Study On Dynamic Characteristics Of Toroidal Droplets On A Substrate

Flow on a solid substrate with a toroidal free interface is a cross-scale multi-field coupling problem with a wide range of applications in 3D printing of bionic structures.The morphology is far from meeting the needs of most practical applications due to its structural instability,randomness and difficulty in maintenance.Based on the application of 3D printed bionic structures on solid substrates,and the widespread application prospects demonstrated by annular droplets in practical applications as the background,this topic investigates the ring formation mechanism,fracture mechanism,and takes the dynamic characteristics of annular droplets on substrates as the research object and provides technical support and theoretical basis for 3D printed bionic structures on solid substrates.For the theoretical study,the definition of toroidal droplets on a flat plate is introduced,followed by the derivation of the equations for the surface area and volume of the liquid ring through wetting and surface energy.The fracture mechanism of the annular droplet on a flat plate is analyzed,the shrinkage instability of the annular droplet is explained by Laplace’s equation,and the shrinkage characteristics of the liquid ring are analyzed by the surface free energy.A theoretical explanation is provided for the instability phenomenon of annular dropletsFor the experimental study,we designed and built an experimental platform for the preparation of annular droplets on flat plates,which mainly includes a substrate preparation device,an annular droplet preparation device on flat plates and a UV light curing device.The substrate preparation device is used to prepare PDMS films on plastic discs;the annular droplet preparation device is used to prepare annular droplets on flat plates;the light curing device is used to light cure the annular droplets on flat plates to realize the controlled preparation of solid annuli on flat plates.In the experimental study of the ring formation mechanism of annular droplets on flat plates,the substrate has a significant effect on the ring formation effect of annular droplets on flat plates,and the plastic substrate has a better effect than the copper substrate and the glass substrate.When varying the rotation speed of the substrate,only when the injection speed is high enough and the rotation speed is large enough,the annular droplets with nearly circular shape can be prepared.With the increase of PDMS film,the annular droplets on the substrate are less likely to split.Regarding the fracture characteristics of the annular droplets on the plate,the number of splitting of the annular droplets on the plate increased gradually with the increase of the R₀/a₀,and the annular droplets on the plate were similar to the annular droplets in liquid-liquid,and the number of broken annular droplets on the plate was linearly related to the R₀/a₀ for the PDMS substrate with the film thickness of 380μmn≈0.465R₀/a₀.In the experimental study of the dynamic characteristics of droplets on a flat plate,the initial aspect ratio of the annular droplets decreases as the volume increases,leading to a decrease in the number of annular droplets splitting,but a gradual increase in the time required for the final stabilization of the annular droplets.As the surface tension of the solution decreases,the initial aspect ratio of the annular droplet on the plate decreases slightly,and unlike the case in the liquid-liquid phase,the time for the annular droplet on the plate to reach the fastest instability increase rate and to reach the final stable state is shortened.It was found that viscosity has a significant effect on the break-up time of the annular droplets on the plate,and viscosity accelerates the break-up process.In practical application,UV light curing technology is combined with the preparation technology of annular droplets on the substrate,and solid rings with different ring widths and heights are prepared by controlling the initial volume of annular droplets on the plate,which lays a theoretical foundation for the controllable preparation of solid rings on the plate,and provides a guiding significance for the application of 3D printing bionic structure in the biomedical field.