We present a design for integrated lab-on-chip microsystems dedicated to mass spectrometry analysis based on the fabrication of watertight microchannels for the circulation of liquids. In this paper, we demonstrate how to fabricate complete polymer microchannels using the negative photoresist SU-8 which has the advantage of being compatible with protein analysis by mass spectrometry. Our method of fabrication requires novel technological steps involving SU-8 multi-layer processing, improved SU-8 adhesion and the use of SU-8 wafer bonding for the watertight closing of the microchannels with a Pyrex wafer. This technique also encompasses the design of various microfluidic elements such as tapered recesses for the housing of capillary tubes allowing the connection of the channels to external systems. Following this, the capillary tubes were used to test the hydrodynamic behaviour of the channels and consequently the efficiency of our technological process in achieving fully watertight structures within our flow rate and pressure specifications.

Evaporation of droplets of three pure liquids (water, 1-butanol, and ethanol) and four binary solutions (5 wt % 1-butanol-water-based solution and 5, 25, and 50 wt % ethanol-water-based solutions) deposited on hydrophobic silicon was investigated. A drop shape analyzer was used to measure the contact angle, diameter, and volume of the droplets. An infrared camera was used for infrared thermal mapping of the droplet's surface. An acoustic high-frequency echography technique was, for the first time, applied to track the alcohol concentration in a binary-solution droplet. Evaporation of pure alcohol droplets was executed at different values of relative humidity (RH), among which the behavior of pure ethanol evaporation was notably influenced by the ambient humidity as a result of high hygrometry. Evaporation of droplets of water and binary solutions was performed at a temperature of 22 °C and a mean humidity of approximately 50%. The exhaustion times of alcohol in the droplets estimated by the acoustic method and the visual method were similar for the water-1-butanol mixture; however, the time estimated by the acoustic method was longer when compared with that estimated by the visual method for the water-ethanol mixture due to the residual ethanol at the bottom of the droplet.

Many applications involving superhydrophobic materials require accurate control and monitoring of wetting states and wetting transitions. Such monitoring is usually done by optical methods, which are neither versatile nor integrable. This letter presents an alternative approach based on acoustic measurements. An acoustic transducer is integrated on the back side of a superhydrophobic silicon surface on which water droplets are deposited. By analyzing the reflection of longitudinal acoustic waves at the composite liquid-solid-vapor interface, we show that it is possible to track the local evolution of the Cassie-to-Wenzel wetting transition efficiently, as induced by evaporation or the electrowetting actuation of droplets.