Backflow-mediated domain switching in nematic liquid crystals

Turk, Janez; Svenšek, Daniel

doi:10.1103/physreve.89.032508

Cited by 5 publications

(2 citation statements)

References 53 publications

(60 reference statements)

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“…The ratio between these time scales measures the unsteadiness of the flow which is τ v τ n ∼ 10 −6 for the parameters given above. In problems involving strong electric fields, the time scales may become comparable [30]. In the present setting, however, the time scales are so different that we can consider that the fluid relaxes instantaneously when compared to the director field relaxation.…”

Section: Materials Parameters and Time Scalesmentioning

confidence: 88%

Dynamics of flowing 2D skyrmions

Coelho¹,

Tasinkevych²,

Gama³

2021

J. Phys.: Condens. Matter

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We investigate, numerically, the effects of externally imposed material flows on the structure and temporal evolution of liquid crystal (LC) skyrmions. The dynamics of a 2D system of skyrmions is modeled using the Ericksen–Leslie theory, which is based on two coupled equations, one for material flow and the other for the director field. As the time scales of the velocity and director fields differ by several orders of magnitude for realistic values of the system parameters, we have simplified the calculations by assuming that the velocity relaxes instantaneously when compared to the relaxation of the director field. Thus, we have used a finite-differences method known as artificial compressibility with adaptive time step to solve the velocity field and a fourth-order Runge-Kutta method for the director field. We characterized the skyrmion shape or configuration as a function of the time and the average velocity of the flow field. We found that for velocities above a certain threshold, the skyrmions stretch in the direction perpendicular to the flow, by contrast to the regime of weak flows where the skyrmions stretch along the streamlines of the flow field. These two regimes are separated by an abrupt (first-order) dynamical transition, which is robust with respect to e.g., the LC elastic anisotropy. Additionally, we have found how the presence of a second skyrmion affects the evolution of the shape of the skyrmions, by comparing the evolution of pairs of skyrmions to the evolution of a single-skyrmion.

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Section: Materials Parameters and Time Scalesmentioning

confidence: 88%

Dynamics of flowing 2D skyrmions

Coelho¹,

Tasinkevych²,

Gama³

2021

J. Phys.: Condens. Matter

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“…Backflow indicates the flow generation due to director reconfiguration. Backflow is demonstrated to importantly affect the dynamics in various dynamical processes, such as (i) the motion of topological defects in the defect coarsening dynamics [31][32][33] , (ii) the manipulation and anisotropic aggregation of colloidal particles 34,35 , (iii) the Non-Stokesian dynamics of colloidal particles 36 , and (iv) flow-enhancement and power requirement in pulsatile flows 37 . Backflow effects have been successfully implemented in driving flows of smectic liquid crystals [38][39][40] .…”

Section: Field Generated Nematic Microflows Via Backflow Mechanismmentioning

confidence: 99%

Field generated nematic microflows via backflow mechanism

Kos

Ravnik

2020

Sci Rep

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Generation of flow is an important aspect in microfluidic applications and generally relies on external pumps or embedded moving mechanical parts which pose distinct limitations and protocols on the use of microfluidic systems. A possible approach to avoid moving mechanical parts is to generate flow by changing some selected property or structure of the fluid. In fluids with internal orientational order such as nematic liquid crystals, this process of flow generation is known as the backflow effect. In this article, we demonstrate the contact-free generation of microfluidic material flows in nematic fluids -including directed contact-free pumping- by external electric and optical fields based on the dynamic backflow coupling between nematic order and material flow. Using numerical modelling, we design efficient shaping and driving of the backflow-generated material flow using spatial profiles and time modulations of electric fields with oscillating amplitude, rotating electric fields and optical fields. Particularly, we demonstrate how such periodic external fields generate efficient net average nematic flows through a microfluidic channel, that avoid usual invariance under time-reversal limitations. We show that a laser beam with rotating linear polarization can create a vortex-like flow structure and can act as a local flow pump without moving mechanical parts. The work could be used for advanced microfluidic applications, possibly by creating custom microfluidic pathways without predefined channels based on the adaptivity of an optical set-up, with a far reaching unconventional idea to realize channel-less microfluidics.

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Numerical prediction of the driving performance of liquid crystal actuators

Tsuji

Chono

2018

Applied Physics Letters

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To study the performance of liquid crystal actuators, we prepared a sandwich cell with a movable upper plate and drove this upper plate in its plane. To predict the driving performance of such an actuator, we proposed a simple one-dimensional model that combines the motion of the upper plate of the liquid crystal cell with the flow of the liquid crystal, and then, we compared the predicted motion of the plate with reported experimental results. The proposed model qualitatively predicted the motion of the upper plate. Using this model, we studied the rotation of the liquid crystal molecules and the velocity profiles between the two plates. When the applied voltage has a frequency of 1 Hz, the molecules between the two plates return completely to their initial angle when the electric field is released; at 10 Hz, the molecules do not return to their initial angle but instead return to approximately 40°; at 100 Hz, they oscillate around 90° with a small amplitude. At 10 Hz, the induced velocity profile is S-shaped, while at 20 Hz, the profile is double-S-shaped; this unusual behavior stems from the so-called kickback effect.

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Backflow-mediated domain switching in nematic liquid crystals

Cited by 5 publications

References 53 publications

Dynamics of flowing 2D skyrmions

Dynamics of flowing 2D skyrmions

Field generated nematic microflows via backflow mechanism

Numerical prediction of the driving performance of liquid crystal actuators

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