Three-phase line/perimeter, contact angle

The introduction of contact angles gives rise to another interfacial characteristic, the line tension. This is the contractile tension acting in the three-phase contact perimeter around drops as in fig. 1.1, and may phenomenologically be considered the one-dimensioncd analogue of the interfacial tension with SI units of N or J m. As it is a typiccd three-phase characteristic, we shall not treat it here, but in sec. 5.6. 

Line tensions (r) are forces acting in the three-phase contact perimeter. Their SI-unit is N. When r > 0 the line tension tries to shorten the perimeter, for r < 0 the trend is the other way around. Line tensions are typiccdly excess qucuitities in that their action comes on top of that of the three interfacial tensions constituting the contact angle, and which are related through Young s law or Neumann s triangle. 

FIGURE 3.2 Spreading of a spherical droplet. At h > fr, the spherical droplet profile is not distorted by the hydrodynamic flow t < is the radins of action of the disjoining pressure r t) is the macroscopic wetting perimeter (the apparent three-phase contact line) R(t) is the tme microscopic wetting perimeter 0(f) is the dynamic contact angle and H(t) is the drop apex. 

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