Estimate the applied force (in Newtons) on a drop according to the
        electromechanical model [1].

            Ly is the length of the actuated electrode along the y-axis
                (perpendicular to the direction of motion) in milimeters. By
                default, use the square root of the actuated electrode area,
                i.e.,
                    Ly=Lx=sqrt(Area)
                To get the force normalized by electrode width (i.e., in units
                of N/mm), set Ly=1.0.

        1. Chatterjee et al., "Electromechanical model for actuating liquids in
           a two-plate droplet microfluidic device," Lab on a Chip, no. 9
           (2009): 1219-1229.

def force(self, Ly=None):
        '''
        Estimate the applied force (in Newtons) on a drop according to the
        electromechanical model [1].

            Ly is the length of the actuated electrode along the y-axis
                (perpendicular to the direction of motion) in milimeters. By
                default, use the square root of the actuated electrode area,
                i.e.,
                    Ly=Lx=sqrt(Area)
                To get the force normalized by electrode width (i.e., in units
                of N/mm), set Ly=1.0.

        1. Chatterjee et al., "Electromechanical model for actuating liquids in
           a two-plate droplet microfluidic device," Lab on a Chip, no. 9
           (2009): 1219-1229.
        '''
        if self.calibration._c_drop:
            c_drop = self.calibration.c_drop(self.frequency)
        else:
            c_drop = self.capacitance()[-1] / self.area
        if self.calibration._c_filler:
            c_filler = self.calibration.c_filler(self.frequency)
        else:
            c_filler = 0
        if Ly is None:
            Ly = np.sqrt(self.area)
        return 1e3 * Ly * 0.5 * (c_drop - c_filler) * self.V_actuation()**2