We designed and experimentally validated a new type of passive valve for centrifugal microfluidic platforms. A liquid column entering an unvented receiving chamber is stopped by the counter-pressure of compressed air. This valve opens under defined conditions at high centrifugal frequencies at which the interface between liquid and air becomes unstable and enables a phase exchange, forwarding the liquid. Burst frequencies of the valve were determined for liquids typically used in biochemical assays: pure water, water with detergent concentrations between 0.01 and 10%, and pure ethanol. Burst frequencies between 8.5 +/- 0.6 and 27.9 +/- 2.0 Hz were measured for different surface tensions. The burst frequencies can be tuned by simple geometrical changes in the valving structure. The valve does not require ultra-precise structures or local surface modifications and is therefore ideal for low-cost microfluidic polymer disks. Potential applications are in the field of multiparameter and panel analysis, such as PCR-genotyping.
This article presents a study on the capillary driven movement of gas bubbles confined in tapered channel configurations. These configurations can be used to transport growing gas bubbles in micro fluidic systems in a passive way, i.e. without external actuation. A typical application is the passive degassing of CO 2 in micro direct methanol fuel cells (lDMFC). Here, a one-dimensional model for the bubble movement in wide tapered channels is derived and calibrated by experimental observations. The movement of gas bubbles is modelled on straight trajectories based on a balance of forces. The bubble geometry is considered as three dimensional. In the development of the model, the effects of surface tension, inertia, viscosity, dynamic contact angle and thin film deposition are considered. It is found that in addition to viscous dissipation, the dynamics related to the contact line-dynamic contact angle and thin film deposition-are essential to describe the gas bubble's movement. Nevertheless, it was also found that both of these effects, as modelled within this work, have similar impact and are hard to distinguish. The model was calibrated against experiments in a parameter range relevant for the application of travelling gas bubbles in passive degassing structures for lDMFCs.
This study presents a new, simple and robust, pneumatically actuated method for the generation of liquid metal micro droplets in the nano-to picoliter range. The so-called StarJet dispenser utilizes a star-shaped nozzle geometry that stabilizes liquid plugs in its center by means of capillary forces. Single droplets of the liquid metal can be pneumatically generated by the interaction of the sheathing gas flow in the outer grooves of the nozzle and the liquid metal. For experimental validation, a print head was build consisting of silicon chips with a star-shaped nozzle geometry and a heated actuator (up to 280°C). The silicon chips are fabricated by Deep Reactive Ion Etching (DRIE). Chip designs with different star-shaped geometries were able to generate droplets with diameters in the range of the corresponding nozzle diameters. The StarJet can be operated in two modes: Either continuous droplet dispensing mode or drop on demand (DoD) mode. The continuous droplet generation mode for a nozzle with 183 lm diameter shows tear-off frequencies between 25 and 120 Hz, while droplet diameters remain constant at 210 lm for each pressure level. Metal columns were printed with a thickness of 0.5-1.0 mm and 30 mm height (aspect ratio [30), to demonstrate the directional stability of droplet ejection and its potential as a suitable tool for direct prototyping of the metal microstructures.
In this paper we present a new concept of creating and using capillary pressure gradients for passive degassing and passive methanol supply in direct methanol fuel cells (DMFCs). An anode flow field consisting of parallel tapered channels structures is applied to achieve the passive supply mechanism. The flow is propelled by the surface forces of deformed CO 2 bubbles, generated as a reaction product during DMFC operation. This work focuses on studying the influence of channel geometry and surface properties on the capillary-induced liquid flow rates at various bubbly gas flow rates. Besides the aspect ratios and opening angles of the tapered channels, the static contact angle as well as the effect of contact angle hysteresis has been identified to significantly influence the liquid flow rates induced by capillary forces at the bubble menisci. Applying the novel concept, we show that the liquid flow rates are up to thirteen times higher than the methanol oxidation reaction on the anode requires. Experimental results are presented that demonstrate the continuous passive operation of a DMFC for more than 15 h.
We present a new microvalve that can be monolithically integrated in centrifugally driven lab-on-achip systems. In contrast to existing operation principles that use hydrophobic patches
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