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At the moment, manipulation of liquid microdroplets is required in various microfluidic and lab-on-a-chip devices, as well as advanced sensors. The platforms used for these purposes should provide the possibility of controlled selective movement and coalescence of droplets, and the manipulation speed should be sufficiently high (more than 10 mm/s). In addition, to facilitate their practical application, such platforms should have a simple planar geometry and low manufacturing cost. We report here a new method for microdroplet manipulation based on the use of non-uniform electrostatic fields. Our platform uses an electrode array embedded in a dielectric planar superhydrophobic substrate (50 × 50 mm). When a voltage is applied to a certain sequence of electrodes, a non-uniform electrostatic field is produced, which acts to attract a droplet on the substrate to the electrodes. This achieves a stepwise movement of the droplet. We realized non-contact, selective and high speed (up to 80 mm/s) movement of the individual droplets along specified trajectories (like a chess game) and their selective coalescence. It allowed us to demonstrate several controllable chemical reactions including an analytical one. In our opinion, this approach has a huge potential for chemical technology applications, especially in advanced sensors. [ABSTRACT FROM AUTHOR]

We report an easy-to-implement device for surface-enhanced Raman scattering (SERS)-based detection of various analytes dissolved in water droplets at trace concentrations. The device combines an analyte-enrichment system and SERS-active sensor site, both produced via inexpensive and high-performance direct femtosecond (fs)-laser printing. Fabricated on a surface of water-repellent polytetrafluoroethylene substrate as an arrangement of micropillars, the analyte-enrichment system supports evaporating water droplet in the Cassie–Baxter superhydrophobic state, thus ensuring delivery of the dissolved analyte molecules towards the hydrophilic SERS-active site. The efficient pre-concentration of the analyte onto the sensor site based on densely arranged spiky plasmonic nanotextures results in its subsequent label-free identification by means of SERS spectroscopy. Using the proposed device, we demonstrate reliable SERS-based fingerprinting of various analytes, including common organic dyes and medical drugs at ppb concentrations. The proposed device is believed to find applications in various areas, including label-free environmental monitoring, medical diagnostics, and forensics. [ABSTRACT FROM AUTHOR]