Study On Characteristics Of The Photothermal Effect Induced Two-phase Flow And Heat Transfer In Microchannels

Microfluidics integrates the functions of mixing, separation, purificationand detection together, enabling precise, automated manipulation of tiny volumes of fluid. Because of a variety of advantages in that they can reduce sample and reagent volumes, shorten reaction times, provide high-throughput, automation and low cost, microfluidic systems are regarded as valuable instrumental platform for chemical and biological applications. More recently, the incorporation of modern optics into microfluidics creates a new area of optofluidics, enabling precise non-contact manipulation of fluids since the light can be easily focused to a tiny spot with microscale or even nanoscale. The photothermal effect is one of the important interactions in optofluidics, which can directly control fluid, having a simple system and low lost. Since the microfluidics based on the photothermal effect have a wide foreground and a good developing potence, it has attracted much attention. However, the main research is focused on the realization of the principle and the function of the device, the characteristics of the microcfluidics based on the photothermal effect still remain unclear. A deep understanding of its underlying mechanism is required for better designing and developing the optofludic devices. As a result, the characteristics of the mass and heat transfer caused by the photothermal effect in microchannels will be investigated in this work.In this work, the numerical simulation and visualization experimental method were used to study gradually the photothermal effect induced phase change micropump which mainly include the dynamic behaviors of coalescence and evaporation. Firstly, the moving liquid column coalescing with a sessile droplet in microchannel is simulated. Particular attention is paid to the dynamic interfacial phenomena during the coalescence and the effect on advancing the water-air interface. In addition, effects of the wettability, the sizes and shape of the microchannel and droplet size as well as the position on the coalescence behaviors and liquid column movement are also studied. Then the the dynamic behaviors of the liquid column coalescing with multi-droplets were visually investigated. The effects of droplet quantity and position, inlet pressure, and microchannel size were alsoexplored. Additionally, the effect of the multi droplets arrangement on the coalescence was investigated by numerical method. Then the photothermal effect of an infrared laser induced evaporation and Marangoni convection of a liquid column in microchannels is numerically studied. The main foucses are on the characteristics of the heat and mass transfer in the process of the evaporation both for the steady and unsteady situation. In the end, the photothermal effect induced phase change driven the fluid in microchannel will be investigated systematically by the visual experiments. The main conclusion obtained in this work are as follows:The coalescence between the moving liquid column and a single droplet in the rectangular and triangular microchannels using the volume of fluid(VOF) method and the coupled level set and volume of fluid(CLSVOF) model. It is found that the coalescence can accelerate the original liquid column movement due to the formation of the lager-curvature meniscus at the interface induced by the coalescence, which lowed the pressure in liquid column and increases the capillary pressure. The velocity increment ratio due to the coalescence is relevant to the viscous force and the surface free energy from the droplet. Simulation results reveal that the velocity increment ratio increases with the contact angle in hydrophobic microchannels but it is reverse in the hydrophilic microchannels both in rectangular and triangular microchannel. The smaller channel size, head pressure and distance between the droplet and inlet, and larger droplet size exhibit a larger acceleration rate as a result of induced lower viscous effect and greater surface free energy. As compared to the rectangular microchannel with the same hydraulic diameter, the triangular microchannel exhibits smaller velocity increment ratio because of stronger viscous effect.For the experiments of the liquid column coalescence with the droplets which were generated by photothermally inducedevaporation and condensation in microchannels, the MFCS was used to supply water into the microchannel with controllable inlet pressure and monitor the flow rate to get the variation of the liquid flow velocity due to the coalescence. The results show that during the coalescence process, the interface had dramatic continuous changes in a very short time. Accordingly, the coalescence has yielded a large velocity increment ratio as a consequence of lowered liquid pressure at the interface. In addition, when the front interface of the liquid flowwas in contact with droplets, air bubbles were easily entrapped in the liquid flow and then attached on the wall as a result of the hydrophobicity of the PDMS nature. It should be mentioned that in three times repeated experiments, though the distribution of droplets was relatively random, the values of the liquid flow velocity appeared almost the same. The simulated results for the effect of droplets arrangement on the coalescence also confirm this view.For the simulation of the photothermal effect of an infrared laser induced evaporation, the volumetric Gaussian heat source was used to model the laser beam and the influence of the mainstream flow on the interface shape and the change of the vapor pressure neat the interface were negligible. The results show that the Marangoni convection induced by the laser non-uniform heating increase the heat transfer coefficient near the interface. The evaporation mass flow rate in the hydrophilic microchannel is greater than that in the hydrophobic microchannel due to the small thermal resistance. The direction of flow patterns in liquid column are totally different since the relative magnitudes of the horizontal and vertical temperature gradients are opposite for these two surfaces. It is also found as the laser power increases, the evaporation mass flow rate, evaporation heat radio and strength of the Marangoni convection are all increased because more heat can be generated. However, the sizes of the convection rolls are almost the same. Regarding the laser spot position, the evaporation mass flow rate and evaporation heat radio both decrease with increasing the distance between the laser spot and interface because of increased thermal resistance. Moreover, the laser spot size had little influence on the both evaporation mass flow and Marangoni convection.For the dynamic behavior of the evaporation of the continuous liquid column with free surface under laser heating in microchannel, the simulated works have taken into account the change of the vapor pressure and the interface configuration. The results show that for the stationary liquid conlum, the interface temperature is increased with time, yet the evaporating heat transfer coefficient and mass transfer coefficient are decreased and the evaporation mass flow rate decreasesfirstly and then keeps constant. The magnitude of the vertical temperature gradient is greater than the horizontal one. When the liquid comlumn flow is drivenby the pressure head, the vapor mass transfer is mainly deponds on the convective way and the mass transfer coefficient was relatively larger than the stationary liquid column interface temperature which is positively related to the speed of liquid column. The evaporating heat and mass transfer coefficient are both increased and then decreased. Even the interface temperature is lower than the temperature when the liquid conlumn is stationary at the same time, the evaporation mass flow rate is geater. The maximum evaporation mass flow rate appeared when the liquid column velocity was reached a peak. The Marangoni convection enlarges the pressure near the interface, hence it products the hindrance effect on the liquid column moving forward.For the photothermal effect induced phase change driving the continuous liquid column in microchannel, the visualization experimental method and image processing technique were used to investigate the velocity of liquid column and the droplets condensation behavior. The results show when the laser spot position is unchanged, the velocity of liquid column is decreased with the forward interface, and then oscillateduntil it becomes zero at last. This is because the more and more sparse distribution of the condensate droplets at the front of the interface due to the reducing evaporation rate and the droplets which can coalesce with the liquid column need be bigger size. The length of the liquid column moving is increased with increasing the laser power. When the laser spot is moved with the interface, the liquid column velocity increases firstly and then be constant. The smaller distance between the laser spot size and interface and the larger laser power exhibite the denser condensate droplets with smaller size and the larger actuating speed.