. As rain falls on a superhydrophobic surface like the lotus leaf, the water droplets roll easily off the leaf surfaces (see Figure 3). As the droplets travel along the leaves, they pick up any dirt or other matter they encounter along the way. This process keeps the lotus leaves dry, clean and free of pathogens such as bacteria and fungi.

The self-cleaning and water-repellent qualities of superhydrophobic surfaces have the potential for many practical applications. House paints, roof tiles and various surface coatings are already on the market (see Figure 3). These products are examples of "biomimicry." By understanding how the lotus leaf and other plants create superhydrophobic surfaces by using a two-tiered surface layer, engineers have created human-made surfaces that "mimic" the properties of the natural ones. One of the most interesting uses of human-made superhydrophobic surfaces are fabrics made by Nano-Tex™ and other manufacturers that repel tomato sauce, coffee and even red wine. Researchers are also developing fabrics that can stay dry for days underwater, swimsuits that cannot become wet, and ship hulls with dramatically reduced drag.

Condensation and Sperhydrophobic Surfaces

In nature, when dew condenses on lotus leaves, the dew soon rolls off the leaf, just as water droplets falling onto the leaf do. However, when scientists began to study the superhydrophobic properties of the lotus leaf, they discovered water behaved differently in laboratories. Water poured or dropped on leaves in a controlled setting still demonstrated superhydrophobic properties. However, when water vapor was allowed to condense onto a lotus leaf in the lab, the water droplets were "sticky" and clung to the leaves.

Figure 4. The Wenzel wetting state. When water condenses on a rough surface, it condenses from within the texture and forms a droplet in the "sticky" Wenzel wetting state.

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When water vapor condenses on a rough surface, it forms from inside the texture of the surface. As this water droplet grows it enters the "sticky" Wenzel wetting state in which the droplet is "pinned" to the surface (see Figure 4). These droplets do not display the easy motion of droplets on a superhydrophobic surface. In the lab, droplets remain in this "sticky" state, but in nature water vapor condensation, such as dew, soon transitions into a second wetting state known as the Cassie-Baxter wetting state.

Figure 5. Comparison of the Wenzel and Cassie-Baxter droplet wetting states. In the Wenzel state, the water droplet fills the spaces in the rough surface. In the Cassie-Baxter state, the water droplet sits on top of the rough surface with a layer of air underneath.

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When water is dropped or poured onto superhydrophobic surfaces, the water droplets sit lightly on the very tips of the surface protrusions, leaving a layer of air between the droplets and the leaf surface. Water droplets in this Cassie-Baxter state demonstrate the extreme water repellency that characterize superhydrophobicity (see Figure 5).

The transition between the Wenzel and Cassie-Baxter states requires an input of energy. In nature, this energy might be provided by the mechanical energy supplied by wind or water movement. When dew forms on a lotus leaf in the "sticky" Wenzel state, the movement of the leaf provides enough energy to "unpin" the water droplet and allow it free movement to roll off the leaf. In a laboratory setting, this mechanical energy must be provided by the experimenter, such as placing the lotus leaf on a stereo speaker or shaking the lotus leaf by hand..Superhydrophobicity — The Lotus Effect - Lesson - www.TeachEngineering.org