Omnidirectional transport and navigation of Janus particles through a nematic liquid crystal film

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Omnidirectional transport and navigation of Janus particles through a nematic liquid crystal film

Dinesh Kumar Sahu, Swapnil Kole, Sriram Ramaswamy, and Surajit Dhara

Phys. Rev. Research 2, 032009(R) – Published 8 July 2020

Abstract

We create controllable active particles in the form of metal-dielectric Janus colloids which acquire motility through a nematic liquid crystal film by transducing the energy of an imposed perpendicular AC electric field. We achieve complete command over trajectories by varying the field amplitude and frequency, piloting the colloids at will in the plane spanned by the axes of the particle and the nematic. The underlying mechanism exploits the sensitivity of electro-osmotic flow to the asymmetries of the particle surface and the liquid crystal defect structure. We present a calculation of the dipolar force density produced by the interplay of the electric field with director anchoring and the contrasting electrostatic boundary conditions on the two hemispheres, which accounts for the dielectric-forward (metal-forward) motion of the colloids due to induced puller (pusher) force dipoles. These findings open unexplored directions for the use of colloids and liquid crystals in controlled transport, assembly, and collective dynamics.

Received 29 November 2019
Revised 20 February 2020
Accepted 17 June 2020

DOI:https://doi.org/10.1103/PhysRevResearch.2.032009

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Applications of soft matter Electric field effects Topological defects

Complex fluids Janus particles Living matter & active matter Nematic liquid crystals

Polymers & Soft Matter

Authors & Affiliations

Dinesh Kumar Sahu¹, Swapnil Kole², Sriram Ramaswamy ², and Surajit Dhara ^1,*

¹School of Physics, University of Hyderabad, Hyderabad 500 046, India
²Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560 012, India

^*sdsp@uohyd.ernet.in

Article Text

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Supplemental Material

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References

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Issue

Vol. 2, Iss. 3 — July - September 2020

Subject Areas

Soft Matter

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Images

Figure 1
(a) Quadrupolar distortion of the nematic director around a spherical particle. The red circle represents the Saturn-ring defect. (b) Janus quadrupolar particles in a planar cell with an AC field. The dark hemisphere represents metal. (c) Optical microscope texture of a Janus quadrupolar particle in the NLC between crossed polarizer (P) and analyzer (A) in the $x y$ plane. Inset: Texture with a $λ$ plate (530 nm). (d) Time-coded trajectories ( $T_{\min} = 0$ s to $T_{\max} = 5$ s), labeled from I to VIII, under an AC electric field (1.54 $V μ m^{- 1}$ , 30 Hz). Trajectories of selected particles are grouped. Pink arrows denote the direction of motion. (See Supplemental Material Movie S1 [25]). The direction of the electric field ( $\vec{E}$ ) and the director ( $\hat{n}$ ) for all trajectories are shown in the center. Cell thickness: 5.2 $μ m$ .
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Figure 2
Quadrupolar director field surrounding a Janus particle. The applied electric field $E$ is transverse to the director. The red circle denotes the Saturn-ring defect. The black hemisphere represents metal.
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Figure 3
Liquid-crystal-enabled electro-osmotic flows around a quadrupolar Janus particle in two orthogonal planes parallel to the electric field. Red circles represent Saturn-ring defects. The black hemisphere represents metal. The large curved arrows around the metal hemisphere indicate stronger flows. The off-center force dipoles $F$ (shown in green) are (a) pullers and (b) pushers. The propulsion direction of the particles is indicated by a pink arrow for (a) trajectory III/VII and (b) trajectory I/V.
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Figure 4
(a) Change in the direction of motion of a quadrupolar Janus particle upon altering the field amplitude recursively between 1.5 $V μ m^{- 1}$ (purple) to 2.0 $V μ m^{- 1}$ (yellow), keeping the frequency constant at 30 Hz. (See Supplemental Material Movie S2 [25].) Arrows indicate the direction of motion. (b) Time-coded trajectories of a particle at different field amplitudes and a fixed frequency (30 Hz). (c) The angle between the trajectory and the director ( $θ$ ) decreases linearly with the field at a slope of $- 43 . 2^{\circ} \pm 2 . 3^{\circ} μ {m V}^{- 1}$ . (d) Change in the direction of motion of a particle upon altering the frequency recursively between 30 Hz (purple) and 50 Hz (yellow), keeping the field amplitude constant at $1.5 V μ m^{- 1}$ . (See Supplemental Material Movie S3 [25].) (e) Time-coded trajectories of a particle at different frequencies and a fixed amplitude (1.5 $V μ m^{- 1}$ ). (f) $θ$ increases linearly with $f$ at a slope of $1^{\circ} \pm 0 . 05^{\circ} {Hz}^{- 1}$ . By “relative coordinate” we mean relative to the starting point of each trajectory. Error bars represent the standard deviation of the mean value. Cell thickness: 5.2 $μ m$ .
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Figure 5
(a) A particle with $φ = 135^{\circ}$ is piloted to a predetermined place by changing the amplitude of the field (see Supplemental Material Movie S4 [25]). The field amplitude is increased in a continuous manner from 1.2 to 2.0 $V μ m^{- 1}$ , keeping the frequency fixed at 30 Hz. (b) A particle with $φ = 315^{\circ}$ is transported to a predetermined place by increasing the frequency in a continuous manner from 20 to 90 Hz, keeping the amplitude fixed at 1.6 $V μ m^{- 1}$ (see Supplemental Material Movie S5 [25]). Arrows near the particles show the initial direction of motion. Cell thickness: 5.2 $μ m$ .
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Figure 6
(a) Dependence of the velocity components $V_{x}$ and $V_{y}$ of a Janus particle [trajectory II; Fig. 1] on the electric-field amplitude $E$ at $f = 30$ Hz. Solid lines show least-squares fits to $V \propto E^{2}$ . The slopes of $V_{x}$ and $V_{y}$ are 3.2 and 2.2 $μ m^{3} V^{- 2} s^{- 1}$ , respectively. (b) Frequency-dependent ( $E = 1.54 V μ m^{- 1}$ ) velocity components $V_{x}$ and $V_{y}$ . Red and blue lines show the theoretical fit to Eq. (2). Fit parameters are $τ_{e} = 0.25$ s, $τ_{p} = 0.032$ s for $V_{x}$ and $τ_{e} = 0.08$ s, $τ_{p} = 0.04$ s for $V_{y}$ . Error bars represent the standard deviation of the mean value. Cell thickness: 5.2 $μ m$ .
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