Non-equilibrium ordering of liquid crystalline (LC) films driven by external gradients in surfactant concentration

Soumita Maiti; Sangchul Roh; Itai Cohen; Nicholas L Abbott

doi:10.1016/j.jcis.2022.12.124

Non-equilibrium ordering of liquid crystalline (LC) films driven by external gradients in surfactant concentration

J Colloid Interface Sci. 2023 May:637:134-146. doi: 10.1016/j.jcis.2022.12.124. Epub 2022 Dec 24.

Authors

Soumita Maiti¹, Sangchul Roh², Itai Cohen³, Nicholas L Abbott⁴

Affiliations

¹ Smith School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY 14853, USA. Electronic address: sm2766@cornell.edu.
² Smith School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY 14853, USA. Electronic address: sr974@cornell.edu.
³ Laboratory of Atomic and Solid-State Physics, Cornell University, Ithaca, NY 14853, USA; Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, NY 14853, USA. Electronic address: itai.cohen@cornell.edu.
⁴ Smith School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY 14853, USA. Electronic address: nla34@cornell.edu.

PMID: 36696789
DOI: 10.1016/j.jcis.2022.12.124

Abstract

Hypothesis: Gradients in the concentration of amphiphiles play an important role in many non-equilibrium processes involving complex fluids. Here we explore if non-equilibrium interfacial behaviors of thermotropic (oily) liquid crystals (LCs) can amplify microscopic gradients in surfactant concentration into macroscopic optical signals.

Experiments: We use a milli-fluidic system to generate gradients in aqueous sodium dodecyl sulfate (SDS) concentration and optically quantify the dynamic ordering of micrometer-thick nematic LC films that contact the gradients.

Findings: We find that the reordering of the LCs is dominated by interfacial shearing by Marangoni flows, thus providing simple methods for rapid mapping of interfacial velocities from a single optical image and investigating the effects of confinement of surfactant-driven interfacial flows. Additionally, we establish that surface advection and surfactant desorption are the two key processes that regulate the interfacial flows, revealing that the dynamic response of the LC can provide rapid and potentially high throughput approaches to measurement of non-equilibrium interfacial properties of amphiphiles. We also observe flow-induced assemblies of microparticles to form at the LC interface, hinting at new non-equilibrium approaches to microparticle assembly. We conclude that dynamic states adopted by LCs in the presence of surfactant concentration gradients provide new opportunities for engineering complex fluids beyond equilibrium.

Keywords: Interfacial (Marangoni) flows; Liquid crystals; Optical mapping; Surfactants.