Biomedical Engineering Reference
In-Depth Information
Figure 1. Liquid drop on a solid surface: Y is the Young contact angle; γ LG is the interfacial tension
liquid-gas; γ SV and γ SL are interfacial tensions solid-gas and solid-liquid.
1. Wetting and Wettability
The term “wetting” describes a displacement of a solid-gas (air) interface with a
solid-liquid interface. That is a process in which free energy decreases in a system
consisting of three contacting phases. The term “wettability” describes the ability of
a surface to maintain a contact with a liquid. The degree of wetting and wettability
is determined by a force balance between adhesive and cohesive molecular forces.
The forces in equilibrium at a solid-liquid boundary are commonly described by
the Young equation shown in Fig. 1a as a quasi-static wetting situation:
γ SG =
cos Y , (1)
where γ SG , γ SL and γ LG are interfacial solid-gas, solid-liquid and liquid-gas ten-
sions, respectively, and Y is the equilibrium contact angle. Since the quantities
γ SG and γ SL are generally inaccessible to experiments, in contrast to γ LG , the Young
equation is often used for solving the inverse problem, namely to determine the dif-
ference SG
γ SL +
γ LG ·
γ SL ) , which is referred to as wetting tension or adhesive tension ,
by means of experimental values for static or quasi-static contact angle and inter-
facial tension γ LG . The magnitude of the contact angle depends on the strength of
molecular interactions between liquid molecules inside the drop as well as between
a liquid and a solid surface.
The surface tension of a liquid is a measure of its resistance to increase its sur-
face area. Liquids with relative large intermolecular forces such as water tend to
have relatively high surface tensions, that is, high resistances to increasing their
surfaces areas. When a small droplet of a pure liquid is put in contact with a flat
surface, two distinct equilibrium regimes may be found: partial wetting with a fi-
nite contact angle, or complete wetting with a zero contact angle [1]. However, if the
contact angle is larger than 90 , the liquid does not wet the surface and the situation
is referred to as non-wetting . A more detailed description of advancing, receding,
Young's contact angles as well as problems of experimental and theoretical verifi-
cation of equilibrium contact angle is recently provided in [2].
2. Three Phase Contact (TPC) Zone
The three-phase contact (TPC) zone is the region where three immiscible coexisting
phases meet, unless one of them wets the interface between the other two. The
intersection of the three interfaces forms the three phase contact zone, the so-called
TPC line as shown in Fig. 2.
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