Lehmann rotation of cholesteric droplets driven by Marangoni convection

Abstract

We show experimentally and theoretically that the Lehmann effect recently observed by Yoshioka and Araoka (Nat. Commun., 2018,9, 432) in emulsified cholesteric liquid crystal droplets under temperature gradients is due to Marangoni flows rather than to the thermomechanical or chemomechanical couplings often invoked to explain the phenomenon.

Role of an oscillatory electric field on the Lehmann rotation of cholesteric droplets

“…In this formula, δn δθ = ∂n ∂θ − e z × n, by denoting by e z the unit vector along the vertical z-axis and θ the polar angle in cylindrical coordinates (r, θ, z), Ω = 1 2 ∇×v is the local rotation rate, A is the symmetric strain rate tensor This general formula is derivated in Ref. [13] and remains valid when an electric field is applied as can be easily checked by adding the electric energy to the elastic and surface anchoring energies in the derivation and by replacing the elastic molecular field h by h + h E , where Auernhammer et al by using tracer colloids (from Ref. [14]).…”

“…Other models were recently proposed such as the meltinggrowth model to explain the rotation in the coexistence region [12] and a pure hydrodynamical model to explain the rotation in the emulsions [13]. Despite these efforts, the Lehmann effect is not yet completely understood [11] and further experimental investigations are still desirable.…”

Differential rotation in cholesteric pillars under a temperature gradient

“…In addition, the assumed director field could have a slight incompatibility with the real structure. Particularly, in the present study, the interface between LC and the solvent has a weak anchoring property 16,18,26,27 , which can deform the helical structure around the edge region of the pillar. For these reasons, the rotational torque and behaviour in Region-III may become different from the theoretical consideration.…”

“…photovariable helical winding 7 , dynamic topological solitons [8][9][10] or steady rotational motions in droplets [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] , upon various external stimuli, such as temperature gradient, electric field, material flow and light irradiation. Recently, highly stable and efficient rotation of Ch LC droplets under a temperature gradient have been reported in dispersion systems [12][13][14][15][16][17][18][19] . Although the detailed mechanism of the stable and efficient rotation in these systems is not yet clear, two main rotational modes have been suggested: one is the collective rotation of the local director and the other is the rigid-body rotation of the whole droplet [12][13][14][17][18][19] .…”

“…Recently, highly stable and efficient rotation of Ch LC droplets under a temperature gradient have been reported in dispersion systems [12][13][14][15][16][17][18][19] . Although the detailed mechanism of the stable and efficient rotation in these systems is not yet clear, two main rotational modes have been suggested: one is the collective rotation of the local director and the other is the rigid-body rotation of the whole droplet [12][13][14][17][18][19] . However, in the present open 1 RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.…”