Design And Fabrication Of Microfluidic Chip With Porous Photothermal Evaporative Separator And Its Phase-change Induced Concentration Performance

In the chemical production,materials synthesis and biochemical analysis,the target component concentration in the sampling solution greatly affects the accuracy of detection,synthesis and time cost.Before the detection and reaction,an increase in the target component concentration can improve the sensitivity,accuracy and reduce the reagent consumption.Photothermal effect induced evaporation owns the advantages of fast response,high sensitivity and so on,which exhibits promising potential in the field of microfluidics.However,the application of the photothermal effect induced evaporation into microfluidics for sample concentration still suffers from limited vapor transport and instability of two-phase flow in the microchannel.Therefore,the development of highly efficient photothermal evaporation layer and the design of new microfluidics for phase change induced concentration are critically important for improving the performance of the concentration caused by the photothermally induced evaporation.Toward the issues existing in current concentration driven by photothermally induced evaporation,a microfluidic chip with a porous photothermal evaporation membrane separator was designed and fabricated for sample concentration in this thesis.This chip consisted of a microchannel and a porous photothermal evaporation layer,i.e.,a solid wall of the microchannel was replaced by the porous photothermal evaporation layer,which directly faced the surrounding.When the light was irradiated to this evaporation layer,the heat generated by the photothermal conversion could increase the temperature of the solution,making the solvent evaporate.The generated vapor could directly transport to the surrounding via this porous photothermal evaporation layer.Such design can effectively avoid the issue of limited vapor transport and instability of two-phase flow in the microchannel.Obviously,the design and fabrication of the porous photothermal evaporation layer and its evaporation characteristics are the core of realizing this new type chip.As a result,the study using the porous photothermal evaporation layer as a clue was performed in this thesis.The outcome of this thesis are summarized as below.(1)Based on commercial carbon paper to function as the porous photothermal evaporation layer,a microfluidic chip with this layer as a membrane separator was designed and fabricated.The effects of the light intensity,liquid flow rate,sample concentration and porous layer thickness and microchannel depth on the performance were investigated.Experimental results indicated that the microfluidic chip with the porous photothermal evaporation membrane separator could effectively realize the evaporative concentration.In addition,it was found that the light intensity could greatly affect the heat generation by the photothermal conversion and thereby the solution temperature.Larger light intensity resulted in higher temperature,which enhanced the evaporation and thus improved the concentration factor.As for the liquid flow rate,small liquid flow rate allowed for a longer residence time,which increased the amount of the evaporated vapor and thus increased the concentration factor.The change of the sample concentration could affect the solvent evaporation.As the sample concentration increased,the evaporation rate of solvent decreased,making the concentration factor reduce.Increasing the thickness of the porous layer,the concentration factor increased.The increase of the microchannel depth led to a reduction of the concentration factor.(2)A PDMS porous film with high photothermal conversion was developed by using salt template method.Using this method,the pore structure of the PDMS porous film could be regulated by properly selecting the salt type as a template.The surface morphology,pore structure,light absorption,photothermal conversion efficiency and wettability were characterized.It was found that the light absorption of the PDMS porous film was three times as the commercial carbon paper in the wavelength of200-800 nm.In case the light with the intensity of 200 m W/cm²was irradiated to the PDMS porous film and carbon paper,the photothermal evaporation efficiency of PDMS porous film is nearly 30%higher than that of commercial carbon paper,and the surface PDMS porous film has better light absorption performance and photothermal conversion performance.After the PDMS porous films were modified by plasma activation and PVA grafting,the films exhibited hydrophilic nature for a long time.(3)The prepared PDMS porous films were employed as the porous photothermal evaporation layer,experimental results showed that the performance of the microfluidic chip with the PDMS porous film as an evaporative membrane separator was better than that with commercial carbon paper.The effects of the pore structure of the PDMS porous film,light intensity,liquid flow rate and sample concentration were explored.It was shown that the thin layer structure on the surface of the porous material has a negative effect on the sample solution transport,and when the thickness of the porous material is larger with too small pore structure,the pore structure will lead to the increase of solute transport resistance from the porous layer to the flow channel,and reduce the evaporation rate of solvent molecules.When the PDMS porous film was fabricated by using C₆H₅Na₃O₇as the template,because it has large pore volume and size,which facilitated the mass transfer,the performance of the microfluidic chip with the PDMS porous film exhibited the optimal performance.Under the light intensity of200 m W/cm²and liquid flow rate of 5?L/min,the microfluidic chip could yield a concentration factor of nearly 4.Such performance meet the requirements in the fields of chemical production,materials synthesis and biochemical analysis and so on.