DOI: 10.1002/elan.201900293 Electrochemical Detection of Droplets in Microfluidic Devices: Simultaneous Determination of Velocity, Size and Content Teo Lombardo, [a] Lidia Lancellotti, [a] Christelle Souprayen, [a] Catherine Sella, [a] and Laurent Thouin* [a] Abstract: Microfluidic devices were designed to electro- chemically detect in a two-phase flow the velocity, size and content of aqueous droplets containing redox species. The principle of these determinations is based on the analysis of a unique chronoamperometric response re- corded during the passage of a droplet over channel microelectrodes. Two configurations of electrochemical cell with different geometries were investigated both theoretically and experimentally. Velocity and size of droplets, as well as internal recirculating convection within droplets, were evaluated from chronoamperomet- ric curves by specific transition times depending on the cell configuration. In addition, the droplet content was probed from the Faradaic current controlled by mass transport and by internal hydrodynamic regime. For droplet velocity and size, experimental data were system- atically compared to optical measurements. All the results demonstrated the high performance of the electrochem- ical detection reached under these conditions. They successfully validate the concept of self-consistent electro- chemical detections of aqueous droplets within micro- channels for the simultaneous determination of their velocity, size and content. Keywords: Droplet · microfluidics · amperometry · mass transport · microelectrode 1 Introduction Over the last few years, droplet microfluidics has attracted wide interest in the microfluidic community [1–5]. Due to their flexibility, droplet-based microfluidic systems are potent high-throughput platforms [6] at the femtoliter scale for (bio)chemical [7] and biological [8] assaying, screening [9], synthesis [10] and single-cell testing [11]. Unlike in continuous microfluidics, a two-phase flow is implemented where samples and reagents are generally carried as discrete aqueous droplets within an immiscible oil, eliminating dispersion effects and reducing signifi- cantly the volume of solutions being handled. Each droplet can be considered as a microchamber for space- limited reactions and activation of complex processes. Under these conditions, benefits result from dimensional scaling that enables rapid fluid mixing and accurate control of reactions in decreased reaction times. The mixing of components is enhanced and does not require any special structures as in continuous laminar flow [12]. However, the generation and manipulation of droplets must be accurate with a precise control of their velocity, size and composition [13]. Operations on droplets often lead to discontinuous changes of pressure (i.e., during droplet formation, coalescence, splitting, mixing and sorting) producing droplets that are sensitive to changes in flow condition and fluid composition. As droplets move in and out of microchannels they also alter the hydro- dynamic resistance which causes variations of flow rates throughout the overall microfluidic circuit. The size and velocity of droplets, and distances between droplets, may thus vary spatially and temporally. The velocity can be also affected by an interplay of various physico-chemical parameters like flow rate and interfacial tension [14–15]. Hence, droplet characterization in real time is crucial, in particular for applications based on continuous droplet flow [16] involving multipoint detection [17] or multistep droplet systems [10]. In this context, optical detections are the most widely used. Optical adsorption and fluorescence spectroscopy are usually employed to analyze droplet content [18–19]. Optical droplet detectors can be integrated by a light source and detector aligned with the channels [20]. Videos recorded by high-speed CCD cameras or by laser diffraction are used to evaluate position, size and velocity of droplets [15]. Although optical detections lead to accurate determinations, video analysis require image recording and extensive data processing that do not facilitate real-time monitoring. Furthermore, optical in- strumentation is bulky, which limits the development of point-of-care devices. Only a few attempts have been reported to miniaturize optical flow cells [21]. On the other hand, electrical detections provide a scalable, low power and cost effective alternatives. Both the resistance and capacitive components of the signal can [a] T. Lombardo, L. Lancellotti, C. Souprayen, C. Sella, L. Thouin Département de chimie, Ecole normale supérieure, Université PSL, Sorbonne Université, CNRS, 75005, Paris, France E-mail: laurent.thouin@ens.fr Full Paper www.electroanalysis.wiley-vch.de © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2019, 31, 1 – 10 1 These are not the final page numbers! ��