Study On Characteristics Of The Two-Phase Flow And Heat Transfer In The Droplet Manipulation By The Photothermal Effect

Microfluidics integrates the multiple functions of mixing,separation,dilution,reaction and detection and so on,into a chip with the area from several to dozen square centimeters,which provides several advantages of small sample and reagent volumes,short reaction time,high resolution and sensitivity,easy integration and low emission.In particular,as one of the most important branches of microfluidics,the droplet microfluidics offers additional advantages,such as the reduction of the surface inhibition,no diffusion of the sample and avoidance of the cross-contamination,high throughput operation and so on.Hence,the droplet microfluidics is promising in the applications of biomedicine,environmental monitoring,chemical analysis,drug diagnosis and other fields.The droplet manipulation is the key step for droplet microfluidics,which can be realized by several approaches.Among them,the droplet manipulation based on the photothermal effect possesses the outstanding features,including high sensitivity,fast response,accurate control and various functions.However,existing works are mainly focused on the function realization of the photothermal effect based microfluidic devices.The underlying mechanism of the two-phase flow and heat transfer remains unclear,but which plays an important in its performance.For these reasons,this thesis is to study the two-phase flow and heat transfer characteristics of the photothermal effect based droplet manipulation,whose background relies on the photothermal effect based droplet microfluidics for the Polymerase Chain Reaction(PCR)applications.The obtained results will provide a foundation for the design and optimization of this system.The photothermal effect based droplet microfluidics for the PCR applications usually covers several steps,including the droplet formation,droplet migration and coalescence,temperature control,etc.In this thesis,the two-phase flow and heat transfer behaviors in the photothermal effect based droplet microfluidics are studied by the visual experiments and numerical simulations.Firstly,the characteristics of the droplet formation in a cross-junction microchannel is studied using the visualization experiment,and then the characteristics of droplet formation manipulated by the photothermal effect of an infrared laser is explored.The effects of the laser power,spot position and flow rates on the droplet formation are also investigated.Secondly,the migration and coalescence of the droplets driven by the photothermal effect are visually investigated,and the effects of the laser power,spot position and droplet size are explored.Thirdly,the coalescence of two droplets with different temperatures in the oil phase,which is a key process in the droplets coalescence driven by the photothermal effect,is simulated using the volume of fluid(VOF)method.Effects of the temperature difference,oil viscosity,oil conductivity,droplet size and surface-tension temperature coefficient on the dynamic flow and heat transfer are analyzed.Fourthly,the thermocapillary flow and heat transfer behaviors of the water droplet suspended in the oil phase and heated by an infrared focused laser are studied using the simulation method;and the effects of the laser power,spot size,the conductivity and viscosity of the oil,and the surface-tension coefficient are investigated.Fifthly,the photothermally induced two-phase flow and heat transfer characteristics of the water droplets in the microchannel are studied and the effects of the laser power,spot position,spot size,thermal conductivity and viscosity of the oil,and surface-tension temperature coefficient are explored.At last,a new type droplet microfluidic system for the PCR application is designed,where the droplet is fixed by the anchor and heated by a focused laser.The heat transfer characteristics of the droplet is analyzed.The main conclusions obtained in this thesis are presented as follows:The droplet formation in a cross-junction microchannel and manipulated by a focused infrared laser is visually investigated.The results show that the droplet volume reduces with the increase of the continuous phase flow rate and the decrease of the disperse phase flow rate.When the focused infrared laser is projected at the front interface of the droplet during the shedding process,the laser induced thermocapillary flow retards the breakage of water phase,leading to the increase of the droplet volume.The increase ratio of the droplet volume is increased with the increase of the laser power.When the laser is projected at the location far away from the interface,the viscosity of the disperse phase is reduced due to the increase of the temperature,which results in small shear force to the water and thereby the reduction of the droplet volume as compared to the case without the laser.The manipulation of the droplet migration and coalescence by a focused laser is studied using the visualization experiment.Results indicate that when the laser spot position is fixed,the droplet migration can be actuated by the photothermally induced thermocapillary flow.The initial moving velocity of the droplet is increased with increasing the distance between the laser spot and the droplet center.During the migration process,the moving velocity is firstly increased.When the back interface of the droplet is near to the laser spot,the moving velocity starts to decline.It is found that the moving velocity increases with increasing the laser power and reducing the droplet size.In addition,it is also found that the laser induced droplet migration can lead to the droplet coalescence by the squeeze.Aiming at the coalescence of two droplets with different temperatures encountered in the photothermal effect induced coalescence,the volume of fluid(VOF)method is employed to study the characteristics.The results show that the coalescence of two droplets with different temperatures lead to a self-propelled migration of the coalesced droplet caused by the pressure difference and thermocapillary flow triggered by the temperature gradient.When the surface-tension temperature coefficient is negative,the migration direction of the self-propelled droplet will reverse due to the shift of the high temperature region triggered by thermocapillary flow within the droplet.However,the migration direction of self-propelled droplet remains unchanged when the surface-tension temperature coefficient is positive.Besides,the travel distance of the self-propelled droplet is increased with the decrease of the oil phase viscosity and thermal conductivity.Increasing the droplet size and the surface-tension temperature coefficient leads to the reduction of the migration distance.The two-phase flow and heat transfer of a droplet suspended in the oil phase and heated by the photothermal effect are studied using the numerical simulation.It is shown that the droplet temperature rise exhibits good linear relationship with the laser power.The temperature distribution inside the droplet and the response time become more uniform and shorter,respectively,as the laser spot size increases;and the numbers of Nu and Ma are reduced with increasing the laser spot size.The Nu number and Ma number are firstly increased and then reduced with increasing the thermal conductivity of the oil phase.As the dynamic viscosity of oil phase increases,the temperature rise increases,the temperature distribution inside the droplet becomes more uniform,and the Nu number and Ma number become smaller and larger,respectively.The two-phase flow and heat transfer of the droplet in the microchannel heated by a focused laser are numerically studied.Results show that the temperature within the droplet facing the downstream region is higher than that facing the upstream region due to the heat transfer to the oil phase.Higher laser power leads to stronger thermocapillary flow and higher droplet temperature.The droplet temperature rise exhibits good linear relationship with the laser power.Larger laser spot size leads to lower droplet temperature rise and more uniform temperature distribution and weaker thermocapillary flow.The effects of laser power and laser spot size on the heat transfer coefficient are weak and the greatest difference is less than 3%.When the laser spot moves to downstream,the heat transfer coefficient and Nu number are both reduced,and the Ma is firstly increased and then reduced.The heat transfer coefficient exhibits good linear relationship with the velocity of oil phase;and both of the Ma number and the temperature rise are reduced with increasing the velocity of oil phase.A novel photothermal effect based droplet microfluidic system with the anchor for the PCR application is proposed.Thermal analysis is performed to demonstrate the feasibility of the proposed microfluidic system.Results indicate that uniform temperature distribution within the droplet can be achieved through the selection of the carrying phase with suitable thermal conductivity.The droplet temperature rise is linear with the laser power.Rapid temperature response is achieved.It is indicated that accurate temperature control can be realized by adjusting the laser power.When the laser is applied,the proposed system can finish one PCR thermal cycle in 28.6 s,and 48 cycles in less than 23 min,which can save more than half of the operating time as compared to traditional PCR microfluidic chips.