Spatial Site-Patterning of Wettability in a Microcapillary Tube

Wettability Alteration of a Thiolene-Based Polymer (NOA81): Surface Characterization and Fabrication Techniques

Wettability plays a significant role in controlling multiphase flow in porous media for many industrial applications, including geologic carbon dioxide sequestration, enhanced oil recovery, and fuel cells. Microfluidics is a powerful tool to study the complexities of interfacial phenomena involved in multiphase flow in well-controlled geometries. Recently, the thiolene-based polymer called NOA81 emerged as an ideal material in the fabrication of microfluidic devices, since it combines the versatility of conventional soft photolithography with a wide range of achievable wettability conditions. Specifically, the wettability of NOA81 can be continuously tuned through exposure to UV-ozone. Despite its growing popularity, the exact physical and chemical mechanisms behind the wettability alteration have not been fully characterized. Here, we apply different characterization techniques, including contact angle measurements, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) to investigate the impact of UV-ozone on the chemical and physical properties of NOA81 surfaces. We find that UV-ozone exposure increases the oxygen-containing polar functional groups, which enhances the surface energy and hydrophilicity of NOA81. Additionally, our AFM measurements show that spin-coated NOA81 surfaces have a roughness less than a nanometer, which is further reduced after UV-ozone exposure. Lastly, we extend NOA81 use cases by creating (i) 2D surface with controlled wettability gradient and (ii) a 3D column packed with monodisperse NOA81 beads of controlled size and wettability.

Liquid crystal microcapillary-based sensors for affordable analytical applications

Stimuli-responsive properties of liquid crystals (LCs), when combined with their optical properties, offer sensitive and rapid sensing applications. Here, we propose and demonstrate a microcapillary-based method to be applied for the online detection of amphiphilic species, which can be further used for tracking biological and chemical species in aqueous media. Specifically, we used compartments (300-1400 μm) of nematic 4-cyano-4'-pentylbiphenyl (5CB) that were positioned into cylindrical glass microcapillaries that promote homeotropic anchoring. The flat surfaces of the cylindrical LC compartments were in contact with an aqueous media. We characterized the equilibrium and nonequilibrium response of LCs upon a change in their anchoring at the aqueous interfaces. Upon anchoring transition, we observed the formation of a positively charged defect at the proximity of the interface that moved to the center of the LC compartment and reached equilibrium, a four-petal configuration. This transition was observed to take an average of 41 ± 19 min., which we related to the motion of the defect due to the imbalance of the elastic forces. During the transition, we observed metastable states which could be removed via thermal treatment. We showed the capillary sensors to be useful considering their ease of additional quantification. We also show that the sensors are reversible that facilitate temporal and cumulative quantification. The findings reported in this study can further be used to develop sensors for specific purposes that require continuous tracking of the chemical and biological species that is critical for the health and safety of the individuals and society.

Functionalized multiscale visual models to unravel flow and transport physics in porous structures

The fluid flow, species transport, and chemical reactions in geological formations are the chief mechanisms in engineering the exploitation of fossil fuels and geothermal energy, the geological storage of carbon dioxide (CO₂), and the disposal of hazardous materials. Porous rock is characterized by a wide surface area, where the physicochemical fluid-solid interactions dominate the multiphase flow behavior. A variety of visual models with differences in dimensions, patterns, surface properties, and fabrication techniques have been widely utilized to simulate and directly visualize such interactions in porous media. This review discusses the six categories of visual models used in geological flow applications, including packed beds, Hele-Shaw cells, synthesized microchips (also known as microfluidic chips or micromodels), geomaterial-dominated microchips, three-dimensional (3D) microchips, and nanofluidics. For each category, critical technical points (such as surface chemistry and geometry) and practical applications are summarized. Finally, we discuss opportunities and provide a framework for the development of custom-built visual models.

Inner Surface Design of Functional Microchannels for Microscale Flow Control

Fluidic flow behaviors in microfluidics are dominated by the interfaces created between the fluids and the inner surface walls of microchannels. Microchannel inner surface designs, including the surface chemical modification, and the construction of micro-/nanostructures, are good examples of manipulating those interfaces between liquids and surfaces through tuning the chemical and physical properties of the inner walls of the microchannel. Therefore, the microchannel inner surface design plays critical roles in regulating microflows to enhance the capabilities of microfluidic systems for various applications. Most recently, the rapid progresses in micro-/nanofabrication technologies and fundamental materials have also made it possible to integrate increasingly complex chemical and physical surface modification strategies with the preparation of microchannels in microfluidics. Besides, a wave of researches focusing on the ideas of using liquids as dynamic surface materials is identified, and the unique characteristics endowed with liquid-liquid interfaces have revealed that the interesting phenomena can extend the scope of interfacial interactions determining microflow behaviors. This review extensively discusses the microchannel inner surface designs for microflow control, especially evaluates them from the perspectives of the interfaces resulting from the inner surface designs. In addition, prospective opportunities for the development of surface designs of microchannels, and their applications are provided with the potential to attract scientific interest in areas related to the rapid development and applications of various microchannel systems.

Effect of triblock copolymer surfactant composition on flow-induced phase inversion emulsification in a tapered channel

HYPOTHESIS

Phase inversion emulsification (PIE) is a process that inverts the dispersed and continuous phases of an emulsion and is useful for preparing emulsions that are challenging to produce using conventional techniques. A recent work has shown that PIE can be induced by flowing an emulsion through a tapered channel. Although prior studies have shown that flow-induced PIE (FIPIE) is influenced by the flow conditions and wetting properties of the channel surface, little is known about the effect of surfactant structure on FIPIE. We hypothesize that FIPIE is affected by the composition and structure of the surfactant used for emulsion stabilization.

EXPERIMENTS

We use Pluronics, a series of ABA triblock copolymers composed of hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(propylene oxide) (PPO) with various lengths (A = PEO, B = PPO), as model surfactants to test this hypothesis. We observe that triblock copolymer surfactants with long PEO blocks suppress FIPIE. A scaling analysis based on a polymer brush model qualitatively agrees with the experimental observation. We also show that for small molecular weight Pluronics, FIPIE is significantly suppressed when Pluronics with large PPO blocks are used.

FINDINGS

Our results strongly indicate that the steric repulsion provided by the PEO blocks as well as the dilatational elasticity provided by the PPO blocks are key factors that control the FIPIE process.

Flow-induced phase inversion of emulsions in tapered microchannels

Phase inversion emulsification (PIE) is a process of generating emulsions by inverting the continuous and dispersed phases of pre-existing emulsions. Although PIE is conventionally performed in batch processes, flowing emulsions through precisely engineered channels (i.e. flow-induced phase inversion emulsification (FIPIE)) can induce PIE and potentially enable continuous processing. In this study, we demonstrate flow-induced phase inversion of oil-in-water (O/W) emulsions using microfluidic channels with gradual constriction. We investigate the importance of wetting properties and geometric characteristics of microfluidic channels on FIPIE. We show that two dimensionless groups, Ca and the ratio of droplet-size to channel dimensions determine the outcome of the process. In situ observation of individual droplets undergoing FIPIE reveals that the rupture of films of the continuous (water) phase between oil droplets and a wetting oil layer on the surface of microchannels is the most crucial step for phase inversion. Finally, we compare our experimental observations with a scaling relationship that is based on the force balance between disjoining pressure and Laplace pressure, which provides insights into the underlying physical phenomena responsible for the rupture of the aqueous film and the occurrence of FIPIE. We believe our work provides critical insights and parameters for designing channels and pores that can be used for continuous PIE.