Photothermally Induced Phase Change And Two-Phase Flow Characteristics In Microchannels

Microfluidics has been rapidly developed in the past few decades.Unlike traditional laboratory,several functions such as sample preparation,controlled reaction,analysis and detection can be integrated into a tiny chip with the area of only a few square centimeters by microfluidic technologies.Due to its intrinsic advantages of low reagent consumption,fast response,high sensitivity,microfluidics has been widely applied in the fields of biochemical research,life science,medical detection,etc.Recently,optofluidics,as the fusion of traditional microfluidics and modern optics,realizes a series of novel functions on the microfluidic platform through the interactions between fluid and light.As one of the most commonly utilized interactions,photothermally induced phase change of fluid is frequently applied in optofluidic devices for fluid manipulation due to its outstanding features,including local control,fast response,simple operation,etc.However,present works mainly focus on the device design and function implementation,the underneath mechanism of phase change and two-phase flow still remains unclear.To address this issue,a comprehensive experimental study was carried out to investigate the photothermally induced phase change and two-phase flow characteristics and interfacial behaviors in the microchannel in this thesis.The obtained results can lay a solid foundation for the design and performance enhancement of the photothermally based new type optofludic devices.The photothermally induced phase change and two-phase flow is a complex process,covering the phase change,two-phase flow and interfacial phenomenon.To investigate this multi-physics coupled problem,a focused laser heating on a liquid column in the microchannel was firstly studied.Effects of laser power and spot position,microchannel geometric construction and surface wettability were investigated.The results revealed that the phase change behaviors in the microchannel exhibited two basic modes: micro pump mode and chemical separation mode both based on the photothermal effect induced phase change.Based on this point,these two in-channel phase change modes were studied,respectively.For the micro pump mode,the characteristics of the phase change actuation by a moving laser spot was studied.The influence of the laser power and laser spot moving speed on the fluid flow velocity was also studied.The relationship between the maximum fluid flow velocity and laser heating condition was revealed.Besides,the actuating characters in microchannels with complex structures were also investigated.Based on the photothermal fluid actuation,an in-channel pulsating flow actuation method was proposed to simulate the oscillating biochemical signals in the in-vitro environment.In the smoothed microchannel,square waveforms were triggered by controlling the laser switching and motion.In the microchannel with sawtooth-shaped baffles,triangular waveforms with high pulse intensity and high signal-noise ratio were triggered.Effects of the heating condition and baffle design were investigated.For the chemical separation mode,the phase change characteristics of a single slug and bubble assisted slug in the microchannel were studied,respectively.Effects of the flow pattern,heating condition,fluid type were investigated.Finally,an optofluidic membrane chemical separator was designed for effective chemical separation in the microchannel,the separation performance of the designed separator was evaluated under different operation conditions.The main outcomes of this thesis are summarized as follows.The phase change behaviors induced by a focused laser as a local heating source in the microchannel exhibited two basic modes,i.e.the micro pump mode and the chemical separation mode.When the laser power was relatively low and the spot positon was relatively far away from the interface,the phase change mode tended to be micro pump mode as a result of the photothermally induced evaporation-condensation-coalescence process.When the laser power was relatively high and the spot position was near the interface,the phase change mode tended to be chemical separation mode as a result of the photothermally induced evaporation-condensation process.It was found that with the increase of the depth-width ratio of the microchannel,the interface advancing speed was slowed down under low laser power,and the condensed slug could hardly be formed under high laser power.Besides,air bubbles were easily entrapped into the liquid body at large depth-width ratio,which greatly influenced the stability of the phase change process.The change of the surface wettability changed the phase change behaviors in the microchannel.When the laser power was relatively low,the interface was advanced via the coalescence in the hydrophobic microchannel,while the interface continuously receded in the hydrophilic microchannel.When the laser power became relatively high,the formation of the condensed slug was observed in the hydrophobic microchannel,while the interface still receded in the hydrophilic microchannel with faster speed.By controlling the focused laser to move with the interface along the microchannel,the fluid could be actuated to continuously flow forward in the microchannel.At a certain laser power,increasing the moving speed of the spot could lift the flow speed of the liquid,and the maximum moving speed could be lifted by increasing the laser power.Through the change of the spot moving trajectory,the local phase change intensity could be adjusted to realize the manipulation of the flow direction in the complex microchannel networks.In the microchannel with irregular section area,the photothermally induced phase change exhibited the ability of driving fluid flow across the obstacles.A pulsating flow generation method was presented based on the photothermally induced phase change.In smoothed microchannel,the pulsating flow with a square waveform could be triggered by periodically changing the laser operation mode including power switch and motion control.By controlling laser power and the mobile-immobile interval time,the flow rate waveform could be adjusted.In the microchannel with sawtooth-shaped baffles,triangular pulsating flow with intensive peak flow rate and high signal-noise ratio could be triggered by the photothermally induced phase change.The peak flow rate and pulse frequency could be adjusted by changing the input laser power and the geometric constructions of the baffle design.By utilizing a focus laser continuously heating on the liquid column,in-channel chemical separation could be achieved through the evaporation-condensation phase change process.By continuous laser heating,the saline solution could be supersaturated and the solute crystal could be separated,and the evaporation rate of saline solution was lower than that of pure water.It was found that the increase of the laser power and decrease of the distance between the spot and interface could lift the evaporation and condensation rate,by which the separation was enchanced.The bubble-assisted flow pattern provided the individual hot/cold interfaces for chemical separation,which was able to prevent the mixing of separated solvent and the solution.However,the evaporation was weakened due to the condensation slug.An optofluidic membrane chemical separator based on the photothermally induced phase change was designed for efficient chemical separation in the microchannel.When the separator was operated by an IR laser,the separation performance could be enhanced by increasing the laser power and solution concentration or decreasing the flow rate.After coating Ag nanoparticles on the wall surface of separation chamber,the separator could be operated with 532 nm visible laser.By doing this,the selective absorption to the IR laser of the working fluid could be eliminated,and the performance of the separator could be further boosted.