Numerical Simulation Of Droplet Generation And Experimental Study On Sealing Surface Modification Of COC Chip

Droplet microfluidic technology is one of the important engineering technologies to realize absolute quantification of nucleic acid detection(digital PCR),and microfluidic control can not be separated from the support of chip laboratory,so the design and manufacture of microfluidic chip is the key to its further development.Microfluidic chips are fabricated by photolithography and injection molding,and then packaged to obtain a completely closed chip.The surface performance of the sealing surface is very important because it affects droplet generation,chip bonding and cell adhesion.Therefore,the modification of the sealing surface is of great significance to improve the chip performance.In this paper,the cyclic olefin copolymer(COC)microfluidic chip was taken as the research object,and the influence of wettability of microchannel wall on droplet formation was predicted by numerical simulation.Then,the surface modification was carried out by plasma to complete the experimental verification,and the variation rules of droplet formation,fluid wetting and bonding strength before and after modification were studied,which provided technical reference for the preparation,application and performance optimization of microfluidic chip.The main work is as follows:(1)The flow behavior of water and fluorine oil in microchannels was numerically simulated by using the VOF multiphase flow model.The results show that the weaker the wettability of the chip wall,the easier it is to form water-in-oil droplets,and the enhancement of drag reduction characteristics on the wall,resulting in a 29.3%lower generation frequency and an 8.3%increase in diameter of droplets at 90°compared with 120°.With the increase of wettability,the interface between water phase and fluorine oil phase changes from convex to concave,and the formation of droplets changes from water-in-oil to oil-in-water at 30°;With the increase of capillary number of continuous phase,the influence of wall wettability on droplet formation decreases.(2)COC was modified by air/CF₄ RF discharge low temperature plasma,and the water contact angle of COC surface was changed from 90°±1.8°to 21°±1.5°and then to 120°±1.3°at room temperature.The optimal power and time for hydrophilic and hydrophobic water treatment were 70W-20s and 200W-100s,respectively.(3)The modified COC surface was characterized by contact angle measurement(CA),x-ray electron spectroscopy(XPS),scanning electron microscope(SEM)and Fourier transform infrared spectroscopy(FT-IR).The results show that the modification process is mainly the introduction of functional groups,supplemented by etching,which only modifies the surface and does not affect the deep structure of the material.Microscopically,it also shows that the characteristic size of the chip microstructure has not changed significantly before and after surface modification.(4)The modified COC chips with surface contact angles of 30°,90°and 120°are studied experimentally,and the experimental results are in good agreement with the simulation results.The results show that after air plasma treatment,the wettability of the chip is significantly improved,and the bonding strength is increased by more than6 times,but oil-in-water droplets are formed After being activated by air plasma and then treated by CF₄ plasma,the hydrophobicity of the chip recovered and surpassed its intrinsic value,and water-in-oil was regenerated.