On a superhydrophobic surface, an aqueous droplet can maintain its quasi-spherical shape without wetting the surface. This geometry creates a nearly isothermal micro-scale container in which chemical reactions and analysis can be conducted using small volumes (&lt;10 µL) of reagents. Due to their small size and thermal isolation from the substrate, convection in these droplets is suppressed and so mixing within these micro-containers occurs only by diffusion. In this paper, we present a method to induce, control and quantify convective mixing within small size droplets poised on multifunctional superhydrophobic surfaces. We prepared superhydrophobic surfaces using a 3D printing process to create arrays of PDMS posts onto which functionalized nanoparticles were partially embedded. On these surfaces, an aqueous droplet was placed and imaged using a high speed camera to quantify convection. By modifying the functionality of the nanoparticles, two reactions were studied. In one system, singlet oxygen was generated by illuminating sensitizer particles such that singlet oxygen was generated at the solid-liquid interface. Anthracene dipropanate dianion dissolved in the drop trapped the 1O2 and the concentration change was measured by UV spectroscopy. In a second system, the protein binding interaction between NeutrAvidin (dissolved in the droplet) and biotin (bound to the surface) was quantified using a fluorescently labeled protein. The binding profile was plotted as a function of time, and the kinetic rate constant was calculated. By comparing the rates with and without convection, we show that the reactions are limited by diffusion in static droplets and that convection leads to significantly higher reaction rates.

Reactions in Individual droplets on a superhydrophobic surface: Effect of convection

251st National Meeting (2016)

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