Analysis of flow patterns emerging during evaporation in parallel microchannels

The evaporation processes of 2-propanol and water in cyclo olefin polymer (COP) and silicon microchannels of square cross-section are studied with a high-speed camera. The COP channels with a cross-section of 50 μm × 50 μm are rather smooth, whereas the 30 μm × 30 μm silicon channels have comparatively rough surfaces. For the COP channels, two different evaporation modes are identified, both with oscillating liquid–vapor menisci. One of these modes is characterized by an extremely rapid evaporation and a corresponding discontinuous shift of the meniscus. In the silicon channels four different evaporation modes are observed. Oscillatory motion of the liquid fronts also dominates here, and depending on the total mass flow and the wall temperature the oscillations in different channels are synchronized or desynchronized. Besides the flow patterns also the velocity trajectories of the evaporating liquid fronts are analyzed in detail and show a rather good reproducibility over different channels and different cycles. Compared to most other studies reported in this field, bubble nucleation is found to be of secondary importance for the evaporation processes.