Coplanar dual-frequency EWOD used as an actuator in a microsystem. Liquid surface detection at laser wavelength scale in a microsystem. Laser interferometry used at drop apex. Contact line friction coefficient identified from apex motion and vibrations model. Here is presented an interferometry technique to quantify damping of electrowetting induced shape oscillations of a sessile microdrop. This characterization method and its devoted chip can be inserted into a microsystem to quantify contact line friction and hence surface ageing in digital electrowetting applications (lab-on-chips, microfluidic lenses and displays). Here, the contact line friction is proposed as a criteria to distinguish sensor surfaces contaminated by adsorbed bio-molecules.