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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]

The study on cells in vitro usually encounters the signals with oscillatory or periodic temporal profiles, which requires the pulsating flow control. Aiming at this target, a novel approach to generate the pulsating waveform of fluid flow by the photothermally induced phase change in the microchannel with the sawtooth-shaped baffles was developed in this work. It was demonstrated that the periodical immobilization-motion-immobilization flow behavior was formed by the combined photothermally induced interfacial behaviors and the in-channel baffle structures, which successfully generated stable oscillating flow with good response. To further explore the pulsating flow behaviors, effects of the baffle design and input laser power were also investigated. It was shown that increasing the baffle width and interval distance between them could yield large pulse peak flow rate but low frequency, while both the pulse peak flow rate and frequency increased with the input laser power. These results indicate that via the change of the baffle design parameters and laser power, the pulse flow waveform can be well controlled for the in vitro cellular studies. [ABSTRACT FROM AUTHOR]