No abstract

A low voltage electroosmotic (eo) pump suitable for high density integration into microfabricated fluidic systems has been developed. The high density integration of the eo pump required a small footprint as well as a specific on-chip design to ventilate the electrolyzed gases emerging at the platinum (Pt) electrodes. For this purpose, a novel liquid-gas (lg) separator was invented. This lg-separator separated the gas bubbles from the liquid and guided them away from the eo pump. Its operational principle was solely based on the geometry of tapered sidewalls. An eo pump sandwiched by two lg separators (microchannels in the range of 10 lm, footprint of 100 lm 9 15 lm) was experimentally investigated. The lg-separator was able to reliably separate and ventilate an emerging gas flow of 2 pl s -1 . The eo pump achieved flow rates of 50 pl s -1 at actuation voltages of 5 V.

Advances in micro and nano fabrication technologies have enabled fabrication of smaller and more sensitive devices for applications not only in solid-state physics but also in medicine and biology. The demand for devices that can precisely transport material, specifically fluids are continuously increasing. Therefore, integration of various technologies with numerous functionalities in one single device is important. Scanning probe microscope (SPM) is one such device that has evolved from atomic force microscope for imaging to a variety of microscopes by integrating different physical and chemical mechanisms. In this article, we review a particular class of SPM devices that are suited for fluid dispensing. We review their fabrication methods, fluid-pumping mechanisms, real-time monitoring of dispensing, physics of dispensing, and droplet characterization. Some of the examples where these probes have already been applied are also described. Finally, we conclude with an outlook and future scope for these devices where femtolitre or smaller volumes of liquid handling are needed.

A micro pump was integrated into a scanning force microscope probe for circulating liquid through its hollow cantilever and tip. The interior cross section of the cantilever was 2.25 lm  3.75 lm. All fluidic parts were made of SiO 2 , while the tip apex was made of Si 3 N 4 . The key fabrication techniques were silicon wafer bonding and wet-oxidation. The pumping mechanism was relying on the enhanced evaporation at an enlarged water/air interface at the exit of the microchannel. Capillary forces continuously wetted this interfacial area, thus drawing the liquid through the system. At room temperature, a pump rate of 11 pl s À1 was experimentally evaluated. The observed temperature dependence of the pump rate could be qualitatively understood by a plain model calculation.

The deposition of electrodes as the final step in the microfabrication of a fluidic system avoids incompatibilities with the microfabrication, i.e., high-temperature steps, or the process environment, i.e., CMOS fabrication. The employed strategy to deposit and structure silver (Ag) electrodes in microfluidic capillaries (cross-sectional length less than 10 μm) is presented. First, the adhesion of the Ag layer to the silicon dioxide (SiO 2 ) surface of the capillary was improved with an intermediate mercapto silane layer. Second, the Ag electrodes were electrolessly deposited with a modified Tollens reagent. The high conductivity of the deposited Ag layer indicated high density and purity. Third, the electrodes were structured by controlling the capillary filling of the electroless solution within the fluidic system using microfluidic stop valves. Experiments in a microfluidic system with a capillary dimension of 3 μm showed successful deposition and microfluidic structuring of the Ag electrode, as well as postdeposition, void-free filling by changing the solution's surface tension.[2010-0261]