Biomedical Engineering Reference
In-Depth Information
on the complexity of the phenomenon, up to modern spreading theories, active re-
search is continuing. Many theoretical models have shown good agreement with
experimental results for a number of liquid-solid systems. However, due to a wide
range of hydrophobic surfaces and liquids involved, it is difficult to create a general
model. The use of surfactants as additives further complicates the matter.
To improve wetting, especially on extremely hydrophobic surfaces, surfactants
have been broadly employed for decades. The investigation of spreading of surfac-
tant solutions has led to new approaches and discoveries. As surfactant structure
and performance may greatly differ, proposed models became more complex. This
is particularly the case for trisiloxane surfactants—the 'superspreaders', whose su-
perb behaviour is poorly understood. Another area of interest is spreading on porous
substrates, especially when surfactants are used.
Here we provide a detailed review of significant achievements in the field of
spreading. Acknowledged models are presented, after a comprehensive theoretical
part, which could be very useful for beginners in this scientific area. In addition,
later approaches are discussed for a number of spreading related areas, such as
surfactants on liquid films, on porous substrates and spreading instabilities. As
this fascinating phenomenon is still systematically explored, further discoveries are
soon expected.
B. Wetting of Ideal and Real Surfaces
Wetting is the contact between a liquid and a solid (or another liquid), when the
two are brought into contact. When a liquid drop is placed onto a solid surface, it
will wet the surface to a certain extent, which depends on forces that act on the
three-phase (l-s-g) contact line, the so-called triple line [1, 2], until the equilibrium
is reached. The wetting process is normally characterised by the change in shape
of the drop and, consequently, its geometrical parameters (radius, height, contact
angle). Once the equilibrium, which is defined as the state with the minimal excess
of the total free energy of the system, is reached, the shape of the drop stops varying
and wetting parameters no longer change. Young (1805) was the first to suggest the
constancy of the equilibrium contact angle ( θ eq ) of a liquid drop on a solid surface,
as a function of the surface free energy, the interfacial free energy and the surface
tension of the liquid. In years to come, Dupré incorporated thermodynamic aspects
and this relationship is nowadays known as the Young-Dupré equation:
γ sg
γ sl
cos θ eq =
,
(1)
γ lg
where γ sg , γ sl and γ lg are interfacial tensions at solid-gas, solid-liquid and liquid-
gas interfaces, respectively (Fig. 1). The first of these is better known as the surface
energy of the solid, while the last is commonly referred to as the surface tension of
the liquid. Surface energy is one of the basic properties of a solid surface and varies
greatly for different materials. The value of the surface energy is the consequence
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