FIG. 5.

Relationship between interfacial tensions and the contact angle of the oil droplet on the agar-water surface (θ). The following equation was used to keep forces in balance: (tension at air-water interface) = (tension at oil-water interface) + (tension at oil-air interface × cosine θ). Tension at the oil-air interface is a fixed value because it is rarely influenced by surfactants (27). Therefore, as the air-water tension decreases from added surfactant, as does the tension at the oil-water interface, the contact angle, θ, of the oil droplet will change to compensate for the inequal effect of the surfactant on those two interfaces.