LED-based opto-wetting and fluidic transport for droplet mixing

Light energy is utilized as an extensive tool for actuation of fluid flow in microfluidic devices. We report a novel method of droplet actuation using LED (light-emitting diode) that can be utilized for microfluidic mixing applications. This low power LED which emits light of peak wavelength 395 nm delivering luminous power of less than 300 millicandela (mcd) was used to create an interfacial tension gradient on solid–liquid interface containing photoresponsive molecule of azobenzene (C 12 H 10 N 2 ). LED-induced photoisomerization of azobenzene creates a spatial gradient in interfacial energy on the solid–liquid interface, which enhances the droplet movement. The concept of LED-enhanced fluidic actuation was demonstrated using droplets of water, urine, blood serum over photoresponsive substrate. The substrates were prepared by coating the surface of silicon substrate with azobenzene dissolved in silicone resin solution. Detailed experimental characterization of LED-actuated droplet velocity was performed, where the effect of parameters such as frequency of light actuation, droplet volume, concentration of azobenzene was evaluated. Droplet transport of DI water (de-ionized) of volume 60 µL was achieved at a velocity of 160 µm/s corresponding to LED actuation frequency of 1 Hz. In vitro quantification of blood calcium and urine pH was performed using fabricated chip scale opto-wetting platforms. The proposed work could be useful in the development of droplet microfluidic chips, where combinatorial mixing can be achieved using an array of LEDs.