Interfacial Assembly at the Liquid-liquid Interface for Soft-devices Fabrication

Abstract

In the domain of natural materials, governed by principles of energy minimization and interfacial activity, spontaneous self-arrangement manifests at the liquid-liquid interface (LLI). Known as liquid interfacial self-assembly (LISA), this phenomenon offers a versatile platform with applications spanning material synthesis, device fabrication, energy conversion and storage, and biomedical advancements. Understanding the underlying mechanisms comprehensively is crucial for extending LISA approaches and advancing their practical utility in soft-device fabrication. This thesis aims to develop various soft devices, such as all liquid robots and all liquid optical fiber, based on the LISA of polymers and colloids. In the meantime, to bring a deeper understanding of the LISA, different assembly mechanism was investigated and developed. The key achievements are summarized as follows:

First, by harnessing traditional electrostatic interactions between positively charged dual-aminopropyl terminated polydimethylsiloxane (NH₂-PDMS-NH₂) and the negatively charged polyanion Poly(sodium 4-styrene sulfonate) (PSS), the author initiated spontaneous interfacial self-assembly of NH₂-PDMS-NH₂ and PSS at the water-silicone oil interface, establishing a novel all-liquid three-dimensional (3D) printing system. With the versatile all-liquid 3D printing system, a water fiber was fabricated in the oil phase, which enables a refractive index (RI) difference across the interface. Consequently, the structured water fiber with a higher RI enables real-time light communication with a bandwidth as high as 100 Mbps. With the merits of its liquid nature, the all-liquid optical fiber enables easy manipulation and reparation in complicated working conditions.

In the second section, the author introduced competing self-assembly at the liquid interface, thus presenting a novel approach termed magneto-responsive and reconfigurable interfacial self-assembly (MRRIS), enabling the reversible control of the properties of the liquid interface. The MRRIS process leverages two key principles: (1) Ferrofluids, composed of magnetic Fe₃O₄ nanoparticles (NPs), oleic acid, and a carrier liquid, which respond to external magnetic fields, thereby altering their shapes; and (2) Structured liquids, stabilized by a jammed assembly of solid NPs, which maintain nonspherical shapes and withstand external mechanical loads. The competing assembly between Fe₃O₄ NPs and silica NPs brings two unique functions: the responsiveness of the liquid interface to an external magnetic field and the structural stability of the liquid interface. This approach has demonstrated efficacy in both shaping and reshaping the liquid, as well as in fabricating all-liquid robots capable of transporting soft liquid cargo.

In the third section, to extend the application of LISA across materials lacking interfacial activity, the author introduces an innovative approach involving the introduction of an additional solvent into the biphasic system, termed Chaperone Solvent-Assisted Assembly (CSAA). This method revolves around a target polymer, denoted as X, and three solvents: α, β, and γ. Solvents α and β act as poor solvents for X and are immiscible with each other, while solvent γ serves as a good solvent for X and is miscible with both α and β, playing the role of the 'chaperone' solvent. The cross-interface diffusion of solvent γ initiates the LISA of interfacially nonactive polymer X. CSAA exhibits versatility across various polymer types, encompassing both conventional engineering polymers (e.g., polyacrylonitrile, polystyrene, polyvinyl chloride) and newly developed polymers (e.g., perylene diimide (PDI)-based copolymers). Unlike conventional LISA methods driven by pair interactions (e.g., electrostatic interaction, host-guest recognition), which are susceptible to environmental complexities, CSAA demonstrates resilience to changes in pH and ionic strength, rendering it particularly valuable in challenging conditions. The adaptable nature of CSAA facilitates the fabrication of diverse polymer films for a wide array of applications, including the synthesis of thermo-responsive interfacial membranes, the production of photoelectric polymer thin films, and the realization of all-liquid 3D printing.

Date of Award	16 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yu CHAI (Supervisor)