Abstract
Colloidal stability and growth processes (e.g., coalescence, Ostwald ripening) of oil droplets ultrasonically dispersed in water in the absence of any stabilizing agents (e.g., surfactants, amphiphilic polymers) (surfactant-free oil-in-water emulsions) were examined to evaluate the stabilization mechanism of oil-in-water emulsions and roles of stabilizing agents on colloidal stability. For example, surfactant-free oil droplets composed of longer-chain hydrocarbons were more stable in water than those of shorter-chain hydrocarbons. The addition of long-chain hydrocarbons (e.g., n-hexadecane, squalane) or hydrophobic polymers (e.g., polystyrene) into oil droplets enhanced colloidal stability of surfactant-free oil droplets dispersed in water. This paper summarizes advances on surfactant-free emulsion systems; (i) colloidal stability and growth processes of surfactant-free oil droplets dispersed in water, (ii) development of single-droplet detection technique for monitoring the droplet growth, (iii) the colloidal stabilization of oil-in-water emulsions with the addition of long-chain hydrocarbons or hydrophobic polymers into droplets, and with sequential megasonic irradiation and (iv) nano- and micro-scale material fabrication using surfactant-free emulsions
