Microfluidic control over Nematic Liquid Crystal Flows – A short review

The authors investigate how fluids with nematic order (nematic liquid crystal) form orientational structures in shear flow. This work originates from the article: “Microfluidic control over topological states in channel-confined nematic flows” published in Nature Communications and written by Simon Čopar, Žiga Kos, Tadej Emeršič & Uroš Tkalec.

Abstract – Microfluidic Control over Nematic Liquid Crystal Flows

Orientational order of nematic liquid crystals (NLCs), unlike isotropic liquids, enhances the influence of their rheological properties, and thus requires precise tuning of the flow parameters to control the orientational patterns.

Discontinuous nematic transitions arise from perpendicular structure at low flow rates, to flow-aligned structure at high flow rates, in microfluidic channels with perpendicular surface alignment. The precise tuning of the driving pressure is used to stabilize and to manipulate the topologically protected chiral intermediate state that occurs before the transition from homeotropic to flow-aligned.

The mechanisms underlying the transition are identified. A phenomenological model was built 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 introducing a chiral dopant. Finally, the transitions can be triggered through channel geometry and fine control of the flow rate.

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Introduction – Microfluidic control over Nematic Liquid Crystal Flows

Nematic liquid crystals (NLCs) can be described as fluids with the orientational order of their anisotropic building blocks. This property impacts fluid rheological behavior when confined to thin planar cells or a network of channels [1-2].

Confined nematic microflows can be employed in various applications from optofluidic [3-5] to guided material transport through microfluidic networks [6-7]. The combination of nematic orientational order and the flow induces structural organization and hydraulic conductivity in porous networks [8-9] and shapes the molecular field by Poiseuille flows [10].

Microfluidics allows for rheological fluid restriction: the focus can be given to the study of the fluid properties in a confined environment [11]. This is of interest for complex fluids such as non-Newtonian, anisotropic and active fluids [12-15]. Indeed, complex fluids require much finer control in order to observe the desired non-equilibrium states in shear flow.

Aims & Objectives

• Investigation of nematic liquid crystals flows in thin microfluidic channels with perpendicular surface anchoring of nematic molecules on the channel walls.
• Report of topologically protected flow regime with a broken mirror symmetry in a well-controlled and reproducible manner.
• Study of the mechanisms connecting the behaviour of the NLC with its material parameters by numerical modelling.

This video shows chiral bowser states with a soliton in between form. Courtesy of Uroš Tkalec. https://creativecommons.org/licenses/by/4.0/.

This video describes a chiral bowser states with a soliton in between form which gradually goes back to the normal bowser state. Courtesy of Uroš Tkalec. https://creativecommons.org/licenses/by/4.0/.

Key findings – Microfluidic control over nematic liquid crystal flows

Brief materials & methods

A PDMS chip with a rectangular cross section with height h≈12 μm, width w≈100 μm and length L≈20 mm was used throughout the whole experiment.

The authors have used the OB1 Mk3 (Elveflow) pressure-driven flow controller to ensure a fine tuning of the flow and thus, a fine tuning of the resulting rheological properties of the fluid as represented in Fig 1. The flow rates ranged from 0.05 to 0.85 μL/h. Controller inbuilt flow profile routines gradually increased and subsequently maintained the applied driving pressure.

Glimpse of results

The authors identified the orientational phase transition from a perpendicular structure at low flow rates, to a flow-aligned structure at high flow rates, in microfluidic channels with perpendicular surface alignment as presented in Fig 2.

This work demonstrated techniques of experimental control of a NLC flow in microconfinement to predictably produce the desired orientational regimes and transitions between them. The study was focused more specifically on the secondary orientational transition in the shadow of the main flow-aligning transition of nematic materials in homeotropic channels. This delicate state breaks the mirror symmetry, and presents rich dynamics of its left and right-handed domains and the solitons between them.

The flow velocity is the main parameter driving the transitions. It can be tuned by shape of the channel and by the driving pressure applied. A topological description of the director profiles was obtained before and after the transition. Numerical simulations demonstrated the effect of the twist elastic constant and its key role in stabilizing the chiral state.

Find out more…

To know more about these exciting results, please check the original paper : “Microfluidic control over topological states in channel-confined nematic flows” published in Nature Communications and written by Simon Čopar, Žiga Kos, Tadej Emeršič & Uroš Tkalec.

This work explores how fluids with nematic order form orientational structures in shear flow, where precise control of otherwise possibly highly irregular flow conditions is utilized by a microfluidic confinement. This accurate control is ensured by the use of pressure-driven flow controller OB1 Mk3 Elveflow.

References