Growth Rates of Hydrogen Microbubbles in Reacting Femtoliter Droplets

Interfacial hydrogen evolution reaction from Ouzo-effect-generated bulk nano/micro droplets of liquid organic hydrogen carriers

HYPOTHESIS

Organosilanes as liquid organic hydrogen carriers (LOHCs) offer a promising solution for the safe storage and transport of hydrogen gas as a clean energy source. However, the dehydrogenation reaction of organosilanes in the presence of water faces the challenge of sluggish kinetics in conventional bulk reactions. Dispersing organosilanes as stable nanodroplets in basic water offers a potential strategy to increase the interfacial area, thereby enhancing H2 production efficiency.

EXPERIMENTS

Organosilane nanodroplets were generated through spontaneous emulsification via the Ouzo effect in a ternary organosilane-water-acetone system. The reaction between the organosilane nano/microdroplets and the alkaline aqueous phase led to H2 generation. This study investigates how the composition and size distribution of these droplets influence H2 production yield. To gain deeper insight into the reaction mechanisms, single reacting microdroplets were analyzed using side-view imaging and confocal microscopy.

FINDINGS

Organosilane nano/microdroplets formed from the Ouzo effect in the presence of a co-solvent. H2 formation yields from interfacial reactions of these droplets reached up to 25%, whereas single reacting microdroplets achieved a maximum yield of 3.5%. This study demonstrates that spontaneous emulsification in ternary mixture using the Ouzo effect can enhance reaction kinetics and product yields. Furthermore, detailed insights into the behavior of H2 bubbles, from their nucleation within a microdroplet to their growth and eventual detachment, were obtained through the analysis of single reacting microdroplets.

Surfactant-Mediated Interfacial Hydrogen Evolution Reaction

Hydrogen is a highly promising clean energy source without greenhouse gas emissions. Liquid organic hydrogen carriers (LOHCs) offer a promising alternative for convenient storage and transportation. This study investigates the interfacial hydrogen evolution reaction between polymethylhydrosiloxane (PMH), a representative LOHC, and water, focusing on controlling reaction kinetics by modifying interfacial properties with surfactants. The hydrogen production rate at a planar interface between PMH and water catalyzed by sodium hydroxide revealed that surfactants such as Tween 20 and sodium dodecyl sulfate (SDS) can slow down the hydrogen formation by 5 to 20 times, possibly due to an overcrowded interface effect. In contrast, cationic surfactants, such as hexadecyltrimethylammonium bromide (CTAB) and other quaternary ammonium surfactants, act as pseudo phase-transfer catalysts and accelerate the hydrogen formation rate up to 3-fold at a concentration of 0.05 times their critical micelle concentration. As the PMH microdroplets were dispersed in the surfactant aqueous solution, the conversion yields of hydrogen with cationic surfactants reached up to 45%, which is significantly higher than the yields achieved with Tween 20 or SDS. The effects of the surfactant type were further confirmed by following hydrogen bubble growth in a single PMH droplet. Overall, our findings demonstrate that selecting an appropriate surfactant can provide an effective control over the interfacial reaction rate of dehydrogenation of LOHCs. This offers strategies for manipulating liquid-liquid interfaces and controlling in-demand hydrogen production.

Spontaneous Rise of Hydrogen Microbubbles in Interfacial Gas Evolution Reaction

Liquid organic hydrogen carrier is a promising option for the transport and storage of hydrogen as a clean energy source. This study examines the stability and behavior of organic drops immobilized on a substrate during an interfacial hydrogen-evolution reaction (HER) at the drop surface and its surrounding aqueous solution. Hydrogen microbubbles form within the drop and rise to the drop apex. The growth rate of the hydrogen in-drop bubble increases with the concentration of the reactant in the surrounding medium. The drop remains stable till the buoyancy acting on the in-drop bubble is large enough to overcome the capillary force and the external viscous drag. The bubble spontaneously rises and carries a portion drop liquid to the solution surface. These spontaneous rising in-drop bubbles are detected in measurements using a high-precision sensor placed on the upper surface of the aqueous solution, reversing the settling phase from phase separation in the reactive emulsion. The finding from this work provides new insights into the behaviors of drops and bubbles in many interfacial gas evolution reactions in clean technologies.

Topological wetting states of microdroplets on closed-loop structured surfaces: Breakdown of the Gibbs equation at the microscale

Microdroplets are a class of soft matter that has been extensively employed for chemical, biochemical, and industrial applications. However, fabricating microdroplets with largely controllable contact-area shape and apparent contact angle, a key prerequisite for their applications, is still a challenge. Here, by engineering a type of surface with homocentric closed-loop microwalls/microchannels, we can achieve facile size, shape, and contact-angle tunability of microdroplets on the textured surfaces by design. More importantly, this class of surface topologies (with universal genus value = 1) allows us to reveal that the conventional Gibbs equation (widely used for assessing the edge effect on the apparent contact angle of macrodroplets) seems no longer applicable for water microdroplets or nanodroplets (evidenced by independent molecular dynamics simulations). Notably, for the flat surface with the intrinsic contact angle ~0°, we find that the critical contact angle on the microtextured counterparts (at edge angle 90°) can be as large as >130°, rather than 90° according to the Gibbs equation. Experiments show that the breakdown of the Gibbs equation occurs for microdroplets of different types of liquids including alcohol and hydrocarbon oils. Overall, the microtextured surface design and topological wetting states not only offer opportunities for diverse applications of microdroplets such as controllable chemical reactions and low-cost circuit fabrications but also provide testbeds for advancing the fundamental surface science of wetting beyond the Gibbs equation.

Continuous synthesis of BaFe2O4 and BaFe12O19 nanoparticles in a droplet microreactor for efficient detection of antihistamine drugs in oral fluid using surface-assisted laser desorption/ionization mass spectrometry

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) has received considerable attention as a complementary approach to matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), offering substantial potential for analyzing molecules in the low-mass region. Herein, we propose a facile method, a microreactor for the synthesis of two types of barium ferrite (BaFe2O4 and BaFe12O19) nanoparticles (NPs) within moving droplets for detecting antihistamine (AH) drugs in oral fluid (OF). The synthesized BaFe2O4 and BaFe12O19 NPs exhibited small particle size, good ultraviolet absorption, and excellent performance in SALDI-MS, as determined by survival yield measurements. The limits-of-detection for AH drugs were in the range of 1 pg mL-1 to 100 ng mL-1, and spot-spot reproducibility of the SALDI substrates was satisfactory. Moreover, when analyzing cetirizine in OF, the obtained recoveries of cetirizine were 101% and 99% using BaFe2O4 and BaFe12O19 NP, respectively. Furthermore, the proposed method was validated by analyzing OF samples from a healthy volunteer who consumed a 5 mg levocetirizine tablet for seven days. SALDI-MS analysis confirmed the successful detection of endogenous components, the parent ion of cetirizine, and other exogenous substances. This study reports an advanced application of droplet microreactor technology for designing and synthesizing a wide range of novel and efficient SALDI-MS substrates for various applications.

An antioxidation strategy based on ultra-small nanobubbles without exogenous antioxidants

Antioxidation is in demand in living systems, as the excessive reactive oxygen species (ROS) in organisms lead to a variety of diseases. The conventional antioxidation strategies are mostly based on the introduction of exogenous antioxidants. However, antioxidants usually have shortcomings of poor stability, non-sustainability, and potential toxicity. Here, we proposed a novel antioxidation strategy based on ultra-small nanobubbles (NBs), in which the gas-liquid interface was employed to enrich and scavenge ROS. It was found that the ultra-small NBs (~ 10 nm) exhibited a strong inhibition on oxidization of extensive substrates by hydroxyl radicals, while the normal NBs (~ 100 nm) worked only for some substrates. Since the gas-water interface of the ultra-small NBs is non-expendable, its antioxidation would be sustainable and its effect be cumulative, which is different to that using reactive nanobubbles to eliminate free radicals as the gases are consumptive and the reaction is unsustainable. Therefore, our antioxidation strategy based on ultra-small NB would provide a new solution for antioxidation in bioscience as well as other fields such as materials, chemical industry, food industry, etc.

Effects of Gas Type, Oil, Salts and Detergent on Formation and Stability of Air and Carbon Dioxide Bubbles Produced by Using a Nanobubble Generator

Recent developments in ultrafine bubble generation have opened up new possibilities for applications in various fields. Herein, we investigated how substances in water affect the size distribution and stability of microbubbles generated by a common nanobubble generator. By combining light scattering techniques with optical microscopy and high-speed imaging, we were able to track the evolution of microbubbles over time during and after bubble generation. Our results showed that air injection generated a higher number of microbubbles (<10 μm) than CO₂ injection. Increasing detergent concentration led to a rapid increase in the number of microbubbles generated by both air and CO₂ injection and the intensity signal detected by dynamic light scattering (DLS) slightly increased. This suggested that surface-active molecules may inhibit the growth and coalescence of bubbles. In contrast, we found that salts (NaCl and Na₂CO₃) in water did not significantly affect the number or size distribution of bubbles. Interestingly, the presence of oil in water increased the intensity signal and we observed that the bubbles were coated with an oil layer. This may contribute to the stability of bubbles. Overall, our study sheds light on the effects of common impurities on bubble generation and provides insights for analyzing dispersed bubbles in bulk.