Investigation of the Petal Effect

Plant leaves and petals provide an example of surfaces with high CA and high and low CA hystereses. Bhushan and Her [4] studied two kinds of superhydrophobic rose petals: (1) Rosa Hybrid Tea, cv. Bairage and (2) Rosa Hybrid Tea,

Droplet on Rosa, cv. Bairage

Droplet on Rosa, cv. Bairage

0° tilt angle 180° tilt angle

Fig. 2.3 Optical micrographs of water droplets on Rosa, cv. Bairage at 0° and 180° tilt angles. Droplet is still suspended when the petal is turned upside down [4]

0° tilt angle 180° tilt angle

Fig. 2.3 Optical micrographs of water droplets on Rosa, cv. Bairage at 0° and 180° tilt angles. Droplet is still suspended when the petal is turned upside down [4]

Table 2.1 Surface roughness statistics for the two rose petals [4]

Peak-to-base

Midwidth

Peak radius Bump density

height (im)

(im)

(im) (1/10,000 im2)

Rosa, cv. Bairage

6.8

16.7

5.8 23

(high adhesion)

Rosa, cv. Showtime

8.4

15.3

4.8 34

(low adhesion)

Table 2.2 Wetting regimes of a surface with

a single level of hierarchy of roughness

State

Cassie-Baxter

Wenzel

Impregnating cassie

Cavities

Air

Water under droplet Water everywhere

CA

High

High

High

CA hysteresis

Low

Can be high

Low

cv. Showtime, referred to as Rosa, cv. Bairage and Rosa, cv. Showtime, respectively. Figure 2.2 shows optical micrographs and scanning electron microscopy (SEM) images and atomic force microscope (AFM) surface height maps of two rose petals. Figure 2.3 shows a sessile and a suspending water droplet on Rosa, cv. Bairage demonstrating that it can simultaneously have high CA and high adhesion and high CA hysteresis.

The surface roughness of the two rose petals was measured with the AFM, and the results for the peak-to-base height of bumps, the midwidth, peak radius, and bump density are summarized in Table 2.1. The data indicates that the low adhesion specimen (Rosa, cv. Showtime) has a higher density and height of the bumps, indicating that the penetration of water between the micro-bumps is less likely. Wetting of a rough surface with a single level of hierarchy of roughness details can follow several scenarios (Table 2.2).

Table 2.3 Different regimes of wetting of a surface with dual roughness

Air in microstructure Water under droplet Water in microstructure impregnating microstructure

Air in Lotus, high CA, nanostructure low CA hysteresis

Water under Cassie (air-filled droplet in microstructure, water in nanostructure nanostructure), high CA, low CA hysteresis

Water Cassie filled nanostructure impregnating nanostructure

Rose, high CA, Rose filled high CA hysteresis microstructure

Wenzel (water in micro- Wenzel filled and nanostructure), microstructure high CA, high or low CA hysteresis

Wenzel filled Wenzel filled micro nanostructure and nanostructure

Petal Effect Wetting

Cassie filled nanostiotture Weuzel filled nauostmchire Wenzel filled micro/uanosliuchire

Fig. 2.4 Schematics of nine wetting scenarios for a surface with hierarchical roughness

Cassie filled nanostiotture Weuzel filled nauostmchire Wenzel filled micro/uanosliuchire

Fig. 2.4 Schematics of nine wetting scenarios for a surface with hierarchical roughness

For a hierarchical structure with small bumps on top of the larger bumps, a larger number of scenarios are available, and they are summarized in Table 2.3 and Fig. 2.4. Water can penetrate either in the micro- or nanostructure, or into both. In addition, the micro- or nanostructure can be impregnated by water or air. The regimes with water penetrating into the microstructure can have high solid-water adhesion and therefore high CA hysteresis.

Bhushan and Her [4] conducted a series of carefully designed experiments to decouple the effects of the micro- and nanostructures. They synthesized micro-structured surfaces with pillars out of epoxy resin. The epoxy surfaces were

Hierarchical structures with n-hexatriacontane Microstructures with

23 |im pitch 105 jitn pitch 210 pm pitch

Hierarchical structures with n-hexatriacontane Microstructures with

23 |im pitch 105 jitn pitch 210 pm pitch

Low magnification images

rt-hexatriacontans (0.1 jig/mm!) n-hexatriacontsne (0.2 ngfmm*)

rt-hexatriacontans (0.1 jig/mm!) n-hexatriacontsne (0.2 ngfmm*)

Hierarchical Microstructures

High magnification images

Fig. 2.5 SEM micrographs of the microstructures and nanostructures fabricated with two different masses of n-hexatriacontane for hierarchical structure. All images were taken at 45° tilt angle. All samples are positive replicas, obtained from negative replica with dental wax and Si micropatterned master template (14 im diameter and 30 im height) fabricated with epoxy resin coated with n-hexatriacontane [4]

High magnification images

Fig. 2.5 SEM micrographs of the microstructures and nanostructures fabricated with two different masses of n-hexatriacontane for hierarchical structure. All images were taken at 45° tilt angle. All samples are positive replicas, obtained from negative replica with dental wax and Si micropatterned master template (14 im diameter and 30 im height) fabricated with epoxy resin coated with n-hexatriacontane [4]

reproduced from model Si templates and were created by a two-step molding process producing a dual replica (first a negative replica and then a positive replica of the original Si template). Surfaces with a pitch (the periodicity of the structure of the pillars) of 23, 105, and 210 im and with the same diameter (14 im) and height (30 im) of the pillars were produced. After that, nanostructures were created on the microstructured sample by self-assembly of the alkane n-hexatri-acontane (CH3(CH2)34CH3) deposited by a thermal evaporation method. Alkanes of varying chain lengths are common hydrophobic compounds of plant waxes. On smooth surfaces, alkanes can cause a large contact angle and a small contact angle hysteresis for water droplets. To fabricate the nanostructure, various masses of n-hexatriacontane were coated on a microstructure. The nanostructure is formed by three-dimensional platelets of n-hexatriacontane. Platelets are flat crystals, grown perpendicular to the surface. They are randomly distributed on the surface, and their shapes and sizes show some variation. Figure 2.5 shows selected images. When different masses of wax are applied, the density of the nanostructure is changed.

For surfaces with a small pitch of 23 mm, while the mass of n-hexatriacontane is changed, there are only small changes in the static contact angle and contact

Table 2.4 CA and CA hysteresis for surfaces with various micro- and nanoroughness (based on

Mass of n-hexatriacontane (ig/ Pitch mm2)

23 im

105 im

210 im

hysteresis

CA CA

hysteresis

CA CA

hysteresis

152 87

153 20 160 5 168 4

135 45 135 42 150 12 166 3

Fig. 2.6 Schematic of a wetting regime map as a function of microstructure pitch and the mass of nanostructure material. The mass of nanostructure material equal to zero corresponds to microstructure only (with the Wenzel and Cassie regimes). Higher mass of the nanostructure material corresponds to higher values of pitch, at which the transition occurs

Fig. 2.6 Schematic of a wetting regime map as a function of microstructure pitch and the mass of nanostructure material. The mass of nanostructure material equal to zero corresponds to microstructure only (with the Wenzel and Cassie regimes). Higher mass of the nanostructure material corresponds to higher values of pitch, at which the transition occurs

Effects Energy Conservation

angle hysteresis values, which means that they are always in the "Lotus" wetting regime. On the surface with a 210 lm pitch value, as the mass of n-hexatria-contane is increased, the static contact angle is increased, and the reverse trend was found for the contact angle hysteresis. This was interpreted as evidence that the nanostructure is responsible for the CA hysteresis and low adhesion between water and the solid surface. The results are summarized in Table 2.4. The wetting regimes are shown schematically in Fig. 2.6 as a function of the pitch of the microstructure and the mass of n-hexatriacontane. A small mass of the nano-structure material corresponds to the Cassie and Wenzel regimes, whereas a high mass of nanostructure corresponds to the Lotus and rose regimes. The Lotus regime is more likely for larger masses of the nanostructure material. Figure 2.7 shows a droplet on a horizontal surface of a hierarchical structure with 23 and 105 im pitch and n-hexatriacontane (0.1 ig/mm2). Air pockets are observed in the first case and not observed in the second case, indicating the difference between the two regimes [4].

To further verify the effect of wetting states on the surfaces, evaporation experiments with a droplet on a hierarchical structure coated with two different

Fig. 2.7 a Droplet on a horizontal surface of hierarchical structure with 23 im pitch and n-hexatriacontane (0.1 ig/mm2) showing air pocket formation and b droplet on a hierarchical structure with 105 im pitch and n-hexatriacontane (0.1 ig/mm2) and 0.2 ig/mm2 showing no air pocket and air pocket formation, respectively. Also shown is the image taken on the inclined surface with hierarchical structure with 0.1 ig/mm2 showing that droplet is still suspended [4]

Hierarchical Structured Surface

amounts of n-hexatriacontane were performed. Figure 2.8 shows the optical micrographs of a droplet evaporating on two different hierarchical structured surfaces. On the n-hexatriacontane (0.1 ig/mm2) coated surface, an air pocket was not visible at the bottom area of the droplet. However, the droplet on the surface has a high static contact angle (152°) since the droplet still cannot completely impregnate the nanostructure. The footprint size of the droplet on the surface has only small changes from 1820 to 1791 im. During evaporation, the initial contact area between the droplet and hierarchical structured surface does not decrease until the droplet evaporates completely, which means complete wetting between droplet and microstructures. For the n-hexatriacontane (0.2 ig/mm2) coated surface, the light passes below the droplet, and air pockets can be seen, so to start with the

Complete Wetting Solid Surface

Fig. 2.8 Optical micrographs of droplet evaporation on the hierarchical structured surfaces with 105 im pitch value. n-Hexatriacontane (0.1 ig/mm2) coated sample has no air pocket formed between the pillars in the entire contact area until evaporation was completed. Hierarchical structure with n-hexatriacontane (0.2 ig/mm2) has air pocket, and then the transition from the ''Lotus'' regime to the "Rose petal'' regime occurred [4]

Fig. 2.8 Optical micrographs of droplet evaporation on the hierarchical structured surfaces with 105 im pitch value. n-Hexatriacontane (0.1 ig/mm2) coated sample has no air pocket formed between the pillars in the entire contact area until evaporation was completed. Hierarchical structure with n-hexatriacontane (0.2 ig/mm2) has air pocket, and then the transition from the ''Lotus'' regime to the "Rose petal'' regime occurred [4]

droplet is in the Cassie-Baxter regime. When the radius of the droplet decreased to 381 im, the air pockets are not visible anymore. The footprint size of the droplet on the surface changed from 1177 to 641 im, since droplet remained on only a few pillars until the end of the evaporation process.

The experimental observations of the two types of rose petals show that hierarchically structured plant surfaces can have both adhesive and non-adhesive properties at the same time with high CA. This is due to the existence of various modes of wetting of a hierarchical surface, so that water can penetrate either into macro- or nanoroughness, or into both. Water penetration into the microroughness tends to result in high adhesion with the solid surface, whereas the presence of the nanoroughness still provides high CA. As a result, two distinct modes of wetting are observed, one can be called the ''Lotus'' mode (with low adhesion) and the other is the ''rose'' mode with high adhesion.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment