Jetting and Droplet Formation Driven by Interfacial Electrohydrodynamic Effects Mediated by Solitons in Liquid Crystals

Soumik Das, Noe Atzin, Xingzhou Tang, Ali Mozaffari, Juan de Pablo, and Nicholas L. Abbott

Phys. Rev. Lett. 131, 098101 – Published 1 September 2023

Abstract

Solitons are highly confined, propagating waves that arise from nonlinear feedback in natural (e.g., shallow and confined waters) and engineered systems (e.g., optical wave propagation in fibers). Solitons have recently been observed in thin films of liquid crystals (LCs) in the presence of ac electric fields, where localized LC director distortions arise and propagate due to flexoelectric polarization. Here we report that collisions between LC solitons and interfaces to isotropic fluids can generate a range of interfacial hydrodynamic phenomena. We find that single solitons can either generate single droplets or, alternatively, form jets of LC that subsequently break up into organized assemblies of droplets. We show that the influence of key parameters, such as electric field strength, LC film thickness, and LC-oil interfacial tension, map onto a universal state diagram that characterizes the transduction of soliton flexoelectric energy into droplet interfacial energy. Overall, we reveal that solitons in LCs can be used to focus the energy of nonlocalized electric fields to generate a new class of nonlinear electrohydrodynamic effects at fluid interfaces, including jetting and emulsification.

Received 14 March 2023
Accepted 7 July 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.098101

Physics Subject Headings (PhySH)

Ballistic transport Collective behavior Distortions & defects Elastic forces Electric field effects Functional materials Localization Pattern formation

Liquid crystals Solitons

Polymers & Soft Matter

Authors & Affiliations

Soumik Das ¹, Noe Atzin², Xingzhou Tang ², Ali Mozaffari², Juan de Pablo², and Nicholas L. Abbott ^1,*

¹Smith School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853, USA
²Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA

^*nla34@cornell.edu

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Issue

Vol. 131, Iss. 9 — 1 September 2023

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Images

Figure 1
(a),(b) Schematic illustration depicting the (a) azimuthal and (b) polar LC director orientations within a soliton propagating along the direction of the dotted arrow (c)–(f) Timed snapshots (crossed polars and a red plate compensator inserted at 45° from the polars) showing formation of a LC droplet following collision of a soliton with the LC-isotropic oil interface. Insets in (f) show bright-field and cross-polar images of the LC droplet. The applied voltage is 517 Hz, 50 V. Green arrows indicate the direction of gold deposition. Scale bar $30 μ m$ . (g) Number of generated LC drops as a function of number of solitons hitting the interface at different applied electric fields. (h) Size distribution of generated LC drops.
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Figure 2
(a)–(f) Bright-field images showing the nematic CCN-47–squalane interface following the collision of a single soliton with the interface. The brown arrows in (a) and (b) indicate the incident soliton traveling at an angle 45° with respect to the interface. The dashed line in (a) indicates the CCN-47–squalane interface. The dashed line in (b) and (c) indicates the centerline of the soliton. The left wing of the soliton collides first with the interface and forms a jet indicated by the brown curved arrow in (c). The droplet generated by the soliton collision is indicated by arrows in (e) and (f). Scale bar $15 μ m$ . The black arrows in (c) and (d) point to LC director perturbations in the LC that are generated by the right wing of the soliton. (g),(h) Cross-polar images showing the director perturbations in the LC near the interface after a collision with a soliton. The white arrows in (g) and (h) indicate the propagating soliton and the LC jet, respectively. The yellow arrow in (h) shows the director perturbations created by the right wing of the soliton. Scale bar $15 μ m$ . Image contrast in (a)–(h) has been enhanced for visualization. (i) Sizes of LC drop generated for two different LC film thicknesses. The red and blue bars correspond to $8 − μ m$ - and $12 − μ m$ -thick LC films.
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Figure 3
(a),(b) LC droplet sizes generated by solitons as a function of $γ_{LC − oil}$ and two values of $d_{LC}$ : (a) $8 μ m$ and (b) $12 μ m$ . The numbers above the bars indicate the ratio of $E_{surface}$ to $E_{flexo}$ . (c) LC droplet sizes generated by solitons, as a function of applied voltage. The blue bar corresponds to the chaotic emulsification regime. (d) State diagram showing the final state of the interface as a function of the relative magnitudes of normalized interfacial and flexoelectric energies.
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Figure 4
(a) Formation of LC droplets in the presence of the chaotic striped phase. Inset in (a) shows the droplets. Scale bar $10 μ m$ . (b)–(g) Bright-field images showing the nematic CCN-47–squalane interface following a collision with a soliton at glancing (75°) incidence. The dashed rectangle in (b) shows the incident soliton moving in a direction indicated by the black arrow. After the collision of the soliton with the LC interface, a long filament of LC is formed parallel to the interface, as shown in (c)–(e). In (e), the onset of breakup of the filament is evident (black arrow). The LC droplets are indicated by black arrows in (e)–(g). During growth of the LC jet, director perturbations [red arrows in (e) and (f)] propagate along the LC side of the interface. Brown arrows in (d)–(g) indicate the collision of a second soliton with the LC interface. Scale bar $30 μ m$ . The optical contrast in the images was enhanced for ease of visualization. (h)–(k) Snapshots (cross polar) showing the reflection of a soliton that collides with the CCN-47–squalane interface at normal incidence. Scale bar $30 μ m$ .
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