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With the promising potential in non-contact fluid manipulation, the photothermally induced phase change along with the interfacial behaviors have been frequently applied in various microfluidic devices. The property of the working fluid greatly affects the photothermal conversion and thus phase change behaviors. For this reason, the IR laser induced phase change of the NaCl solution in the microchannel was visually studied and compared with distilled water in this work. Experimental results indicated that the NaCl solution exhibited the distinct phase change behaviors. The existence of dissolved ions could weaken the photothermal conversion, leading to lowered temperature rise and evaporation rate as compared to the distilled water. In particular, different from the distilled water, the condensed droplets were formed at the region relatively far away from the evaporating interface due to lower evaporation rate. No coalescence between the condensed droplets and liquid slug was observed. Besides, the effects of the NaCl concentration, laser power and spot position were also investigated. It was found that higher NaCl concentration resulted in lower temperature rise and evaporation rate. With the increase of the input laser power and the decrease of the distance between the laser spot and the interface, the temperature rise and evaporation rate could be improved. The obtained results can be further applied in the design and operation of the microfluidic devices based on the photothermally induced phase change. [ABSTRACT FROM AUTHOR]

In this work, the condensation behaviors of distilled water actuated by photothermal effect of the infrared laser with the wavelength of 1550 nm in a microchannel were investigated through the visualization method and image processing technique. The impacts of the laser power and the laser spot position on the interface movement and slug formation were also explored. It is found that the interface tended to be advanced as a result of the photothermally induced evaporation–condensation–coalescence process at lower laser powers while the interface was firstly advanced at the beginning and then moved backward after the liquid slug formation in front of the interface at high laser powers. Experimental results also showed that higher laser power and smaller distance between the initial gas–liquid interface and fixed laser spot yielded faster advancing of the gas–liquid interface. For cases with the liquid slug formation, the growth rate of the liquid slug increased with increasing the laser power and decreasing the distance of the laser spot position away from the initial interface position as a result of a high evaporation rate. [ABSTRACT FROM AUTHOR]

The dynamic behavior of the moving liquid column coalescing with a sessile droplet in triangular microchannels is numerically investigated by using coupled volume of fluid with level set interface tracking method implemented in ANSYS Fluent 14.5 in conjunction with the continuum surface force model. It is found that for both hydrophobic and hydrophilic microchannels, the coalescence between the moving liquid column and droplet can accelerate the original liquid column movement as a result of the induced curvature that lowers the liquid pressure at the interface. As compared to the rectangular microchannel with the same hydraulic diameter, the triangular microchannel exhibits smaller velocity increment ratio because of stronger viscous effect. Simulation results also reveal that the velocity increment ratio increases with the contact angle in hydrophobic microchannels, but it is reverse in the hydrophilic microchannels. The effects of the droplet size, lengthways and transverse positions are also investigated in this work. It is shown that larger droplet and smaller distance between the droplet and inlet or the substrate center can result in larger velocity increment ratio as a result of higher surface energy and lower viscous dissipation energy, respectively. The results obtained in this study create a solid theoretical foundation for designing and optimizing microfluidic devices encountering such a typical phenomenon. [ABSTRACT FROM AUTHOR]

In this work, the characteristics of the meniscus evaporation from a microchannel were studied, which was induced by the photothermal effect of the infrared laser with the wavelength of 1550 nm. The evaporation rate and mass transfer coefficient at the interface were determined by the proposed image process technique coupled with an infrared camera. Experimental results showed that once the infrared laser was applied, the interface temperature was rapidly increased but with non-uniform distribution as a result of such a tiny local heating source. Accompanying with the rapid temperature rise, the meniscus evaporation was also immediately actuated. After a certain laser heating period, the interface temperature was increased to a stable value with relatively uniform distribution. In the meantime, the evaporation rate increased and became steady. Besides, the effects of the laser power and laser spot position on the evaporation rate and mass transfer coefficient at the interface were also explored. It was shown that both the interface temperature and evaporation rate linearly increased with the laser power as a result of more heat generated. Smaller distance between the laser spot and front interface yielded higher interface temperature and evaporation rate because of smaller heat transfer resistance resulting from smaller transport length. Reducing the distance could make the increase of the evaporation rate to become more significant. With respect to the mass transfer coefficient, it is interesting to find that the mass transfer coefficients under all cases were almost the same, about 0.4 m/s. [ABSTRACT FROM AUTHOR]