where \(h\) is the actuator height at rest,\(l_{e}\left(\alpha\right)\) and \(l_{p}\left(\alpha\right)\) are
the length of the zipped and unzipped portions of the pouch,
respectively.
Since the liquid volume is constant during deformation, so is the area\(A\) in each 2D cross section. \(l_{e}\left(\alpha\right)\) and\(l_{p}\left(\alpha\right)\) are a function of the area \(A\) and of
the angle \(\alpha\)