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• A microfluidic evaporator with a photothermal porous layer was designed for continuous sample concentration. • Concentration performance of the evaporator were tested under various operation conditions including light intensity, flow rate and different sample type. • By regulating operation parameters the designed evaporator could reach a maximum concentration factor of 2.9 under continuous working condition with minimum flow/thermal disturbance to the bulk solution. On microfluidic platform, sample concentration plays a crucial role in on-chip detection, analysis and synthesis, etc. Evaporation-induced solvent removal is frequently applied to various application scenarios due to its simplicity and compatibility. However, the enclosed microfluidic channels often hinder the solvent vapor discharge during the evaporation process due to the formation of two-phase flow and interfacial instability. Herein, we propose a novel microfluidic evaporator incorporated with a photothermal porous layer for continuous sample concentration. By absorbing the sample fluid into the porous structure, the evaporator is capable of achieving volumetric heating of the sample fluid thus enhancing the solvent evaporation while the porous network ensured efficient vapor venting. Moreover, the thermal resistance of porous layer shields the heat transfer towards the bulk fluid therefore prevented the sample solution from overheating during the operation process. The results demonstrate the designed microfluidic evaporator exhibits good sample concentration capability under continuous flow conditions. [ABSTRACT FROM AUTHOR]

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]

• Proposed device produces monodisperse droplets massively, confirmed experimentally. • The device is based on three-dimensional axisymmetric fractal-structure. • The impact of machining deviation on flow rate and droplet size is analyzed. • Obtained relationships: mass-flow, droplets' size deviation and device outlet size. In this paper, a three-dimensional axisymmetric fractal structure device was proposed for massive production of monodisperse droplets. Its channel number could increase to nm for an m -stage fractal device with n -channel distributors. A fractal device with 36 channels was constructed, and the droplet generation experiment was carried out. The experiment showed that the coefficient of variations (CVs) of the generated droplets were all lower than 4.0% at four different flow rates. Additionally, the relationship between the mass flow rate and fractal structure variation was examined through theoretical and data regression analysis. To elaborate the overall structure deviation, the accumulated deviation was introduced, the formula of accumulated deviation and droplet size was given. The results showed that the droplet products were highly monodisperse. The droplet size deviation was within 2% when the accumulated deviation of dispersed- and continuous-phase distributors was less than 4% and 10%, respectively. [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]