Electro-osmosis of nematic liquid crystals under weak anchoring and second-order surface effects

Abstract

Advent of nematic liquid crystals flows have attracted renewed attention in view of microfluidic transport phenomena. Among various transport processes, electroosmosis stands as one of the efficient flow actuation method through narrow confinement. In the present study, we explore the electrically actuated flow of a nematic fluid with ionic inclusions taking into account the influences from surface induced elastic and electrical double layer phenomena. Influence of surface effects on the flow characteristics is known to get augmented in micro-confined environment and must be properly addressed. Towards this, we devise the coupled flow governing equations from fundamental free energy analysis considering the contributions from first and second-order elastic, dielectric, flexoelectric, ionic and entropic energies. We have further considered weak anchoring surface conditions with second order elasticity which helps us to more accurately capture the director deformations along the boundaries. The present study focuses on the influence of surface charge and elasticity effects in the resulting linear electroosmosis through a slit-type microchannel whose surface are considered to be chemically treated in order to display a homeotropic-type weak anchoring state. An optical periodic stripe configuration of the nematic director has been observed especially for higher electric fields wherein the Ericksen number for the dynamic study is restricted to the order of unity.

Electrokinetic flow and electric conduction of salt‐free solutions in a capillary

“…where ψ (ρ ) was obtained in Eq. (6) or (9). An expression for the average fluid velocity in the capillary is…”

“…The results of the dimensionless relative surface potential Zeζ /kT = ψ (R) − ψ (0) calculated from Eqs. (6) and (9) for the exact and approximate solutions, respectively, are plotted versus σ in Fig. 3 for the salt-free solution.…”

“…(6) and (18) for the exact solution and Eqs. (9) and (19) for the approximate solution are plotted versus σ in Fig. 5.…”

“…A charged solid wall bounding an ionic solution forms an electric double layer at the interface, in which the diffuse ions carry a total charge equal in magnitude but opposite in sign to that of the wall. When an ionic solution filled in a charged narrow channel is subject to an external electric field or pressure gradient that interacts with the double layer, electrokinetic phenomena such as electroosmosis and streaming potential will occur [1][2][3][4][5][6][7][8][9]. Electrokinetic flows are of many fundamental and practical interests in a variety of scientific and technological areas.…”

Charge-, salt- and flexoelectricity-driven anchoring effects in nematics

“…Furthermore, the use of ions in liquid crystals was demonstrated in a recent experimental paper to affect electrostatically controlled anchoring of charged colloidal particles on a surface [16], and also indirectly by the balance of screened electrostatic interactions and elastic interactions to produce specific crystal structures [17]. Ions also influence the isotropic-nematic phase transition in lyotropic liquid crystals [18] and influences nonlinear electroosmosis in nematic liquid crystals [19]. On the other hand, flexoelectricity is predicted to stabilize blue plases [20,21] and is known to influence director profiles in nematic cells [22][23][24], colloidal transport [25], and flow [26,27].…”