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9 Lotus Leaf Effect -- Hydrophobicity and Self-Cleaning

9 Lotus Leaf Effect -- Hydrophobicity and Self-Cleaning

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Biology on the Nanoscale

Fig. 11.63 Hydrophobicity as a prerequisite for the lotus effect, the rose petal effect, and the water

strider. (a) Lotus effect. A water droplet collects dirt from the surface of a lotus leaf. (b) Close-up

of a lotus leaf (Nelumbo nucifera), an example of a superhydrophobic plant surface which originates from the multilength-scale structuring by micrometer-sized bumps and nanoscale waxy hairs

[11.178]. (c, d) Scanning electron micrograph (SEM) of the surface of a red rose petal showing

a periodic array of micropapillae and nanofolds on each papilla top [11.180]. (e) Water strider

(Gerris remigis) walking on the surface of a lake (see [11.178]). (Reprinted with permission from

[11.178] (a) (b) (e) and [11.180] (c) (d). © 2008 Materials Research Society (a) (b) (e) and © 2008

American Chemical Society (c) (d))

In the lotus leaf the surface has hydrophobic hierarchical micro- and

nanostructures (Fig. 11.63b) with a coexistence of micrometer-sized bumps and

nanoscale waxy hairs [11.176, 11.178]. Similar hierarchical micro- and nanostructures are found on the surfaces of rose petals (Fig. 11.63c, d). Hydrophobicity

permits water striders (Gerris remigis) with hierarchical micro–nanostructured legs

to stroll effortlessly on water (Fig. 11.63e), where a single leg can hold up to 1.52

mN, which is 15 times the entire water strider’s body weight [11.178].


Lotus Leaf Effect – Hydrophobicity and Self-Cleaning


Fig. 11.64 (a, b) Model artificial lotus leafs. (a) Scanning electron micrograph (SEM) of a surface

containing micrometer-sized posts with a smooth surface. (b) Post from (a) covered with nanosized

methylsilicone fibrils (∼40 nm in diameter) yielding superhydrophobic properties [11.179]. (c)

Schematic representation of a flat hydrophilic surface where a water droplet just passes through

strongly adhering dirt particles. (d) A different situation occurs on a topographically structured

superhydrophobic substrate with weak adherence of dirt particles and spherical rolling droplets

which pick up the dirt particles and hence clean the surface [11.178]. (Reprinted with permission

from [11.179] (a) (b) and [11.178] (c) (d). © 2008 Materials Research Society)

In a model artificial lotus leaf (Fig. 11.64a, b) with micrometer smooth posts,

water contact angles of θA /θR = 176◦ /156◦ are measured (Fig. 11.64a), whereas

with nanoscopic roughness on the posts (Fig. 11.64b) nearly perfect hydrophobicity

θA /θR => 176◦ / > 176◦ is found. From this it is concluded that the nanohairs on

the bumps of the lotus leaf substantially enhance hydrophobicity by increasing the

local receding contact angle [11.179].

This superhydrophobic effect is the origin of self-cleaning [11.177]. As the spherical water droplets roll around easily on the leaf, they encounter debris and other

particulates which minimally adhere to the surface with multiple length scales of

roughness and therefore can be readily carried away by the traversing water droplet



Biology on the Nanoscale

(see Fig. 11.64d). The self-cleaning effect is unavailable on a hydrophilic surface

with non-spherical water droplets (Fig. 11.64c).

11.10 Food Nanostructures

Milk, cereals, meat, etc., all these products are based on nanostructures such as gels,

emulsions, foams, or combinations thereof [11.181]. Structural studies of foods can

promote the understanding of the individual components and their effect on the overall microstructure of complex food products. Five out of ten of the world’s largest

food and beverage companies are investing in some form of this research [11.182].

Two studies on meat and milk proteins and an example from food packaging will be

outlined in the following.

The protein forming the thick filaments in meat (muscle tissue) is myosin.

The characteristics of myosin gel formation on the nanoscale (Fig. 11.65) are

of importance for the structure of fabricated fish and meat products like surimi

(minced fish) and sausages. Individual myosin molecules (Fig. 11.65a) associate

Fig. 11.65 Association and aggregation of myosin protein molecules to networks. (a)

Transmission electron microscopy (TEM) of myosin molecules at pH 6.0 and 0.6 M KCl. (b) TEM

of head-to-head interactions of myosin at pH 6 and 0.6 M KCl after heat treatment at 40◦ C. (c)

Scanning electron microscopy (SEM) of a filamentous myosin gel at pH 5.5 and 0.25 M KCl after

heat treatment at 60◦ C and (d) an aggregated myosin gel at pH 6.0 and 0.6 M KCl after heat

treatment at 60◦ C. (Reprinted with permission from [11.181]. © 2000 Materials Research Society)




Fig. 11.66 Transmission electron micrographs (TEM) of 6% β-lactoglobulin gels at pH 5.4 (a)

without and (b) with the addition of 0.06% monolaurin emulsifier. (Reprinted with permission

from [11.181]. © 2000 Materials Research Society)

into different supramolecular assemblies (Fig. 11.65b), depending on the ionic

strength. On heating of individual myosins, a filamentous network (Fig. 11.65c)

and from the supramolecular assemblies and aggregated network (Fig. 11.65d) are


Dairy products such as yoghurts and cheeses are based on a structure of protein

gels in combination with the structure of emulsions, where emulsifiers are added

to stabilize the fat. The effect of the emulsifier monolaurin on the gel formation

of β-lactoglobulin, the most important gel-forming milk protein in whey, is a more

open gel network with larger pores (Fig. 11.66b).

For food packaging materials polymers play an important role with an increasing

demand to improve the barrier properties of the packaging materials concerning

permeation of, e.g., water vapor or oxygen, in order to extend the shelf life of the

packed foodstuff. Deposition of SiOx barrier layer with a nanosized thickness of

100 or 30 nm on polyethyleneterephthalate (PET) or poly (propylene) (PP) foils,

respectively, can reduce the oxygen permeability by up to a factor of 65 [11.183]

(see also Sect. 6.9).

11.11 Cosmetics

Cosmetics are intended to improve the appearance of the skin, the hair, or the teeth.

Today many cosmetic products aim at hydrating the skin, reducing or slowing the

signs of aged skin, or protecting the skin against the multitude of daily aggressions that it encounters. As discussed below, nanoparticles, such as liposomes or

nanosomes, may be used for the efficient transport of cosmetic ingredients into

skin [11.185]. Ancient hair dyeing formulae have been found to be based on the

formation of PbS (galena) nanocrystallites [11.188].



Biology on the Nanoscale

11.11.1 Skin Care

The emergence of deeper furrowing and wrinkling is related to a mechanical failure

of the skin as shown by loss of tension and elasticity. The alteration of complexion

is related to skin vasculature (see Fig. 11.67). The reduction in the water holding

capacity of the outer layer of the epidermis, the stratum corneum (Fig. 11.67a, b),

which only contains dead, strongly keratinized cells, also influences the general

aspect of the skin.

Nanoparticles with sizes from 10 nm to a few hundred nanometers can be used

as carriers for cosmetic components. Liposomes (Fig. 11.68b) can encapsulate

water-soluble ingredients in their polar cavity and oil-soluble ingredients in their

hydrophobic cavity. Liposomes have been used since the mid-1980s, when Christian

Dior first introduced its Capture line to the market [11.185]. Nanotopes are formed


Fig. 11.67 (a) Skin appearance, texture, and mechanical strength depend on the structural organization of the layers of the skin (right). The stratum corneum, the outermost layer of the skin, is in

contact with the environment. The epidermis comprises a layer of basal cells, the stratum spinosum,

and the stratum granulosum. Under the epidermis is the dermis, which supports the skin’s vasculature, nerves, hair follicles, and sebum-producing glands. (b) Transmission electron micrograph of

the stratum corneum. Intercellular connections are via protein “rivets” called corneodesmosomes.

Cells achieve rigidity and strength from a network of aligned keratin fiber bundles. (Reprinted with

permission from [11.189]. © 2007 Materials Research Society)




Fig. 11.68 Nanosized liposomes can encapsulate and transport water-soluble ingredients in their

polar cavity and oil-soluble ingredients in their hydrophobic cavity. (Reprinted with permission

from [11.185]. © 2007 Materials Research Society)

from a combination of lecithin, a phospholipid, and cosurfactants, which are aligned

in an alternating arrangement. An example of a commercialized nanotechnology

product is L’Oreal’s Plenitude Revitalift, introduced in 1998, an antiwrinkle cream

that uses a 200 nm technology (nanosomes to carry lipophilic material) to incorporate vitamin A into a polymer capsule to help deliver active ingredients to the skin’s

deeper layers [11.185]. The liposomes are able to bind to microorganisms responsible for skin disorders, sculp irritation, and underarm and foot odor, and hence can

be used to selectively treat these disorders (see [11.184]).

While recognizing the value of these molecular-level advances, there is concern about the safety of nanotechnology for workers and consumers. Therefore, any

new material now delivered to the market place must go through extensive testing

to ensure that it is safe as recommended. This increase in safety requirements is

due to increased regularity requirements, especially in the European Union where

the REACH program (Registration, Evaluation, Authorization, and Restriction

of Chemicals) requires cradle-to-grave documentation of all raw materials (see


For reducing the visibility of wrinkles, light diffusing pigments were developed

from 1000 nm SiO2 spheres coated with TiO2 nanoparticles with a refractive index

similar to that of the skin and Fe2 O3 nanocrystals with a coloration resembling that

of the skin. By this pigment the reflection of light out of a wrinkle is enhanced by

factors of 4–10 compared to that from the skin [11.186].



Biology on the Nanoscale

Fig. 11.69 Scanning electron

microscopy of a sunscreen

agent with SiO2 spheres

(20–50 nm) coated with TiO2

particles smaller than 10 nm

for optimum UV absorption.

(Reprinted with permission

from [11.186]. © 2001

Springer Verlag)

Efficient sun screen agents are produced making use of 20–50 nm SiO2 spheres

coated with TiO2 nanoparticles smaller than 10 nm (Fig. 11.69). These agents are

extremely good in UV absorption and they are invisible when applied to the skin

because particles smaller than 60 nm practically do not scatter light.

11.11.2 Encapsulating a Fragrance in Nanocapsules

This is a method to avoid evaporation of volatile components at an early stage

of usage and to increase shelf life of a product, offering a long-term usage for

household purposes or as a fragrance in many cosmetic products or washing agents

[11.187] (Fig. 11.70).

Fig. 11.70 Transmission

electron micrograph (TEM)

of poly (methyl metacrylate)

(PMMA) nanocapsules for

the hydrophobic fragrance

1,2-dimethyl-1-phenylbutyramide (DMPBA).

(Reprinted with permission

from [11.187]. © 2009





11.11.3 PbS Nanocrystals in Ancient Hair Dyeing

A 2000-year-old recipe sold to this day as a remedy for graying hair is a mixture of

lead oxide, PbO, and slaked lime, Ca(OH)2 , with a small amount of water [11.188].

The treatment of hair with this mixture (see Fig. 11.71a–c) gives rise to blackening due to the precipitation of 5 nm galena (PbS) crystallites as identified by x-ray

scattering and by transmission electron microscopy (Fig. 11.71d, e). The progress

of the crystallite precipitation starting from the hair surface was documented by

scanning electron microscopy energy-dispersive x-ray emission (SEM-EDX) for the

Fig. 11.71 Optical micrographs of hair (a–c; thickness = 10 μm), showing progressive blackening

during treatment with Ca(OH)2 and PbO in water (25◦ C, pH = 12.5). (a) Untreated, (b) and (f)

after 6 h, (c) and (g) after 72 h. (f, g) Pb concentration as obtain from scanning electron microscopy

energy-dispersive x-ray emission (SEM-EDX) with the Pb concentration increasing from dark

blue to green). (d) High-resolution transmission electron micrograph (HRTEM) of a longitudinal

section of a 3-day-treated hair. (e) HRTEM of a PbS nanocrystallite. (Reprinted with permission

from [11.188]. © 2006 American Chemical Society)



Biology on the Nanoscale

quantification of Pb uptake (Fig. 11.71f, g). As deduced from the x-ray scattering

results, the α-helical coiled structure of the hair keratin protein is preserved after the

blackening reaction [11.188].

The PbS crystallites are formed by in-diffusion of Pb ions into the hair keratin

structure where the sulfur for the formation of galena is provided by the natural

amino acids, cystine and methionine [11.188]. The 5 nm sized PbS crystallites represent a kind of substitute of the natural black hair color which is due to the much

larger 300 nm melanin pigment clusters within the colorless keratin-based cortex of

the hair.

11.12 Summary

Nanoscience plays an important role in the investigation of biological phenomena

because the size of inorganic nanoparticles as probes and the spatial resolution of nanotools match the sizes of macromolecular components (in the range

of 2–200 nm) employed in living systems. The minimum size of a living system may have a diameter of 40–50 nm which is in the size range of a virus

or of small biological structures. Cells in living organisms contain a system of

nanoscale machines, molecular motors, and membrane channels. Nanoparticles

can be employed for the bioanalysis of DNA and proteins and for the characterization of these species nanomechanical techniques may be used. Biomimetics

are technologies inspired by the favorable properties of biological systems, such

as nanoscale efficiency, self-organization, and adoptability at relatively low cost.

Nanostructures are characteristic for bone, teeth, and for the photonic biostructures

which yield colors to butterflies and beetles. Nanostructures, furthermore, enable

the lotus leaf self-cleaning effect in plants and are typical for food and for cosmetic

















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