Controllable particle migration in liquid crystal flows

Abstract

We observe novel positional control of a colloidal particle in microchannel flow of a nematic liquid crystal. Lattice Boltzmann simulations show multiple equilibrium particle positions, the existence and position of...

Defect-influenced particle advection in highly confined liquid crystal flows

“…This can be explained with the migration ( i.e. lateral movement perpendicular to the flow direction) to the weak attractor region that we observed in our previous work on controllable particle migration 33 in practically unconfined conditions using a much wider duct and lower confinement ratio 2 R / L x = 0.15. For direct comparison we provide in Table 2 an approximate conversion between particle Ericksen numbers, as used our previous publication, and Ericksen numbers based on the smallest duct dimension, as used in this work.…”

“…4, for instance in the third row, and retains a similar shape at higher Ericksen numbers (see images for 2 R / L x = 0.4, Er = 18.10, 2 R / L x = 0.6, Er = 21.25, and 2 R / L x = 0.8, Er = 22.32). The case for 2 R / L x = 0.4, Er = 18.10 forms an exception in that the particle moves very slightly away from the centre into a stable off-centre position, while in the other cases the particle remains at the centre of the duct, which can be also understood with the migration to the previously observed weak attractor region at similar Ericksen numbers 33 (see Table 2 for conversion of Ericksen numbers). A noticeable difference is that with increasing confinement the defect ring appears compressed in the smallest duct dimension due to the relative proximity of the walls (see image and Movie S2 (ESI†) for 2 R / L x = 0.8, Er = 22.32).…”

“…To achieve this, we assign the lattice spacing Δ x , the algorithmic time step Δ t and the reference pressure p * from unity in lattice Boltzmann units (LBU) to their corresponding values in SI units. The principle of this parameter mapping was also shown in our previous work 33 using a different characteristic length scale.…”

“…At higher Ericksen numbers Er = 64.85 and Er = 86.06 the particle migrates readily to one of the walls 33 and the shape of the defect changes markedly (see Fig. 7 first and third row, second and third column).…”

“…The Ericksen number characterises the ratio of viscous to elastic forces and is defined aswhere u is a characteristic flow velocity, in our case the velocity at the centre of the duct U c , η is the dynamic viscosity, Λ is a characteristic length scale which is set by the narrowest gap size L x (see Table 2 for Λ = 2 R , which allows direct comparison with our previous work 33 ), and κ is the bulk elastic constant of the liquid crystal.…”