Hydrophobicity and Contact Angle
The Figure below shows a drop of water on the surface of two different materials: hydrophilic (a) and hydrophobic (b). You can see how the water is spread more on the hydrophilic surface than on the hydrophobic one. The contact angle, q, is a measure of this property; a hydrophilic material has a small contact angle; for a material which hydrophobic, on the contrary, the contact angle is greater, generally more than 90o.
Generally, for a Pickering emulsion with water as continuous phase, a hydrophilic material will work better than a hydrophobic one. For an oily continuous phase, on the contrary, a hydrophobic material is preferable.
Kinetic Study of Emulsion Formation
Although the principles regulating Pickering emulsions are known, there is little knowledge about the kinetics of these processes; that is of the time necessary for the emulsion to reach equilibrium. A study on this topic was performed recently by professor Manoharan and his coworkers, from the Harvard School of Engineering and Applied Sciences .
The team studied the evolution of an emulsion with time; this emulsion was made of oily and watery fluids (decane, and water containing glycerol and salt, respectively) in the presence of microspheres of polystyrene.
Advanced Microscopic Technique
To observe the interaction of the polystyrene spheres with the oil-water interface, Professor Manoharan and his coworkers used holographic microscopy; this is an innovative microscopic technique, based on the interference and the diffraction of an appropriate laser radiation with the observed object(s). This allowed them to observe the behavior of the particle and of the fluids with high spatial and time resolution.