Abstract: An electrostatic actuator (10) comprising: a first electrode (12); a second electrode (14), which is flexible or arcuate in shape; an insulator (16), the second electrode being coupled to the first electrode via a first coupling so as to define a free portion of the second electrode that can move, the first coupling between the electrodes defining a first zipping crevice (ZC), wherein the actuator further comprises a mobile dielectric (19) located within the first zipping crevice to increase the field force between the electrodes in the region of the first zipping crevice, the mobile dielectric being arranged to be moved with the zipping crevice as it propagates along the actuator due to flexing of the second electrode towards the first electrode.