We present a method to move and control drops of water on superhydrophobic surfaces using magnetic fields. Small water drops ͑volume of 5-35 l͒ that contain fractions of paramagnetic particles as low as 0.1% in weight can be moved at relatively high speed ͑7 cm/s͒ by displacing a permanent magnet placed below the surface. Coalescence of two drops has been demonstrated by moving a drop that contains paramagnetic particles towards an aqueous drop that was previously pinned to a surface defect. This approach to microfluidics has the advantages of faster and more flexible control over drop movement.

Förster resonant energy transfer (FRET) with upconverting nanoparticles (UC-NPs) as donors and quantum dots (QDs) as acceptors has been regarded as a promising tool for biosensing applications. In this work, we use time-resolved fluorescence spectroscopy to analyze the UCNP-to-QD FRET and we focus on the most relevant parameter of the FRET phenomenon, UCNP-QD distance. This distance is controlled by a nanometric silica shell around the UCNP surface. We theoretically reproduce the experimental results applying FRET theory to the distribution of emitting erbium ions in the UCNP. This simple model allows us to estimate the contribution of every erbium 1 ion to the final FRET response and to explore different strategies to improve FRET efficiency. Abbreviations FRET, QD, UCNP KeywordsFörster resonance energy transfer, upconversion, quantum dot

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A droplet-based electrochemical digital magnetofluidics system has been developed. The system relies on the magnetic movement, in air, of different aqueous microdroplets containing magnetic microparticles--serving as the 'sample', 'blank', 'wash' and 'reagent' solutions--into and out of a three-electrode assembly. The movement of all droplets was controlled using the magnetic fields generated by three separate external magnets positioned below the superhydrophobic surface. Square-wave voltammetry was used for rapid measurements of dopamine in multiple successive microdrops with minimal cross talk. The ability of the droplet-based electrochemical microfluidic system to manipulate microliter solutions was also illustrated in bioassays of glucose, involving the merging of enzyme (GOx) and substrate droplets, followed by chronoamperometric measurements of the hydrogen peroxide product in the merged droplet. Variables of the new electrochemical digital magnetomicrofluidic technique were examined and optimized. The new droplet-based electrochemical microfluidic system offers a promising platform for automated clinical diagnostics and drug discovery.

Serial femtosecond crystallography (SFX) with X-ray free electron lasers (XFELs) allows structure determination of membrane proteins and time-resolved crystallography. Common liquid sample delivery continuously jets the protein crystal suspension into the path of the XFEL, wasting a vast amount of sample due to the pulsed nature of all current XFEL sources. The European XFEL (EuXFEL) delivers femtosecond (fs) X-ray pulses in trains spaced 100 ms apart whereas pulses within trains are currently separated by 889 ns. Therefore, continuous sample delivery via fast jets wastes >99% of sample. Here, we introduce a microfluidic device delivering crystal laden droplets segmented with an immiscible oil reducing sample waste and demonstrate droplet injection at the EuXFEL compatible with high pressure liquid delivery of an SFX experiment. While achieving ~60% reduction in sample waste, we determine the structure of the enzyme 3-deoxy-D-manno-octulosonate-8-phosphate synthase from microcrystals delivered in droplets revealing distinct structural features not previously reported.