Reacting surface nanodroplets and sensitive chemical detection
Creators: Li, Zhengxin
Publisher: name: University of Alberta Library
Year: 2022

<p>Due to the large surface-to-volume ratio, micro-sized droplets (i.e. a few microns in radius) provide a unique compartmentalized environment for biphasic processes (i.e. biphasic reaction and liquid-liquid microextraction) that are reported to be cost-effective, streamlined, and high-throughput.<br>
However, we still lack a quantitative understanding of the mechanisms and kinetics of chemical reactions and mass transfer with micro-sized droplets, especially for droplets reacting with reactants from the external phase.</p>

<p>This Ph.D. thesis focused on reactions and mass transfer with surface nanodroplets, the liquid microdomains sitting on a solid surface and immersed in an immiscible surrounding liquid.<br>
As the three-phase contact line of nanodroplets was immobilized on the solid substrate, the evolution of droplet morphology and droplet size can be followed in situ by optical observation methods with high temporal and spatial resolution.<br>
Based on our achieved research progress on surface nanodroplets, systematic research was performed to understand the effects of external flow, droplet size, and many other conditions on the reaction kinetics between droplets and an external phase.</p>

<p>The mass transfer of reactants and products between the bulk and the interface can be a determining factor of the kinetics of biphasic reactions (i.e. reactions involving reactants from two immiscible phases), as reactants isolated in immiscible phases need to be transported together to react, and products need to be removed from the reacting site.<br>
Since the external flow enhanced the mass transfer between the droplet and surrounding phase, and interface-crossing mass transfer is more efficient for a smaller droplet with higher a surface-to-volume ratio, we expected that the biphasic reaction would be faster in smaller droplets or under faster flow rates.</p>

<p>To address our assumptions, the on-drop neutralization between oleic acid surface nanodroplets and sodium hydroxide from an external flow was followed in situ by an optical microscope.<br>
By tracing the droplet shrinkage from produce dissolution, we confirmed that the overall kinetics can be enhanced by increasing the external flow rate or reducing the droplet size.<br>
Our theoretical analysis attributes the faster kinetics to 2 factors:<br>
i. Product diffusion in the Prandtl-Blasius boundary layer, which should be faster at a higher linear flow rate (or Peclet number Pe) and in smaller droplets;<br>
ii: Surface-to-volume-ratio-associated product accumulation inside the droplet.<br>
The quantitative analysis coupling two factors above predict the overall kinetics scale with Pe^(1.5)R^2 (R is the droplet radius), consistent with our experimental results.</p>

<p>Another case of the faster reaction in smaller droplets was from the dehydrocoupling of siloxane by water diffusing from the external phase into the droplet.<br>
From the reaction, generated hydrogen gas triggered the formation of hydrogen bubbles inside the droplet.<br>
Followed by a confocal microscope, the bubble formation was found to be faster in smaller droplets.<br>
Additionally, a decreasing reaction rate profile from the droplet surface to the droplet center was found, as reflected by the slower bubble formation far from the droplet rim.<br>
Our theoretical analysis attributed the non-uniform reaction kinetics to the balance between water diffusion and consumption by the reaction.</p>

<p>Due to the slow diffusion, the concentration of water decrease dramatically from the interface and was found to be depleted in a few microns.<br>
For micro-sized droplets, such a diffusion length was already enough for reactant to penetrate the droplet, however, for a macroscopic drop (millimetre size or larger), such a small diffusion length only allows for reactions nearing the interface.<br>
In this case, the efficiency of the reaction is dramatically limited by the interface-crossing mass transfer.</p>

<p>Taking the advantages of surface pinning and efficient mass transfer, this thesis further explored the application of surface nanodroplets in liquid-liquid extraction and chemical detection.<br>
In a long capillary tube, octanol surface nanodroplets were prepared to extract triclosan and chlorpyrifos from the aqueous flow.<br>
After extraction, octanol droplets were collected by capillary force, and analyzed by ultraviolet-visible spectroscopy (UV-Vis).<br>
A good linear relationship between analyte concentration and absorbance, with a limit of detection (LOD) of ~ 10^(-9) M, which is comparable to prior arts, could be achieved both for triclosan and chlorpyrifos.<br>
The versatility of the setup was further confirmed by coupling with gas chromatography-mass spectrometry (GC-MS), and fluorescence microscopy, suggesting a powerful method for rapid extraction and detection in tandem with offline analytic instruments.</p>