Nanofluidic actuation of molecules and colloids by controlling multiphasic intermembrane transports

Abstract

Surface-dominated physicochemical phenomena assume a pivotal role in mediating the migration of fluids and suspended or dissolved solutes within nanopores, exhibiting exquisite sensitivity to external stimuli. These nanofluidic phenomena propel not only the fluids themselves but also transport dissolved solutes. Activating nanopores provides precise control of dissolved solutes by modulating the nanopore’s characteristics, such as pore dimensions and surface properties. The solutes encompass nanometer-scale colloids and highly diffusive molecules in diverse configurations, spanning biomolecular to engineered materials. Nanofluidic actuation — encompassing gating, accumulation, and pumping mechanisms — offers both fundamental and advanced methods of microfluidic manipulation. Gating provides foundational controllability over fluidic transport through on-off switching. Accumulation proves a tool to manage low-concentration solutes for targeted ion detection and sample pre-concentration. Meanwhile, the ion pump is utilized to heighten concentration differences of diffusive molecules. These nanofluidic actuations offer the robustness and versatility of fluidic devices, enriching their unique functions and applications. Microfluidics predominantly favors liquid-only systems due to their viscosity-driven benefits. However, the extension of microfluidics with the gas phase overcomes its traditional liquid-centric precepts, embracing a more expansive understanding of "fluid." Incorporating gases into microfluidics involves trade-offs between durable operation and effective transport. However, phase separation in membrane-integrated microfluidic devices enables various functions in both durable and effective manners. These functions enrich versatilities across multiple fields, including biology, chemistry, and physics. The primary objective of this research is to explore an innovative approach for manipulating small molecules and colloids within a microfluidic chip using liquid-gas interphase transport mechanisms. The improvement of precision and labor-efficiency could lead to significant advancements. Spontaneous processes such as pervaporation and diffusion are designed in a guided manner to achieve improvements without relying on external energy sources. Therefore, the primary focus is on designing and fabricating a membrane-integrated microfluidic device. Nanostructures are further integrated to obtain the capability of molecular and colloidal manipulation. The devices are utilized for various purposes depending on the target. First, pervaporation is utilized to control the transport of small molecules along nanoslits. The local and independent switching of humidity conditions near nanoslits facilitates the concentration, separation, and actuation of small molecules. Pervaporation-induced flow of solvent and diffusion of small molecules along nanoslits are analyzed via fluorescent signals and theoretical modeling simultaneously to investigate critical parameters. These parameters are concluded to provide insightful control of small molecules. Second, a pervaporation-assisted method for fabricating a particle assembly membrane (PAM) is developed. The concentrating property of pervaporation-induced flow is utilized to in-situ assemble sub-micron to nano-sized particles in microchannels. The assembled particles containing nanopore networks serve as nanoporous membranes. Forced assembly due to liquid flow enables the adoption of various types of particles in terms of size, surface functionality, and wettability. Furthermore, the heterogeneous structure in parallel and serial configurations offers possibilities for numerous applications. Third, gas dissolution is employed to modulate the functions of ionic diodes. Asymmetrically charged heterogeneous PAMs serve as ionic diodes. A control channel is placed in the middle of the ionic diode to supply gas molecules through diffusion via a gas-permeable film. Gases and gas- dissolved solutions are introduced to construct a concentration gradient in a switchable and programmable manner. The diodes exhibit different responses to bias when gas is supplied from the control channel, resulting in the modulation of rectification ratios. Several demonstrations are also conducted, including chemical reactions in an accumulated state and ion signal amplification. In summary, the methods of fabrication and control serve as the basis for designing liquid-gas interphase transport in an on-chip manner. Furthermore, this research accelerates molecular, colloidal, and ionic manipulation in microfluidic devices, thereby enriching micro/nanofluidics and various fields. Spontaneous working mechanisms, especially, enhance the key value of microfluidics, offering low cost and portability. A wide range of target molecules enables the development of multi-functional devices for practical applications, such as cell culture, chemical reactions/sensing, and colloidal delivery.