An evaporation driven pump for microfluidics applications

We present an evaporation driven micro-pump for micro fluidic applications on a foil. In such a device, the evaporation rate is controlled by the geometry of the channel outlet and its temperature. The evaporation is also influenced by environmental parameters such as air humidity and temperature. Due to the mass conservation at the system level, the evaporation speed at the outlet determines the mass flow in the microchannel. The outlet contains multiple holes forming a porous hexagonal structure. For analyzing the evaporation rate in this device, diffusion based models for both a single droplet and an array of droplets are presented. To calculate the evaporation flux in the case of an array of multiple pores at the outlet, an evaporation correction factor is estimated mathematically. The outcome is validated by experiments. The experimental device is made by laminating three polyethylene terephthalate (PET) sheets and has a total thickness of about 300 µm. The device is fabricated by laser processes. Typical results show that with 1 to 61 pores (diameter=250µm, pitch=500µm), flow rates of 9.01×10-3 to 1.60×10-1 µL/min are achieved, as assessed by a 2-Dimensional Particle Tracking Velocimetry (2D-PTV) method. The results are compared with theoretical values based on a sessile droplet evaporation equation including an evaporation correction factor. The results show a good agreement.