Experimental Study On Photothermal Effect Induced Evaporation Characteristics In Microchannels

The advancement of micro/nanofabrication technology allows almost all functions of conventional chemistry and biology laboratories to be incorporated into a chip, forming so-called lab-on-a-chip or microfluidics, which promises to revolutionize many aspects of analytical chemistry and biochemistry. More recently, the incorporation of optics into microfluidics results in the emergence of a new interdiscipline of optofluidics, which leverages the advantages of both disciplines. The photothermal effect is one of the important interactions between the optics and fluids, by which the light energy can be converted into the thermal energy using photothermal materials or direct absorption. The use of the photothermal effect to design new microfluidics has realized many functions on a chip. In particular, the phase change caused by the photothermal effect has also created some new microdevices. However, the characteristics of the evaporation caused by the photothermal effect still remain unclear. As a result, a deep understanding of the evaporation characteristics caused by the photothermal effect in microchannels is of importance.In this work, visualization experimental method and image processing technique were used to study the characteristics of the interface evaporation in and from a microchannel. Here the photothermal effect was induced by focused infrared laser with the wavelength of 1550 nm. At first, a T-shaped microchannel with an open micro-chamber was made. The meniscus evaporation characteristics of water at the open end was studied, including the evaporation rate, interface temperature variation, interface mass transfer mechanism. The influence of laser power, laser spot position and channel size on meniscus evaporation was studied. Then, the subject was changed to the water column inside the microchannel. The evaporation characteristics of water column inside the microchannel was studied. The emphasis was paid to the influence of the laser power, laser spot position and the initial length of the water column on evaporation. At last, the evaporation characteristics of Na Cl solution inside a microchannel was explored. The influence of laser power, spot position and concentration of Na Cl solution was studied. The Na Cl crystal growth process was observed and the change in supersaturation before crystallization was also investigated. Main conclusions are presented as follows.1)For the meniscus evaporation from the open end of a microchannel, it is found that once the laser was applied, the interface temperature rapidly rose but with non-uniform distribution. As the laser heating continued, not only the interface temperature increased to a stable value but also the temperature distribution became uniform. Both maximal interface temperature rise and evaporation rate linearly increased with the laser power. The variations of the interface temperature and evaporation rate with the distance between the laser spot and front interface showed a parabolic decrease with increasing the distance. With the microchannel width increased, the evaporation rate at the interface decreased. Regarding the mass transfer coefficient at the interface, it is found that the mass transfer coefficients in all cases were almost the same, about 0.4 m/s.2) For the evaporation of a water column inside the microchannel, once laser was switched on, the water temperature near the interface rapidly rose, leading to the interface evaporation. During the evaporating process, the front interface gradually moved towards the back-end, and the front interface did not coalescence with the condensate droplets before the interface. The maximal water temperature rise near the interface linearly increased with the laser power. The evaporation rate parabolically decreased with the laser spot position. What’s more, when the initial length of water column was very short, both interfaces of water column would evaporate. The evaporation at the back interface could be ignored if the initial length of water column was more than 1000 μm.3) Regarding the evaporation of a Na Cl solution column inside the microchannel, the evaporation rate of saturated Na Cl solution was much lower than that of water under the same conditions. During the evaporating process, Na Cl solution gradually concentrated due to the evaporation of solvent. Na Cl crystal growth began when the supersaturation of the solution reached about 90%. The maximal solution temperature rise near the interface linearly increased with the laser power. The evaporation rate parabolicly decreased with the laser spot position. The maximal solution temperature rise near the interface linearly increased with the solution concentration. The evaporation rate parabolically decreased with the solution concentration.