This paper presents a novel approach of mixing two laminar flowing streams in microchannels. The mixer consists of a pair of electrodes disposed along a fluidic channel. By energizing the electrodes with a DC-biased (2.5 V) AC voltage (20 V_pp), an electrokinetic flow is induced with a flow profile perpendicular to that of the incoming laminar streams of liquids to be mixed. As a result, the flow lines of the incoming streams and the induced flow are forced to crossover and very efficient stirring and mixing at short mixing length can be achieved. The mixer can be operated from the AC-electroosmotic (ACEO) (σ = 1 mS/m, f = 100 kHz) to the AC-electrothermal (ACET) (σ = 500 mS/m, f = 500 kHz) flow regimes. The mixing efficiency in the ACEO regime was 92%, with a mixing length of 600 µm ( = 2 µL/min), an estimated mixing time of 69 ms and an induced ACEO flow velocity of ∼725 µm/s. The mixing efficiency in the ACET regime was 65% for a mixing length of ∼1200 µm. The mixer is efficient and suitable for mixing reagents in a fluid media from low to high conductivity as required in diverse microfluidic applications.

We demonstrate in this letter that variable focus lenses can be realized by pressure-induced deformation of a liquid interface. Large deflections can be achieved by exploiting the capillary pressure barrier generated by high surface tension liquids at the exit of a small well. Modulation of the internal pressure of a liquid interface produces a change in its radius of curvature thus a shift in the plane of focus. Using liquids with high surface tension like water and polymeric low surface energy substrates, liquid-air lenses were realized with a wide focal length tunability (optical power for 2 mm lenses from 130 to 350 diopters). Using the liquid-liquid interface of water and a nonmiscible liquid, pressure-induced tunable lenses were also demonstrated. The application of the lenses in imaging was also shown.