Cite this: Lab Chip, 2013, 13, 332 Received 13th July 2012, Accepted 16th November 2012 Fast, active droplet interaction: coalescence and reactive mixing controlled by electrowetting on a superhydrophobic surface3 DOI: 10.1039/c2lc41193h www.rsc.org/loc Angelo Accardo, a Federico Mecarini, a Marco Leoncini, a Fernando Brandi, b Emanuela Di Cola, c Manfred Burghammer, c Christian Riekel c and Enzo Di Fabrizio* ad A novel electrowetting-on-dielectrics (EWOD) device in open planar geometry allows probing of droplet mixing on a super- hydrophobic substrate under quasi contact-free conditions. We demonstrate a droplet-based microreactor with integrated con- vective-flow mixing for the reactive-mixing of CaCl 2 /Na 2 CO 3 solutions. The device provides unique conditions for scattering, spectroscopy and imaging probes requiring an unobstructed droplet-access. Droplet-based digital microfluidics applications are rapidly expanding in the biological and chemical fields, due to the possibility of resolving complex processes into a programmable sequence of discrete steps with volumes considerably smaller than those used in continuous flow microfluidics. 1–3 The quasi- homogeneous evaporation of droplets on superhydrophobic surfaces 4 allows high-sensitive probing of residues by spectro- scopic and scattering techniques such as surface enhanced Raman spectroscopy (SERS) 5,6 and X-ray scattering. 7–11 Combining the advantages of digital microfluidics with enhanced droplet mobility on superhydrophobic surfaces due to ultralow friction 12 and quasi- homogeneous evaporation 4 opens therefore new possibilities for probing of small reaction volumes. As compared to droplet-based microfluidics or flat EWOD (electrowetting-on-dielectrics) devices, 1 an open, planar architecture based on a superhydrophobic surface allows exploring solute-concentration kinetics and residue forma- tion of droplets as well as avoid wall-induced shearing effects in mixing processes. (Fig. 1A–C) These features could be used in studies on droplet coalescence and could also provide new insights on reactive-mixing processes. Furthermore, the funda- mental issue of non-equilibrium thermodynamic models applied to irreversible processes in small-scale systems, such as nucleation or chemical reactions, plays an important role in this context. 13,14 Such models are based on the assumption of local fluctuations a Istituto Italiano di Tecnologia, Nanostructures Department, Via Morego, Genova 16163, Italy. E-mail: angelo.accardo@iit.it; federico.mecarini@iit.it; marco.leoncini@iit.it; enzo.difabrizio@iit.it b Istituto Italiano di Tecnologia, Nanophysics Department, Via Morego, Genova, 16163, Italy. E-mail: fernando.brandi@iit.it c European Synchrotron Radiation Facility, B.P. 220, F-38043 Grenoble Cedex, France. E-mail: emanuela.di_cola@esrf.fr; burghammer@esrf.fr; riekel@esrf.fr d BIONEM lab University of Magna Graecia, Campus Salvatore Venuta, Viale Europa 88100, Germaneto-Catanzaro, Italy 3 Electronic supplementary information (ESI) available. See DOI: 10.1039/c2lc41193h Fig. 1 A: Principle of the SHEWOD device showing a droplet in the high contact- angle (suspended) state (V = 0). Once the voltage (V?0) is applied, the contact angle decreases and the droplet spreads (see Supporting Information3). B: Multiple electrode SHEWOD device showing schematically the coalescence of two droplets. The scanning electron microscopy image reveals the super- hydrophobic nanopatterned layer on top of the substrate/electrodes. C: Image of SHEWOD device with water and nanocolloidal PMMA (white) droplets on the synchrotron radiation beamline stage. Lab on a Chip COMMUNICATION 332 | Lab Chip, 2013, 13, 332–335 This journal is ß The Royal Society of Chemistry 2013