Post date: May 1, 2012 4:58:33 AM

Why this topic?

Although this topic is not directly related to the dough and bubbles, discussion of the drops on the surfaces touches the questions essential for their understanding (e.g., hydrophobicity, surface and interfacial tension, pores and texture, and their effect on the physical behaviors). In addition, the Lotus and "rose" effects are fascinating and their discussion can be enormously insightful. Finally, discussion of the surface texture can contribute to our interest in microscopy,

The related term is the "fakir drop"

<< Que´ re´ and Bico have shown that wettability at the liquid/solid interface can be lowered if air is trapped below the liquid in the holes of the rough surface. In such a situation, the liquid positions itself on the surface in a similar fashion as an Indian “fakir” lying on the points of a bed of sharp nails >>.

Single Drop Of Water On Lotus Leaf

Water drop does not wet the surface of the Lotus leaf [or does not adhere to it], and it "slides" down the slope. The surface is called "superhydrophobic" (another example of this is water striders’ nonwetting legs ).

Lotus plants (Nelumbo nucifera) stay dirt-free, an obvious advantage for an aquatic plant living in typically muddy habitats, and they do so without using detergent or expending energy. The plant's cuticle, like that of other plants, is made up of soluble lipids embedded in a polyester matrix – wax – but the degree of its water repellency is extreme (superhydrophobic). This is accomplished through the micro-topography of their leaf surfaces, which while showing a variety of structures, all share a similar mathematical set of proportions associated with superhydrophobicity (from http://www.asknature.org, including the following picture).

This computer-generated image of the lotus surface explains the "fakir drop" allegory (http://en.wikipedia.org/wiki/Wetting )

This (fakir state) is also called the Cassie-Baxter state [see A. B. D. Cassie and S. Baxter, Trans. Faraday Soc. 40, 546 (1944)].

Here is a schematic drawing of the Cassie-Baxter state:

Water "slides" on the tops of the "pins", air underneath. The friction is low.

And this is an opposite case, the Wenzel state [ R. N. Wenzel, Ind. Eng. Chem. 28, 988 (1936)]

Water fills the space between the pins.

Liquid in the Cassie-Baxter state is more mobile than in the Wenzel state.

"Water droplets on rugged hydrophobic surfaces typically exhibit one of the following two states: (i) the Wenzel state in which water droplets are in full contact with the rugged surface (referred as the wetted contact) or (ii) the Cassie state in which water droplets are in contact with peaks of the rugged surface as well as the “air pockets” trapped between surface grooves (the composite contact)" (see Koishi,T et al, PNAS, 106 (2009), pp. 8435-8440;

http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1091&context=chemzeng ).

You can download and watch the movies of the computer simulation showing how the Cassie and Wenzel states can coexist or transit one to another:

• Download Movie 1 (avi)

• Download Movie 2 (avi)

• Download Movie 3 (avi)

The hydrophobic (water-repelling) properties of a surface can be enhanced by creating a texture with different length scales of roughness. The red rose takes advantage of this by using a hierarchy of micro- and nanostructures on each petal to provide sufficient roughness for superhydrophobicity. More specifically, each rose petal has a collection of micropapillae on the surface and each papillae, in turn, has many nanofolds.

The term “petal effect” describes the fact that a water droplet on the surface of a rose petal is spherical in shape (which implies that the surface is strongly hydrophobic) , but, surprisingly, it cannot roll off even if the petal is turned upside (the surface is hydrophobic, but this not directly related to its water-repelling qualities).

This picture from Wikipedia explains the difference between these two cases.

Petal effect Lotus effect

All these observations hinted to the role of micro- and nano-scale texture on the hydrophobic (water-repelling) properties of the surfaces, and became ...

Here is one of many possible applications

(http://www.greenshieldfinish.com/):

textile finish inspired by nature

GreenShield^® is New Technology

Microscopic roughness is the technology behind GreenShield which leverages the enormous surface area of each nanoparticle enabling the particles to efficiently deliver the appropriate chemistry to the fabric. Through the use of nano-particles on the surface of a fabric GreenShield creates a pocket of air allowing water and oil droplets to roll – carrying dirt and stains off the fabric for a self cleaning effect.

Demonstration of red wine, water and soy sauce on GreenShield: