A Study Of Impact Dynamics Of Droplet On The Rotating Substrate

The phenomenon of droplets impacting on solid surface is ubiquitous in nature,and is widely used in various industrial fields such as aerospace,biomedicine,and electronics manufacturing.At present,the spin-coating method is widely used in many research fields such as thin film solar cell fabrication.Dynamic spin coating is essentially a phenomenon of droplets impacting a rotating surface.On the other hand,silver nanowire is a research topic of intense study in the field of flexible transparent electrode because of its good conductivity and light transmittance.In this thesis,the dynamic process of droplets impacting on the solid surface is the key research object.The process of droplets impacting the rotating surface is studied experimentally.At the same time,by adding silver nanowires(Ag NWs)into the droplet,the droplet impact process is related to the directional arrangement of silver nanowires.The main work and conclusions are:(1)Build a visualization system for the droplet impact process,and observe the droplet impact process from the side and top views.For the case where the droplet impact the rotating surface,it is found that the droplet will spread unevenly and produce unstable fingering pattern if the impact point is slightly out of center.Once concentric alignment has been improved,a single droplet can spread evenly upon impacting on the rotating substrate.This work inspires a single drop spin coating method,which can reduce material waste by up to 90%.(2)The effects of Weber number and rotation speed on the droplet spreading on the rotating substrate are explored.It is found that the spreading process can be divided into two stages: the impact-spreading and the centrifugal-spreading.The Weber number mainly affects the initial impact-spreading stage,and the rotation speed mainly affects the centrifugal-spreading stage.It is also found that the diameters of the film rim first increase to a peak value and then decrease.The larger the weber number,the smaller the peak value of the rim diameters.(3)The arrangement of the silver nanowires after the impacting process is investigated.It is found that the silver nanowires on the substrate tend to be arranged in the radial direction for droplets impacting on both the fixed and the rotating substrate.When impacting on the fixed surface,due to the retraction of droplets after maximum spreading,the alignment of Ag NWs may be spoiled and become irregular.For droplets impacting on the rotating surface,there are obvious radial alignment phenomena because no recoil is involved.This suggests the Ag NWs response to the flow field inside the droplet to a certain extent.The numerical simulation using a commercial multiphysics software package COMSOL is carried out to compare the process of droplet impact on the stationary substrate.