Contact angle associated with thin liquid films in emulsions
The surface tension of liquid film can be very different from the surface tension of the liquid in bulk, because of contributions from the disjoining pressure. The authors described a method to measure the surface tension of a liquid film easily by measuring the contact angle formed between the film and the plateau border. The authors showed how the contact angle is related to the disjoining pressure and demonstrated how temperature and different salt concentrations changes the surface tension of liquid film.
The tension of the film, γf is related to the bulk interfacial tension γ by
where h is the film thickness, he is the equilibrium thickness and π is the disjoining pressure. Whenever ΔF < 0, the two bulk surfaces will, on macroscopic scale, intersect an angle 2θ (see figure 1), such that
ΔF essentially determines whether the oil droplets will repel or spontaneously flocculate as in figure 2. For ΔF < 0, we can precisely determine the value of ΔF just by measuring the macroscopic contact angle by equation (3).
The authors proceeded to study concentrated oil in water emulsions and found that ΔF is affected by both temperature and different salt concentration. For example, with alkyl sulphates in the presence of LiCl, NaCl or KCl, the contact angle angle reached at a given concentration increases in the order of Li+ << Na+ < K+. The order reversed when alkyl sulphates were replaced with alkyl carbonates.
Figure 3 shows the huge effect temperature has on the contact angles of dodecane droplets in water. At high temperature > 65oC, contact angle 00 and the oil droplets behave as discrete spheres. At a temperature of 25oC on the other hand, the oil droplets flocculate with contact angle of about 700. The floc consists of highly deformed oil droplets with two types of border: 'linear' channels where three films meet at an angle of 1200 and 'tetrahedral' corner with four films meeting an angle of 109.470 in accordance to Plateau's laws.
It will be interesting to understand how ΔF is affected by salt concentrations and the authors presented a simple way to probe the value of ΔF. A simple DLVO theory cannot account for the contact angle observed when the cations are replaced with Li+, Na+ and K+ since all of them are single charged and DLVO theory predicts that the behaviour will be the same for all the cations. An understanding on the parameters that determine flocculation of colloids is important because increasingly, colloidal self-assembly has been used to create various colloidal structures such as colloidal crystal and inverse opals which exhibit interesting photonic behaviours.
1. Contact angles associated with thin liquid films in emulsions, Aronson and Princen, Nature 1980