Stationary devices employing quadrature phase analysis light scattering are provided, to aid in the determination of the magnitude and polarity of electrophoretic mobility and zeta potential of particles in colloids. The devices use an optical quadrature interferometer with an electrophoresis sample chamber containing sample particles undergoing electrophoresis, the optical quadrature interferometer being configured to generate a quadrature signal. The phase of the quadrature signal may be analyzed at the frequency of the sample chamber electric field to estimate displacements and directions of the particles. The estimates can be used to determine a central value of the magnitude of the electrophoretic mobility, as well as its polarity. Particles having low electrophoretic mobility, or that may be adversely affected by high electric fields, can be analyzed, and constraints on vibration and light source coherence length may be relaxed. A phase modulator or frequency shifter is not required.