Electrocoalescence of aqueous droplets is investigated as a tool for microfluidic processing. Where droplets are separated by only thin lamellae, coalescence can be induced on demand within a fraction of a millisecond at low potentials (few volts). The authors show that in their approach electrocoalescence proceeds through an electric-field-induced dynamic instability of the oil/water interface. When the electrode geometry and applied potential are optimized, individual lamellae can be targeted for rupture within highly ordered droplet arrangements.

We demonstrate that high dispersed phase volume fraction emulsions (i.e., gel emulsions) can be prepared in situ for microfluidic applications. Previously, the production of gel-like emulsions in microfluidic devices, where the droplet size is less than the length-scale of the channel, required multistep splitting of larger droplets in a branched microchannel network. Instead, we employ an abrupt change in the aspect ratio of a single microchannel to rapidly destabilize a confined coflowing stream, forming highly monodisperse droplets (coefficient of variance <1.5%). Using this emulsification mechanism, gel emulsions can be prepared in a single production step.

The dynamics of capillary-driven flow was studied for water and water–glycerol mixtures in open hydrophilic microchannels (embedded in a hydrophobic matrix). The position of the advancing meniscus was recorded as a function of time using high speed microscopy and compared with the Washburn equation. The square of the position of the liquid front increased linearly with time, as predicted by Washburn. For a channel of the same depth, irrespective of the shape of the channel cross-section (rectangular or curved), the liquid flow was faster with decreasing channel width. A modified Washburn equation, accounting for the different flow profile in the open, noncylindrical channels, was developed. The theoretical prediction was in good agreement with the experimental data for a no-slip boundary condition at the liquid–air interface.

According to K. Hashimoto and co-workers, the wettability of TiO2 surfaces can be altered by irradiation. Surfaces become hydrophilic (water contact angle, ∼0°) after UV irradiation and gradually revert to a more hydrophobic state (contact angles, 50−70°) when left in the dark or exposed to visible light. Such changes have been observed for both anatase and rutile surfaces (single crystals and polycrystals) and presumably are not directly related to the photocatalytic activity of TiO2. We report here similar changes of the contact angle of water on flat titania wafers and on densely packed layers of micron-sized titania particles. Wettability changes can be reversibly cycled, and the effect is rather robust. The hydrophobic-to-hydrophilic conversion is faster than the inverse one (at comparable UV and visible light intensities). The contact angle change observed on wafers (∼50°) is twice as large as that on the particle layer, but this is mainly related to the effect of roughness. Using a photomask, we have patterned successfully regular arrays of hydrophobic circles (10 μm in diameter) on a hydrophilic TiO2 matrix and vice versa. These findings are of significant interest for the design of intelligent surfaces even though the detailed mechanism of the transition is yet to be elucidated.