We describe a new actuation technique for gravity experiments based on a liquid field mass. The characterizing idea is to modulate the gravity force acting on a test mass by controlling the level of a liquid in a suitable container. This allows to obtain a periodical gravity force without moving parts (except the liquid level) close to the TM. We describe in detail the most relevant aspects of the liquid actuator and discuss how it can be used in gravity experiments. In particular, we analyze an application to test the inverse square law in the mm to cm distance region.