Liquid crystal-enabled electro-osmosis through spatial charge separation in distorted regions as a novel mechanism of electrokinetics

Abstract

Electrically controlled dynamics of fluids and particles at microscales is a fascinating area of research with applications ranging from microfluidics and sensing to sorting of biomolecules. The driving mechanisms are electric forces acting on spatially separated charges in an isotropic medium such as water. Here, we demonstrate that anisotropic conductivity of liquid crystals enables new mechanism of highly efficient electro-osmosis rooted in space charging of regions with distorted orientation. The electric field acts on these distortion-separated charges to induce liquid crystal-enabled electro-osmosis. Their velocities grow with the square of the field, which allows one to use an alternating current field to drive steady flows and to avoid electrode damage. Ionic currents in liquid crystals that have been traditionally considered as an undesirable feature in displays, offer a broad platform for versatile applications such as liquid crystal-enabled electrokinetics, micropumping and mixing.

Figure 1. Nonlinear electro-osmotic flows in isotropic and LC electrolytes.

(a) ICEO around a conductive particle in water. (b) and (c) LCEO caused by director distortions around a non-conductive particle with perpendicular (b) and tangential (c) anchoring. The electric field accumulates ions at the opposite sides of the particle. The separated charges are forced by the electric field into LCEO motion with directions shown by large curved arrows. The LCEO flows are quadrupolar in both (b) and (c), but of opposite directionality, of a puller (b) and pusher (c) type. (d) Dipolar symmetry of the director around a perpendicularly anchored particle breaks the fore-aft symmetry of LCEO flows, pumping the LC from right to left.

Figure 2. Streamlines and velocity fields of LCEO around glass spheres.

(a,b) perpendicular anchoring with quadrupolar director patterns; (c,d) bipolar sphere with tangential anchoring. Velocities in (b) and (d) are of opposite polarity, creating patterns of a puller (b) and a pusher (d) type. (e,f) sphere with perpendicular anchoring and dipolar director pattern; note pumping from right to left in (f). Particle diameter 2a=50 μm, AC electric field E=26 mV μm⁻¹.

Figure 3. Quantitative parameters of LCEO around glass spheres.

(a) Maximum LCEO velocity around tangentially anchored (circles) and normally anchored (diamonds) spheres increases linearly with the diameter 2a; E=26 mV μm⁻¹; in both cases the director pattern is quadrupolar. (b) Maximum LCEO velocity around a tangentially anchored sphere of diameter 2a=50 μm grows as E². (c) Volume of LC flow around a sphere with a hedgehog (2a=50 μm), Fig. 2f, passing along the x (Q_x(x)) and y (Q_y(y)) axes. The LC is pumped from right to left.

Figure 4. Fourier analysis of velocity for quadrupolar and dipolar distortions.

(a) Radial and (b) azimuthal coefficients for LCEO flows triggered by a quadrupolar director pattern, Fig. 2b; (c) velocity field reconstructed with the three harmonics that show a close similarity to the experimental data in Fig. 2b; (d–f), the same, but for a sphere with a dipolar director and LCEO pumping, compare with Fig. 2f. Particle diameter 2a=50 μm.

Figure 5. Electric field in the nematic cell with two aluminium foil electrodes.

(a) Electric field pattern; (b) Dependencies of the local field on the location within the nematic slab. Cell thickness h=60 μm. Thickness of each glass plates 1.1 mm.