  Suspended droplets of cholesteric (chiral nematic) liquid crystals
spontaneously rotate in the presence of a heat flux due to a temperature
gradient, a phenomenon known as Lehmann effect. So far, it is not clear whether
this effect is due to the chirality of the phase and the molecules or only to
the chirality of the director field. Here, we report the continuous rotation in
a temperature gradient of nematic droplets of a lyotropic chromonic liquid
crystal featuring a twisted bipolar configuration. The achiral nature of the
molecular components leads to a random handedness of the spontaneous twist,
resulting in the coexistence of droplets rotating in the two senses, with
speeds proportional to the temperature gradient and inversely proportional to
the droplet radius. This result shows that a macroscopic twist of the director
field is sufficient to induce a rotation of the droplets, and that the phase
and the molecules do not need to be chiral. This suggests that one can also
explain the Lehmann rotation in cholesteric liquid crystals without introducing
the Leslie thermomechanical coupling -- only present in chiral mesophases. An
explanation based on the Akopyan and Zeldovich theory of thermomechanical
effects in nematics is proposed and discussed.
