Protein molecules absorb and crystallise on He, N 2 , O 2 , and CO 2 gas bubbles as soft templates with prompted nucleation on gas–liquid–solid interface. Crystals attached on gas bubble form curved crystal surface and float in the solution.

Novel organic synthesis routes that circumvent the need for a catalyst and reduce unwanted by-products are highly sought by industry. A novel microfluidic plasma reactor that generates a dielectric barrier discharge (DBD) plasma in the vicinity of the gas-liquid interface and facilitate a chemical reaction at the interface of microbubbles has been used for trans-stilbene epoxidation. Three different operating strategies were implemented to optimise the transfer of species selectivity: single pass, multi-pass and continuous recirculation. The effect of initial trans-stilbene concentration, oxygen content in the feed gas mixture and reaction time on the epoxide formation was studied to optimise the chemical reaction. The optimum operating conditions were found to be short bubble-liquid contact times (~2 s) with frequent exposure to freshly generated microbubbles containing reactive species by continuous liquid recirculation, and under these conditions the overall epoxide yield was ~94% with an overall epoxide selectivity of 10:1.

Reactive species produced by atmospheric-pressure plasma (APP) are useful in many applications including disinfection, pretreatment, catalysis, detection and chemical synthesis. Most highly reactive species produced by plasma, such as ·OH, 1O2 and $$ {\text{O}}_{2}^{ \cdot - } $$, are short-lived; therefore, in situ generation is essential to transfer plasma products to the liquid phase efficiently. A novel microfluidic device that generates a dielectric barrier discharge (DBD) plasma at the gas–liquid interface and disperses the reactive species generated using microbubbles of ca. 200 µm in diameter has been developed and tested. As the bubble size affects the mass transfer performance of the device, the effect of operating parameters and plasma discharge on generated bubbles size has been studied. The mass transfer performance of the device was evaluated by transferring the reactive species generated to an aqueous solution containing dye and measuring percentage degradation of the dye. Monodisperse microbubbles (polydispersity index between 2 and 7%) were generated under all examined conditions, but for gas flow rate exceeding a critical value, a secondary break-up event occurred after bubble formation leading to multiple monodisperse bubble populations. The generated microbubble size increased by up to ~ 8% when the device was operated with the gas plasma in the dispersed phase compared to the case without the plasma due to thermal expansion of the feed gas. At the optimal operating conditions, initial dye concentration was reduced by ~ 60% in a single pass with a residence time of 5–10 s. This microfluidic chip has the potential to play a significant role in lab-on-a-chip devices where highly reactive species are essential for the process.