  Theoretical and numerical models of active Janus particles commonly assume
that the metallo-dielectric interface is parallel to the driving applied
electric field. However, our experimental observations indicate that the
equilibrium angle of orientation of electrokinetically driven Janus particles
varies as a function of the frequency and voltage of the applied electric
field. Here, we quantify the variation of the orientation with respect to the
electric field and demonstrate that the equilibrium position represents the
interplay between gravitational, electrostatic and electrohydrodynamic torques.
The latter two categories are functions of the applied field (frequency,
voltage) as well as the height of the particle above the substrate. Maximum
departure from the alignment with the electric field occurs at low frequencies
characteristic of induced-charge electrophoresis and at low voltages where
gravity dominates the electrostatic and electrohydrodynamic torques. The
departure of the interface from alignment with the electric field is shown to
decrease particle mobility through comparison of freely suspended Janus
particles subject only to electrical forcing and magnetized Janus particles in
which magnetic torque is used to align the interface with the electric field.
Consideration of the role of gravitational torque and particle-wall
interactions could account for some discrepancies between theory, numerics and
experiment in active matter systems.
