Characteristics Of Mass And Heat Transfer And Interface Dynamics In Droplet Evaporation Actuated By Photothermal Effect

The open droplet microfluidics can manipulate mini discrete droplets in a small open space,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.The open droplet microfluidics has been applied to many fields including clinical medicine,chemistry,materials science and analytical chemistry and so on,as it offers not only the advantages inheriting from conventional microfluidics,including mini volume of the 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 the 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 merits including remote control,tunability of the intensity,prominent spatial and temporal resolution.However,the research on the photothermal effect-induced phase change of the droplet on the substrate in an open space is still limited and the underlying mechanism of the heat and mass transfer as well as the interfacial behavior in the photothermally heated droplet remains unclear.To address the issue,a systemic study has been carried out to investigate the photothermally induced phase change of the droplets.The aim of this thesis is to reveal the characteristics of the heat and mass transfer and the interface dynamics of the photothermally induced droplet evaporation,which can provide the theoretical basis for the design and optimization of novel photo-actuation droplet analysis/detection chips.The photothermally induced phase change of the droplets is a complex process of the heat and mass transfer and interfacial behaviors,involving the light-heat conversion,phase change,the Marangoni flow,the gas phase flow near the interface,the interface behaviors and their couplings.The most significant feature lies in the localized laser heating.The phase change process is affected by the laser power,laser spot position,substrate wettability,droplet volume and fluid properties and so on,which is rather complex.In this thesis,the laser-induced phase change of the droplets on the hydrophobic and superhydrophobic surfaces is first studied.The non-uniform temperature distribution and the evaporation rate are acquired.Effects of laser power,droplet volume,surface wettability and microstructures are investigated.Secondly,the laser-induced phase change on the hydrophilic surface is investigated and a method for excellently manipulating levitated femtolitre/picolitre droplets by a laser beam is developed.The influence of the interface morphology on the vapor flow near the interface is explored.The preconditions for the creation and maintenance of the levitated droplets are analyzed.In particular,the extraordinary motility of the levitated droplets is investigated.In addition,laser-induced non-uniform temperature field induces the Marangoni flow.The effects of surface wettability,laser power,particle concentration and particle size on the Marangoni flow and the particle deposition patterns are investigated.The light-induced Marangoni flow could inhibit the“Coffee-ring effect”,resulting in the particle enrichment at the specific position.Next,for the droplet-based chemical separation,the laser-induced rapid phase change and the shrink of the triple-phase contact line on the smooth hydrophobic surface are utilized to achieve the chemical separation of NaCl in droplet.Effects of laser power,droplet volume,working medium and solute concentration are investigated.Finally,a photo-actuation droplet analysis/detection chip based on the interface dynamics of the laser-heated droplet is proposed to implement the directional migration,coalescence,mixing enhancement and screening.Effects of the operating conditions,such as laser power,laser position,droplet size and surface wettability on the performance of the photo-actuation droplet analysis/detection chip are also investigated.The main conclusions of this paper were as follows:The laser-induced phase change of the droplets on the hydrophobic and superhydrophobic surfaces is studied.The interface behavior of the original droplet was significantly affected by the interface reforming between the condensed droplets formed by the localized heating source-induced non-uniform temperature field and the original one.On the hydrophobic substrate,the droplet interface temperature rapidly increased,resulting in the fast evaporation.In the initial stage,the triple-phase contact line briefly extended by the coalescence between the condensed droplets and the original one,and then withdrew with the CCA mode until it disappears.By adjusting the laser power,the interface temperature and the evaporation rate could be precisely controlled.On the superhydrophobic substrate,the Cassie-to-Wenzel transition accompanying with the obvious extension of the triple-phase contact line was actuated by the continuous condensation in the microstructures of the superhydrophobic surface and coalescence between the condensed droplets and the original one.The extension process was accelerated as a result of the intensified light-caused evaporation.Noticeably,it was demonstrated that the dimensionless final migration distance under different conditions were similar.The laser-induced phase change of the droplets on the hydrophilic surface is studied.As the laser heating went on,the original droplet on the hydrophilic surface briefly extended and then evaporated with the CCR mode until they disappeared.In particular,the unique light-induced vapor flow in association with the interface morphology with the small contact angle was responsible for creation and manipulation of levitated femtolitre/picolitre droplets.However,the already-formed levitated droplets could be stably sustained with the low laser power and the hydrophobic interface morphology.The levitated droplets formed by this method showed extraordinary motility because of the bonding effect of the light-dominated vapor trap.The highly-accurate two-dimensional labyrinth movement of the levitated droplets with designed trajectories above the free surface was easily realized by scanning the light.The experimental results showed that the critical velocity of the droplet reached 100 times of its radius.The influence of the photothermal effect-induced Marangoni flow on the particle motility and deposition pattern are investigated.The localized heat source-induced non-uniform temperature field along the droplet interface resulted in the intense Marangoni flow in the droplet.Higher the laser power,lower the surface tension of the water.Therefore,the direction of the light-induced Marangoni flow along the interface was from the laser position to the triple-phase contact line,and that inside the droplet was from the triple-phase contact line to the center and then to the laser position at the droplet interface.On the hydrophobic substrate,the combination of the light-induced Marangoni flow and the continue retreat of the triple-phase contact line leaded to the final concentration of the particles at the laser position,forming the particle enrichment which was significantly different from the“Coffee-ring effect”.On the hydrophilic surface,the Marangoni vortex containing many particles gradually breaks down in the last stage of droplet evaporation,resulting in the particle deposition including a particle enrichment at the specific area and a particle ring at the triple-phase contact line.The photothermal effect-induced evaporation of the NaCl solution on the smooth hydrophobic surface are investigated.By utilizing a focus laser to continuously heat a droplet deposited on the hydrophobic surface,the droplet-based on-site chemical separation could be achieved by the phase change and the continuous retraction of the triple-phase contact line.With the mass loss of solvent of the droplet,the solute concentration of NaCl gradually increased to be supersaturated and then induced the crystallization.At the same time,due to the inclusion of droplet interface,the crystal of NaCl finally concentrated in the laser position.The laser power could precisely control the separation rate of NaCl solute,enabling the rapid separation of trace NaCl solute?3.4mmol/L?.Comparing with the natural evaporation,the photothermally chemical separation rate increased more than 30 times.Moreover,it was proved that the chemical separation process in the droplet can be applied to measure the initial solute concentration.A hydrophilic patterns assisted optical droplet-based reaction/analysis platform was developed,which enabled the high-throughput,efficient,flexible multi-step droplets migration,coalescence,mixing and sorting by the photo-induced Cassie-to-Wenzel transition and Marangoni flow.Through the asymmetric evaporation-condensation caused by the asymmetric temperature field generated by the off-center laser heating,the directional extension of droplet interface and the coalescence of the droplets on the neighboring hydrophilic anchors were induced.Notably,the rapid mixing of the reagents was realized by the light-induced Marangoni flow inside the droplet.The coalescence temperature,the coalescence time,the coalescence sequence and the temperature of the merged droplet could be well controlled by the light power and light position.Finally,an ion detection by processing a chromogenic reaction using the developed droplet platform was presented as a proof of concept,in which the ferrous ions(Fe²⁺)in the droplet was detected by the precipitate of Prussian Blue?Fe₄[Fe?CN?₆]₃?.The detection range was84.0 mol/L-28.8 mmol/L...