504
F. Gentile et al.
medicine. A mathematical description of SHSs was provided, on the basis of which
the devices could be designed and fabricated; extremely diluted biological solutions
were therefore analyzed, and Rhodamine molecules were detected in the very low
abundance range of 10 −18 M.
The working principle is based on the use of small drops of de-ionized water, containing the moieties at study, which have to be positioned upon the substrate and
there let to evaporate. In sight of a simple balance of forces, the line of contact at
the solid interface would recede with time, and thus the footprint of the drop would
also gradually reduce. When the drop gets sufficiently small, a transition to more
a stable state occurs, whereby the drop is firmly attached to the substrate, and the
scale-down of the area of contact is prevented. Thereupon, few molecules would be
accumulated into a very small region, assuring an increased density and, accordingly,
the attainment of the limits of detection (Fig. 14.1).
14.2 Super-Hydrophobicity: The Physical Model
Nature has endowed the leafs of some plants with the noticeable capability of
repelling water, and thus if a drop is positioned upon these, it would assume a shape
that resembles that of a sphere [24]. Conventionally we say that all the surfaces that
behave like these leaves are super hydrophobic. The most practical property of SHSs
is a reduced friction coefficient on account of which they can be used for a number
of different applications, including, on the medical side, the manipulation and control of diluted solutions of biological interest [25, 26]. SHSs typically feature two
distinguishing ingredients, that are (i) a quasi periodic hierarchical structure with
dimensions comprised in the micro to nano meter range (and thus the geometry), and
(ii) a superficial, chemical or physical, modification of the surface. Understanding
the mechanisms whereby the combination of these reduces wettability is essential
for the rational design of artificial SHSs, as those described later in the text. In the
following sections, the laws describing the wetting phenomena are briefly recapitulated (I); why an hexagonal lattice of pillars would mimic natural SHSs better than
other configurations is explained (II); the dynamics of slowly evaporating droplets
and the influence of certain parameters on that is discussed (III); and, lastly, the
parameters describing the lattice above are optimized on the basis of a mathematical
criterion (IV).
14.2.1 The Physics of Drops and Surfaces
It is well known that the physics of micrometric or sub millimetric drops is correctly
governed by surface tension solely. The angle of contact ε at the solid/air/liquid
interface indicates the propensity of a droplet of whether wetting or not a surface.
Surfaces where ε > 150 ∪ are super-hydrophobic. Interestingly, perfectly flat sur-
F. Gentile et al.
medicine. A mathematical description of SHSs was provided, on the basis of which
the devices could be designed and fabricated; extremely diluted biological solutions
were therefore analyzed, and Rhodamine molecules were detected in the very low
abundance range of 10 −18 M.
The working principle is based on the use of small drops of de-ionized water, containing the moieties at study, which have to be positioned upon the substrate and
there let to evaporate. In sight of a simple balance of forces, the line of contact at
the solid interface would recede with time, and thus the footprint of the drop would
also gradually reduce. When the drop gets sufficiently small, a transition to more
a stable state occurs, whereby the drop is firmly attached to the substrate, and the
scale-down of the area of contact is prevented. Thereupon, few molecules would be
accumulated into a very small region, assuring an increased density and, accordingly,
the attainment of the limits of detection (Fig. 14.1).
14.2 Super-Hydrophobicity: The Physical Model
Nature has endowed the leafs of some plants with the noticeable capability of
repelling water, and thus if a drop is positioned upon these, it would assume a shape
that resembles that of a sphere [24]. Conventionally we say that all the surfaces that
behave like these leaves are super hydrophobic. The most practical property of SHSs
is a reduced friction coefficient on account of which they can be used for a number
of different applications, including, on the medical side, the manipulation and control of diluted solutions of biological interest [25, 26]. SHSs typically feature two
distinguishing ingredients, that are (i) a quasi periodic hierarchical structure with
dimensions comprised in the micro to nano meter range (and thus the geometry), and
(ii) a superficial, chemical or physical, modification of the surface. Understanding
the mechanisms whereby the combination of these reduces wettability is essential
for the rational design of artificial SHSs, as those described later in the text. In the
following sections, the laws describing the wetting phenomena are briefly recapitulated (I); why an hexagonal lattice of pillars would mimic natural SHSs better than
other configurations is explained (II); the dynamics of slowly evaporating droplets
and the influence of certain parameters on that is discussed (III); and, lastly, the
parameters describing the lattice above are optimized on the basis of a mathematical
criterion (IV).
14.2.1 The Physics of Drops and Surfaces
It is well known that the physics of micrometric or sub millimetric drops is correctly
governed by surface tension solely. The angle of contact ε at the solid/air/liquid
interface indicates the propensity of a droplet of whether wetting or not a surface.
Surfaces where ε > 150 ∪ are super-hydrophobic. Interestingly, perfectly flat sur-