Numerical Study On Flow And Heat/mass Transport During The Sessile Droplet Evaporation Actuated By Photothermal Effect

Open droplet microfluidics refers to the manipulation of mini discrete droplets on a open substrate,which integrates various functions into an open chip with the surface area of only a few square centimeters to accomplish various biochemical reactions and analyses.Open droplet microfluidics has been applied to many fields including clinical medicine,chemistry,materials science,analytical chemistry and so on,as it offers not only the advantages inheriting from conventional microfluidics,including mini volume of reagents,precise control,high specific surface area and easy integration,but also the merits of convenient on-line analysis,excellent compatibility,no cross-contamination of reagents,etc.Recently,the ideas of the droplet manipulation in open droplet microfluidics via the interactions between the light and fluids have been demonstrated.Among the light-fluid interactions,the photothermal effect has exhibited the significant potential in the manipulation of droplet temperature field,evaporation rate,interface behaviors and internal flow due to its intrinsic features of fast response,non-contact,easy manipulation,prominent spatial and temporal resolution.However,current researches on the photothermal effect-induced evaporation of the droplet on the substrate in an open space are mainly focused on the phase change characteristics and interfacial behaviors.The underlying mechanism of the flow and heat/mass transfer in the photothermally heated droplet remains unclear.Therefore,numerical study on the photothermally induced phase change of the droplet as well as the accompanied flow and heat/mass transfer is performed in this thesis.The objective is to reveal the flow and heat/mass transfer mechanism and characteristics during the photothermal effect induced droplet evaporation,which can provide the theoretical basis for the design and optimization of novel droplet microfluidics based on the photothermal effect.The photothermally induced phase change of the droplet in accompany with flow and heat/mass transport is a complex process,covering the photo-thermal conversion,non-uniform temperature distribution,phase change,Marangoni flow,gas flow near the interface,heat and mass transfer and interfacial behaviors.In this thesis,the single-component droplet evaporation,flow and heat/mass transfer caused by the focused laser as a localized heat source was first numerical studied.Here,a numerical model describing the evaporation of a single-component droplet caused by the photothermal effect was created,by which the phase change rate,non-uniform temperature distribution as well as the induced flow and heat transfer were obtained.Then,the effect of the Marangoni flow induced by the localized photothermal effect caused non-uniform temperature distribution on the particle deposition was numerically studied.How the localized photothermal effect induced Marangoni flow manipulates the coffee-ring effect was analyzed.Finally,a numerical model describing the localized photothermal effect induced two-component droplet evaporation was established,with which the effect of the laser power and concentration and other parameters on the droplet evaporation was studied.Main conclusions of this thesis are as follows:(1)A transient model describing the localized photothermal effect(acting on the droplet center)induced single-component droplet evaporation was established,which accounted for the photothermal conversion,phase change,flow and heat/mass transfer of gas phase,internal flow and heat transfer.It was found that compared with the experimental data and numerical results obtained from pure gas diffusion model,the flow and heat/mass transport of gas phase under the localized photothermal effect showed significant effect on the droplet evaporation,which could not be ignored.Under the effect of localized photothermal conversion,the temperature gradient across the interface of the droplet could produce a strong Marangoni effect,which greatly enhanced the flow and heat transfer in the droplet.As a result,the temperature in the center of the droplet was reduced but the temperature at the edge of the droplet was increased.In this case,the Marangoni effect reduced the maximum interface temperature,which reduced the average droplet interface temperature and the evaporation rate.This effect became more obvious when the contact angle is larger.The study on the droplet evaporation behaviors under various conditions showed that the variations of the contact angle and droplet volume with time showed similar evolution behaviors under different laser powers,initial droplet volumes and relative humilities in the case of constant contact radius mode.A theoretical relationship correlating the contact angle and droplet volume with time during the photothermal effect induced evaporation of a droplet on the hydrophilic substrate under the constant contact radius mode was achieved.(2)Aiming at the evaporative deposition process of the droplets containing micro-nano suspended particles,a mixture model with unidirectional coupling method was developed to study the particle deposition characteristics.It was found that the non-uniform temperature field induced by the localized heat source formed a violent Marangoni flow inside the droplet,which could change the particle deposition behaviors and inhibit the "coffee ring effect".Because of the localized heating effect originated from the focused laser acting on the droplet center,the interface temperature was the highest at the droplet center and decreased along the interface to the edge of the droplet.Such a non-uniform temperature distribution made the fluid flow from the droplet center to the edge along the interface and then flow back to the center in the droplet and finally flow up to the interface at the center,which resulted in a dense deposition pattern.It was also found that there existed a critical transition power.When the laser power was lower than this critical transition power,the Marangoni flow in the droplet decreased with the increase of laser power,and the "coffee ring effect" was more obvious;when the laser power was higher than this critical transition power,the Marangoni flow in the droplet increased with the increase of the laser power,and the "coffee ring effect" was effectively suppressed.By adjusting the laser power and laser spot size,the Marangoni flow intensity,particle deposition time and pattern could be flexibly controlled.Increasing the laser power and reducing the laser spot size could enhance the Marangoni flow and thereby the particles concentration at the center.The deposition time could also be reduced.(3)The evaporation characteristics and interface behavior of the two-component droplets(ethanol-water and glycerol-water)heated by the localized photothermal effect were numerical studied.It was found that the localized photothermal effect had a significant influence on the evaporation and internal flow of the two-component droplet.The non-uniform heating mode caused by a focused laser led to a non-uniform temperature distribution at the interface,building up a strong thermocapillary convection.Meanwhile,because of non-uniform temperature distribution,the evaporation rate across the interface was also non-uniform,which could result in non-uniform concentration distribution and thereby the solute Marangoni flow.Both of them were able to form a uniform and stable circulation in the droplet.For the ethanol-water droplet,because the evaporation rate of ethanol was larger than water but its photothermal conversion was lower than water,the evaporation rate increased and the maximum interface temperature reduced as the ethanol concentration increased.As for the glycerol-water droplet,because glycerol has a high boiling point,it can be treated as a non-evaporative component.However,glycerol has a higher photothermal conversion than water.Therefore,as the glycerol concentration increased,although the interface temperature increased,the water evaporation rate from the droplet reduced.Besides,the variation of the surface tension during the evaporation of the ethanol-water droplet was significant,the dynamic wetting behaviors of the droplet need to be considered.The density and viscosity of the glycerol-water mixture was highly dependent on the glycerol concentration,which would greatly affect the internal flow and heat transfer inside the droplet.