Entry chosen by Kelly Miller
The zeta potential is a parameter that characterizes electrochemical equilibrium of interfaces. It is the electric potential in the interfacial double layer at the location of the slipping plane (see figure 1 below) versus a point in the bulk fluid away from the interface. In other words, zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle.
Image Reference: http://www.dispersion.com/zeta-potential.html
The zeta potential is a value that can be related to the stability of colloidal dispersions as it plays a key role in the theory of aggregative stability. The zeta potential determines the electrostatic repulsion between particles. The higher the zeta potential, the stronger the repulsion and the more stable the system becomes. For example, it is the high zeta potential of the fat droplets in milk that prevents them against aggregation. Adding acid to milk reduces the zeta potential and this leads to cheese formation from aggregate droplets.
The Zeta potential is a property of an electric structure that is built up at interfaces referred to as the electric double layer. There is a build up of an electric surface charge at a charged surface which creates an electrostatic field that then affects the ions in the bulk of the liquid. This electrostatic field creates a countercharge and this screens the electric surface charge. The net electric charge in the screening diffuse layer is equal in magnitude to the net surface charge but is of opposite charge. Consequently, the complete structure is electrically neutral.
Along with the Debye length, another important distance to consider within the electric double layer is the "slipping plane" which is associated with tangential motion of the liquid relative to the surface. While ions in the slipping plane can move tangentially to the charged surface, ions underneath the slipping plane remain attached to the surface (and this layer is referred to as the Stern Layer). Electric potential corresponding to the slipping plane is the " Zeta potential ".