Microfluidic control over topological states in channel-confined nematic flows

Abstract

Compared to isotropic liquids, orientational order of nematic liquid crystals makes their rheological properties more involved, and thus requires fine control of the flow parameters to govern the orientational patterns. In microfluidic channels with perpendicular surface alignment, nematics discontinuously transition from perpendicular structure at low flow rates to flow-aligned structure at high flow rates. Here we show how precise tuning of the driving pressure can be used to stabilize and manipulate a previously unresearched topologically protected chiral intermediate state which arises before the homeotropic to flow-aligned transition. We characterize the mechanisms underlying the transition and construct a phenomenological model to describe the critical behaviour and the phase diagram of the observed chiral flow state, and evaluate the effect of a forced symmetry breaking by introduction of a chiral dopant. Finally, we induce transitions on demand through channel geometry, application of laser tweezers, and careful control of the flow rate.

In Situ Visualization of the Structural Evolution and Alignment of Lyotropic Liquid Crystals in Confined Flow

“…[ 50 ] In simple shear flow, at high shear rates where viscous forces dominant (large Er ), the director was aligned in the flow direction, whereas at low shear rates (small Er ) where elastic forces are dominant, lyotropic chromonic liquid crystals were found oriented perpendicularly to the flow direction. [ 45,46 ] For pressure‐driven channel flows, such as the ones investigated in this work, where one characteristic velocity would determine the Er of the channel flow, [ 51 ] it is important to note that locally in the channel cross section there is a velocity gradient and as a consequence the shear rate varies locally. [ 52 ] Thus, near the channel walls, where the velocity gradients are high, viscous forces would be expected to dominate resulting in a director orientation in the flow direction.…”

Periodic Arrays of Chiral Domains Generated from the Self-Assembly of Micropatterned Achiral Lyotropic Chromonic Liquid Crystal

“… 7 − 10 For instance, chiral symmetry breaking happens when LC molecules have bent shapes 11 − 13 or are placed in confined geometries. 14 − 17 Among the various kinds of LC types, lyotropic chromonic LCs (LCLCs), dissolved in water, are frequently used to explore spontaneous chiral symmetry breaking. 18 − 23 LCLC molecules have rigid planar shapes due to polyaromatic cores, which spontaneously aggregate “face-to-face” through π–π stacking interaction to form columns when they are dissolved in an aqueous medium.…”

Novel optofluidic concepts enabled by topological microfluidics-INVITED

“…In addition to the exotic and emergent dynamical attributes of anisotropic fluids [13,14], LCs furnish topological defects due to spontaneous symmetry breaking [15,16]: such defects offer strong prospects as labile and reconfigurable optofluidic elements [17,18]. Conflicting boundary conditions, in combination with geometric constraints and appropriate external fields (for instance hydrodynamic), allow tailored generation of topological defects in microfluidic environments [19][20][21], thus paving the way for the next generation of dynamic, flowstabilized, defect-based optofluidic platforms [22].…”