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Recent advances in droplet manipulation methods by electric fields and signals require a deeper understanding of water droplet charge. In this paper, we have investigated the electrophoretic motion of individual water microdroplets injected into non-polar silicone and paraffin oil by video optical microscopy on an individual droplet basis to determine droplet charge. It was found that the initial surface charge density of surfactant free droplets directly after injection from a micropipette is positive and of the order of 10-6 C/m2, regardless of pH and ion concentration in the range from pH 4 to pH 10 and from 0.01 mmol/l to 1.5 mol/l, respectively. The experimental results together with molecular dynamics simulations show that the nature and polarity of the charge can be explained by anisotropic orientation of water molecules at the interface rather than selective adsorption of ions. Furthermore, we showed that slip at the liquid-liquid boundary must be taken into account when interpreting electrophoretic measurements of droplets. [ABSTRACT FROM AUTHOR]

Glass capillary based microfluidicdevices are able to create extremelyuniform droplets, when formed under the dripping regime, at low setupcosts due to their ease of manufacture. However, as they are rarelyparallelized, simple methods to increase droplet production from asingle device are sought. Surfactants used to stabilize drops in suchsystems often limit the maximum flow rate that highly uniform dropscan be produced due to the lowering interfacial tension causing jetting.In this paper we show that by simple design changes we can limit theinteractions of surfactants and maximize uniform droplet production.Three flow-focused configurations are explored: a standard glass capillarydevice (consisting of a single round capillary inserted into a squarecapillary), a nozzle fed device, and a surfactant shielding device(both consisting of two round capillaries inserted into either endof a square capillary). In principle, the maximum productivity ofuniform droplets is achieved if surfactants are not present. It wasfound that surfactants in the standard device greatly inhibit dropletproduction by means of interfacial tension lowering and tip-streamingphenomena. In the nozzle fed configuration, surfactant interactionswere greatly limited, yielding flow rates comparable to, but lowerthan, a surfactant-free system. In the surfactant shielding configuration,flow rates were equal to that of a surfactant-free system and couldmake uniform droplets at rates an order of magnitude above the standardsurfactant system. [ABSTRACT FROM AUTHOR]