Angle of Contact Relation (Sla cosθ + Ssl = Ssa)
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Sla cosθ + Ssl = Ssa
What each symbol means
| Symbol | What it stands for |
|---|---|
| θ | Angle of contact (degrees or radians) |
| Sla | Surface tension at liquid-air interface (N/m) |
| Ssa | Surface tension at solid-air interface (N/m) |
| Ssl | Surface tension at solid-liquid interface (N/m) |
When to use this
Derived from equilibrium of surface tension forces at the contact line. If Ssl > Sla, then cosθ < 0, so θ is obtuse (liquid does not wet solid, e.g., mercury on glass). If Ssl < Sla, θ is acute (liquid wets solid, e.g., water on glass). Wetting agents reduce θ; waterproofing agents increase θ.