Spectral tuning of liquid microdroplets standing on a superhydrophobic surface using electrowetting A. Kiraz, a Y. Karadağ, and A. F. Coskun Department of Physics, Koç University, Rumelifeneri Yolu, 34450 Sariyer, Istanbul, Turkey Received 20 February 2008; accepted 23 April 2008; published online 13 May 2008 Using electrowetting, we demonstrate reversible spectral tuning of the whispering gallery modes of glycerol/water microdroplets standing on a superhydrophobic surface by up to 4.7 nm at 400 V. Our results can inspire electrically tunable optical switches and filters based on microdroplets on a superhydrophobic surface. The sensitivity of the observed spectral drift to the contact angle can also be used to measure the contact angles of microdroplets on a superhydrophobic surface. © 2008 American Institute of Physics. DOI: 10.1063/1.2927373 Optical communication systems require largely tunable optical microcavities to function as building blocks of largely tunable optical switches and filters. 1 Liquid microdroplets standing on a superhydrophobic surface possess unique fea- tures which can make them technologically favorable in such applications. First, due to their liquid nature, these micro- droplets are easily deformable. 2,3 Second, thanks to the su- perhydrophobic surface, their position stabilization is granted. They do not require complex position stabilization techniques such as electrodynamic levitation 4 or optical tweezing. 5 Third, they are cost effective and disposable. They do not pose any microfabrication challenges. Fourth, the interaction of these microdroplets with the superhydro- phobic surface inspires unique shape deformation mecha- nisms. To this end, here we demonstrate spectral tuning using electrowetting. Electrowetting is the increase in the wetting of a super- hydrophobic surface by conducting liquid microdroplets due to an external electric field. 6 As a result of an applied volt- age, a space charge layer of counter ions is built near the superhydrophobic surface in the microdroplet. This leads to a decrease in the effective solid-liquid interfacial tension which in turn decreases the contact angle. Electrowetting has recently become a powerful tool in microfluidics research inspiring various important applications including lab-on-a- chip devices in which microdroplets are translated along arbitrary paths, 7 adjustable microlenses, 8 and electronic displays. 9 In this letter, we report the spectral tuning of the whispering gallery modes WGMs of glycerol/water micro- droplets standing on a superhydrophobic surface using elec- trowetting. The decrease in the contact angle leads to an increase in the equatorial radius of the microdroplets result- ing in the redshift of the WGMs which circulate in the equa- torial plane parallel to the surface. We demonstrate spectral tuning by up to 4.7 nm as a result of a maximum applied voltage of 400 V. We also show that the observed spectral tuning mechanism is reversible. The sketch of the experimental setup is shown in Fig. 1. Two cover glasses having conducting, indium tin oxide ITO coatings on one of their surfaces were used in the sample chamber. The nonconductive surface of one of the cover glasses was spin coated with hydrophobically coated silica nanoparticles Degussa A.G., LE2 using a 50 mg / ml ethanol dispersion. Glycerol/water microdroplets were sprayed onto the superhydrophobic surface using an ultra- sonic nebulizer from a 10 / 90 glycerol/water solution con- taining 5 M rhodamine B and 135 mM KCl. Water content in the microdroplets quickly evaporated on the superhydro- phobic surface under ambient temperature and pressure and a relative humidity of 40%, revealing microdroplet diam- eters ranging from a few up to 20 m with resulting glyc- erol contents 90%. Following microdroplet generation, the second cover glass was glued on the first cover glass with its ITO coated surface facing the superhydrophobic surface. The spacing between the two cover glasses was kept at minimum by squeezing during the sealing of the chamber. After the adhesive was hardened, spacings of d 1 =30–60 m were typically obtained, and the chamber between two cover glasses was totally sealed. All the data presented in this letter were taken from microdroplets in the same sample chamber with a spacing of d 1 =40 m. In all results presented, a posi- tive voltage was applied between nodes A and B, V AB  0. Similar spectral shifts in the WGMs were observed as a func- tion of |V AB | for all microdroplets when the sign of V AB was reversed. Spectral positions of the WGMs were determined with fluorescence spectroscopy experiments. Individual mi- crodroplets were excited in the vicinity of their rims with a green, solid state laser  =532 nm using an air objective a Electronic mail: akiraz@ku.edu.tr. FIG. 1. Color online Illustration of the experimental setup. C, cover glass; S, superhydrophobic coating; ITO, indium tin oxide coating; O, microscope objective. The sketch is not to scale. c 1 and c 2 indicate capacitances per unit area. APPLIED PHYSICS LETTERS 92, 191104 2008 0003-6951/2008/9219/191104/3/$23.00 © 2008 American Institute of Physics 92, 191104-1 Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp